Synergistic combination therapy delivered via layer-by-layer nanoparticles induces solid tumor regression of ovarian cancer

The majority of patients with high grade serous ovarian cancer (HGSOC) develop recurrent disease and chemotherapy resistance. To identify drug combinations that would be effective in treatment of chemotherapy resistant disease, we examined the efficacy of drug combinations that target the three antiapoptotic proteins most commonly expressed in HGSOC-BCL2, BCL-XL, and MCL1. Co-inhibition of BCL2 and BCL-XL (ABT-263) with inhibition of MCL1 (S63845) induces potent synergistic cytotoxicity in multiple HGSOC models. Since this drug combination is predicted to be toxic to patients due to the known clinical morbidities of each drug, we developed layer-by-layer nanoparticles (LbL NPs) that co-encapsulate these inhibitors in order to target HGSOC tumor cells and reduce systemic toxicities. We show that the LbL NPs can be designed to have high association with specific ovarian tumor cell types targeted in these studies, thus enabling a more selective uptake when delivered via intraperitoneal injection. Treatment with these LbL NPs displayed better potency than free drugs in vitro and resulted in near-complete elimination of solid tumor metastases of ovarian cancer xenografts. Thus, these results support the exploration of LbL NPs as a strategy to deliver potent drug combinations to recurrent HGSOC. While these findings are described for co-encapsulation of a BCL2/XL and a MCL1 inhibitor, the modular nature of LbL assembly provides flexibility in the range of therapies that can be incorporated, making LbL NPs an adaptable vehicle for delivery of additional combinations of pathway inhibitors and other oncology drugs.

Abstract P5-01-02: Single cell RNA transcriptomics reveals tumor promoting mammary cell subpopulation upon replication stress in BRCA1 mutant breast cancer mouse model

Women with BRCA1 (B1) mutation have an exceptionally high risk of developing breast cancer. The only effective preventive strategy currently offered to these women is the life altering prophylactic mastectomy. In light of limited treatment options available, it is critical that new therapeutic and preventive strategies be identified. Design of such strategies requires an understanding of early events in the breast cells that drive tumorigenesis. B1 heterozygous mouse models can help us identify these early changes in mammary tissue as the cells become tumor cells. However, despite the well-established association between B1 heterozygosity and cancer predisposition in humans, there are currently no such B1 mouse models that faithfully recapitulate this high risk of tumor formation. B1 heterozygous mice are not tumor-prone. This makes it difficult to use these models to study the role of B1 heterozygosity and to identify early tumor promoting changes in the breast tissue. We have now established a mouse model that induces mammary tumors in B1 heterozygous (Brca1wt/flx,Trp53flx/flx,K14cre) mice upon replication stress (RS), thus giving us a tool to study early tumor promoting changes in B1 heterozygous breast tissue. Our approach is based on our published work that reveals haploinsufficiency for RS suppression in B1 heterozygous cells. Given the importance of RS development in tumorigenesis, this effect would be a logical contributor to B1 mutant cancer development. Indeed, increasing RS in B1 heterozygous mice resulted in accelerated mammary tumorigenesis. RS in this mouse model was delivered by injecting DNA-adduct forming 4-nitroquinoline-1-oxide via mammary intraductal injection over a course of 7 weeks. RS served as an efficient and abnormally rapid driver of tumor formation (30 days post completion of injection regimen) in B1 heterozygous, but not B1 wild type mice. B1 heterozygous mice formed mammary adenocarcinoma and ductal carcinoma in-situ. Immunofluorescence based tissue section analysis and transcriptomic analysis reveals that adenocarcinomas formed in B1 heterozygous mice carry a basal epithelial phenotype like those found in human breast cancer. Such an accelerated tumor model system could prove to be invaluable in understanding the earliest events in B1 mutant breast cancer.Furthermore, our scRNAseq-based analysis has revealed early changes that occur in the breast tissue as different cell types (luminal and basal) respond to RS, and have identified new cell populations that emerge exclusively in B1 heterozygous mammary tissue undergoing RS. For this analysis, cells were collected from naïve mammary tissue, tissue collected midway through injections, and post-tumor tissue. This analysis identified a unique population of trans-differentiated cells expressing prognostic markers that have correlation to poor outcome in human breast cancer. This RS-induced mammary cell population in B1 heterozygous tissue also expresses both luminal progenitor and basal epithelial markers. Interestingly, this population was enriched for proliferation markers like Top2a, Ube2c, mKi67, and Ccnb2. Given that such proliferation markers are a hallmark of cancer stem cells, we suspect that this transdifferentiated population, which is primarily enriched in B1 heterozygous mammary tissue undergoing RS, marks some of the early cancer promoting changes in the breast tissue.Altogether, our integrative approach reveals that B1 heterozygosity in combination with RS leads to accumulation and proliferation of a specific mammary cell population that contributes to breast tumorigenesis. Identification of such early drivers is critical for the design of effective preventive and therapeutic strategies for women with B1 mutation.

Citation Format: Shailja Pathania, Joshua Rivera, Delan Khalid, Monica Manne, Stevenson Tran, Kemmie Kibaja, Carman MC Li, Joan Brugge. Single cell RNA transcriptomics reveals tumor promoting mammary cell subpopulation upon replication stress in BRCA1 mutant breast cancer mouse model [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-01-02.

Abstract P4-10-12: Characterizing the tumor and immune microenvironment through treatment to predict response to neoadjuvant HER2-targeted therapy using the Digital Spatial Profiler

Background: While introduction of HER2-targeted therapies has dramatically improved outcomes for patients with HER2-positive disease, even with the addition of HER2-targeted agents, 40-50% of patients do not achieve a pCR (pathologic complete response) following neoadjuvant therapy implying that clinical or molecular differences may be present in responders versus non-responders. While recent bulk expression studies have identified several biomarkers associated with response to HER2-targeted therapies in the neoadjuvant setting, these studies are limited in their ability to assign observed changes to specific geographic or phenotypic cell populations, such as the malignant tumor core or the surrounding microenvironment.

Methods: Here we used the Digital Spatial Profiler (DSP, NanoString Technologies, Inc.) to profile regions-of-interest containing pancytokeratin (panCK)+ tumor cells and infiltrated immune cells that are co-localized with the tumor cells. Using this technology, we assayed archival tissue from 28 patients with HER2-positive breast cancer from the TRIO-B07 (NCT00769470) clinical trial, who were treated with trastuzumab, lapatinib, or both, followed by standard chemotherapy plus HER2-targeted therapy. Tissue specimens were collected from the pre-treatment diagnostic biopsy (Baseline) and after one cycle of targeted therapy (Runin). To study regional heterogeneity, we selected an average of four panCK-enriched tissue regions from each sample. Using DSP, we performed multiplexed quantification of 38 tumor and immune protein markers and 96 RNA markers on the selected tissue regions and compared our findings to bulk mRNA expression data from the same cohort.

Results: Within the panCK-enriched regions, DSP revealed significant treatment-associated decreases in HER2 protein levels and the downstream PI3K-Akt signaling pathway in Runin compared to Baseline samples. In tandem, we observed a significant increase in infiltrating leukocytes, with CD45, a pan-leukocyte marker, and CD8, a marker for T cells that mediate tumor cell killing, showing the most dramatic changes. These changes in Runin compared to Baseline were more significant in the subset of cases that achieved a pCR versus those that do not, independent of ER status. Comparison of Runin samples to matched Baseline samples from the same patient enabled improved prediction of patient outcome (pCR) compared with analysis of a single timepoint alone. We also found that the DSP panCK enrichment strategy captures additional signal not observed in bulk expression data. For instance, using bulk expression, a decrease in HER2 RNA levels between Baseline and Runin was evident but there was no difference in the degree of decrease in HER2 mRNA between pCR and no pCR cases. Using DSP, we observed that the significant decrease in HER2 levels at Runin is more pronounced in cases that achieved a pCR. Across both tumor and immune markers, regional heterogeneity increased at Runin compared to Baseline.

Conclusions: In this study, we used DSP and a panCK enrichment strategy to retrospectively delineate the changes that occurred in tumor cells and co-localized immune cells during HER2-targeted therapy. In comparison to traditional or multiplexed IHC, DSP allows for simultaneous profiling of a large number of markers, enabling the characterization of multiple cancer signaling pathways and immune markers on a single tissue specimen. This study demonstrates the utility of pancytokeratin-enriched spatial proteomic profiling to characterize treatment-associated changes and identify predictive biomarkers.

NanoString’s Digital Spatial Profiler is for Research Use Only. Not to be used for diagnostic procedures.

Citation Format: Katherine Lee McNamara, Jennifer L. Caswell-Jin, Zhicheng Ma, Jason J. Zoeller, Michelle Kriner, Zoey Zhou, Jason Reeves, Margaret Hoang, Joseph Beechem, Dennis J. Slamon, Michael F. Press, Joan Brugge, Sara A. Hurvitz, Christina Curtis. Characterizing the tumor and immune microenvironment through treatment to predict response to neoadjuvant HER2-targeted therapy using the Digital Spatial Profiler [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-10-12.

Abstract ES6-1: Oxidative stress responses in cancer

Generation of reactive oxygen species (ROS), a natural byproduct of oxygen metabolism, occurs in all aerobic organisms at a controlled rate. Cancer cells are subjected to numerous cellular insults, including dysregulated oncogenes and loss of their natural extracellular matrix (ECM) interactions, that lead to the generation of high levels of ROS and selection for upregulation of anti-oxidant programs that allow adaptation to elevated ROS. Although cancer cells possess enhanced canonical antioxidant programs that neutralize ROS, they nevertheless exhibit prominent ROS levels in response to these insults, suggesting the existence of additional programs that allow cancer cells to tolerate elevated ROS.

My presentation will include a discussion of the evolving concepts about the role of oxidative stress and anti-oxidants in distinct processes associated with tumor initiation and progression and the potential for targeting anti-oxidants for therapeutic intervention. In addition, I will describe studies from our laboratory using 3-dimensional culture systems to model early events in breast cancer which have highlighted the critical role of oxidative stress in eliminating aberrantly proliferating breast epithelial cells, as well as the role of endogenous anti-oxidants in promoting proliferation and survival of pre-malignant cells. I will also discuss studies providing evidence for an unconventional mechanism whereby cancer cells co-opt the ROS-activated neuronal Ca2+-permeable TRPA1 channel in order to adapt to oxidative environments.

Citation Format: Brugge J. Oxidative stress responses in cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr ES6-1.

Abstract 989: Organoid cultures from normal and cancer-prone human breast tissues preserve complex epithelial lineages and can form chimeric mammary glands in vivo

Use of cell culture models to investigate the development of cancer associated with familial breast cancer syndromes is challenging because many culture methodologies do not maintain all of the lineages that are present in vivo, particularly the progenitor cells that may be the specific cancer cell-of-origin. We evaluated the ability of organoid culture technology to preserve stem/progenitor and differentiated cell types via long-term propagation of normal human mammary tissues. Tissue samples were obtained from elective reduction mammoplasties or from prophylactic mastectomies for cancer prevention, and were histologically normal. These were grown using conditions similar to organoid cultures for other epithelial cell types, as has been recently described (Sachs et al., Cell 2018). We found that basal stem and luminal progenitor cells could be cultured long-term, and differentiated in culture to generate mature basal and luminal cell types. Analysis of matched organoid cultures and native tissues by mass cytometry (CyTOF) for 38 markers at single-cell resolution confirmed the presence of multiple mammary epithelial cell types in the organoids, and demonstrated that protein expression patterns of the tissue of origin were largely preserved in culture. In addition, we generated a panel of over 40 mammary organoids derived from patients harboring inherited mutations in the cancer predisposition gene BRCA1 and from unaffected controls. Despite interindividual variability, an expansion in luminal progenitor populations associated with BRCA1 mutation could be detected in cultured organoids. Human mammary organoids heterozygous for BRCA1 could be engrafted into the murine mammary gland, resulting in mixed ductal and acinar structures. These studies indicate that this new model system is well suited for studies of aberrant phenotypes associated with BRCA1 mutation and approaches to prevent cancer development in these genetically predisposed tissues.

Citation Format: Jennifer M. Rosenbluth, Ronald Schackmann, Carman Li, Norman Sachs, Deborah Dillon, Andrea Richardson, Jane Brock, Judy Garber, Gary Kenneth Gray, Jason Zoeller, Mackenzie Boedicker, Hendrick Johannes Kuiken, Hans Clevers, Joan Brugge. Organoid cultures from normal and cancer-prone human breast tissues preserve complex epithelial lineages and can form chimeric mammary glands in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 989.

Abstract 3301: Using a novel single-cell lineage-tracing technique to uncover the mechanisms driving nongenetic cancer relapse

Despite favorable initial response to platinum chemotherapy, the majority of patients with ovarian cancer will develop recurrent disease and succumb to it within 5 years of diagnosis. Initial recurrence is frequently platinum-sensitive and women can respond to multiple lines of platinum-based chemotherapy before eventually developing platinum-refractory disease. The observation that the tumor cells that survive the initial therapy give rise to a chemosensitive progeny is inconsistent with a simple Darwinian model of selection of genetically resistant clones. Tumor dormancy, stochastic cell state shifts and stem cell-like populations are among the mechanisms hypothesized to mediate re-emergence of platinum-sensitive ovarian cancer. However, given the lack of high-throughput methods to simultaneously track cell state and lineage, it is not currently feasible to distinguish the relative contribution of each of these factors. To address this need, we generated a new lentiviral barcode library, termed Watermelon. Unlike existing barcoding libraries that enable the tracking of multiple cancer cells solely at the DNA level, the Watermelon library encodes transcribed barcodes, which allows one to map the lineage to the transcriptional profile of each individual cell. This unique barcoding scheme, which is coupled with a genetic H2B retention system, enables simultaneous tracing of lineage as well as the transcriptional and proliferative states of each cell in the population during drug treatment. We have established and characterized an in vitro platinum-sensitive relapse model and are applying the Watermelon system together with time-lapse imaging to study the mechanisms underlying time-to-relapse variation and cell-fate decisions. By broadening our knowledge of the cellular and molecular pathways that affect noninherited drug resistance, our work will facilitate the future development of novel therapies that delay or even prevent the emergence of platinum-refractory ovarian cancer.

Citation Format: Yaara Oren, Aviv Regev, Joan Brugge. Using a novel single-cell lineage-tracing technique to uncover the mechanisms driving nongenetic cancer relapse [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3301.

Abstract 5478: The TRPA1 Ca2+-permeable channel mediates a non-canonical redox adaptation in cancer cells

Generation of reactive oxygen species (ROS), a natural byproduct of oxygen metabolism, occurs in all aerobic organisms at a controlled rate. Cancer cells are subjected to numerous cellular insults, including dysregulated oncogenes and dissociation from their natural extracellular matrix (ECM) niches, leading to the generation of high levels of ROS. Although cancer cells possess enhanced canonical antioxidant programs that neutralize ROS, they nevertheless exhibit prominent ROS levels in response to these insults, suggesting the existence of additional programs that allow cancer cells to tolerate elevated ROS.

In this study, we provide evidence for an unconventional mechanism for redox adaptation involving the ROS-activated, Ca2+-permeable TRPA1 channel, which normally functions as an irritant receptor in sensory neurons but is highly upregulated in breast, lung, malignant neural sheath, and other tumors. We found that TRPA1 overexpression enhances survival and proliferation of MCF-10A cells under conditions of matrix-detachment and this effect was dependent on TRPA1 activation by ROS generated as loss of matrix anchorage. In MCF-10A acini, TRPA1 expression induced Ca2+ entry in ECM-deprived cells in the luminal space and promoted their survival and proliferation. Conversely, TRPA1 knockdown inhibited Ca2+ responses to ROS generated through ECM detachment and induced clearance of cells from the luminal space in breast cancer spheroids. TRPA1 was also activated by ROS-inducing chemotherapies and drove chemoresistance in breast, lung and malignant peripheral nerve sheath tumor cells, and its downregulation suppressed breast xenograft tumor growth and enhanced chemosensitivity. TRPA1 mediated these effects independent of antioxidant responses, but through activation of cellular survival and anti-apoptotic programs involving the RAS-ERK/AKT/mTOR pathways.

Together, our findings describe an unexpected mechanism whereby cancer cells co-opt the neuronal TRPA1 channel in order to adapt to oxidative environments. This TRPA1-induced response is distinct from canonical redox adaptation mechanisms that rely on antioxidant program. As TRPA1 inhibitors are currently in clinical trials for pain and respiratory therapies, these studies raise the possibility of using these inhibitors as therapeutic chemosensitizers in TRPA1-enriched tumors.

Citation Format: Nobuaki Takahashi, Hsing-Yu Chen, Isaac Harris, Daniel Stover, Roderick Bronson, Thomas Deraedt, Karen Cichowski, Alana Welm, Gordon Mills, Joan Brugge. The TRPA1 Ca2+-permeable channel mediates a non-canonical redox adaptation in cancer cells [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 5478. doi:10.1158/1538-7445.AM2017-5478

Rational combination therapy with PARP and MEK inhibitors capitalizes on therapeutic liabilities in RAS mutant cancers

Mutant RAS has remained recalcitrant to targeted therapy efforts. We demonstrate that combined treatment with poly(adenosine diphosphate-ribose) polymerase (PARP) inhibitors and mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitors evokes unanticipated, synergistic cytotoxic effects in vitro and in vivo in multiple RAS mutant tumor models across tumor lineages where RAS mutations are prevalent. The effects of PARP and MEK inhibitor combinations are independent of BRCA1/2 and p53 mutation status, suggesting that the synergistic activity is likely to be generalizable. Synergistic activity of PARP and MEK inhibitor combinations in RAS mutant tumors is associated with (i) induction of BIM-mediated apoptosis, (ii) decrease in expression of components of the homologous recombination DNA repair pathway, (iii) decrease in homologous recombination DNA damage repair capacity, (iv) decrease in DNA damage checkpoint activity, (v) increase in PARP inhibitor-induced DNA damage, (vi) decrease in vascularity that could increase PARP inhibitor efficacy by inducing hypoxia, and (vii) elevated PARP1 protein, which increases trapping activity of PARP inhibitors. Mechanistically, enforced expression of FOXO3a, which is a target of the RAS/MAPK pathway, was sufficient to recapitulate the functional consequences of MEK inhibitors including synergy with PARP inhibitors. Thus, the ability of mutant RAS to suppress FOXO3a and its reversal by MEK inhibitors accounts, at least in part, for the synergy of PARP and MEK inhibitors in RAS mutant tumors. The rational combination of PARP and MEK inhibitors warrants clinical investigation in patients with RAS mutant tumors where there are few effective therapeutic options.

Abstract 3843: Design of effective combination therapies for high-grade serous ovarian cancer using patient-derived xenograft models

Despite advances in understanding the genetics and molecular biology of high grade serous ovarian cancer, there have not been any significant improvements in the outcome of patients treated with approved therapies to date. The overall aim of our studies is to identify synergistic drug combinations for the treatment of high-grade serous ovarian (HGS-Ov) cancer and the biomarkers that predict sensitivity for future translation in clinical trials. In particular, this study focuses on the vulnerabilities of patient-derived ovarian cancer cells to inhibition of anti-apoptotic proteins and the design of novel combination therapies to overcome drug resistance and improve the response of selected patients to a particular set of targeted therapies.

Previous studies from our lab provided evidence that inhibition of BCL-2/XL can significantly enhance the sensitivity of tumor cells to targeted therapies in ovarian and breast cancer cell line models (Muranen T et al., Cancer Cell 2012). In this study, we evaluated the efficacy of this combination treatment in ascites cells derived from 15 high-grade serous patient-derived ovarian cancer xenografts (PDXs). We found that inhibitors of PI3K/mTOR (GNE-493) and BCL-2/XL (ABT-737) act synergistically ex vivo, with an ∼15 fold variability among the 15 patient-derived samples. A large scale in vivo experiment is ongoing to evaluate the efficacy of this combined treatment in six PDX models.

To identify biomarkers that predict drug sensitivity we performed proteomic Reverse Phase Protein Array (RPPA) and immunoblot analyses. We found that baseline levels of the pro-apoptotic protein BIM correlate with sensitivity to BCL-2/XL inhibition. This motivated us to examine the basis for low BIM expression. In three of the least sensitive PDX models, we found that low levels of BIM correlated with ERK activation based on increased ERK phosphorylation and that inhibition of MEK by PD-0325901 in these models caused upregulation of BIM, as predicted from previous published reports which demonstrated that phosphorylation of BIM by ERK causes its degradation (Ley R, et al. J Biol Chem. 2003). We then examined the sensitivity of these PDX models to ABT-737 and PD-0325901, MEK162, or CI-1040 ex vivo and observed strong synergy resulting in reduction of cell viability and increased cell death. These results suggest that low BIM/high phosphoERK is a potential biomarker for sensitivity of ovarian tumors to combined MEK and BCL-2/XL inhibition. We now plan to examine the efficacy of drug combinations that target MEK and BCL-2/XL in these PDX models in vivo.

Our studies promise to lead to the identification of new drug combination therapies for HGS-Ov cancer treatment and predictive biomarkers to stratify patients that can benefit from these targeted therapies

Citation Format: Claudia Iavarone, Ioannis Zervantonakis, Hsing-Yu Chen, Sangeetha S. Palakurthi, Joyce F. Liu, Ursula Matulonis, Ronny Drapkin, Gordon Mills, Joel Leverson, Deepak Sampath, Joan Brugge. Design of effective combination therapies for high-grade serous ovarian cancer using patient-derived xenograft models. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3843.

Abstract 2736: Regulation of cytokinesis by polo-like kinase 4

Introduction. The serine/threonine mitotic kinase, polo-like kinase 4 (PLK4), is known to play a critical role in centrosome duplication in preparation for cell division. Based on our preliminary observations with a PLK4 kinase inhibitor, we investigated the possibility that PLK4 may also play a role in regulation of cytokinesis.

Experimental Procedures. Immunofluorescence was used to localize PLK4 and phospho-PLK4 in cultured human breast, ovarian, and colorectal cancer cell lines throughout the cell cycle without and with CFI-400945 PLK4 inhibitor and MG-115 protease inhibitor. Flow cytometry and videomicroscopy were used to analyze the consequences of PLK4 inhibition on cytokinesis.

Results. Using immunofluorescence, PLK4 was localized to centrosomes; however, we also found that phospho-PLK4 was cleaved and distributed to kinetochores (metaphase and anaphase), cleavage furrow (telophase), and middle body (cytokinesis) during cell division in colorectal, ovarian, and breast cancer cells. Distribution of phospho-PLK4 to the cleavage furrow and middle body raised the possibility that this kinase plays a functional role in cytokinesis. Using either CFI-400945 PLK4 kinase inhibitor or the MG-115 protease inhibitor, we found that PLK4 accumulated in centrosomes with inhibition of translocation of PLK4 to the middle body. This change in subcellular distribution of PLK4 was associated with generation of large, multi-nucleated tumor cells or tumor cells with polyploidy. Videomicroscopy confirmed that treatment with CFI-400945 PLK4 inhibitor was associated with prevention of cellular abscission in treated cells.

Conclusions. These observations demonstrate a role for phospho-PLK4 in facilitation of cytokinesis. A regulatory role for PLK4 in cytokinesis makes it a potential target for therapeutic intervention in appropriately selected cancers.

Citation Format: Michael F. Press, Bin Xie, Simon Davenport, Yu Zhou, Neil O’Brien, Michael Palazzolo, Tak Mak, Joan Brugge, Dennis J. Slamon. Regulation of cytokinesis by polo-like kinase 4. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2736.

Abstract PR01: Adaptive resistance of patient-derived ovarian cancer cells to PI3K/mTOR inhibition

Relapse due to drug resistance remains a major challenge for successful cancer treatment despite advances in chemotherapy efficacy and the development of targeted therapies. We have previously reported a therapy-induced adaptive response that is associated with treatment resistance and are investigating whether this adaptive response program could be targeted to increase treatment efficacy (Muranen et al, Cancer Cell 2012). The PI3K pathway is amplified in high-grade serous ovarian tumor and represents an attractive target in this cancer subtype. We found that both in vitro and in vivo exposure of patient-derived ovarian cancer cells to the PI3K/mTOR dual inhibitor GNE-493 increases the levels of the anti-apoptotic Bcl-2 protein, and multiple tyrosine kinase receptors, such as EGFR and HER2. Concurrent treatment of these cancer cells with GNE-493 and ABT737 (a Bcl-2 and Bcl-xl dual inhibitor) causes a reduction in cell number and increased cell death. Ongoing studies are underway to comprehensively characterize the adaptive response signature of a panel of patient-derived ovarian cancer cells and study whether targeting multiple different upregulated proteins (e.g. EGFR) may increase PI3K/mTOR treatment efficacy.

Therapy resistance has also been associated with exposure of tumor cells to pro-survival factors secreted by supporting cells in the tumor microenvironment (Straussman et al, Nature 2012 and Wilson et al, Nature 2012). We found that the growth-factors EGF, inflammatory cytokine IL-6 and ascites fluid from ovarian cancer patients induces up-regulation of Bcl-2 as well as IGFR, which masked the adaptive response to PI3K/mTOR inhibitors. We are performing ongoing studies using orthotopic patient-derived ovarian cancer xenografts to investigate the patterns of residual disease in vivo with the ultimate goal to identify whether Bcl-2 targeted drugs will enhance the efficacy of PI3K/mTOR inhibition and whether there is differential sensitivity at distinct anatomical sites (pancreas, diaphragm, peritoneal wall).

Our studies provide a framework to identify adaptive response programs and tumor microenvironment factors in patient-derived ovarian cancer cells that limit treatment efficacy of PI3K/mTOR pathway inhibition and to develop therapies to abrogate this resistance.

Citation Format: Ioannis Zervantonakis, Hsing-Yu Chen, Taru Muranen, Joyce Liu, Ronny Drapkin, Ursula Matulonis, Joan Brugge. Adaptive resistance of patient-derived ovarian cancer cells to PI3K/mTOR inhibition. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Drug Sensitivity and Resistance: Improving Cancer Therapy; Jun 18-21, 2014; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(4 Suppl): Abstract nr PR01.

Abstract 2976: Intratumoral cellular heterogeneity of epithelial ovarian carcinoma and its impact on tumor behavior

One of the challenging features of cancer is intratumoral heterogeneity (ITH). The existence of ITH can have profound consequences on tumor progression and drug sensitivity. However it is been essentially impossible to monitor the extent to which individual clonal populations contribute to and affect tumor behavior because of our inability to culture tumor cells from patients in vitro without loosing initial heterogeneity resulting from clonal selection under culture conditions.

Through our collaboration with Dr. Tan Ince, we have access to cell culture medium (WIT-OC media) that allows primary tumor cells to undergo continuous population doublings without selecting for or against specific clones under culture conditions. By utilizing this media we have generated 80 single cell clones from a Clear cell Carcinoma (CCC) patient sample and monitored the extent of phenotypic and genomic heterogeneity among 12 of the clonal populations. These clones showed striking heterogeneity in copy number alterations as well as phenotypic heterogeneity in vitro as assessed by anoikis, growth in soft agar, doubling time, and growth in 3D reconstituted basement membrane cultures. We also examined correlations between in vitro properties and tumor formation in vivo using clones tagged with Gaussia-Luciferase (Gluc), a secreted luciferase that is secreted from cells and can be assayed in whole blood. We found that only three clones were transplantable by themselves; however, the clone which showed the most rapid expansion in the mouse was limited in it tumorigenic capacity, generating only ascitic tumor populations. To address whether clonal heterogeneity could contribute to tumor progression, we transplanted mixtures of clones and utilized barcoded clones to analyze the clonal composition of tumor masses. Interestingly, mixtures of the 12 clones resulted in the generation of more aggressive tumors, including the formation of solid tumors and metastases to lungs, liver and brain and allow in vivo growth; these metastatic growths contain distinct mixtures of individual clonal populations. Moreover, clones that could not initiate tumor formation individually could generate metastatic tumors, suggesting that there is cooperativity between the clones to enhance growth and induce metastasis.

The study has provided evidence that cell lines generated from individual clones vary significantly in their functional activities in vitro and in vivo. Moreover, by studying the single cell clones, we have found evidence for the existence of interclonal crosstalk during tumor progression and are currently studying the nature and impact of such crosstalk on tumor progression and metastasis.

Citation Format: Suha Naffar-Abu Amara, Laura Selfors, Marit Krohn, Tan Ince, Gordon Mills, Joan Brugge. Intratumoral cellular heterogeneity of epithelial ovarian carcinoma and its impact on tumor behavior. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2976. doi:10.1158/1538-7445.AM2014-2976

Abstract OT1-1-07: Phase II trial of paclitaxel combined with trastuzumab and pertuzumab as preoperative therapy for HER2 positive inflammatory breast cancer

Background: Inflammatory breast cancer (IBC) is a rare and virulent disease, accounting for 2-5% of breast cancer (BC) in the United States. The molecular analysis of IBC reveals a propensity to segregate into more proliferative intrinsic subtypes, such as HER2 positive (+) (Van Laere, et al, Br Ca Res Treat 2006). Studies demonstrate a 40-50% incidence of HER2+ disease among IBC, which is >2x the incidence in non-IBC (Turpin E, et al, Br Ca Res Treat 2002). The prevalence of HER2+ IBC and the availability of agents targeting the HER2 domain support investigation into the optimal preoperative regimen incorporating HER2 targeting agents. This preoperative trial evaluates combination pertuzumab (P) paclitaxel (T) and trastuzumab (H) in patients (pts) with newly diagnosed HER2+ IBC.

Methods: Pts with HER2+ Stage III (cT4d, any N, M0) IBC (male or female) are eligible if they have not received prior therapy for BC, have adequate organ function, ECOG PS ≤ 1, cardiac ejection fraction ≥ 50%, and willing to undergo 2 research biopsies (bx) of the affected breast for correlative assays. Staging studies, research bx and whole blood sample are obtained at baseline. Pts receive a loading dose of P 840mg and H 4mg/kg IV (day (d) 1, week (wk) 1). On d8, wk2, a 2nd breast bx and whole blood sample are obtained and wkly H 2mg/kg with T 80mg/m2 IV are administered for a total of 16 wks. P 420mg is given concurrently every 21d for 5 doses beginning on d21, wk3. Following adequate tumor response, pts proceed to modified radical mastectomy followed by doxorubicin (A) 60mg/m2 with cyclophosphamide(C) 600mg/m2 every 21d for 4 cycles. Following AC, the chest wall and regional lymph nodes are treated with radiation concurrently with a loading dose of P 840mg and H 8mg/kg followed by P 420mg and H 6mg/kg every 21d for 12 doses. Adjuvant endocrine therapy is given with P and H if estrogen and/or progesterone receptor positive.

Correlatives: To investigate whether the pre-treatment status of the tumor predicts HER2-resistance, next-generation sequencing technology will be employed to explore gene expression patterns including PAM50-based intrinsic subtyping. Both de novo and acquired mechanisms of resistance to HER2 therapy will be explored including analysis of residual disease obtained at mastectomy. Exploration of early adaptive molecular changes seen in d8, wk2 tumor samples will provide information useful in developing improved therapeutic strategies.

Statistics: The primary endpoint is pathologic complete response (pCR) following therapy with PTH. A Simon minimax two-stage design is being used. If the proportion of pts having pCR is ≤ 0.15 then PTH is considered minimally effective, versus alternative hypothesis that PTH is worthy of further study if proportion pCR ≥ 0.40. In the 1st stage, if ≤ 2/16 pts have pCR, the study is stopped; if ≥ 3 pts have pCR, the study proceeds. In the 2nd stage, PTH is rejected if ≤ 7 of 27 pts have a pCR (α = 0.039; β = 0.05). Up to 30 pts will be enrolled. Secondary objectives are disease-free survival, time to tumor failure and overall survival. Time-to-event distributions are summarized using Kaplan-Meier and 2-sided 90% CI for the medians. Clinical trial information: NCT01796197.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr OT1-1-07.

Abstract PR7: Visualizing the dynamic and heterogeneous responses of single cells to inhibitors of the growth factor signaling network

The growth factor signaling network, including the Akt, ERK, mTOR and other pathways, is deregulated in many diseases and is a prominent target for therapeutic inhibitors. The effectiveness of these inhibitors is limited by the existence of homeostatic feedback loops that act to restore signaling and attenuate the effects of inhibition. To obtain the single-cell data necessary to fully characterize these rapid and heterogeneous responses, we developed a library of mammary epithelial cells stably expressing over 90 fluorescent protein-based reporters. This library comprises reporters for: cellular processes including cell death, cell cycle, protein translation, and metabolism; kinases including ERK, Akt and AMPK; and microRNAs involved in the regulation of cellular differentiation and stress response. Using high-throughput live-cell microscopy and computational image analysis, we monitored the responses of these pathways to inhibitors of EGFR, ErbB2, PI3K, mTOR, MEK, Raf, and Bcl-2. This analysis revealed unexpectedly dynamic fluctuations in key pathways including ERK, Akt, and glycolysis. In one example, inhibition of EGFR transformed constitutive ERK activity into intermittent bursts of activity ranging in duration from 20-90 minutes with high cell-to-cell variability. Characterization of ERK activity pulses revealed that their frequency depended on the level of EGF receptor activity and that their duration was essential for controlling the commitment to cell proliferation. In sharp contrast, inhibition of MEK altered the amplitude, rather than the frequency, of ERK activity. Using these live-cell measurements in combination with high content immunofluorescence, we constructed a computational model of feedback loops controlling ERK activity pulses. Quantitative mapping of ERK output to proliferation rate revealed a distinct and exquisitely sensitive threshold of ERK output required for proliferation. Potential strategies for reducing ERK output below this threshold and effectively blocking proliferation include dual inhibition of upstream regulators or intermittent high-dose inhibition. Single-cell quantitative analysis of inhibitor responses is now being extended to other pathways to identify signaling dynamics and information transfer properties that enable effective therapeutic targeting of the network.

This proffered talk is also presented as Poster A19.

Citation Format: John Albeck, Yin P. Hung, Gregory Leya, Gary Yellen, Gordon Mills, Joan Brugge. Visualizing the dynamic and heterogeneous responses of single cells to inhibitors of the growth factor signaling network [abstract]. In: Proceedings of the AACR Special Conference on Chemical Systems Biology: Assembling and Interrogating Computational Models of the Cancer Cell by Chemical Perturbations; 2012 Jun 27-30; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2012;72(13 Suppl):Abstract nr PR7.

Abstract LB-418: A live-cell miRNA reporter library for identifying non-genetic drug resistance networks at single-cell resolution

The effectiveness of targeted cancer therapies is limited by cellular resistance. Emerging research has demonstrated that a major source of drug resistance is non-genetic: pre-existing heterogeneity in protein expression levels, or induction of adaptive feedback loops, can protect cells from the effects of kinase inhibitors and other drugs. Traditional genomic or proteomic techniques may fail to identify key resistance pathways because they do not stem from changes in genetic sequences, are often transient in nature, and may operate in only a subset of cells within a population. To address this challenge, we have developed a library of over 80 live-cell reporters that, in combination with high-throughput automated microscopy, enable the identification of transiently active resistance pathways in drug-treated cells. These reporters are based on miRNA regulatory sequences since numerous miRNAs have been implicated in drug resistance and because miRNAs can act as surrogate indicators for the activation of signaling pathways in which they participate. We have developed and validated a novel ratiometric reporter design for optimal detection of miRNA expression changes at single-cell resolution and have constructed retroviral reporters for miRNAs involved in apoptosis, cell proliferation, cancer stem cells, metabolism, and stress response. Live imaging of mammary epithelial cells treated with inhibitors targeting EGFR, PI3-K, mTOR, and Bcl-2 identified a set of 20-30 miRNA reporters with significant expression level alterations in surviving cells that are consistent with miRNA expression changes characteristic of cancer cells. Bioinformatic analyses of these miRNAs revealed commonly regulated pathways and downstream targets, including Bcl-2 and Cyclin-D. Thus, the reporter library is capable of identifying, in living cells, pathways involved in the cellular response to targeted cancer therapies. By identifying components of the signaling networks controlling inhibitor resistance, we hope to discover new drug targets that can be used to sensitize tumor cells to existing targeted therapies. The reporter library can be easily expressed in most cell lines of interest and should have broad utility for identifying transiently regulated pathways.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-418. doi:1538-7445.AM2012-LB-418

Abstract 4751: Identification of mechanism involved in mesothelial clearance by ovarian tumor spheroids

Metastatic dissemination of ovarian tumor cells to organs in the peritoneal cavity involves the intercalation of dissociated tumor cells into the mesothelial monolayer covering those organs. We established a cell culture model that uses time-lapse video microscopy to investigate the mechanisms involved in the initial interactions between tumor cells and the mesothelium. Tumor cells are cultured in suspension, where they associate into multicellular spheroids. When spheroids are incubated with a mesothelial monolayer, the tumor cells attach to the mesothelial cells and then displace the mesothelial cells and gain access to the underlying substratum. We believe that force generation by the ovarian cancer cells is required for mesothelial clearance, as inhibition of myosin IIA, IIB, or talin I in the ovarian cancer spheroids attenuates mesothelial clearance.

To further elucidate the mechanisms by which ovarian cancer cell aggregates clear a mesothelial monolayer, we have taken several different approaches. First, 21 ovarian cancer cell lines were classified according to their ability to clear a mesothelial monolayer. 14 of the cell lines were able to induce clearance in the monolayer, whereas 8 could not. Western blot analysis of the expression levels of several integrins in the 21 cell lines revealed a positive correlation between α5 integrin expression and clearance ability. Furthermore, blocking α5 integrin function, using a function-blocking antibody, significantly inhibited mesothelial clearance in 6 out of 8 cell lines tested.

In addition, data from RNA microarrays on the 21 cell lines were analyzed to identify genes that are differentially expressed in intercalation-competent and -incompetent tumor cell lines. The microarray analysis revealed several candidate genes, including: SEPT6, GJA1, VIM, and SLIT2 (up) as well as IL23A, INHIBB and LCN2 (down). The functional importance of these genes, and others, in mesothelial clearance is currently being investigated through loss-of-function and gain-of-function genetic manipulations.

Finally, we have initiated a targeted RNAi screen using one of the ovarian cancer cell lines, OVCA433, that aggressively clears the mesothelial monolayer. We plan to include siRNAs that were shown to regulate migration in a previous screen our lab had performed, as well as, other siRNAs directed against genes known to be involved in migration. In a pilot screen using 54 siRNAs, we found several genes that decrease mesothelial clearance, including EPHB2, IGFR1, and MAP3K11. We are expanding the screen to include genes that are differentially expressed in clearance competent and incompetent ovarian tumor cell lines. The hits form these screens will be validated and characterized to identify the cellular pathways that regulate mesothelial clearance. Lastly, we will determine, in vivo, if these genes regulate mesothelial invasion in a mouse model of peritoneal metastasis

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4751. doi:10.1158/1538-7445.AM2011-4751

Joan Brugge: running rings around cancer. Interview by Caitlin Sedwick

Brugge has devoted her career to uncovering how perturbations in normal cellular processes give rise to cancer.

A structure-guided approach to creating covalent FGFR inhibitors

The fibroblast growth factor receptor tyrosine kinases (FGFR1, 2, 3, and 4) represent promising therapeutic targets in a number of cancers. We have developed the first potent and selective irreversible inhibitor of FGFR1, 2, 3, and 4, which we named FIIN-1 that forms a covalent bond with cysteine 486 located in the P loop of the FGFR1 ATP binding site. We demonstrated that the inhibitor potently inhibits Tel-FGFR1-transformed Ba/F3 cells (EC(50) = 14 nM) as well as numerous FGFR-dependent cancer cell lines. A biotin-derivatized version of the inhibitor, FIIN-1-biotin, was shown to covalently label FGFR1 at Cys486. FIIN-1 is a useful probe of FGFR-dependent cellular phenomena and may provide a starting point of the development of therapeutically relevant irreversible inhibitors of wild-type and drug-resistant forms of FGFR kinases.

A new mutational AKTivation in the PI3K pathway

Although multiple members of the phosphatidylinositol-3-kinase pathway (PI3K) are targeted by germline or somatic mutations, functional mutations in the three akt isoforms have proven elusive. This is somewhat surprising, as AKT represents a key node in the PI3K pathway, exhibiting transforming activity when incorporated into the AKT8 retrovirus. A recent report in Nature identifies a transforming E17K PH domain mutation in akt1 in breast (8%), colorectal (6%), and ovarian (2%) cancers. E17K-akt1 transforming activity appears due to PtdIns(3,4)P2- and PtdIns(3,4,5)P3-independent recruitment of AKT1 to the membrane. This novel observation raises important theoretical and clinical questions.

A two-gene expression ratio predicts clinical outcome in breast cancer patients treated with tamoxifen

Tamoxifen significantly reduces tumor recurrence in certain patients with early-stage estrogen receptor-positive breast cancer, but markers predictive of treatment failure have not been identified. Here, we generated gene expression profiles of hormone receptor-positive primary breast cancers in a set of 60 patients treated with adjuvant tamoxifen monotherapy. An expression signature predictive of disease-free survival was reduced to a two-gene ratio, HOXB13 versus IL17BR, which outperformed existing biomarkers. Ectopic expression of HOXB13 in MCF10A breast epithelial cells enhances motility and invasion in vitro, and its expression is increased in both preinvasive and invasive primary breast cancer. The HOXB13:IL17BR expression ratio may be useful for identifying patients appropriate for alternative therapeutic regimens in early-stage breast cancer.

Vav1/2/3-null mice define an essential role for Vav family proteins in lymphocyte development and activation but a differential requirement in MAPK signaling in T and B cells

The Vav family of Rho guanine nucleotide exchange factors is thought to orchestrate signaling events downstream of lymphocyte antigen receptors. Elucidation of Vav function has been obscured thus far by the expression of three highly related family members. We generated mice lacking all Vav family proteins and show that Vav-null mice produce no functional T or B cells and completely fail to mount both T-dependent and T-independent humoral responses. Whereas T cell development is blocked at an early stage in the thymus, immature B lineage cells accumulate in the periphery but arrest at a late “transitional” stage. Mechanistically, we show that the Vav family is crucial for both TCR and B cell receptor (BCR)–induced Ca²⁺ signaling and, surprisingly, is only required for mitogen-activated protein kinase (MAPK) activation in developing and mature T cells but not in B cells. Thus, the abundance of immature B cells generated in Vav-null mice may be due to intact Ras/MAPK signaling in this lineage. Although the expression of Vav1 alone is sufficient for normal lymphocyte development, our data also reveal lineage-specific roles for Vav2 and Vav3, with the first demonstration that Vav3 plays a critical compensatory function in T cells. Together, we define an essential role for the entire Vav protein family in lymphocyte development and activation and establish the limits of functional redundancy both within this family and between Vav and other Rho–guanine nucleotide exchange factors.

Association of the Shc and Grb2/Sem5 SH2-containing proteins is implicated in activation of the Ras pathway by tyrosine kinases

The mammalian shc gene encodes two overlapping, widely expressed proteins of 46 and 52K, with a carboxy-terminal SH2 domain that binds activated growth factor receptors, and a more amino-terminal glycine/proline-rich region. These shc gene products (Shc) are transforming when overexpressed in fibroblasts. Shc proteins become phosphorylated on tyrosine in cells stimulated with a variety of growth factors, and in cells transformed by v-src (ref. 2), suggesting that they are tyrosine kinase targets that control a mitogenic signalling pathway. Here we report that tyrosine-phosphorylated Shc proteins form a specific complex with a non-phosphorylated 23K polypeptide encoded by the grb2/sem-5 gene. The grb2/sem-5 gene product itself contains an SH2 domain, which mediates binding to Shc, and is implicated in activation of the Ras guanine nucleotide-binding protein by tyrosine kinases in both Caenorhabditis elegans and mammalian cells. Consistent with a role in signalling through Ras, shc overexpression induced Ras-dependent neurite outgrowth in PC12 cells. These results suggest that Shc tyrosine phosphorylation can couple tyrosine kinases to Grb2/Sem-5, through formation of a Shc-Grb2/Sem-5 complex, and thereby regulate the mammalian Ras signalling pathway.

Phenotypic and genetic alterations in pre-cancerous cells in the colon

Cell dysplasia in polyps and in ulcerative colitis are thought to be the pre-cancerous lesion leading to invasive colon cancer. Many polyps and dysplastic lesions in ulcerative colitis have phenotypic changes (blood group antigen, cytokeratins, CEA, TAG-72.3 antigen expression) and genetic changes (c-K-ras mutation, enhanced c-myc expression and pp60c-src activity) which are characteristic of invasive cancers. Thus, these early pre-cancerous lesions may be a late stage in the genetic evolution of colon cancer.

Characterization of the altered form of the c-src gene product in neuronal cells

The pp60c-src protein that is expressed at high levels in cultures of neurons from rat embryos displays an altered mobility on SDS-polyacrylamide gels due to a structural difference in the amino-terminal region of the molecule. In this report we show that the expression of this unique form of pp60c-src, designated pp60c-src(+), is not restricted to cultured neuronal cells since the pp60c-src molecules expressed in tissues from avian and rat neural tissues also display a retarded electrophoretic mobility. The amino-terminal region from pp60c-src(+) was found to contain a novel phosphorylated tryptic peptide that contains phosphoserine. However, this phosphorylation does not appear to be responsible for the retarded electrophoretic mobility of pp60c-src(+), since the mobility of this protein is not altered by phosphatase treatment under conditions that remove greater than 95% of the radiolabeled phosphate on pp60c-src(+). The altered electrophoretic form of pp60c-src was also shown to be radiolabeled with [3H]myristate, indicating that pp60c-src is fatty-acylated in neurons, as is pp60c-src in fibroblasts. The pp60c-src molecules synthesized in vitro using rabbit reticulocyte lysates programmed with mRNA from embryonic brain migrated more slowly on SDS-polyacrylamide gels than the pp60c-src protein that was translated in vitro using RNA from embryonic limb tissue. These results suggest the possibility that the c-src mRNA expressed in neurons may undergo a unique form of processing to generate the structurally distinct form of neuronal pp60c-src(+).

A 90,000-dalton binding protein common to both steroid receptors and the Rous sarcoma virus transforming protein, pp60v-src

Previous studies have shown that the avian progesterone receptor, when in the nontransformed 8 S state, is complexed to another cellular protein having a molecular weight of 90,000. In this report, we show that this receptor-binding protein is indistinguishable from the 90,000-dalton protein which associates in a complex with the Rous sarcoma virus transforming protein, pp60v-src. This identity was established by the following criteria. 1) Monoclonal antibodies directed against the pp60v-src-associated 90-kDa protein recognized the 90-kDa progesterone receptor binding protein in an immunoblot assay. Conversely, monoclonal antibodies that recognize the progesterone receptor binding protein bind to the 90-kDa protein which complexes with pp60v-src. 2) Peptide maps prepared from the 90-kDa proteins immunoprecipitated from chicken cells with monoclonal antibodies directed against either the 90-kDa receptor binding protein or the 90-kDa pp60v-src-associated protein were indistinguishable. 3) Preincubation of the progesterone receptor complex with monoclonal antibodies prepared against the pp60v-src-associated protein caused a shift in the sedimentation of the progesterone receptor. Previous studies have established that the pp60v-src-associated protein is indistinguishable from one of the major heat shock proteins which are induced under a variety of stress conditions in eukaryotic cells. These present studies implicate a new role for this 90-kDa protein in the action of steroid hormones.

Mutation of a termination codon affects src initiation

The four Rous sarcoma virus messages gag, gag-pol, env, and src all derive from a full-length RNA precursor. All four messages contain the same 5' leader segment. Three of the messages, gag, gag-pol, and env, use an AUG present in this leader to initiate translation. The src AUG initiation codon lies 3' of the leader segment, 90 bases downstream of the gag initiation codon in the spliced src message. However, in the spliced src message a UGA termination codon lies between the gag AUG and the src AUG. All three codons are in the same reading frame. By using oligonucleotide-directed mutagenesis, the UGA termination codon has been converted to CGA. Cells infected with the mutant (called 1057 CGA) were spindle shaped, distinct from the rounded shape of cells infected with the parental Rous sarcoma virus. The mutant virus initiates src translation at the gag AUG, producing a 63,000-dalton src protein. We suggest that the wild-type src message produces two polypeptides, a very small (nine-amino acid) peptide that is initiated at the gag AUG and the 60,000-dalton src protein that is initiated at the src AUG.

Interaction between the Rous sarcoma virus transforming protein and two cellular phosphoproteins: analysis of the turnover and distribution of this complex

The transforming protein of Rous sarcoma virus (RSV), pp60src, was previously shown to associate with two cellular proteins of Mr 90,000 and 50,000 in RSV-transformed chicken cells. In this report, we demonstrate that this interaction is specific for a discrete population of pp60src molecules. Newly synthesized pp60src was found to preferentially associate with pp90 and pp50 to form a short-lived complex. The half-life of this complex varied from 9 to 15 min in cells transformed by nondefective strains of RSV. This interaction between pp60src, pp50, and pp90 took place in a soluble fraction of the cell, and the complex-bound pp60src molecules were not phosphorylated on tyrosine. These results suggest that pp90 and pp50 may be involved in the processing of pp60src molecules before the association of pp60src with the plasma membrane. The kinetics of dissociation of this complex were shown to be altered in cells infected with viruses containing a temperature-sensitive defect in the src gene. When cells infected with these viruses were grown at the nonpermissive temperature, more than 90% of the pp60src molecules were associated with pp90 and pp50, and little or no dissociation was observed in a 3-h chase period. These results suggest that mutations in the src gene which affect the transforming activity of pp60src also affect the stability of the interaction of pp60src with pp90 and pp50.

Peptide analysis of the transformation-specific antigen from avian sarcoma virus-transformed cells

Sera from rabbits bearing tumors induced by avian sarcoma virus (ASV) were ussed to immunopecipitate virus-specific proteins from extracts of chicken, hamster, and field vole cells transformed by ASV. Two virus-specific proteins having molecular weights of 76,000 and 60,000 were found in all cell lines examined. The 76,000-molecular-weight protein, Pr76, is the precursor to the internal core proteins of ASV. The 60,000-molecular-weight (60K) transformation-specific antigen from each cell line was subjected to peptide analysis, using chymotrypsin and Staphylococcus aureus V8 protease. The resulting peptide maps of the 60K protein from the different ASV-infected cell types were similar for each enzyme, strongly suggesting that the 60K protein is virus coded. Two-dimensional analysis of chymotryptic peptides from Pr76 and 60K reveals that 60K is not related to the gs antigen precursor. Radiolabeling of ASV-transformed cells with inorganic phosphate revealed that 60K is phosphorylated in vivo. The 60K proteins isolated from both ASV-transformed chicken and field vole cells were found to contain one tryptic phosphopeptide. The tryptic phosphopeptides of 60K from both cell lines migrated identically upon two-dimensional peptide analyses, and their migration differed from that of the principal phosphopeptide of Pr76.