Structural optimization of siRNA conjugates for albumin binding achieves effective MCL1-directed cancer therapy

The high potential of siRNAs to silence oncogenic drivers remains largely untapped due to the challenges of tumor cell delivery. Here, divalent lipid-conjugated siRNAs are optimized for in situ binding to albumin to improve pharmacokinetics and tumor delivery. Systematic variation of the siRNA conjugate structure reveals that the location of the linker branching site dictates tendency toward albumin association versus self-assembly, while the lipid hydrophobicity and reversibility of albumin binding also contribute to siRNA intracellular delivery. The lead structure increases tumor siRNA accumulation 12-fold in orthotopic triple negative breast cancer (TNBC) tumors over the parent siRNA. This structure achieves approximately 80% silencing of the anti-apoptotic oncogene MCL1 and yields better survival outcomes in three TNBC models than an MCL-1 small molecule inhibitor. These studies provide new structure-function insights on siRNA-lipid conjugate structures that are intravenously injected, associate in situ with serum albumin, and improve pharmacokinetics and tumor treatment efficacy.

Albumin-binding RNAi Conjugate for Carrier Free Treatment of Arthritis

Osteoarthritis (OA) and rheumatoid arthritis (RA) are joint diseases that are associated with pain and lost quality of life. No disease modifying OA drugs are currently available. RA treatments are better established but are not always effective and can cause immune suppression. Here, an MMP13-selective siRNA conjugate was developed that, when delivered intravenously, docks onto endogenous albumin and promotes preferential accumulation in articular cartilage and synovia of OA and RA joints. MMP13 expression was diminished upon intravenous delivery of MMP13 siRNA conjugates, consequently decreasing multiple histological and molecular markers of disease severity, while also reducing clinical manifestations such as swelling (RA) and joint pressure sensitivity (RA and OA). Importantly, MMP13 silencing provided more comprehensive OA treatment efficacy than standard of care (steroids) or experimental MMP inhibitors. These data demonstrate the utility of albumin 'hitchhiking' for drug delivery to arthritic joints, and establish the therapeutic utility of systemically delivered anti-MMP13 siRNA conjugates in OA and RA.

Editorial summary

Lipophilic siRNA conjugates optimized for albumin binding and "hitchhiking" can be leveraged to achieve preferential delivery to and gene silencing activity within arthritic joints. Chemical stabilization of the lipophilic siRNA enables intravenous siRNA delivery without lipid or polymer encapsulation. Using siRNA sequences targeting MMP13, a key driver of arthritis-related inflammation, albumin hitchhiking siRNA diminished MMP13, inflammation, and manifestations of osteoarthritis and rheumatoid arthritis at molecular, histological, and clinical levels, consistently outperforming clinical standards of care and small molecule MMP antagonists.

Core polymer optimization of ternary siRNA nanoparticles enhances in vivo safety, pharmacokinetics, and tumor gene silencing

Gene silencing with siRNA nanoparticles (si-NPs) is promising but still clinically unrealized for inhibition of tumor driver genes. Ternary si-NPs containing siRNA, a single block NP core-forming polymer poly[(2-(dimethylamino)ethyl methacrylate)-co-(butyl methacrylate)] (DMAEMA-co-BMA, 50B), and an NP surface-forming diblock polymer 20 kDa poly(ethylene glycol)-block-50B (20kPEG-50B) have the potential to improve silencing activity in tumors due to the participation of both 50B and 20kPEG-50B in siRNA electrostatic loading and endosome disruptive activity. Functionally, single block 50B provides more potent endosomolytic activity, while 20kPEG-50B colloidally stabilizes the si-NPs. Here, we systematically explored the role of the molecular weight (MW) of the core polymer and of the core:surface polymer ratio on ternary si-NP performance. A library of ternary si-NPs was formulated with variation in the MW of the 50B polymer and in the ratio of the core and surface forming polymeric components. Increasing 50B core polymer MW and ratio improved si-NP in vitro gene silencing potency, endosome disruptive activity, and stability, but these features also correlated with cytotoxicity. Concomitant optimization of 50B size and ratio resulted in the identification of lead ternary si-NPs 50B4-DP100, 50B8-DP100, and 50B12-DP25, with potent activity and minimal toxicity. Following intravenous treatment in vivo, all lead si-NPs displayed negligible toxicological effects and enhanced pharmacokinetics and tumor gene silencing relative to more canonical binary si-NPs. Critically, a single 1 mg/kg intravenous injection of 50B8-DP100 si-NPs silenced the tumor driver gene Rictor at the protein level by 80% in an orthotopic breast tumor model. 50B8-DP100 si-NPs delivering siRictor were assessed for therapeutic efficacy in an orthotopic HCC70 mammary tumor model. This formulation significantly inhibited tumor growth compared to siControl-NP treatment. 50B8-DP100 si-NPs were also evaluated for safety and were well-tolerated following a multi-dose treatment scheme. This work provides new insight on ternary si-NP structure-function relationships and identifies core polymer optimization strategies that can yield safe si-NP formulations with potent oncogene silencing.

Abstract 2706: Therapeutic silencing of Rictor using siRNA nanoparticles to selectively block mTORC2 signaling in triple negative breast cancer

Triple negative breast cancers (TNBCs) are aggressive malignancies with a high rate of recurrence even after neoadjuvant chemotherapy (NAC) and resection. Nearly 50% of NAC-treated TNBC patients harboring residual disease display mutations in the phosphatidyl inositol-3 kinase/mechanistic target of rapamycin (PI3K/mTOR) pathway, correlating PI3K/mTOR with chemoresistance. The protein kinase mTOR functions within two distinct protein complexes—mTORC1 and mTORC2. Inhibition of mTORC1 is ineffective in TNBCs. However, less is known regarding mTORC2 as a TNBC drug target, though mTORC2 is known to regulate tumor cell survival and motility/metastasis in other cancers. Here, we leverage siRNA technology to block expression of the mTORC2 obligate cofactor, Rictor, to test the hypothesis that selective mTORC2 inhibition will improve TNBC response to chemotherapy.

siRNA-carrying nanoparticle complexes (si-NPs) continue to face systemic siRNA delivery challenges such as limited stability, off-target toxicities, and suboptimal tumor accumulation. Most conventional si-NPs are designed as “binary” systems comprising the siRNA cargo and the polymeric carrier that encapsulates it. Ternary si-NPs containing siRNA, an NP core-forming polymer (50B), and an NP surface-forming polymer (20kPEG-50B) have the potential to improve tumor silencing activity because of the participation of both polymers in siRNA encapsulation and endosome disruptive activity. Through concomitant structure-function optimization of 50B core polymer ratio (50B:20kPEG-50B) and molecular weight, we identified multiple lead ternary si-NPs with potent gene silencing activity, enhanced siRNA loading stability, and minimal toxicity. Following intravenous (i.v) treatment in vivo, all lead ternary si-NPs displayed increased circulation half-life, tumor uptake, and tumor gene silencing relative to the more canonical binary si-NPs. Critically, a single 1 mg/kg i.v. injection of our optimized formulation, 50B8-DP100 si-NPs, resulted in 80% tumor Rictor knockdown in orthotopic MDA-MB-231 tumors in mice.

Therapeutic efficacy of 50B8-DP100 si-NPs, harboring siRNA against Rictor (siRictor-NPs) was then tested in models of TNBC. siRictor-NPs decreased cell growth and enhanced apoptosis in multiple TNBC cell lines. In an orthotopic HCC70 TNBC model, i.v. treatment with siRictor-NPs revealed substantial inhibition of mTORC2 activity in tumors by western analysis. siRictor-NP treatment also inhibited tumor growth to 52% relative to siControl-NP treatment. Preliminary in vitro studies further indicate that combining chemotherapy with siRictor-NPs improves TNBC cell killing over chemotherapy alone. Together, this work provides strong evidence for the therapeutic potential of Rictor knockdown and identifies a novel nanotechnology for the treatment of PI3K-active TNBC.

Citation Format: Shrusti S. Patel, Ella N. Hoogenboezem, Fang Yu, Alex G. Sorets, Fiona K. Cherry, Justin H. Lo, Nora Francini, Richard A. d'Arcy, Rebecca S. Cook, Craig L. Duvall. Therapeutic silencing of Rictor using siRNA nanoparticles to selectively block mTORC2 signaling in triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2706.

Structural Optimization of siRNA Conjugates for Albumin Binding Achieves Effective MCL1-Targeted Cancer Therapy

The high potential for therapeutic application of siRNAs to silence traditionally undruggable oncogenic drivers remains largely untapped due to the challenges of tumor cell delivery. Here, siRNAs were optimized for in situ binding to albumin through C18 lipid modifications to improve pharmacokinetics and tumor delivery. Systematic variation of siRNA conjugates revealed a lead structure with divalent C18 lipids each linked through three repeats of hexaethylene glycol connected by phosphorothioate bonds. Importantly, we discovered that locating the branch site of the divalent lipid structure proximally (adjacent to the RNA) rather than at a more distal site (after the linker segment) promotes association with albumin, while minimizing self-assembly and lipoprotein association. Comparison to higher albumin affinity (diacid) lipid variants and siRNA directly conjugated to albumin underscored the importance of conjugate hydrophobicity and reversibility of albumin binding for siRNA delivery and bioactivity in tumors. The lead conjugate increased tumor siRNA accumulation 12-fold in orthotopic mouse models of triple negative breast cancer over the parent siRNA. When applied for silencing of the anti-apoptotic oncogene MCL-1, this structure achieved approximately 80% MCL1 silencing in orthotopic breast tumors. Furthermore, application of the lead conjugate structure to target MCL1 yielded better survival outcomes in three independent, orthotopic, triple negative breast cancer models than an MCL1 small molecule inhibitor. These studies provide new structure-function insights on optimally leveraging siRNA-lipid conjugate structures that associate in situ with plasma albumin for molecular-targeted cancer therapy.

Combined Dusp4 and p53 loss with Dbf4 amplification drives tumorigenesis via cell cycle restriction and replication stress escape in breast cancer

AIM

Deregulated signaling pathways are a hallmark feature of oncogenesis and driver of tumor progression. Dual specificity protein phosphatase 4 (DUSP4) is a critical negative regulator of the mitogen-activated protein kinase (MAPK) pathway and is often deleted or epigenetically silenced in tumors. DUSP4 alterations lead to hyperactivation of MAPK signaling in many cancers, including breast cancer, which often harbor mutations in cell cycle checkpoint genes, particularly in TP53.

METHODS

Using a genetically engineered mouse model, we generated mammary-specific Dusp4-deleted primary epithelial cells to investigate the necessary conditions in which DUSP4 loss may drive breast cancer oncogenesis.

RESULTS

We found that Dusp4 loss alone is insufficient in mediating tumorigenesis, but alternatively converges with loss in Trp53 and MYC amplification to induce tumorigenesis primarily through chromosome 5 amplification, which specifically upregulates Dbf4, a cell cycle gene that promotes cellular replication by mediating cell cycle checkpoint escape.

CONCLUSIONS

This study identifies a novel mechanism for breast tumorigenesis implicating Dusp4 loss and p53 mutations in cellular acquisition of Dbf4 upregulation as a driver of cellular replication and cell cycle checkpoint escape.

Therapeutic MK2 inhibition blocks pathological vascular smooth muscle cell phenotype switch

Vascular procedures, such as stenting, angioplasty, and bypass grafting, often fail due to intimal hyperplasia (IH), wherein contractile vascular smooth muscle cells (VSMCs) dedifferentiate to synthetic VSMCs, which are highly proliferative, migratory, and fibrotic. Previous studies suggest MAPK-activated protein kinase 2 (MK2) inhibition may limit VSMC proliferation and IH, although the molecular mechanism underlying the observation remains unclear. We demonstrated here that MK2 inhibition blocked the molecular program of contractile to synthetic dedifferentiation and mitigated IH development. Molecular markers of the VSMC contractile phenotype were sustained over time in culture in rat primary VSMCs treated with potent, long-lasting MK2 inhibitory peptide nanopolyplexes (MK2i-NPs), a result supported in human saphenous vein specimens cultured ex vivo. RNA-Seq of MK2i-NP-treated primary human VSMCs revealed programmatic switching toward a contractile VSMC gene expression profile, increasing expression of antiinflammatory and contractile-associated genes while lowering expression of proinflammatory, promigratory, and synthetic phenotype-associated genes. Finally, these results were confirmed using an in vivo rabbit vein graft model where brief, intraoperative treatment with MK2i-NPs decreased IH and synthetic phenotype markers while preserving contractile proteins. These results support further development of MK2i-NPs as a therapy for blocking VSMC phenotype switch and IH associated with cardiovascular procedures.

MLL3 is a de novo cause of endocrine therapy resistance

Background

Cancer resequencing studies have revealed epigenetic enzymes as common targets for recurrent mutations. The monomethyltransferase MLL3 is among the most recurrently mutated enzymes in ER+ breast cancer. The H3K4me1 marks created by MLL3 can define enhancers. In ER+ breast cancer, ERα genome‐binding sites are primarily distal enhancers. Thus, we hypothesize that mutation of MLL3 will alter the genomic binding and transcriptional regulatory activity of ERα.

Methods

We investigated the genomic consequences of knocking down MLL3 in an MLL3/PIK3CA WT ER+ breast cancer cell line.

Results

Loss of MLL3 led to a large loss of H3K4me1 across the genome, and a shift in genomic location of ERα‐binding sites, which was accompanied by a re‐organization of the breast cancer transcriptome. Gene set enrichment analyses of ERα‐binding sites in MLL3 KD identified endocrine therapy resistance terms, and we showed that MLL3 KD cells are resistant to tamoxifen and fulvestrant. Many differentially expressed genes are controlled by the small collection of new locations of H3K4me1 deposition and ERα binding, suggesting that loss of functional MLL3 leads to new transcriptional regulation of essential genes. Motif analysis of RNA‐seq and ChIP‐seq data highlighted SP1 as a critical transcription factor in the MLL3 KD cells. Differentially expressed genes that display a loss of ERα binding upon MLL3 KD also harbor increased SP1 binding.

Conclusions

Our data show that a decrease in functional MLL3 leads to endocrine therapy resistance. This highlights the importance of genotyping patient tumor samples for MLL3 mutation upon initial resection, prior to deciding upon treatment plans.

A requirement for p120-catenin in the metastasis of invasive ductal breast cancer

We report here the effects of targeted p120-catenin (encoded by CTNND1; hereafter denoted p120) knockout (KO) in a PyMT mouse model of invasive ductal (mammary) cancer (IDC). Mosaic p120 ablation had little effect on primary tumor growth but caused significant pro-metastatic alterations in the tumor microenvironment, ultimately leading to a marked increase in the number and size of pulmonary metastases. Surprisingly, although early effects of p120-ablation included decreased cell-cell adhesion and increased invasiveness, cells lacking p120 were almost entirely unable to colonized distant metastatic sites in vivo The relevance of this observation to human IDC was established by analysis of a large clinical dataset of 1126 IDCs. As reported by others, p120 downregulation in primary IDC predicted worse overall survival. However, as in the mice, distant metastases were almost invariably p120 positive, even in matched cases where the primary tumors were p120 negative. Collectively, our results demonstrate a strong positive role for p120 (and presumably E-cadherin) during metastatic colonization of distant sites. On the other hand, downregulation of p120 in the primary tumor enhanced metastatic dissemination indirectly via pro-metastatic conditioning of the tumor microenvironment.

Selective glutamine metabolism inhibition in tumor cells improves antitumor T lymphocyte activity in triple-negative breast cancer

Rapidly proliferating tumor and immune cells need metabolic programs that support energy and biomass production. The amino acid glutamine is consumed by effector T cells and glutamine-addicted triple-negative breast cancer (TNBC) cells, suggesting that a metabolic competition for glutamine may exist within the tumor microenvironment, potentially serving as a therapeutic intervention strategy. Here, we report that there is an inverse correlation between glutamine metabolic genes and markers of T cell-mediated cytotoxicity in human basal-like breast cancer (BLBC) patient data sets, with increased glutamine metabolism and decreased T cell cytotoxicity associated with poor survival. We found that tumor cell-specific loss of glutaminase (GLS), a key enzyme for glutamine metabolism, improved antitumor T cell activation in both a spontaneous mouse TNBC model and orthotopic grafts. The glutamine transporter inhibitor V-9302 selectively blocked glutamine uptake by TNBC cells but not CD8+ T cells, driving synthesis of glutathione, a major cellular antioxidant, to improve CD8+ T cell effector function. We propose a "glutamine steal" scenario, in which cancer cells deprive tumor-infiltrating lymphocytes of needed glutamine, thus impairing antitumor immune responses. Therefore, tumor-selective targeting of glutamine metabolism may be a promising therapeutic strategy in TNBC.

Repurposing of a Thromboxane Receptor Inhibitor Based on a Novel Role in Metastasis Identified by Phenome-Wide Association Study

Although new drug discoveries are revolutionizing cancer treatments, repurposing existing drugs would accelerate the timeline and lower the cost for bringing treatments to cancer patients. Our goal was to repurpose CPI211, a potent and selective antagonist of the thromboxane A₂-prostanoid receptor (TPr), a G-protein-coupled receptor that regulates coagulation, blood pressure, and cardiovascular homeostasis. To identify potential new clinical indications for CPI211, we performed a phenome-wide association study (PheWAS) of the gene encoding TPr, TBXA2R, using robust deidentified health records and matched genomic data from more than 29,000 patients. Specifically, PheWAS was used to identify clinical manifestations correlating with a TBXA2R single-nucleotide polymorphism (rs200445019), which generates a T399A substitution within TPr that enhances TPr signaling. Previous studies have correlated 200445019 with chronic venous hypertension, which was recapitulated by this PheWAS analysis. Unexpectedly, PheWAS uncovered an rs200445019 correlation with cancer metastasis across several cancer types. When tested in several mouse models of metastasis, TPr inhibition using CPI211 potently blocked spontaneous metastasis from primary tumors, without affecting tumor cell proliferation, motility, or tumor growth. Further, metastasis following intravenous tumor cell delivery was blocked in mice treated with CPI211. Interestingly, TPr signaling in vascular endothelial cells induced VE-cadherin internalization, diminished endothelial barrier function, and enhanced transendothelial migration by tumor cells, phenotypes that were decreased by CPI211. These studies provide evidence that TPr signaling promotes cancer metastasis, supporting the study of TPr inhibitors as antimetastatic agents and highlighting the use of PheWAS as an approach to accelerate drug repurposing.

MEK activation modulates glycolysis and supports suppressive myeloid cells in TNBC

Triple-negative breast cancers (TNBCs) are heterogeneous and aggressive, with high mortality rates. TNBCs frequently respond to chemotherapy, yet many patients develop chemoresistance. The molecular basis and roles for tumor cell-stromal crosstalk in establishing chemoresistance are complex and largely unclear. Here we report molecular studies of paired TNBC patient-derived xenografts (PDXs) established before and after the development of chemoresistance. Interestingly, the chemoresistant model acquired a distinct KRASQ61R mutation that activates K-Ras. The chemoresistant KRAS-mutant model showed gene expression and proteomic changes indicative of altered tumor cell metabolism. Specifically, KRAS-mutant PDXs exhibited increased redox ratios and decreased activation of AMPK, a protein involved in responding to metabolic homeostasis. Additionally, the chemoresistant model exhibited increased immunosuppression, including expression of CXCL1 and CXCL2, cytokines responsible for recruiting immunosuppressive leukocytes to tumors. Notably, chemoresistant KRAS-mutant tumors harbored increased numbers of granulocytic myeloid-derived suppressor cells (gMDSCs). Interestingly, previously established Ras/MAPK-associated gene expression signatures correlated with myeloid/neutrophil-recruiting CXCL1/2 expression and negatively with T cell-recruiting chemokines (CXCL9/10/11) across patients with TNBC, even in the absence of KRAS mutations. MEK inhibition induced tumor suppression in mice while reversing metabolic and immunosuppressive phenotypes, including chemokine production and gMDSC tumor recruitment in the chemoresistant KRAS-mutant tumors. These results suggest that Ras/MAPK pathway inhibitors may be effective in some breast cancer patients to reverse Ras/MAPK-driven tumor metabolism and immunosuppression, particularly in the setting of chemoresistance.

Therapeutic inhibition of Mcl-1 blocks cell survival in estrogen receptor-positive breast cancers

Cancers often overexpress anti-apoptotic Bcl-2 proteins for cell death evasion, a recognized hallmark of cancer progression. While estrogen receptor (ER)-α+ breast cancers express high levels of three anti-apoptotic Bcl-2 family members (Bcl-2, Bcl-xL, and Mcl-1), pharmacological inhibition of Bcl-2 and/or Bcl-xL fails to induce cell death in ERα+ breast cancer cell lines, due to rapid and robust Mcl-1 upregulation. The mechanisms of acute Mcl-1 upregulation in response to Bcl-2/Bcl-xL inhibition remain undefined in in ERα+ breast cancers. We report here that blockade of Bcl-2 or Bcl-xL, alone or together, rapidly induced mTOR signaling in ERα+ breast cancer cells, rapidly increasing cap-dependent Mcl-1 translation. Cells treated with a pharmacological inhibitor of cap-dependent translation, or with the mTORC1 inhibitor RAD001/everolimus, displayed reduced protein levels of Mcl-1 under basal conditions, and failed to upregulate Mcl-1 protein expression following treatment with ABT-263, a pharmacological inhibitor of Bcl-2 and Bcl-xL. Although treatment with ABT-263 alone did not sustain apoptosis in tumor cells in culture or in vivo, ABT-263 plus RAD001 increased apoptosis to a greater extent than either agent used alone. Similarly, combined use of the selective Mcl-1 inhibitor VU661013 with ABT-263 resulted in tumor cell apoptosis and diminished tumor growth in vivo. These findings suggest that rapid Mcl-1 translation drives ABT-263 resistance, but can be combated directly using emerging Mcl-1 inhibitors, or indirectly through existing and approved mTOR inhibitors.

Abstract 4728: IFNγ signaling drives EZH2 degradation to induce MHC-II expression in melanoma

Background: The molecular mechanisms governing anti-PD-1 resistance in melanoma are not fully understood. Suboptimal transcriptional and molecular response to interferon-gamma (IFNγ) is commonly identified in tumors with intrinsic or acquired resistance to anti-PD-1. Epigenetic regulators have been shown to reprogram IFNγ responses in some cases, though the mechanism is not clear. The ability of melanoma cells to express MHC-II is a positive clinical predictor to anti-PD-1 therapy, and MHC-II is induced only on cells with robust IFNγ responses. Enhancer of zeste homolog 2 (EZH2), an epigenetic regulator with both methyltransferase-dependent and -independent function, can suppress IFNγ target genes and is frequently overexpressed in melanoma. However, the mechanisms whereby EZH2 interacts with the IFNγ pathway and its role in melanoma-specific MHC-II expression is not known.

Methods: HLA-DR (MHC-II)-proficient (A375, SKMEL28 and SKMEL5) and -deficient (CHL-1 and MEWO) melanoma cell lines were stimulated with IFNγ for up to 48h, and the modulation of EZH2 protein and function were studied temporally. To test the role of EZH2 in MHC-II (HLA-DR) expression in IFNγ responses, genetic (siRNA) and chemical (GSK343) EZH2 inhibition was utilized.

Results: EZH2 was highly expressed in all cell lines tested, regardless of HLA-DR-proficiency. In 2 out of 3 HLA-DR-inducible melanoma cell lines, EZH2 protein expression was downregulated (4-24 hrs) in response to IFNγ stimulation, without effects on EZH2 mRNA expression or downstream tri-methylated H3K27 expression. Blocking proteosomal degradation with MG-132 reversed the IFNγ-induced decrease in EZH2 expression in HLA-DR-proficient cell lines. After siRNA knockdown of EZH2, SKMEL28 cells increased constitutive HLA-DR expression, while A375 cells became more sensitive to IFNγ-induced HLA-DR upregulation. In addition, IFNγ-induced HLA-DR was increased in A375 cells with EZH2 inhibition using GSK343.

Conclusion: This work demonstrates a potential role for EZH2 in suppressing IFNγ responses, including the induction of MHC-II, in melanoma cells. EZH2 is subject to proteosomal degradation in MHC-II-inducible cell lines, which, in part, contributes to the expression of MHC-II.

Citation Format: Jamaal L. James, Susan R. Opalenik, Abigail Toren, Rebecca S. Cook, Justin M. Balko. IFNγ signaling drives EZH2 degradation to induce MHC-II expression in melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4728.

Abstract 1511: MEK activation modulates immunosuppressive MDSCs and metabolic phenotypes in TNBC

Triple-negative breast cancers (TNBCs) are highly heterogeneous and aggressive, with high mortality rates. Although TNBC is typically more responsive to chemotherapy than other breast cancer subtypes, many patients develop chemo-resistance. The molecular processes between tumor and stromal cells involved in developing chemo-resistance are under-explored. Here we report studies of paired TNBC patient-derived xenografts (PDX) established before and after chemo-resistance. Despite significant genetic similarities, the chemo-resistant PDX model harbored a rare constitutively-active KRASQ61R mutation which was not present in the chemo-naive PDX. Further analysis demonstrated that the chemo-resistant KRAS-mutant model exhibited altered gene expression changes including increased expression of CXCR2-ligands CXCL1 and CXCL2, which are responsible for recruiting immune cells to tumors. These expression patterns were largely inhibited in vivo by MEK inhibitor (MEKi) treatment. Moreover, in breast cancer cell lines, CXCL1, CXCL2, and granulocyte macrophage-colony stimulating factor (CSF2, stimulates granulocyte and macrophage differentiation from hematopoietic precursor cells, including immunosuppressive myeloid cells) transcripts were also downregulated by MEKi. Notably, chemo-resistant KRAS-mutant tumors harbored increased Gr1+ and Arginase-1+ cells, consistent with recruitment of immunosuppressive M2-like macrophages and/or myeloid-derived suppressor cells (MDSCs), which was inhibited by MEKi. Further experiments demonstrate that CD45+CD11b+Ly6G+ MDSC accumulation in tumors can be inhibited by MEKi treatment alone, or by CXCR2 inhibition, suggesting that the effects of MEK inhibition on MDSC recruitment are CXCL1/2-dependent. Confirming the translational relevance of these findings, in >200 murine and >1000 human breast tumors, Ras/MAPK transcriptional activity correlated with myeloid-recruiting CXCL1/2 expression and negatively with T-cell recruiting chemokines (CXCL9/10/11), even in the absence of activating KRAS mutations. The association with Ras/MAPK activity was also confirmed using immunofluorescence to quantify MHC-II-low myeloid cells in 80 post-chemotherapy TNBC tumors. Importantly, MEKi and chemotherapy combination treatment reversed immunosuppressive cell accumulation and metabolic phenotypes exemplified by altered optical redox ratios (NAD(P)H/FAD) in the chemo-resistant KRAS mutant tumors, resulting in tumor growth suppression in mice. MEKi treatment also reduced redox ratios in 3D cultures of breast cancer cell lines further suggesting that MEK inhibition targets multiple oncogenic processes in breast cancer. These results suggest that Ras/MAPK pathway inhibitors may be effective in some breast cancer patients to reverse Ras/MAPK-driven tumor metabolism and immunosuppression, particularly in the setting of chemo-resistant disease.

Citation Format: Derek A. Franklin, Joe T. Sharick, Paula I. Ericsson-Gonzalez, Violeta Sanchez, Phillip T. Dean, Susan R. Opalenik, Stefano Cairo, Jean-Gabriel Judde, Michael T. Lewis, Jenny C. Chang, Melinda E. Sanders, Rebecca S. Cook, Melissa C. Skala, Jennifer Bordeaux, Jehovana Orozco Bender, Christine Vaupel, Gary Geiss, Douglas Hinerfeld, Justin M. Balko. MEK activation modulates immunosuppressive MDSCs and metabolic phenotypes in TNBC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1511.

Disruption of the Scaffolding Function of mLST8 Selectively Inhibits mTORC2 Assembly and Function and Suppresses mTORC2-Dependent Tumor Growth In Vivo

mTOR is a serine/threonine kinase that acts in two distinct complexes, mTORC1 and mTORC2, and is dysregulated in many diseases including cancer. mLST8 is a shared component of both mTORC1 and mTORC2, yet little is known regarding how mLST8 contributes to assembly and activity of the mTOR complexes. Here we assessed mLST8 loss in a panel of normal and cancer cells and observed little to no impact on assembly or activity of mTORC1. However, mLST8 loss blocked mTOR association with mTORC2 cofactors RICTOR and SIN1, thus abrogating mTORC2 activity. Similarly, a single pair of mutations on mLST8 with a corresponding mutation on mTOR interfered with mTORC2 assembly and activity without affecting mTORC1. We also discovered a direct interaction between mLST8 and the NH₂-terminal domain of the mTORC2 cofactor SIN1. In PTEN-null prostate cancer xenografts, mLST8 mutations disrupting the mTOR interaction motif inhibited AKT S473 phosphorylation and decreased tumor cell proliferation and tumor growth in vivo. Together, these data suggest that the scaffolding function of mLST8 is critical for assembly and activity of mTORC2, but not mTORC1, an observation that could enable therapeutic mTORC2-selective inhibition as a therapeutic strategy. SIGNIFICANCE: These findings show that mLST8 functions as a scaffold to maintain mTORC2 integrity and kinase activity, unveiling a new avenue for development of mTORC2-specific inhibitors.

Cellular Metabolic Heterogeneity In Vivo Is Recapitulated in Tumor Organoids

Heterogeneous populations within a tumor have varying metabolic profiles, which can muddle the interpretation of bulk tumor imaging studies of treatment response. Although methods to study tumor metabolism at the cellular level are emerging, these methods provide a single time point “snapshot” of tumor metabolism and require a significant time and animal burden while failing to capture the longitudinal metabolic response of a single tumor to treatment. Here, we investigated a novel method for longitudinal, single-cell tracking of metabolism across heterogeneous tumor cell populations using optical metabolic imaging (OMI), which measures autofluorescence of metabolic coenzymes as a report of metabolic activity. We also investigated whether in vivo cellular metabolic heterogeneity can be accurately captured using tumor-derived three-dimensional organoids in a genetically engineered mouse model of breast cancer. OMI measurements of response to paclitaxel and the phosphatidylinositol-3-kinase inhibitor XL147 in tumors and organoids taken at single cell resolution revealed parallel shifts in metaboltruic heterogeneity. Interestingly, these previously unappreciated heterogeneous metabolic responses in tumors and organoids could not be attributed to tumor cell fate or varying leukocyte content within the microenvironment, suggesting that heightened metabolic heterogeneity upon treatment is largely due to heterogeneous metabolic shifts within tumor cells. Together, these studies show that OMI revealed remarkable heterogeneity in response to treatment, which could provide a novel approach to predict the presence of potentially unresponsive tumor cell subpopulations lurking within a largely responsive bulk tumor population, which might otherwise be overlooked by traditional measurements.

Microparticle Depots for Controlled and Sustained Release of Endosomolytic Nanoparticles

Introduction

Nucleic acids have gained recognition as promising immunomodulatory therapeutics. However, their potential is limited by several drug delivery barriers, and there is a need for technologies that enhance intracellular delivery of nucleic acid drugs. Furthermore, controlled and sustained release is a significant concern, as the kinetics and localization of immunomodulators can influence resultant immune responses. Here, we describe the design and initial evaluation of poly(lactic-co-glycolic) acid (PLGA) microparticle (MP) depots for enhanced retention and sustained release of endosomolytic nanoparticles that enable the cytosolic delivery of nucleic acids.

Methods

Endosomolytic p[DMAEMA]_10kD-bl-[PAA_0.3-co-DMAEMA_0.3-co-BMA_0.4]_25kD diblock copolymers were synthesized by reversible addition-fragmentation chain transfer polymerization. Polymers were electrostatically complexed with nucleic acids and resultant nanoparticles (NPs) were encapsulated in PLGA MPs. To modulate release kinetics, ammonium bicarbonate was added as a porogen. Release profiles were quantified in vitro and in vivo via quantification of fluorescently-labeled nucleic acid. Bioactivity of released NPs was assessed using small interfering RNA (siRNA) targeting luciferase as a representative nucleic acid cargo. MPs were incubated with luciferase-expressing 4T1 (4T1-LUC) breast cancer cells in vitro or administered intratumorally to 4T1-LUC breast tumors, and silencing via RNA interference was quantified via longitudinal luminescence imaging.

Results

Endosomolytic NPs complexed to siRNA were effectively loaded into PLGA MPs and release kinetics could be modulated in vitro and in vivo via control of MP porosity, with porous MPs exhibiting faster cargo release. In vitro, release of NPs from porous MP depots enabled sustained luciferase knockdown in 4T1 breast cancer cells over a five-day treatment period. Administered intratumorally, MPs prolonged the retention of nucleic acid within the injected tumor, resulting in enhanced and sustained silencing of luciferase relative to a single bolus administration of NPs at an equivalent dose.

Conclusion

This work highlights the potential of PLGA MP depots as a platform for local release of endosomolytic polymer NPs that enhance the cytosolic delivery of nucleic acid therapeutics.

Biological Consequences of MHC-II Expression by Tumor Cells in Cancer

Immunotherapy has emerged as a key pillar of cancer treatment. To build upon the recent successes of immunotherapy, intense research efforts are aimed at a molecular understanding of antitumor immune responses, identification of biomarkers of immunotherapy response and resistance, and novel strategies to circumvent resistance. These studies are revealing new insight into the intricacies of tumor cell recognition by the immune system, in large part through MHCs. Although tumor cells widely express MHC-I, a subset of tumors originating from a variety of tissues also express MHC-II, an antigen-presenting complex traditionally associated with professional antigen-presenting cells. MHC-II is critical for antigen presentation to CD4+ T lymphocytes, whose role in antitumor immunity is becoming increasingly appreciated. Accumulating evidence demonstrates that tumor-specific MHC-II associates with favorable outcomes in patients with cancer, including those treated with immunotherapies, and with tumor rejection in murine models. Herein, we will review current research regarding tumor-enriched MHC-II expression and regulation in a range of human tumors and murine models, and the possible therapeutic applications of tumor-specific MHC-II.

Abstract A187: RIG-I agonists reinforce antitumor adaptive immunity and decrease Treg activity in breast cancer

RIG-I like receptors, RNA helicases that sense viral oligonucleotide motifs and activate innate immunity, are gaining interest in cancer therapy, given their ability to redirect immune responses within the tumor microenvironment (TME), and increase efficacy of experimental cancer vaccines. RIG-I agonists are not well studied in breast cancers, a type of cancer that is often considered immunologically “silent.” We recently reported that therapeutic delivery of RIG-I agonists increase tumor-infiltrating leukocytes (TILs) and expression of proinflammatory Th1 cytokines in the 4T1 mouse model of aggressive, metastatic breast cancer through tumor cell-intrinsic mechanisms. However, these studies do not rule out the importance of myeloid immune responders (e.g., macrophages and dendritic cells) in propagating the effects of RIG-I agonists against tumor cells in vivo, nor do they rule out the impact of RIG-I agonists on adaptive antitumor immunity, a subject that is relatively understudied. We assessed the effects of the RIG-I agonist SLR20 on the the activity of effector T-lymphocytes (TEff) and regulatory T-lymphocytes (TReg) in the TME. Interestingly, SLR20 treatment of mouse and human breast tumor cells increased expression of FAS and MHC-I on tumor cells, and caused tumor cells to express T-cell chemoattractants (e.g., CXCL10, RANTES), potentially increasing T-cells recruitment to tumors, and increasing tumor cell susceptibility to TEff recognition and killing. Using an ex vivo co-culture assay in which 4T1 mouse mammary tumor cells were co-cultured with CD8+ T-cells harvested from mice pre-inoculated with SLR20-treated 4T1 tumor cells, we measured the rate of CD8+-mediated tumor cell killing. This approach revealed that T-cells harvested from mice inoculated with SLR20-treated cells caused greater tumor cell killing than what was seen by CD8+ T-cells harvested from untreated mice. We also found that conditioned media harvested from 4T1 cells treated with SLR20 increased clonal expansion of CD3/CD28-activated T-cells above what was seen with conditioned media harvested from 4T1 cells treated with a control oligonucleotide or from untreated 4T1 cells. TGFβ-mediated differentiation of CD4+ T-cells into tolerogenic and immunosuppressive TRegs was measured in cultures of CD4+ T-cells treated with cultured media derived from SLR20-treated 4T1 cells. These studies showed that cultured media harvested from 4T1 cells treated with SLR20, but not from untreated 4T1 cells or 4T1 cells treated with a control ligand, diminished TReg differentiation, and decreased CD4+ T-cells surface expression of PD-1, CTLA4, and CCR8. Importantly, in vivo experiments assessing therapeutic treatment of 4T1 tumors with SLR20 revealed greater tumor growth inhibition when SLR20 was combined with PD-L1 targeting antibodies. Taken together, these findings indicate that therapeutic activation of RIG-I signaling operates at the interface of innate and adaptive immunity within breast tumors to redirect the TME from an immunosuppressed state to one that is immunogenic and receptive to clinically relevant checkpoint inhibitors.

Citation Format: David L. Elion, Max E. Jacobson, Donna J. Hicks, Bushra Rahman, Violeta Sanchez, Paula I Gonzales-Ericsson, Olga Fedorova, Anna M. Pyle, John T. Wilson, Rebecca S. Cook. RIG-I agonists reinforce antitumor adaptive immunity and decrease Treg activity in breast cancer [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr A187.

Treatment-Induced Tumor Cell Apoptosis and Secondary Necrosis Drive Tumor Progression in the Residual Tumor Microenvironment through MerTK and IDO1

Efferocytosis is the process by which apoptotic cells are cleared from tissue by phagocytic cells. The removal of apoptotic cells prevents them from undergoing secondary necrosis and releasing their inflammation-inducing intracellular contents. Efferocytosis also limits tissue damage by increasing immunosuppressive cytokines and leukocytes and maintains tissue homeostasis by promoting tolerance to antigens derived from apoptotic cells. Thus, tumor cell efferocytosis following cytotoxic cancer treatment could impart tolerance to tumor cells evading treatment-induced apoptosis with deleterious consequences in tumor residual disease. We report here that efferocytosis cleared apoptotic tumor cells in residual disease of lapatinib-treated HER2⁺ mammary tumors in MMTV-Neu mice, increased immunosuppressive cytokines, myeloid-derived suppressor cells (MDSC), and regulatory T cells (Treg). Blockade of efferocytosis induced secondary necrosis of apoptotic cells, but failed to prevent increased tumor MDSCs, Treg, and immunosuppressive cytokines. We found that efferocytosis stimulated expression of IFN-γ, which stimulated the expression of indoleamine-2,3-dioxegenase (IDO) 1, an immune regulator known for driving maternal-fetal antigen tolerance. Combined inhibition of efferocytosis and IDO1 in tumor residual disease decreased apoptotic cell- and necrotic cell-induced immunosuppressive phenotypes, blocked tumor metastasis, and caused tumor regression in 60% of MMTV-Neu mice. This suggests that apoptotic and necrotic tumor cells, via efferocytosis and IDO1, respectively, promote tumor 'homeostasis' and progression. SIGNIFICANCE: These findings show in a model of HER2⁺ breast cancer that necrosis secondary to impaired efferocytosis activates IDO1 to drive immunosuppression and tumor progression.

Therapeutically Active RIG-I Agonist Induces Immunogenic Tumor Cell Killing in Breast Cancers

Cancer immunotherapies that remove checkpoint restraints on adaptive immunity are gaining clinical momentum but have not achieved widespread success in breast cancers, a tumor type considered poorly immunogenic and which harbors a decreased presence of tumor-infiltrating lymphocytes. Approaches that activate innate immunity in breast cancer cells and the tumor microenvironment are of increasing interest, based on their ability to induce immunogenic tumor cell death, type I IFNs, and lymphocyte-recruiting chemokines. In agreement with reports in other cancers, we observe loss, downregulation, or mutation of the innate viral nucleotide sensor retinoic acid-inducible gene I (RIG-I/DDX58) in only 1% of clinical breast cancers, suggesting potentially widespread applicability for therapeutic RIG-I agonists that activate innate immunity. This was tested using an engineered RIG-I agonist in a breast cancer cell panel representing each of three major clinical breast cancer subtypes. Treatment with RIG-I agonist resulted in upregulation and mitochondrial localization of RIG-I and activation of proinflammatory transcription factors STAT1 and NF-κB. RIG-I agonist triggered the extrinsic apoptosis pathway and pyroptosis, a highly immunogenic form of cell death in breast cancer cells. RIG-I agonist also induced expression of lymphocyte-recruiting chemokines and type I IFN, confirming that cell death and cytokine modulation occur in a tumor cell-intrinsic manner. Importantly, RIG-I activation in breast tumors increased tumor lymphocytes and decreased tumor growth and metastasis. Overall, these findings demonstrate successful therapeutic delivery of a synthetic RIG-I agonist to induce tumor cell killing and to modulate the tumor microenvironment in vivo Significance: These findings describe the first in vivo delivery of RIG-I mimetics to tumors, demonstrating a potent immunogenic and therapeutic effect in the context of otherwise poorly immunogenic breast cancers. Cancer Res; 78(21); 6183-95. ©2018 AACR.

p73 Is Required for Ovarian Follicle Development and Regulates a Gene Network Involved in Cell-to-Cell Adhesion

We report that p73 is expressed in ovarian granulosa cells and that loss of p73 leads to attenuated follicle development, ovulation, and corpus luteum formation, resulting in decreased levels of circulating progesterone and defects in mammary gland branching. Ectopic progesterone in p73-deficient mice completely rescued the mammary branching and partially rescued the ovarian follicle development defects. Performing RNA sequencing (RNA-seq) on transcripts from murine wild-type and p73-deficient antral follicles, we discovered differentially expressed genes that regulate biological adhesion programs. Through modulation of p73 expression in murine granulosa cells and transformed cell lines, followed by RNA-seq and chromatin immunoprecipitation sequencing, we discovered p73-dependent regulation of a gene set necessary for cell adhesion and migration and components of the focimatrix (focal intra-epithelial matrix), a basal lamina between granulosa cells that promotes follicle maturation. In summary, p73 is essential for ovarian folliculogenesis and functions as a key regulator of a gene network involved in cell-to-cell adhesion and migration.

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p73 is required for murine ovarian folliculogenesis and proper corpus luteum formation

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p73 loss leads to defects in progesterone signaling and mammary gland branching

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In murine ovaries, p73 is expressed specifically in granulosa cells

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p73 regulates components of the granulosa cell focimatrix and migration

Genetic and Phenotypic Diversification of Heterogeneous Tumor Populations

Chemotherapy is the most commonly prescribed treatment for patients with aggressive and lethal triple negative breast cancers (TNBCs), which often develop chemoresistance. A recent study combined single nucleus sequencing, single cell RNA sequencing, and evolutionary biology to understand how tumor cells use genetic and phenotypic diversity to evade the selective pressures of neoadjuvant chemotherapy.

Staphylococcus xylosus Cystitis and Struvite Urolithiasis in Nude Mice Implanted with Sustained-release Estrogen Pellets

Female nude mice (J:NU-Foxn1nu; age, 6 wk) were injected with 1 million MCF7 human breast cancer cells in the fourth mammary fat pads and received a 21-d sustained-release estrogen pellet (0.25 mg) subcutaneously in the dorsum of the neck. All mice were maintained in sterile housing and provided sterile water and irradiated rodent chow. Approximately 6 wk after implantation, 4 of the 30 mice showed clinical signs of depression and dehydration. The 2 animals most severely affected were euthanized and presented for necropsy. The urinary bladders of these animals were distended with variable sized white, opaque uroliths. Urinalysis revealed coccal bacteria, erythrocytes, neutrophils and struvite crystals. Urine cultures from both necropsied animals grew heavy, pure growths of Staphylococcus xylosus. The organism was sensitive to all antibiotics tested except erythromycin (intermediate). Analysis of the uroliths revealed 100% struvite composition. Remaining mice in the study were evaluated clinically for hydration status, the ability to urinate, and the presence of palpable stones in the urinary bladder; one additional mouse had a firm, nonpainful bladder (urolithiasis suspected). Given the sensitivity of the organisms cultured from urine samples, the remaining mice were placed on enrofloxacin in the drinking water (0.5 mg/mL). All remaining mice completed the study without further morbidity or mortality. Previous studies have reported the association of estrogen supplementation with urinary bladder pathology, including infection and urolithiasis. Here we present a case of urolithiasis and cystitis in nude mice receiving estrogen supplementation that was associated with Staphylococcus xylosus, which previously was unreported in this context. When assessing these nude mice for urolithiasis, we found that visualizing the stones through the body wall, bladder palpation, and bladder expression were helpful in identifying affected mice.

Harnessing RIG-I and intrinsic immunity in the tumor microenvironment for therapeutic cancer treatment

Cancer immunotherapies that remove checkpoint restraints on adaptive immunity are gaining clinical momentum. Approaches aimed at intrinsic cellular immunity in the tumor microenvironment are less understood, but are of intense interest, based on their ability to induce tumor cell apoptosis while orchestrating innate and adaptive immune responses against tumor antigens. The intrinsic immune response is initiated by ancient, highly conserved intracellular proteins that detect viral infection. For example, the RIG-I-like receptors (RLRs), a family of related RNA helicases, detect viral oligonucleotide patterns of certain RNA viruses. RLR activation induces immunogenic cell death of virally infected cells, accompanied by increased inflammatory cytokine production, antigen presentation, and antigen-directed immunity against virus antigens. Approaches aimed at non-infectious RIG-I activation in cancers are being tested as a treatment option, with the goal of inducing immunogenic tumor cell death, stimulating production of pro-inflammatory cytokines, enhancing tumor neoantigen presentation, and potently increasing cytotoxic activity of tumor infiltrating lymphocytes. These studies are finding success in several pre-clinical models, and are entering early phases of clinical trial. Here, we review pre-clinical studies of RLR agonists, including the successes and challenges currently faced RLR agonists on the path to clinical translation.

Melanoma response to anti-PD-L1 immunotherapy requires JAK1 signaling, but not JAK2

Immunotherapies targeting programmed cell death protein 1 (PD-1) or its ligand, programmed cell death ligand 1 (PD-L1), dramatically improve the survival of melanoma patients. However, only ∼40% of treated patients demonstrate a clinical response to single-agent anti-PD-1 therapy. An intact tumor response to type-II interferon (i.e. IFN-γ) correlates with response to anti-PD-1, and patients with de novo or acquired resistance may harbor loss-of-function alterations in the JAK/STAT pathway, which lies downstream of the interferon gamma receptor (IFNGR1/2). In this study, we dissected the specific roles of individual JAK/STAT pathway members on the IFN-γ response, and identified JAK1 as the primary mediator of JAK/STAT signaling associated with IFN-γ-induced expression of antigen-presenting molecules MHC-I and MHC-II, as well as PD-L1 and the cytostatic response to IFN-γ. In contrast to the crucial role of JAK1, JAK2 was largely dispensable in mediating most IFN-γ effects. In a mouse melanoma model, inhibition of JAK1/2 in combination with anti-PD-L1 therapy partially blocked anti-tumor immunologic responses, while selective JAK2 inhibition appeared to augment therapy. Amplification of JAK/STAT signaling in tumor cells through genetic inhibition of the negative regulator PTPN2 potentiated IFN-γ response in vitro and in vivo, and may be a target to enhance immunotherapy efficacy.

Selective mTORC2 Inhibitor Therapeutically Blocks Breast Cancer Cell Growth and Survival

Small-molecule inhibitors of the mTORC2 kinase (torkinibs) have shown efficacy in early clinical trials. However, the torkinibs under study also inhibit the other mTOR-containing complex mTORC1. While mTORC1/mTORC2 combined inhibition may be beneficial in cancer cells, recent reports describe compensatory cell survival upon mTORC1 inhibition due to loss of negative feedback on PI3K, increased autophagy, and increased macropinocytosis. Genetic models suggest that selective mTORC2 inhibition would be effective in breast cancers, but the lack of selective small-molecule inhibitors of mTORC2 have precluded testing of this hypothesis to date. Here we report the engineering of a nanoparticle-based RNAi therapeutic that can effectively silence the mTORC2 obligate cofactor Rictor. Nanoparticle-based Rictor ablation in HER2-amplified breast tumors was achieved following intratumoral and intravenous delivery, decreasing Akt phosphorylation and increasing tumor cell killing. Selective mTORC2 inhibition in vivo, combined with the HER2 inhibitor lapatinib, decreased the growth of HER2-amplified breast cancers to a greater extent than either agent alone, suggesting that mTORC2 promotes lapatinib resistance, but is overcome by mTORC2 inhibition. Importantly, selective mTORC2 inhibition was effective in a triple-negative breast cancer (TNBC) model, decreasing Akt phosphorylation and tumor growth, consistent with our findings that RICTOR mRNA correlates with worse outcome in patients with basal-like TNBC. Together, our results offer preclinical validation of a novel RNAi delivery platform for therapeutic gene ablation in breast cancer, and they show that mTORC2-selective targeting is feasible and efficacious in this disease setting.Significance: This study describes a nanomedicine to effectively inhibit the growth regulatory kinase mTORC2 in a preclinical model of breast cancer, targeting an important pathogenic enzyme in that setting that has been undruggable to date. Cancer Res; 78(7); 1845-58. ©2018 AACR.

Intrinsic apoptotic pathway activation increases response to anti-estrogens in luminal breast cancers

Estrogen receptor-α positive (ERα+) breast cancer accounts for approximately 70–80% of the nearly 25,0000 new cases of breast cancer diagnosed in the US each year. Endocrine-targeted therapies (those that block ERα activity) serve as the first line of treatment in most cases. Despite the proven benefit of endocrine therapies, however, ERα+ breast tumors can develop resistance to endocrine therapy, causing disease progression or relapse, particularly in the metastatic setting. Anti-apoptotic Bcl-2 family proteins enhance breast tumor cell survival, often promoting resistance to targeted therapies, including endocrine therapies. Herein, we investigated whether blockade of anti-apoptotic Bcl-2 family proteins could sensitize luminal breast cancers to anti-estrogen treatment. We used long-term estrogen deprivation (LTED) of human ERα+ breast cancer cell lines, an established model of sustained treatment with and acquired resistance to aromatase inhibitors (AIs), in combination with Bcl-2/Bcl-xL inhibition (ABT-263), finding that ABT-263 induced only limited tumor cell killing in LTED-selected cells in culture and in vivo. Interestingly, expression and activity of the Bcl-2-related factor Mcl-1 was increased in LTED cells. Genetic Mcl-1 ablation induced apoptosis in LTED-selected cells, and potently increased their sensitivity to ABT-263. Increased expression and activity of Mcl-1 was similarly seen in clinical breast tumor specimens treated with AI + the selective estrogen receptor downregulator fulvestrant. Delivery of Mcl-1 siRNA loaded into polymeric nanoparticles (MCL1 si-NPs) decreased Mcl-1 expression in LTED-selected and fulvestrant-treated cells, increasing tumor cell death and blocking tumor cell growth. These findings suggest that Mcl-1 upregulation in response to anti-estrogen treatment enhances tumor cell survival, decreasing response to therapeutic treatments. Therefore, strategies blocking Mcl-1 expression or activity used in combination with endocrine therapies would enhance tumor cell death.

ErbB3 drives mammary epithelial survival and differentiation during pregnancy and lactation

Background

During pregnancy, as the mammary gland prepares for synthesis and delivery of milk to newborns, a luminal mammary epithelial cell (MEC) subpopulation proliferates rapidly in response to systemic hormonal cues that activate STAT5A. While the receptor tyrosine kinase ErbB4 is required for STAT5A activation in MECs during pregnancy, it is unclear how ErbB3, a heterodimeric partner of ErbB4 and activator of phosphatidyl inositol-3 kinase (PI3K) signaling, contributes to lactogenic expansion of the mammary gland.

Methods

We assessed mRNA expression levels by expression microarray of mouse mammary glands harvested throughout pregnancy and lactation. To study the role of ErbB3 in mammary gland lactogenesis, we used transgenic mice expressing WAP-driven Cre recombinase to generate a mouse model in which conditional ErbB3 ablation occurred specifically in alveolar mammary epithelial cells (aMECs).

Results

Profiling of RNA from mouse MECs isolated throughout pregnancy revealed robust Erbb3 induction during mid-to-late pregnancy, a time point when aMECs proliferate rapidly and undergo differentiation to support milk production. Litters nursed by ErbB3^KO dams weighed significantly less when compared to litters nursed by ErbB3^WT dams. Further analysis revealed substantially reduced epithelial content, decreased aMEC proliferation, and increased aMEC cell death during late pregnancy. Consistent with the potent ability of ErbB3 to activate cell survival through the PI3K/Akt pathway, we found impaired Akt phosphorylation in ErbB3^KO samples, as well as impaired expression of STAT5A, a master regulator of lactogenesis. Constitutively active Akt rescued cell survival in ErbB3-depleted aMECs, but failed to restore STAT5A expression or activity. Interestingly, defects in growth and survival of ErbB3^KO aMECs as well as Akt phosphorylation, STAT5A activity, and expression of milk-encoding genes observed in ErbB3^KO MECs progressively improved between late pregnancy and lactation day 5. We found a compensatory upregulation of ErbB4 activity in ErbB3^KO mammary glands. Enforced ErbB4 expression alleviated the consequences of ErbB3 ablation in aMECs, while combined ablation of both ErbB3 and ErbB4 exaggerated the phenotype.

Conclusions

These studies demonstrate that ErbB3, like ErbB4, enhances lactogenic expansion and differentiation of the mammary gland during pregnancy, through activation of Akt and STAT5A, two targets crucial for lactation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-017-0893-7) contains supplementary material, which is available to authorized users.

Abstract 3702: Quantitative imaging of metabolic changes in macrophage subsets

Macrophages play an important role in tumor therapeutic response. However, current methods cannot image heterogeneities in macrophage activity on a single-cell level over time within intact, living samples. Macrophage metabolism is closely linked to macrophage function, as M1-like macrophages rely on aerobic glycolysis and M2-like macrophages rely on fatty acid oxidation and oxidative phosphorylation. Thus, we propose to develop optical metabolic imaging (OMI) using two-photon microscopy to monitor cell-level changes in macrophage polarization and functional activity. OMI exploits the auto-fluorescence intensities and lifetimes of the metabolic co-enzymes NAD(P)H and FAD, and has been previously used to dynamically image in vivo tumors with cell-level resolution. The optical redox ratio (fluorescence intensity of NAD(P)H divided by FAD) provides a global measure of redox balance within individual cells. The fluorescence lifetimes of NAD(P)H and FAD provide information in protein-binding activities in metabolic reactions. OMI was used to monitor metabolic changes in macrophages on a single-cell level during polarization and after metabolic perturbations. RAW 264.7 macrophages were stimulated towards an M1- or M2-like phenotype, and imaged with OMI at 24, 48, and 72 hours post-polarization. M1- and M2-like macrophages exhibited significant differences (p<0.05) in redox ratio and FAD fluorescence lifetime values at 48 and 72 hours post-polarization. The M1-like and M2-like character of the macrophages were validated with quantitative polymerase chain reaction (qPCR) analysis. Population density modeling of single-cell metabolism established differences in intra-sample heterogeneity across subsets over time. M1- and M2-like macrophages were also treated with 2-deoxy-glucose (glycolysis inhibitor), etomoxir (fatty acid oxidation inhibitor), and cyanide (oxidative phosphorylation inhibitor. Treatment with etomoxir caused a significant decrease in redox ratio in M2-like macrophages (p<0.05) with no significant effect on M1-like macrophages. In contrast, the redox ratio decreased in M1-like macrophages upon inhibition of glycolysis (p<0.05). Comparable to previously published data, inhibition of oxidative phosphorylation in both groups yielded a decreased NAD(P)H mean lifetimes (p<0.05) and increased FAD mean lifetimes (p<0.05). These results establish OMI as a method to image heterogeneity in macrophage populations within intact, living samples. This method could be used to understand macrophage interactions in the microenvironment of intact in vivo tumors.

Citation Format: Tiffany M. Heaster, David L. Elion, Rebecca S. Cook, Melissa C. Skala. Quantitative imaging of metabolic changes in macrophage subsets [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3702. doi:10.1158/1538-7445.AM2017-3702

Two distinct mTORC2-dependent pathways converge on Rac1 to drive breast cancer metastasis

Background

The importance of the mTOR complex 2 (mTORC2) signaling complex in tumor progression is becoming increasingly recognized. HER2-amplified breast cancers use Rictor/mTORC2 signaling to drive tumor formation, tumor cell survival and resistance to human epidermal growth factor receptor 2 (HER2)-targeted therapy. Cell motility, a key step in the metastatic process, can be activated by mTORC2 in luminal and triple negative breast cancer cell lines, but its role in promoting metastases from HER2-amplified breast cancers is not yet clear.

Methods

Because Rictor is an obligate cofactor of mTORC2, we genetically engineered Rictor ablation or overexpression in mouse and human HER2-amplified breast cancer models for modulation of mTORC2 activity. Signaling through mTORC2-dependent pathways was also manipulated using pharmacological inhibitors of mTOR, Akt, and Rac. Signaling was assessed by western analysis and biochemical pull-down assays specific for Rac-GTP and for active Rac guanine nucleotide exchange factors (GEFs). Metastases were assessed from spontaneous tumors and from intravenously delivered tumor cells. Motility and invasion of cells was assessed using Matrigel-coated transwell assays.

Results

We found that Rictor ablation potently impaired, while Rictor overexpression increased, metastasis in spontaneous and intravenously seeded models of HER2-overexpressing breast cancers. Additionally, migration and invasion of HER2-amplified human breast cancer cells was diminished in the absence of Rictor, or upon pharmacological mTOR kinase inhibition. Active Rac1 was required for Rictor-dependent invasion and motility, which rescued invasion/motility in Rictor depleted cells. Rictor/mTORC2-dependent dampening of the endogenous Rac1 inhibitor RhoGDI2, a factor that correlated directly with increased overall survival in HER2-amplified breast cancer patients, promoted Rac1 activity and tumor cell invasion/migration. The mTORC2 substrate Akt did not affect RhoGDI2 dampening, but partially increased Rac1 activity through the Rac-GEF Tiam1, thus partially rescuing cell invasion/motility. The mTORC2 effector protein kinase C (PKC)α did rescue Rictor-mediated RhoGDI2 downregulation, partially rescuing Rac-guanosine triphosphate (GTP) and migration/motility.

Conclusion

These findings suggest that mTORC2 uses two coordinated pathways to activate cell invasion/motility, both of which converge on Rac1. Akt signaling activates Rac1 through the Rac-GEF Tiam1, while PKC signaling dampens expression of the endogenous Rac1 inhibitor, RhoGDI2.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-017-0868-8) contains supplementary material, which is available to authorized users.

Functional Optical Imaging of Primary Human Tumor Organoids: Development of a Personalized Drug Screen

Primary tumor organoids are a robust model of individual human cancers and present a unique platform for patient-specific drug testing. Optical imaging is uniquely suited to assess organoid function and behavior because of its subcellular resolution, penetration depth through the entire organoid, and functional endpoints. Specifically, optical metabolic imaging (OMI) is highly sensitive to drug response in organoids, and OMI in tumor organoids correlates with primary tumor drug response. Therefore, an OMI organoid drug screen could enable accurate testing of drug response for individualized cancer treatment. The objective of this perspective is to introduce OMI and tumor organoids to a general audience in order to foster the adoption of these techniques in diverse clinical and laboratory settings.

Targeting EphA2 impairs cell cycle progression and growth of basal-like/triple-negative breast cancers

Basal-like/triple-negative breast cancers (TNBCs) are among the most aggressive forms of breast cancer, and disproportionally affects young premenopausal women and women of African descent. Patients with TNBC suffer a poor prognosis due in part to a lack of molecularly targeted therapies, which represents a critical barrier for effective treatment. Here, we identify EphA2 receptor tyrosine kinase as a clinically relevant target for TNBC. EphA2 expression is enriched in the basal-like molecular subtype in human breast cancers. Loss of EphA2 function in both human and genetically engineered mouse models of TNBC reduced tumor growth in culture and in vivo. Mechanistically, targeting EphA2 impaired cell cycle progression through S-phase via downregulation of c-Myc and stabilization of the cyclin-dependent kinase inhibitor p27/KIP1. A small molecule kinase inhibitor of EphA2 effectively suppressed tumor cell growth in vivo, including TNBC patient-derived xenografts. Thus, our data identify EphA2 as a novel molecular target for TNBC.

mTORC1 and mTORC2 in cancer and the tumor microenvironment

The mammalian target of rapamycin (mTOR) is a crucial signaling node that integrates environmental cues to regulate cell survival, proliferation and metabolism, and is often deregulated in human cancer. mTOR kinase acts in two functionally distinct complexes, mTOR complex 1 (mTORC1) and 2 (mTORC2), whose activities and substrate specificities are regulated by complex co-factors. Deregulation of this centralized signaling pathway has been associated with a variety of human diseases including diabetes, neurodegeneration and cancer. Although mTORC1 signaling has been extensively studied in cancer, recent discoveries indicate a subset of human cancers harboring amplifications in mTORC2-specific genes as the only actionable genomic alterations, suggesting a distinct role for mTORC2 in cancer as well. This review will summarize recent advances in dissecting the relative contributions of mTORC1 versus mTORC2 in cancer, their role in tumor-associated blood vessels and tumor immunity, and provide an update on mTOR inhibitors.

Abstract P2-04-03: Differential effect of chemotherapy on immune gene expression signatures based on molecular subtype of breast cancer

Background: Neoadjuvant chemotherapy (NAC) is frequently used in triple negative breast cancers (TNBC), and patients who achieve pathological complete response (pCR) following NAC have improved outcomes over those with residual disease (non-pCR). Unfortunately only 30% of TNBC patients achieve pCR, with no good treatment options for other 70% with non-pCR. Tumor infiltrating lymphocytes (TILs) in TNBC are predictive of pCR to NAC, and TILs found in the residual disease further prognosticate patients with residual disease into an improved prognosis subset. These data suggest there is an immune component to TNBC that might be affected by chemotherapy. Given the advent of immunotherapy trials in breast cancer, specifically in combination with NAC, there is an unmet need to gain a better understanding of the immune microenvironment in TNBC.

Objective: We investigated the role of NAC on the immune microenvironment by examining breast cancer samples before (diagnostic biopsy) and after (residual disease) NAC, as these data are pertinent to the design and analysis of ongoing clinical trials in breast cancer combining immunotherapy with concurrent chemotherapy.

Methods: RNA was extracted from 51 paired (pre- and post-NAC) breast cancer specimens with extensive residual disease, and analyzed by nCounter analysis for expression of >750 immune-related genes. Functional immune signature scores were generated, compared between matched pre- and post-NAC, and stratified by breast cancer molecular subtype (luminal (n=4), Her2-enriched (n=5), basal-like (n=17)). DNA extracted from a subset of these samples was utilized for T cell receptor sequencing to explore changes in T cell clonality following NAC in each subtype.

Results: Across all subtypes of breast cancer, immune scores decreased after NAC consistent with a broad decrease in TILs observed histologically. When samples were stratified by molecular subtype using the PAM50 analysis, luminal A/B and basal-like patients demonstrated a decrease in immune signatures after NAC, while Her2-enriched patients exhibited a global increase in immune scores. Importantly, basal-like patients had the greatest immune signature changes, including decreases in T cell functions and CD8+ T cell, T helper and T regulatory signatures (p>0.05). Conversely, these signatures were increased after NAC in the Her2-enriched molecular subtype. Analyses of T-cell clonality are ongoing, but should yield insight into the effect of NAC on the landscape of effector T-cells in the micro-environment.

Conclusions: Our work suggests that NAC decreases immune infiltrate signatures,</del> specifically related to effector T cells in patients who do not achieve pCR. However, we observed an increase after NAC in the same signatures in Her2-enriched patients, suggesting that these patients' tumors may respond differently to chemotherapy on the immune-molecular level. As clinical trials progress in TNBC with the combination of chemotherapy and immune therapy, more work is needed to understand who might benefit from these therapy regimens.

Citation Format: Hicks MJ, Estrada MV, Sanders ME, Salgado R, Cook RS, Arteaga CL, Balko JM. Differential effect of chemotherapy on immune gene expression signatures based on molecular subtype of breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P2-04-03.

Key Survival Factor, Mcl-1, Correlates with Sensitivity to Combined Bcl-2/Bcl-xL Blockade

An estimated 40,000 deaths will be attributed to breast cancer in 2016, underscoring the need for improved therapies. Evading cell death is a major hallmark of cancer, driving tumor progression and therapeutic resistance. To evade apoptosis, cancers use antiapoptotic Bcl-2 proteins to bind to and neutralize apoptotic activators, such as Bim. Investigation of antiapoptotic Bcl-2 family members in clinical breast cancer datasets revealed greater expression and more frequent gene amplification of MCL1 as compared with BCL2 or BCL2L1 (Bcl-xL) across three major molecular breast cancer subtypes, Luminal (A and B), HER2-enriched, and Basal-like. While Mcl-1 protein expression was elevated in estrogen receptor α (ERα)-positive and ERα-negative tumors as compared with normal breast, Mcl-1 staining was higher in ERα⁺ tumors. Targeted Mcl-1 blockade using RNAi increased caspase-mediated cell death in ERα⁺ breast cancer cells, resulting in sustained growth inhibition. In contrast, combined blockade of Bcl-2 and Bcl-xL only transiently induced apoptosis, as cells rapidly acclimated through Mcl-1 upregulation and enhanced Mcl-1 activity, as measured in situ using Mcl-1/Bim proximity ligation assays. Importantly, MCL1 gene expression levels correlated inversely with sensitivity to pharmacologic Bcl-2/Bcl-xL inhibition in luminal breast cancer cells, whereas no relationship was seen between the gene expression of BCL2 or BCL2L1 and sensitivity to Bcl-2/Bcl-xL inhibition. These results demonstrate that breast cancers rapidly deploy Mcl-1 to promote cell survival, particularly when challenged with blockade of other Bcl-2 family members, warranting the continued development of Mcl-1-selective inhibitors for targeted tumor cell killing.Implications: Mcl-1 levels predict breast cancer response to inhibitors targeting other Bcl-2 family members, and demonstrate the key role played by Mcl-1 in resistance to this drug class. Mol Cancer Res; 15(3); 259-68. ©2016 AACR.

Triple-negative breast cancers with amplification of JAK2 at the 9p24 locus demonstrate JAK2-specific dependence

Amplifications at 9p24 have been identified in breast cancer and other malignancies, but the genes within this locus causally associated with oncogenicity or tumor progression remain unclear. Targeted next-generation sequencing of postchemotherapy triple-negative breast cancers (TNBCs) identified a group of 9p24-amplified tumors, which contained focal amplification of the Janus kinase 2 (JAK2) gene. These patients had markedly inferior recurrence-free and overall survival compared to patients with TNBC without JAK2 amplification. Detection of JAK2/9p24 amplifications was more common in chemotherapy-treated TNBCs than in untreated TNBCs or basal-like cancers, or in other breast cancer subtypes. Similar rates of JAK2 amplification were confirmed in patient-derived TNBC xenografts. In patients for whom longitudinal specimens were available, JAK2 amplification was selected for during neoadjuvant chemotherapy and eventual metastatic spread, suggesting a role in tumorigenicity and chemoresistance, phenotypes often attributed to a cancer stem cell-like cell population. In TNBC cell lines with JAK2 copy gains or amplification, specific inhibition of JAK2 signaling reduced mammosphere formation and cooperated with chemotherapy in reducing tumor growth in vivo. In these cells, inhibition of JAK1-signal transducer and activator of transcription 3 (STAT3) signaling had little effect or, in some cases, counteracted JAK2-specific inhibition. Collectively, these results suggest that JAK2-specific inhibitors are more efficacious than dual JAK1/2 inhibitors against JAK2-amplified TNBCs. Furthermore, JAK2 amplification is a potential biomarker for JAK2 dependence, which, in turn, can be used to select patients for clinical trials with JAK2 inhibitors.

Mouse Models of Breast Cancer: Platforms for Discovering Precision Imaging Diagnostics and Future Cancer Medicine

Representing an enormous health care and socioeconomic challenge, breast cancer is the second most common cancer in the world and the second most common cause of cancer-related death. Although many of the challenges associated with preventing, treating, and ultimately curing breast cancer are addressable in the laboratory, successful translation of groundbreaking research to clinical populations remains an important barrier. Particularly when compared with research on other types of solid tumors, breast cancer research is hampered by a lack of tractable in vivo model systems that accurately recapitulate the relevant clinical features of the disease. A primary objective of this article was to provide a generalizable overview of the types of in vivo model systems, with an emphasis primarily on murine models, that are widely deployed in preclinical breast cancer research. Major opportunities to advance precision cancer medicine facilitated by molecular imaging of preclinical breast cancer models are discussed.

Rictor/mTORC2 Drives Progression and Therapeutic Resistance of HER2-Amplified Breast Cancers

HER2 overexpression drives Akt signaling and cell survival and HER2-enriched breast tumors have a poor outcome when Akt is upregulated. Akt is activated by phosphorylation at T308 via PI3K and S473 via mTORC2. The importance of PI3K-activated Akt signaling is well documented in HER2-amplified breast cancer models, but the significance of mTORC2-activated Akt signaling in this setting remains uncertain. We report here that the mTORC2 obligate cofactor Rictor is enriched in HER2-amplified samples, correlating with increased phosphorylation at S473 on Akt. In invasive breast cancer specimens, Rictor expression was upregulated significantly compared with nonmalignant tissues. In a HER2/Neu mouse model of breast cancer, genetic ablation of Rictor decreased cell survival and phosphorylation at S473 on Akt, delaying tumor latency, penetrance, and burden. In HER2-amplified cells, exposure to an mTORC1/2 dual kinase inhibitor decreased Akt-dependent cell survival, including in cells resistant to lapatinib, where cytotoxicity could be restored. We replicated these findings by silencing Rictor in breast cancer cell lines, but not silencing the mTORC1 cofactor Raptor (RPTOR). Taken together, our findings establish that Rictor/mTORC2 signaling drives Akt-dependent tumor progression in HER2-amplified breast cancers, rationalizing clinical investigation of dual mTORC1/2 kinase inhibitors and developing mTORC2-specific inhibitors for use in this setting. Cancer Res; 76(16); 4752-64. ©2016 AACR.

RAS/MAPK Activation Is Associated with Reduced Tumor-Infiltrating Lymphocytes in Triple-Negative Breast Cancer: Therapeutic Cooperation Between MEK and PD-1/PD-L1 Immune Checkpoint Inhibitors

PURPOSE

Tumor-infiltrating lymphocytes (TIL) in the residual disease (RD) of triple-negative breast cancers (TNBC) after neoadjuvant chemotherapy (NAC) are associated with improved survival, but insight into tumor cell-autonomous molecular pathways affecting these features are lacking.

EXPERIMENTAL DESIGN

We analyzed TILs in the RD of clinically and molecularly characterized TNBCs after NAC and explored therapeutic strategies targeting combinations of MEK inhibitors with PD-1/PD-L1-targeted immunotherapy in mouse models of breast cancer.

RESULTS

Presence of TILs in the RD was significantly associated with improved prognosis. Genetic or transcriptomic alterations in Ras-MAPK signaling were significantly correlated with lower TILs. MEK inhibition upregulated cell surface MHC expression and PD-L1 in TNBC cells both in vivo and in vitro. Moreover, combined MEK and PD-L1/PD-1 inhibition enhanced antitumor immune responses in mouse models of breast cancer.

CONCLUSIONS

These data suggest the possibility that Ras-MAPK pathway activation promotes immune-evasion in TNBC, and support clinical trials combining MEK- and PD-L1-targeted therapies. Furthermore, Ras/MAPK activation and MHC expression may be predictive biomarkers of response to immune checkpoint inhibitors.

Abstract B07: Mcl-1-mediated resistance to ABT-263 is combated by mTOR inhibition in luminal breast cancers

In the context of cancer, the intrinsic apoptotic pathway is exploited to favor tumor cell survival through overexpression of anti-apoptotic Bcl-2 family members (Bcl-A1, Bcl-2, Bcl-xL, Bcl-w and Mcl-1). We investigated targeting of anti-apoptotic Bcl-2 proteins in a panel of human luminal breast cancer cell lines. Use of ABT-263, the Bcl-2/Bcl-xL/Bcl-w inhibitor, induced transient tumor cell killing and decreased tumor cell growth in only 1 of 4 cell lines in three dimensional (3D) cultures. Mcl-1 expression and activity were rapidly upregulated upon ABT-263 treatment, highlighting the compensatory nature of Mcl-1. In luminal breast cancers, Mcl-1 was the most frequently amplified anti-apoptotic Bcl-2 family member according to The Cancer Genome Atlas, while nearly 80% of the luminal breast tumor epithelium was positive for Mcl-1 in a tissue microarray. Thus, we hypothesized that Mcl-1 may be a dominant tumor cell survival factor in luminal breast cancers.

Use of the mTOR inhibitor RAD001 (everolimus) to target Mcl-1 indirectly decreased tumor cell growth and increased tumor cell killing in 3 of 4 cell lines, as well as WAP-Myc luminal mammary tumors, demonstrating the effectiveness of Mcl-1 as a therapeutic target in breast cancers. While Mcl-1 inhibition alone did not affect growth of T47D cells, robust growth inhibition and tumor cell killing were seen in all cell lines upon inhibition of Mcl-1 using RAD001 or a EIF4 complex inhibitor (4E1RCat) in combination with ABT-263, suggesting that the induction of Mcl-1 upon ABT-263 treatment may be supported by increased cap-dependent translation. These results demonstrate that the sensitivity of luminal breast cancers to ABT-263 is enhanced by Mcl-1 inhibition, warranting further investigation into inhibition of anti-apoptotic Bcl-2 family proteins, in particular Mcl-1, as a clinical strategy to improve survival of patients with luminal breast cancers.

Citation Format: Michelle M. Williams, Linus Lee, Meghan M. Morrison, Courtney McKernan, Violeta Sanchez, Donna Hicks, Thomas Stricker, Rebecca S. Cook. Mcl-1-mediated resistance to ABT-263 is combated by mTOR inhibition in luminal breast cancers. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr B07.

Abstract A126: Taking out the trash: Efferocytosis in the tumor microenvironment

We have examined the malignant tumor microenvironment (TME) from a perspective that is often noted but overlooked: tissue repair. It has long been observed that tumors resemble ‘wounds that do not heal,’ but the reasons remain obscured. We found that macrophages in the tumor microenvironment orchestrate wound healing and immune tolerance in response to tumor cell death, a feature common to all solid tumors. We used genetic and pharmacological methods to block macrophage-mediated engulfment of dying tumor cells, a process referred to as efferocytosis. We found that MerTK-mediated efferocytosis of dying tumor cells induces macrophage-mediated production of Th2-like cytokines, including IL-4, IL-13, IL-10, and TGF-beta. Blockade of the receptor tyrosine kinase MerTK through pharmacological tyrosine kinase inhibition or through genetic MerTK abalation blocked macrophage-mediated efferocytosis in the tumor microenvironment, impairing M2-like macrophage polarization and expression of Th2-like cytokines in the tumor microenvironment. Because cell death is a major histological feature of the breast during post-partum involution of milk-producing breast epithelia, we studied the impact of MerTK-mediated efferocytosis in a model of post-partum breast cancer (ppBC). Currently, ppBC accounts for nearly 25% of all breast cancers in young (pre-menopausal) women. In contrast to breast cancers diagnosed during pregnancy, which correlate with a favorable prognosis, ppBCs are highly aggressive, metastatic, and life-threatening, even when corrected for molecular breast cancer subtype and for the age of the woman at diagnosis. We used genetic and pharmacological methods to block MerTK-mediated efferocytosis in ppBCs, demonstrating that efferocytosis increased ppBC metastasis by nearly 10-fold, but that genetic or pharmacological MerTK inhibition markedly decreased metastasis in ppBCS. Similarly, targeted inhibition of efferocytosis-induced cytokines, including TGF-beta, substantially reduced metastasis of efferocytosis-competent ppBCs. Collagen deposition, TReg expansion, and Th2 cytokine upregulation were found to be dependent on efferocytosis-induced TGF-beta signaling. These findings in ppBC, in which widespread tumor cell death was initiated by physiological post-partum involution, prompted our interest in the impact of therapeutically-induced cell death and subsequent efferocytosis on pro-malignant leukocyte behaviors in the tumor microenvironment. As many as 70-80% of primary breast tumors treated with cytotoxic chemotherapy show a partial response in the pre-surgical (neoadjuvant) setting, and lack of a pathological complete response (pCR) is a strong predictor of tumor recurrence following surgical tumor excision. We tested the hypothesis that efferocytosis, the immune system's inherent response to cell death, results in tumor repair (increased ‘wound healing’) in response to cytotoxic treatment. In support of this hypothesis, a single dose of doxorubicin or lapatinib induced a transient wave tumor cell death within 24 hours, followed at post-treatment day 4 with increased M2 macrophages and TRegs within the tumor microenvironment, and increased expression of Th2-like cytokines. Importantly, increased metastases were seen in tumor-bearing mice treated once with doxorubicin over what was seen in untreated mice. However, targeted inhibition of efferocytosis using a MerTK kinase inhibitor (TKIs) in combination with doxorubicin blocked expression of Th2-like cytokines, limited the number of M2-like macrophages in the tumor microenvironment, and prevented therapy-enhanced tumor metastasis. These findings support continued investigation into the role of MerTK and efferocytosis in therapeutic tumor responses.

Note:This abstract was not presented at the conference.

Citation Format: Rebecca S. Cook. Taking out the trash: Efferocytosis in the tumor microenvironment. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A126.

Dual inhibition of Type I and Type III PI3 kinases increases tumor cell apoptosis in HER2+ breast cancers

Introduction

Human epidermal growth factor receptor-2 (HER2) gene amplification (HER2+) drives tumor cell growth and survival in ~25 % of breast cancers. HER2 signaling activates the type I phosphoinositide 3-kinase (PI3K), upon which these tumors rely. Consequently, inhibitors of HER2 and type I PI3K block growth and increase apoptosis in HER2+ breast cancers, especially when used in combination. However, the impact of type III PI3K inhibition, particularly in combination with HER2 blockade or type I PI3K inhibition, remains less clear.

Methods

We utilized small molecule kinase inhibitors, locked nucleic acid antisense oligonucleotides (LNA-ASOs), and siRNA to assess proliferation, autophagy, apoptosis, and protein expression in cell culture models of HER2+ breast cancers.

Results

Treatment of HER2+ breast cancer cells with HER2 inhibitors or type I PI3K kinase inhibitors, alone or in combination, blocked type I PI3K signaling, reduced tumor cell growth, and induced autophagy. Knockdown of the type I PI3K, p110α, using an LNA-ASO termed EZN4150 inhibited PI3K-mediated Akt phosphorylation. However, in contrast to catalytic inhibitors of type I PI3Ks, EZN4150 did not induce autophagy, and blocked autophagy in response to inhibitors of HER2 or type I PI3Ks in a dominant fashion. Sequence analysis of EZN4150 revealed significant homology to the gene encoding the type III PI3K, Vps34, a key component for autophagy induction. EZN4150 simultaneously reduced expression of both p110α and Vps34. Combined inhibition of PI3K signaling and autophagy using individual siRNAs against p110α and Vps34 or using pharmacological type I and type III PI3K inhibitors recapitulated what was seen with EZN4150, and robustly enhanced tumor cell killing.

Conclusions

These studies highlight the important role of Vps34-mediated autophagy in limiting the anti-tumor response to inhibitors of HER2 or type I PI3K in HER2+ breast cancers. The type III PI3K Vps34 represents a potential therapeutic target to block treatment-induced autophagy and enhance tumor cell killing.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-015-0656-2) contains supplementary material, which is available to authorized users.

Receptor tyrosine kinase ERBB4 mediates acquired resistance to ERBB2 inhibitors in breast cancer cells

Approximately 25% of breast cancers overexpress and depend on the receptor tyrosine kinase ERBB2, one of 4 ERBB family members. Targeted therapies directed against ERBB2 have been developed and used clinically, but many patients continue to develop resistance to such therapies. Although much effort has been focused on elucidating the mechanisms of acquired resistance to ERBB2-targeted therapies, the involvement of ERBB4 remains elusive and controversial. We demonstrate that genetic ablation of ERBB4, but not ERBB1-3, led to apoptosis in lapatinib-resistant cells, suggesting that the efficacy of pan-ERBB inhibitors was, at least in part, mediated by the inhibition of ERBB4. Moreover, ERBB4 was upregulated at the protein level in ERBB2+ breast cancer cell lines selected for acquired lapatinib resistance in vitro and in MMTV-Neu mice following prolonged lapatinib treatment. Knockdown of ERBB4 caused a decrease in AKT phosphorylation in resistant cells but not in sensitive cells, suggesting that ERBB4 activated the PI3K/AKT pathway in lapatinib-resistant cells. Importantly, ERBB4 knockdown triggered apoptosis not only in lapatinib-resistant cells but also in trastuzumab-resistant cells. Our results suggest that although ERBB4 is dispensable for naïve ERBB2+ breast cancer cells, it may play a key role in the survival of ERBB2+ cancer cells after they develop resistance to ERBB2 inhibitors, lapatinib and trastuzumab.