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.

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.

Abstract 511: Rictor/mTORC2 drives formation, progression and therapeutic resistance of HER2-amplified breast cancers

The phosphatidyl inositol-3-kinase (PI3K)/Akt signaling pathway is aberrantly activated in nearly 60% of breast cancers, through HER2 amplification, PIK3CA mutation, PTEN inactivation, and other alterations. The mTOR complexes mTORC1 and mTORC2 operate as activators (mTORC2) or effectors (mTORC1) of Akt. We found that mRNA upregulation in the mTORC2 cofactors RICTOR and MAPKAP1 correlated with poor survival in breast cancer patients, while upregulation in the mTORC1 cofactor RPTOR did not. Genomic gains of RICTOR and MAPKAP1 occurred more frequently in breast cancers with PI3K pathway alterations than genomic RPTOR gains. Importantly, Rictor protein levels were increased in invasive over non-invasive breast tumors, while Raptor levels were not. RICTOR gene ablation in a transgenic mouse model of HER2-amplified breast cancer delayed tumorigenesis and decreased lung metastasis, Akt-S473 phosphorylation, cell growth and survival. In human HER2-amplified breast cancer cells, Rictor loss, but not Raptor loss, decreased Akt-S473 phosphorylation, reducing cell survival and motility/invasion. Interestingly, Rictor/mTORC2 loss or treatment with a dual mTORC1/2 inhibitor improved lapatinib-induced cell killing in parental and lapatinib-resistant tumor cells to a greater extent than mTORC1 inhibition. Akt re-activation rescued cell survival, but not motility/invasion, in Rictor-depleted cells. However, Rictor loss caused accumulation of the Rac inhibitor RhoGDI2, thus impairing Rac1-dependent invasion. We conclude that HER2-amplified breast cancers use Rictor/mTORC2 to drive Akt-mediated cell survival and Rac1-mediated cellular invasion. These studies support additional studies into mTORC2-specific inhibitors.

Citation Format: Meghan M. Morrison, Bayley Jones, Violeta Sanchez, Monica V. Estrada, Donna Hicks, Michelle Williams, Dana Brantley-Sieders, Rebecca Cook. Rictor/mTORC2 drives formation, progression and therapeutic resistance of HER2-amplified breast cancers. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 511. doi:10.1158/1538-7445.AM2015-511

Abstract 4198: mTORC2 directs breast morphogenesis through Rictor-dependent PKCα/Rac1 signaling independent of Akt

Akt phosphorylation is a major driver of cell survival, motility, and proliferation in development and disease, causing increased interest in upstream regulators of Akt like mTOR complex 2 (mTORC2). We used genetic disruption of Rictor to impair mTORC2 activity in mouse mammary epithelia, which decreased Akt phosphorylation, ductal length, secondary branching, cell motility, and cell survival. These effects were recapitulated with a pharmacological dual inhibitor of mTORC1/mTORC2, but not upon genetic disruption of mTORC1 function via Raptor knock-out. Surprisingly, Akt re-activation was insufficient to rescue survival, branching, or motility of mTORC2-impaired mammary epithelial cells (MECs). However, activation of the mTORC2 substrate protein kinase C (PKC)-α fully rescued branching, invasion, and survival of MECs after genetic or pharmacological mTORC2 inhibition. PKCα-mediated signaling through the small GTPase Rac1 was necessary for mTORC2-dependent mammary morphogenesis, revealing a novel role for Rictor/mTORC2 in survival and motility of untransformed MECs through an Akt-independent, PKCα/Rac1-dependent mechanism. Ongoing analyses of mammary specific Rictor-deletion in genetically engineered mouse models of breast cancer support a key role for mTORC2 in tumor progression and metastasis.

Citation Format: Dana M. Brantley-Sieders, Meghan M. Morrison, Donna J. Hicks, Rebecca S. Cook. mTORC2 directs breast morphogenesis through Rictor-dependent PKCα/Rac1 signaling independent of Akt. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4198. doi:10.1158/1538-7445.AM2015-4198

Abstract B49: mTORC2 directs breast morphogenesis through Rictor-dependent PKCα/Rac1 signaling independent of Akt

Akt phosphorylation is a major driver of cell survival, motility, and proliferation in development and disease, causing increased interest in upstream regulators of Akt like mTOR complex 2 (mTORC2). We used genetic disruption of Rictor to impair mTORC2 activity in mouse mammary epithelia, which decreased Akt phosphorylation, ductal length, secondary branching, cell motility, and cell survival. These effects were recapitulated with a pharmacological dual inhibitor of mTORC1/mTORC2, but not upon genetic disruption of mTORC1 function via Raptor knock-out. Surprisingly, Akt re-activation was insufficient to rescue survival, branching, or motility of mTORC2-impaired mammary epithelial cells (MECs). However, activation of the mTORC2 substrate protein kinase C (PKC)-α fully rescued branching, invasion, and survival of MECs after genetic or pharmacological mTORC2 inhibition. PKCα-mediated signaling through the small GTPase Rac1 was necessary for mTORC2-dependent mammary morphogenesis, revealing a novel role for Rictor/mTORC2 in survival and motility of untransformed MECs through an Akt-independent, PKCα/Rac1-dependent mechanism.

Citation Format: Meghan M. Morrison, Christian D. Young, Shan Wang, Violeta M. Sanchez, Cook S. Rebecca, Donna J. Hicks, Dana M. Brantley-Sieders. mTORC2 directs breast morphogenesis through Rictor-dependent PKCα/Rac1 signaling independent of Akt. [abstract]. In: Proceedings of the AACR Special Conference: Targeting the PI3K-mTOR Network in Cancer; Sep 14-17, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(7 Suppl):Abstract nr B49.

Abstract B09: Targeting the anti-apoptotic protein Mcl-1 with the mTOR inhibitor RAD001 sensitizes luminal breast cancers to the Bcl-2/Bcl-xL inhibitor ABT-263

In an effort to enhance therapeutic tumor cell killing in luminal breast cancers, we investigated the inhibition of the anti-apoptotic Bcl-2 family proteins Bcl-2, Bcl-xL and Mcl-1. Anti-apoptotic Bcl-2 proteins are inhibitors of the intrinsic apoptotic pathway that sequester and inhibit pro-apoptotic Bcl-2 family members. Sustained overexpression of anti-apoptotic Bcl-2 family members frequently occurs in cancers and supports tumor initiation, tumor progression, therapeutic resistance and poor patient survival. While only 2/324 and 1/324 luminal breast cancers curated by The Cancer Genome Atlas (TCGA) had gene amplification of BCL2 and BCL2L1 (encoding Bcl-xL) respectively, 21/324 (<8%) displayed MCL1 amplification. Wap-Myc transgenic mouse mammary tumors, which are highly enriched for the luminal A transcription signature, displayed profoundly increased Mcl-1 levels as compared to normal mammary tissue, suggesting a role for Mcl-1 in luminal breast cancer biology. Using a panel of four MCL1-amplified breast cancer cell lines, we found that two were marginally sensitive to treatment with ABT-263, a BH3-mimetic that targets Bcl-2 and Bcl-xL. Conversely, ectopic overexpression of Mcl-1 decreased sensitivity to ABT-263. ABT-263 induced minimal cell killing in MCF7 xenografts and had no effect on tumor growth. Similarly, ABT-263 did not affect tumor growth in luminal-like Wap-Myc mouse mammary tumors. Thus, luminal breast cancer cells expressing increased Mcl-1 are poorly responsive to Bcl-2/Bcl-xL inhibition.

Upregulation of Mcl-1 expression and activity, an established ABT-263-resistance mechanism in leukemias and lymphomas, was observed in MCL1-amplified luminal breast cancer cell lines treated with ABT-263 in culture and in vivo. ABT-263 also increased the anti-apoptotic activity of Mcl-1 towards the pro-apoptotic Bcl-2 family member BIM, suggesting that Mcl-1 upregulation compensates for inhibition of Bcl-2 and Bcl-xL by ABT-263. There are currently no Mcl-1 specific inhibitors. However, Mcl-1 undergoes cap-dependent translation, and thus inhibition of cap-dependent translation has been proposed as a method to inhibit the protein expression of Mcl-1. Importantly, the mTOR pathway is required for efficient cap-dependent translation in many cancer cells. We found that the rapalogue RAD001/everolimus, an mTOR inhibitor, decreased Mcl-1 protein levels in MCL1-amplified luminal breast cancer cell lines. Furthermore, RAD001 blocked Mcl-1 protein upregulation in response to ABT-263, producing increased cell killing and decreased cell growth in all MCL1-amplified luminal breast cancer cell lines. Treatment of luminal Wap-Myc mammary tumors with RAD001 decreased tumor volume by 40%, while treatment with ABT-263 had no effect on tumor volume. These data suggest that Mcl-1 targeting may be superior to Bcl-2/Bcl-xL targeting in luminal breast cancers, particularly in luminal breast cancers with MCL1-amplification. Furthermore, we have identified the mTOR pathway as a potential therapeutic target for blocking Mcl-1 expression in luminal breast cancers.

Citation Format: Michelle M. Williams, Linus Lee, Meghan M. Morrison, Andrew J. Williams, Violeta Sanchez, Donna Hicks, Rebecca Cook. Targeting the anti-apoptotic protein Mcl-1 with the mTOR inhibitor RAD001 sensitizes luminal breast cancers to the Bcl-2/Bcl-xL inhibitor ABT-263. [abstract]. In: Proceedings of the AACR Special Conference: Targeting the PI3K-mTOR Network in Cancer; Sep 14-17, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(7 Suppl):Abstract nr B09.

mTOR Directs Breast Morphogenesis through the PKC-alpha-Rac1 Signaling Axis

Akt phosphorylation is a major driver of cell survival, motility, and proliferation in development and disease, causing increased interest in upstream regulators of Akt like mTOR complex 2 (mTORC2). We used genetic disruption of Rictor to impair mTORC2 activity in mouse mammary epithelia, which decreased Akt phosphorylation, ductal length, secondary branching, cell motility, and cell survival. These effects were recapitulated with a pharmacological dual inhibitor of mTORC1/mTORC2, but not upon genetic disruption of mTORC1 function via Raptor deletion. Surprisingly, Akt re-activation was not sufficient to rescue cell survival or invasion, and modestly increased branching of mTORC2-impaired mammary epithelial cells (MECs) in culture and in vivo. However, another mTORC2 substrate, protein kinase C (PKC)-alpha, fully rescued mTORC2-impaired MEC branching, invasion, and survival, as well as branching morphogenesis in vivo. PKC-alpha-mediated signaling through the small GTPase Rac1 was necessary for mTORC2-dependent mammary epithelial development during puberty, revealing a novel role for Rictor/mTORC2 in MEC survival and motility during branching morphogenesis through a PKC-alpha/Rac1-dependent mechanism.

The protein kinase mTOR is frequently activated in breast cancers, where it enhances cancer cell growth, survival, and metastastic spread to distant organs. Thus, mTOR is an attractive, clinically relevant molecular target for drugs designed to treat metastatic breast cancers. However, mTOR exists in two distinct complexes, mTORC1 and mTORC2, and the relative roles of each complex have not been elucidated. Moreover, as pathways that regulate normal tissue growth and development are often highjacked to promote cancer, understanding mTOR function in normal mammary epithelial development will likely provide insight into its role in tumor progression. In this study, we assessed the role of mTORC1 and mTORC2 complexes in normal mammary epithelial cell branching, survival, and invasion. Interestingly, while mTORC1 was not required for branching, survival and invasion of mammary epithelial cells, mTORC2 was necessary for these processes in both mouse and human models. Furthermore, we found that mTORC2 exerts its effects primarily through downstream activation of a PKC-alpha-Rac1 signaling axis rather than the more well-studied Akt signaling pathway. Our studies identify a novel role for the mTORC2 complex in mammary morphogenesis, including cell survival and motility, which are relevant to breast cancer progression.

Abstract B28: Rictor/mTORC2 drives formation, progression, and therapeutic resistance of HER2-amplified breast cancers

Although HER2-targeted therapies substantially improve the outcome for patients with HER2-amplified breast cancer, resistance to HER2 targeting is frequently observed. HER2-amplified tumors depend on the HER2-mediated activation of the phosphatidylinositol-3kinase (PI3K)/Akt pathway, which becomes reactivated in many therapeutically resistant HER2-amplified tumors. We investigated the mTOR complexes mTORC1 and mTORC2 as potential therapeutic targets in HER2-amplified breast cancers, based on their roles as activators (mTORC2) or effectors (mTORC1) of Akt. We found that genomic gains in Rictor, encoding a necessary mTORC2 cofactor, correlated with poor survival in breast cancer patients, while genomic gains in Raptor, an mTORC1 cofactor, did not. Rictor protein levels were increased in invasive breast cancers as compared to ductal carcinoma in situ (DCIS). Conditional Rictor gene ablation decreased growth, cell survival and invasion of human HER2-amplified breast cancer cells by blocking Akt signaling. Treatment of HER2 positive cells with the HER2/EGFR inhibitor Lapatinib caused Rictor upregulation. However, Rictor knockdown improved cell killing in Lapatinib-treated cells and impaired Akt signaling and cell survival in Lapatinib-resistant HER2 positive breast cancer cells. Together, these results demonstrate the pivotal role of mTORC2 in HER2-amplified breast cancers and support efforts to develop mTORC2-targeted therapies.

Citation Format: Meghan M. Morrison, Bayley A. Jones, Donna J. Hicks, Violeta Sanchez, Valeria M. Estrada, Michelle M. Williams, Dana Brantley-Sieders, Rebecca S. Cook. Rictor/mTORC2 drives formation, progression, and therapeutic resistance of HER2-amplified breast cancers. [abstract]. In: Proceedings of the AACR Special Conference: Targeting the PI3K-mTOR Network in Cancer; Sep 14-17, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(7 Suppl):Abstract nr B28.

Activating PIK3CA Mutations Induce an Epidermal Growth Factor Receptor (EGFR)/Extracellular Signal-regulated Kinase (ERK) Paracrine Signaling Axis in Basal-like Breast Cancer

Mutations in PIK3CA, the gene encoding the p110α catalytic subunit of phosphoinositide 3-kinase (PI3K) have been shown to transform human mammary epithelial cells (MECs). These mutations are present in all breast cancer subtypes, including basal-like breast cancer (BLBC). Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), we identified 72 protein expression changes in human basal-like MECs with knock-in E545K or H1047R PIK3CA mutations versus isogenic MECs with wild-type PIK3CA. Several of these were secreted proteins, cell surface receptors or ECM interacting molecules and were required for growth of PIK3CA mutant cells as well as adjacent cells with wild-type PIK3CA. The proteins identified by MS were enriched among human BLBC cell lines and pointed to a PI3K-dependent amphiregulin/EGFR/ERK signaling axis that is activated in BLBC. Proteins induced by PIK3CA mutations correlated with EGFR signaling and reduced relapse-free survival in BLBC. Treatment with EGFR inhibitors reduced growth of PIK3CA mutant BLBC cell lines and murine mammary tumors driven by a PIK3CA mutant transgene, all together suggesting that PIK3CA mutations promote tumor growth in part by inducing protein changes that activate EGFR.

Efferocytosis produces a prometastatic landscape during postpartum mammary gland involution

Breast cancers that occur in women 2-5 years postpartum are more frequently diagnosed at metastatic stages and correlate with poorer outcomes compared with breast cancers diagnosed in young, premenopausal women. The molecular mechanisms underlying the malignant severity associated with postpartum breast cancers (ppBCs) are unclear but relate to stromal wound-healing events during postpartum involution, a dynamic process characterized by widespread cell death in milk-producing mammary epithelial cells (MECs). Using both spontaneous and allografted mammary tumors in fully immune-competent mice, we discovered that postpartum involution increases mammary tumor metastasis. Cell death was widespread, not only occurring in MECs but also in tumor epithelium. Dying tumor cells were cleared through receptor tyrosine kinase MerTK-dependent efferocytosis, which robustly induced the transcription of genes encoding wound-healing cytokines, including IL-4, IL-10, IL-13, and TGF-β. Animals lacking MerTK and animals treated with a MerTK inhibitor exhibited impaired efferocytosis in postpartum tumors, a reduction of M2-like macrophages but no change in total macrophage levels, decreased TGF-β expression, and a reduction of postpartum tumor metastasis that was similar to the metastasis frequencies observed in nulliparous mice. Moreover, TGF-β blockade reduced postpartum tumor metastasis. These data suggest that widespread cell death during postpartum involution triggers efferocytosis-induced wound-healing cytokines in the tumor microenvironment that promote metastatic tumor progression.

Abstract B048: The role of mTORC2 in mammary morphogenesis and HER2-mediated tumorigenesis

Breast cancer is the most common malignancy among western women. Approximately 20% of breast cancers exhibit overexpression of human epithelial growth factor receptor 2 (HER2), a marker of aggressive disease. While HER2-targeted therapies have improved patient survival in this subset, resistance to these molecularly-targeted therapies often occurs, underscoring the need for increased understanding of the signaling pathways required for HER2-mediated transformation and tumor malignancy. As such, transformation by HER2 requires signaling through the phosphoinositide 3-kinase (PI3K)/Akt signaling cascade, that regulates cell growth/survival, metabolism and motility. While targeting PI3K/Akt in HER2-positive breast cancers has been met with some clinical success, it is also associated with extreme toxicity. Understanding factors both upstream and downstream of PI3K is necessary to improve patient morbidity and mortality and will allow us to understand the earliest events driving HER2-related tumorigenesis.

The serine/threonine kinase mammalian target of rapamycin (mTOR) controls key cellular processes, including growth, survival and metabolism. mTOR operates downstream and within the PI3K/Akt pathway within two structurally and functionally distinct complexes known as mTORC1 and mTORC2, whose activity and substrate specificity are regulated by complex-specific cofactors. Specifically, the protein Raptor is a required cofactor for the rapamycin-sensitive mTORC1 complex, which is activated downstream of PI3K/Akt and mediates cell growth and metabolism. The protein Rictor is a required cofactor for mTORC2, which controls cell survival, polarity, and cytoskeletal dynamics. mTORC2 supports Akt activation through direct phosphorylation on S473, which is necessary for maximal PI3K/Akt signaling. The clinical efficacy of rapamycin in luminal breast cancers underscores the importance of mTOR signaling in breast cancers and supports its role as an effector of PI3K. However, little is known about the distinct role of mTORC2 in breast cancer. Interestingly, emerging evidence from prostate and glioblastoma models suggest a direct link between mTORC2 and PI3K-driven cancer progression.

Our preliminary data shows that loss of Rictor/mTORC2, but not Raptor/mTORC1, in primary mammary epithelial cells (PMECs) impairs branching morphogenesis, proliferation, survival and phosphorylation of Akt (S473), leading us to hypothesize that mTORC2 drives PI3K/Akt-mediated cell survival of normal and HER2-transformed breast epithelial cells. We show that loss of Rictor/mTORC2 decreases three-dimensional growth, survival, migration and phosphorylation of Akt (S473) in HER2-positive breast cancer cells and HER2-transformed MCF10A mammary epithelial cells. Additionally, Rictor loss sensitizes HER2-positive breast cancer cells to the HER2 inhibitor, lapatinib. These and additional results from in vivo studies would support investigations into novel treatment combinations to improve the outcome for patients with HER2-amplified breast cancers.

Impact: mTORC2 remains understudied in breast cancers. This study will be the first to examine the role of mTORC2 in mammary morphogenesis and in HER2-transformed breast cancers, providing unprecedented knowledge and mechanistic understanding of mTORC2 in breast cancers. Because HER2 is overexpressed in nearly 20% of all breast cancers, these findings will impact a significant number of patients.

Citation Format: Meghan M. Morrison, Dana Brantley-Sieders, Donna Hicks, Violeta Sanchez, Rebecca Cook. The role of mTORC2 in mammary morphogenesis and HER2-mediated tumorigenesis. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr B048.

Abstract A053: ErbB3 downregulation enhances luminal breast tumor response to antiestrogens

HER3/ErbB3 is required in luminal mammary epithelial cells (MECs). Given that breast cancer phenotypes may reflect biological traits of the MECs from which they originate, we tested the hypothesis that HER3/ErbB3 drives luminal breast cancer growth. We found higher ERBB3 expression and frequent ERBB3 gene copy gains in clinical Luminal A/B breast cancers over other molecular subtypes. ErbB3 increased growth of luminal breast cancer cells, while ErbB3 targeting using U3-1287, a fully-human monoclonal anti-ErbB3 antibody, decreased three-dimensional colony growth, increased apoptosis, and decreased tumor growth in vivo. In response to the anti-endocrine fulvestrant, ErbB3 expression increased in clinical breast tumors, as did phosphatidylinositol 3-kinase (PI3K)/mTOR signaling. However, ErbB3 targeting with U3-1287 blocked compensatory PI3K/mTOR signaling in fulvestrant-treated tumor cells, transcriptional responses to fulvestrant, tumor cell survival and tumor growth. U3-1287 produced these effects in tumor cells with and without genetic PI3K pathway aberrations (PIK3CAH1047R, PIK3CAE545K, PTEN loss). Although fulvestrant increased phosphorylation of all ErbB family RTKs and insulin-like growth factor-1 receptor, phospho-RTK upregulation was not seen in tumors treated with U3-1287 and fulvestrant. Therefore, luminal breast cancer cells utilize ErbB3 for growth, survival, and to limit response to fulvestrant, thus defining a novel role for ErbB3 in breast cancer.

Citation Format: Meghan M. Morrison, Jamie Stanford, Christian Young, David Vaught, Andrew Williams, michelle williams, Chuck M. Perou, Carlos L. Arteaga, Rebecca S. Cook. ErbB3 downregulation enhances luminal breast tumor response to antiestrogens. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr A053.

Abstract A118: The mTORC2 component Rictor directs branching morphogenesis and survival in the mammary epithelium

The intracellular serine/threonine kinase mTOR functions in two distinct complexes that integrate many cellular cues to regulate diverse processes in development and disease. Raptor-dependent mTOR Complex-1 (mTORC1) mediates cell growth and metabolism, whereas the less well-studied Rictor-dependent mTOR Complex-2 (mTORC2) promotes cell survival and cytoskeletal organization. Previous studies link mTORC1 to malignant transformation of mammary epithelial cells (MECs). The role of mTOR and the relative contributions of mTORC1 versus mTORC2 in postnatal mammary morphogenesis, however, remain unclear. We report that mTORC2 ablation via Rictor loss impaired branching morphogenesis, MEC survival, MEC migration, and mTORC2-mediated signaling through Akt and Rac1-GTPase in vivo and ex vivo. Constitutively active Rac1 (CA-Rac1) rescued branching, invasion, and survival of mTORC2-impaired MECs, but CA-Akt did not. These effects were recapitulated by rapamycin treatment. Surprisingly, genetic mTORC1 disruption via Raptor gene targeting did not affect ascinar mammary morphogenesis in organoids or in vivo, but did cause decreased signaling through the mTORC1 effector ribosomal protein S6 and transiently slowed cellular proliferation. These data demonstrate the distinct, non-overlapping properties of mTORC1 and mTORC2 during post-natal mammary development, roles which may be preserved in breast cancers.

Citation Format: Meghan M. Morrison, Donna Hicks, Rebecca S. Cook, Dana M. Brantley-Sieders. The mTORC2 component Rictor directs branching morphogenesis and survival in the mammary epithelium. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr A118.

ErbB3 downregulation enhances luminal breast tumor response to antiestrogens

Aberrant regulation of the erythroblastosis oncogene B (ErbB) family of receptor tyrosine kinases (RTKs) and their ligands is common in human cancers. ErbB3 is required in luminal mammary epithelial cells (MECs) for growth and survival. Since breast cancer phenotypes may reflect biological traits of the MECs from which they originate, we tested the hypothesis that ErbB3 drives luminal breast cancer growth. We found higher ERBB3 expression and more frequent ERBB3 gene copy gains in luminal A/B breast cancers compared with other breast cancer subtypes. In cell culture, ErbB3 increased growth of luminal breast cancer cells. Targeted depletion of ErbB3 with an anti-ErbB3 antibody decreased 3D colony growth, increased apoptosis, and decreased tumor growth in vivo. Treatment of clinical breast tumors with the antiendocrine drug fulvestrant resulted in increased ErbB3 expression and PI3K/mTOR signaling. Depletion of ErbB3 in fulvestrant-treated tumor cells reduced PI3K/mTOR signaling, thus decreasing tumor cell survival and tumor growth. Fulvestrant treatment increased phosphorylation of all ErbB family RTKs; however, phospho-RTK upregulation was not seen in tumors treated with both fulvestrant and anti-ErbB3. These data indicate that upregulation of ErbB3 in luminal breast cancer cells promotes growth, survival, and resistance to fulvestrant, thus suggesting ErbB3 as a target for breast cancer treatment.

Targeting eNOS in pancreatic cancer

Mortality from pancreatic ductal adenocarcinoma cancer (PDAC) is among the highest of any cancer and frontline therapy has changed little in years. Activation of endothelial nitric oxide synthase (eNOS, NOS3, or NOS III) has been implicated recently in the pathogenesis of PDACs. In this study, we used genetically engineered mouse and human xenograft models to evaluate the consequences of targeting eNOS in PDACs. Genetic deficiency in eNOS limited the development of preinvasive pancreatic lesions and trended toward an extended lifespan in mice with advanced pancreatic cancer. These effects were also observed upon oral administration of the clinically evaluated NOS small molecule inhibitor N(G)-nitro-L-arginine methyl ester (l-NAME). Similarly, other transgenic models of oncogenic KRas-driven tumors responded to l-NAME treatment. Finally, these results were recapitulated in xenograft models of human pancreatic cancer, in which l-NAME was found to broadly inhibit tumorigenic growth. Taken together, our findings offer preclinical proof-of-principle to repurpose l-NAME for clinical investigations in treatment of PDACs and possibly other KRas-driven human cancers.

Prevalence of modifiable risk factors for sudden infant death syndrome in British Forces Germany

A questionnaire survey was conducted amongst parents in the military community in British Forces Germany to investigate the prevalence of known and suspected risk factors for Sudden Infant Death Syndrome. Over a thousand questionnaires were returned (response rate 58%) and these showed that the prevalence of babies being placed in the prone position to sleep is now extremely low and the use of room thermometers to help control ambient temperature is widespread. However 29% of the mothers had smoked in pregnancy and 44% of households with a new-born baby had at least one parent who smoked. Additional health promotion activity aimed at reducing the prevalence of smoking in pregnancy and amongst the parents of new-born babies is recommended.

The interaction of a Huntington disease factor with receptors for the neurotoxin kainic acid

A factor from mammalian and human brain, which inhibits the rate of migration of leukocytes obtained from sufferers from Huntington disease (Walls and Ruwoldt, 1984), inhibited the specific binding of the neurotoxin [3H]kainic acid to rat brain synaptic membranes. The factor was present in sucrose-particulate but not in soluble fractions from rat sub-cortical tissue, and was destroyed by tryptic digestion. Whereas an ammonium sulfate fraction of direct saline extracts of brain (Walls and Ruwoldt, 1984) gave poor chromatography on HPLC, prior separation of a sucrose-particulate fraction resulted in much improved chromatography. There was a good concordance between leukocyte migration inhibitory activity and [3H]kainic acid binding inhibitory activity. The factor may be an endogenous modulator of the kainic acid subset of receptors for the excitatory neurotransmitter glutamic acid.

Uptake of gamma-aminobutyric acid and L-glutamic acid by synaptosomes from postmortem human cerebral cortex: multiple sites, sodium dependence and effect of tissue preparation

The uptake of gamma-aminobutyric acid (GABA) and L-glutamic acid by synaptosomes prepared from frozen postmortem human brain was shown to be effected via distinct high and low affinity sites. At approximately 17 h postmortem delay, the kinetic parameters for GABA uptake were: high affinity site, Km 7.1 +/- 2.5 microM, Vmax 18.7 +/- 4.8 nmol.min-1 per 100 mg protein; low affinity site, Km 2 +/- 1 mM, Vmax 425 +/- 250 nmol.min-1 per 100 mg protein (means +/- S.E.M., n = 13). Kinetic parameters for L-glutamate uptake were: high affinity site, Km 7.5 +/- 1.0 microM, Vmax 85 +/- 8 nmol.min-1 per 100 mg protein; low affinity site, Km 1.8 +/- 1.2 mM. Vmax 780 +/- 175 nmol.min-1 per 100 mg protein (n = 11). A detailed kinetic analysis of high affinity GABA uptake was performed over a range of sodium ion concentrations. The results were consistent with a coupling ratio of one Na+ ion to one GABA molecule; a similar result was found with rat brain synaptosomes. However, rat and human synaptosomes differed in the degree to which the substrate affinity of the high affinity GABA uptake site varied with decreasing Na+ ion concentration. High affinity GABA uptake was markedly affected by the method used to freeze and divide the tissue, but did not vary greatly in different cortical regions. There was some decline of high affinity GABA uptake activity with postmortem delay, apparently due to a loss of sites rather than a change in site affinity.

The apical and basal plasma membranes of the human placental syncytiotrophoblast contain different erythrocyte membrane protein isoforms. Evidence for placental forms of band 3 and spectrin

Using immunochemical techniques, we identified forms of erythrocyte membrane proteins in apical and basal plasma membranes of human placental trophoblast. A wheat germ agglutinin-binding intrinsic protein was present in the microvillous (maternal facing) but not the basal (fetal facing) membrane of the syncytiotrophoblast epithelium. Conversely, erythrocyte-related proteins of the basal membrane included two intrinsic membrane proteins, a 95,000 Mr band 3 isoform and a form of spectrin. These four proteins were all absent from the microvillous membrane. The basal membrane spectrin isoform was also present in basal membrane skeletons. A 70,000 Mr polypeptide which reacted with antibodies to band 3 was present in both microvillous and basal plasma membranes. Therefore, certain isoforms of red cell membrane proteins are polarized between the two surfaces of the human placental syncytiotrophoblast. We propose that the localization of spectrin to the basal membrane is related to the less bundled organization of microfilaments at this membrane compared with that of the microvillous membrane. The band 3 isoforms are candidates for participation in maternofetal anion transport.

Substituent effects on the redox reactions of tetraphenylporphyrins

The effect of substituents on pi radical reactions of para-substituted tetraphenylporphyrins was investigated by cyclic voltammetry in methylene chloride. In all cases electron donating substituents produced a more difficult reduction and an easier oxidation. Plots of E1/2 vs. a yielded Hammett linear free energy relationships for cation radical and dication formation and anion radical and dianion formation. An average reaction constant of p=0.07+/-0.01 V was obtained. This was true for tetraphenylporphyrins containing the central metals VO, Mn, Fe, Co, Ni, Cu, and Zn, as well as the free base H2 (p-X)TPP. The value of p appears not to be directly affected by the central metal oxidation state or the overall charge on the complex.

Characterization of the binding of 125-I-labeled epidermal growth factor to human fibroblasts

Epidermal growth factor (EGF) was labeled with 125-I by a lactoperoxidase technique. The unlabeled, monoiodinated and diiodinated species were separated by DEAE-cellulose chromatography and found to possess equivalent biological activities. The binding of monoiodinated epidermal growth factor to human fibroblasts was specific in that unrelated polypeptides did not affect the binding reaction. The binding reaction was a saturable process and was time- and temperature-dependent. A Scatchard analysis of the binding data indicated that each cell was capable of binding approximately 100, 000 molecules of 125-I-EGF. The apparent dissociation constant for the binding reaction was calculated to be 2.7 to 4.3 times 10-minus 10 M. Subsequent to the binding of 125-I-EGF to the fibroblasts, the growth factor was degraded by a cell-mediated proteolysis and [125-I]monoiodotyrosine appeared in the medium. The extent of degradation was reduced by the protease inhibitors, tosyl-L-lysine chloromethyl ketone and the benzyl ester of guanidobenzoic acid. Active binding sites of 125-I-egf appeared to be present in some but not all cell types. These results demonstrated that cells derived from a number of species (human, mouse, rat, and chick) possessed receptors that interacted with this mouse-derived growth factor.