TRPS1 modulates chromatin accessibility to regulate estrogen receptor alpha (ER) binding and ER target gene expression in luminal breast cancer cells

Common genetic variants in the repressive GATA-family transcription factor (TF) TRPS1 locus are associated with breast cancer risk, and luminal breast cancer cell lines are particularly sensitive to TRPS1 knockout. We introduced an inducible degron tag into the native TRPS1 locus within a luminal breast cancer cell line to identify the direct targets of TRPS1 and determine how TRPS1 mechanistically regulates gene expression. We acutely deplete over 80 percent of TRPS1 from chromatin within 30 minutes of inducing degradation. We find that TRPS1 regulates transcription of hundreds of genes, including those related to estrogen signaling. TRPS1 directly regulates chromatin structure, which causes estrogen receptor alpha (ER) to redistribute in the genome. ER redistribution leads to both repression and activation of dozens of ER target genes. Downstream from these primary effects, TRPS1 depletion represses cell cycle-related gene sets and reduces cell doubling rate. Finally, we show that high TRPS1 activity, calculated using a gene expression signature defined by primary TRPS1-regulated genes, is associated with worse breast cancer patient prognosis. Taken together, these data suggest a model in which TRPS1 modulates the genomic distribution of ER, both activating and repressing transcription of genes related to cancer cell fitness.

The Androgen Receptor Does Not Directly Regulate the Transcription of DNA Damage Response Genes

The clinical success of combined androgen deprivation therapy (ADT) and radiotherapy (RT) in prostate cancer created interest in understanding the mechanistic links between androgen receptor (AR) signaling and the DNA damage response (DDR). Convergent data have led to a model where AR both regulates, and is regulated by, the DDR. Integral to this model is that the AR regulates the transcription of DDR genes both at a steady state and in response to ionizing radiation (IR). In this study, we sought to determine which immediate transcriptional changes are induced by IR in an AR-dependent manner. Using PRO-seq to quantify changes in nascent RNA transcription in response to IR, the AR antagonist enzalutamide, or the combination of the two, we find that enzalutamide treatment significantly decreased expression of canonical AR target genes but had no effect on DDR gene sets in prostate cancer cells. Surprisingly, we also found that the AR is not a primary regulator of DDR genes either in response to IR or at a steady state in asynchronously growing prostate cancer cells.

IMPLICATIONS

Our data indicate that the clinical benefit of combining ADT with RT is not due to direct AR regulation of DDR gene transcription, and that the field needs to consider alternative mechanisms for this clinical benefit.

TRPS1 modulates chromatin accessibility to regulate estrogen receptor (ER) binding and ER target gene expression in luminal breast cancer cells

Breast cancer is the most frequently diagnosed cancer in women. The most common subtype is luminal breast cancer, which is typically driven by the estrogen receptor α (ER), a transcription factor (TF) that activates many genes required for proliferation. Multiple effective therapies target this pathway, but individuals often develop resistance. Thus, there is a need to identify additional targets that regulate ER activity and contribute to breast tumor progression. TRPS1 is a repressive GATA-family TF that is overexpressed in breast tumors. Common genetic variants in the TRPS1 locus are associated with breast cancer risk, and luminal breast cancer cell lines are particularly sensitive to TRPS1 knockout. However, we do not know how TRPS1 regulates target genes to mediate these breast cancer patient and cellular outcomes. We introduced an inducible degron tag into the native TRPS1 locus within a luminal breast cancer cell line to identify the direct targets of TRPS1 and determine how TRPS1 mechanistically regulates gene expression. We acutely deplete over eighty percent of TRPS1 from chromatin within 30 minutes of inducing degradation. We find that TRPS1 regulates transcription of hundreds of genes, including those related to estrogen signaling. TRPS1 directly regulates chromatin structure, which causes ER to redistribute in the genome. ER redistribution leads to both repression and activation of dozens of ER target genes. Downstream from these primary effects, TRPS1 depletion represses cell cycle-related gene sets and reduces cell doubling rate. Finally, we show that high TRPS1 activity, calculated using a gene expression signature defined by primary TRPS1-regulated genes, is associated with worse breast cancer patient prognosis. Taken together, these data suggest a model in which TRPS1 modulates the activity of other TFs, both activating and repressing transcription of genes related to cancer cell fitness.

CD46 targeted 212Pb alpha particle radioimmunotherapy for prostate cancer treatment

We recently identified CD46 as a novel prostate cancer cell surface antigen that shows lineage independent expression in both adenocarcinoma and small cell neuroendocrine subtypes of metastatic castration resistant prostate cancer (mCRPC), discovered an internalizing human monoclonal antibody YS5 that binds to a tumor selective CD46 epitope, and developed a microtubule inhibitor-based antibody drug conjugate that is in a multi-center phase I trial for mCRPC (NCT03575819). Here we report the development of a novel CD46-targeted alpha therapy based on YS5. We conjugated ²¹²Pb, an in vivo generator of alpha-emitting ²¹²Bi and ²¹²Po, to YS5 through the chelator TCMC to create the radioimmunoconjugate, ²¹²Pb-TCMC-YS5. We characterized ²¹²Pb-TCMC-YS5 in vitro and established a safe dose in vivo. We next studied therapeutic efficacy of a single dose of ²¹²Pb-TCMC-YS5 using three prostate cancer small animal models: a subcutaneous mCRPC cell line-derived xenograft (CDX) model (subcu-CDX), an orthotopically grafted mCRPC CDX model (ortho-CDX), and a prostate cancer patient-derived xenograft model (PDX). In all three models, a single dose of 0.74 MBq (20 µCi) ²¹²Pb-TCMC-YS5 was well tolerated and caused potent and sustained inhibition of established tumors, with significant increases of survival in treated animals. A lower dose (0.37 MBq or 10 µCi ²¹²Pb-TCMC-YS5) was also studied on the PDX model, which also showed a significant effect on tumor growth inhibition and prolongation of animal survival. These results demonstrate that ²¹²Pb-TCMC-YS5 has an excellent therapeutic window in preclinical models including PDXs, opening a direct path for clinical translation of this novel CD46-targeted alpha radioimmunotherapy for mCRPC treatment.

IGF1R and Src inhibition induce synergistic cytotoxicity in HNSCC through inhibition of FAK

Head and neck cancer is the sixth most common cancer worldwide with a 5-year survival of only 65%. Targeting compensatory signaling pathways may improve therapeutic responses and combat resistance. Utilizing reverse phase protein arrays (RPPA) to assess the proteome and explore mechanisms of synergistic growth inhibition in HNSCC cell lines treated with IGF1R and Src inhibitors, BMS754807 and dasatinib, respectively, we identified focal adhesion signaling as a critical node. Focal Adhesion Kinase (FAK) and Paxillin phosphorylation were decreased as early as 15 min after treatment, and treatment with a FAK inhibitor, PF-562,271, was sufficient to decrease viability in vitro. Treatment of 3D spheroids demonstrated robust cytotoxicity suggesting that the combination of BMS754807 and dasatinib is effective in multiple experimental models. Furthermore, treatment with BMS754807 and dasatinib significantly decreased cell motility, migration, and invasion in multiple HNSCC cell lines. Most strikingly, treatment with BMS754807 and dasatinib, or a FAK inhibitor alone, significantly increased cleaved-PARP in human ex-vivo HNSCC patient tissues demonstrating a potential clinical utility for targeting FAK or the combined targeting of the IGF1R with Src. This ex-vivo result further confirms FAK as a vital signaling node of this combinatorial treatment and demonstrates therapeutic potential for targeting FAK in HNSCC patients.

Role of the runt-related transcription factor (RUNX) family in prostate cancer

Prostate cancer (PCa) is a very complex disease that is a major cause of death in men worldwide. Currently, PCa dependence on the androgen receptor (AR) has resulted in use of AR antagonists and antiandrogen therapies that reduce endogenous steroid hormone production. However, within two to three years of receiving first-line androgen deprivation therapy, the majority of patients diagnosed with PCa progress to castration-resistant prostate cancer (CRPC). There is an urgent need for therapies that are more durable than antagonism of the AR axis. Studies of runt-related transcription factors (RUNX) and their heterodimerization partner, core-binding factor subunit b (CBFβ), are revealing that the RUNX family are drivers of CRPC. In this review, we describe what is presently understood about RUNX members in PCa, including what regulates and is regulated by RUNX proteins, and the role of RUNX proteins in the tumor microenvironment and AR signaling. We discuss the implications for therapeutically targeting RUNX, the potential for RUNX as PCa biomarkers, and the current pressing questions in the field.

Molecular Imaging of Prostate Cancer Targeting CD46 Using ImmunoPET

PURPOSE

We recently identified CD46 as a novel therapeutic target in prostate cancer. In this study, we developed a CD46-targeted PET radiopharmaceutical, [⁸⁹Zr]DFO-YS5, and evaluated its performance for immunoPET imaging in murine prostate cancer models.

EXPERIMENTAL DESIGN

[⁸⁹Zr]DFO-YS5 was prepared and its in vitro binding affinity for CD46 was measured. ImmunoPET imaging was conducted in male athymic nu/nu mice bearing DU145 [AR^-, CD46⁺, prostate-specific membrane antigen-negative (PSMA^-)] or 22Rv1 (AR⁺, CD46⁺, PSMA⁺) tumors, and in NOD/SCID gamma mice bearing patient-derived adenocarcinoma xenograft, LTL-331, and neuroendocrine prostate cancers, LTL-331R and LTL-545.

RESULTS

[⁸⁹Zr]DFO-YS5 binds specifically to the CD46-positive human prostate cancer DU145 and 22Rv1 xenografts. In biodistribution studies, the tumor uptake of [⁸⁹Zr]DFO-YS5 was 13.3 ± 3.9 and 11.2 ± 2.5 %ID/g, respectively, in DU145 and 22Rv1 xenografts, 4 days postinjection. Notably, [⁸⁹Zr]DFO-YS5 demonstrated specific uptake in the PSMA- and AR-negative DU145 model. [⁸⁹Zr]DFO-YS5 also showed uptake in the patient-derived LTL-331 and -331R models, with particularly high uptake in the LTL-545 neuroendocrine prostate cancer tumors (18.8 ± 5.3, 12.5 ± 1.8, and 32 ± 5.3 %ID/g in LTL-331, LTL-331R, and LTL-545, respectively, at 4 days postinjection).

CONCLUSIONS

[⁸⁹Zr]DFO-YS5 is an excellent PET imaging agent across a panel of prostate cancer models, including in both adenocarcinoma and neuroendocrine prostate cancer, both cell line- and patient-derived xenografts, and both PSMA-positive and -negative tumors. It demonstrates potential for clinical translation as an imaging agent, theranostic platform, and companion biomarker in prostate cancer.

AR phosphorylation and CHK2 kinase activity regulates IR-stabilized AR-CHK2 interaction and prostate cancer survival

We have previously demonstrated that checkpoint kinase 2 (CHK2) is a critical negative regulator of androgen receptor (AR) transcriptional activity, prostate cancer (PCa) cell growth, and androgen sensitivity. We have now uncovered that the AR directly interacts with CHK2 and ionizing radiation (IR) increases this interaction. This IR-induced increase in AR-CHK2 interactions requires AR phosphorylation and CHK2 kinase activity. PCa associated CHK2 mutants with impaired kinase activity reduced IR-induced AR-CHK2 interactions. The destabilization of AR - CHK2 interactions induced by CHK2 variants impairs CHK2 negative regulation of cell growth. CHK2 depletion increases transcription of DNAPK and RAD54, increases clonogenic survival, and increases resolution of DNA double strand breaks. The data support a model where CHK2 sequesters the AR through direct binding decreasing AR transcription and suppressing PCa cell growth. CHK2 mutation or loss of expression thereby leads to increased AR transcriptional activity and survival in response to DNA damage.

PRAS40 Phosphorylation Correlates with Insulin-Like Growth Factor-1 Receptor-Induced Resistance to Epidermal Growth Factor Receptor Inhibition in Head and Neck Cancer Cells

EGFR inhibitors have shown poor efficacy in head and neck squamous cell carcinoma (HNSCC) with demonstrated involvement of the insulin-like growth factor-1 receptor (IGF1R) in resistance to EGFR inhibition. IGF1R activates the PI3K-Akt pathway, which phosphorylates proline-rich Akt substrate of 40 kDa (PRAS40) to cease mTOR inhibition resulting in increased mTOR signaling. Proliferation assays separated six HNSCC cell lines into two groups: sensitive to EGFR inhibition or resistant; all sensitive cell lines demonstrated reduced sensitivity to EGFR inhibition upon IGF1R activation. Reverse phase protein microarray analysis and immunoblot identified a correlation between increased PRAS40 phosphorylation and IGFR-mediated resistance to EGFR inhibition. In sensitive cell lines, PRAS40 phosphorylation decreased 44%-80% with EGFR inhibition and was restored to 98%-196% of control by IGF1R activation, while phosphorylation was unaffected in resistant cell lines. Possible involvement of mTOR in this resistance mechanism was demonstrated through a similar pattern of p70S6K phosphorylation. However, addition of temsirolimus, an mTORC1 inhibitor, was insufficient to overcome IGF1R-mediated resistance and suggested an alternative mechanism. Forkhead box O3a (FOXO3a), which has been reported to complex with PRAS40 in the cytoplasm, demonstrated a 6-fold increase in nuclear to cytoplasmic ratio upon EGFR inhibition that was eliminated with concurrent IGF1R activation. Transcription of FOXO3a-regulated TRAIL and PTEN-induced putative kinase-1 (PINK1) was increased with EGFR inhibition in sensitive cell lines; this effect was diminished with IGF1R stimulation. IMPLICATIONS: These data suggest PRAS40 may play an important role in IGF1R-based therapeutic resistance to EGFR inhibition, and this likely occurs via inhibition of FOXO3a-mediated proapoptotic gene transcription.

Discovery of a novel long noncoding RNA overlapping the LCK gene that regulates prostate cancer cell growth

Background

Virtually all patients with metastatic prostate cancer (PCa) will relapse and develop lethal castration-resistant prostate cancer (CRPC). Long noncoding RNAs (lncRNAs) are emerging as critical regulatory elements of many cellular biological processes, and may serve as therapeutic targets for combating PCa progression. Here, we have discovered in a high-throughput RNAi screen a novel lncRNA in PCa, and assessed the oncogenic effects of this lncRNA.

Methods

Rapid amplification of cDNA ends and sequencing was utilized to identify a previously unannotated lncRNA lying within exon six and the 3’UTR of the lymphocyte-specific protein tyrosine kinase (LCK) gene. The levels of HULLK in the presence or absence of hormone and/or enzalutamide or coregulator inhibitors were measured by quantitative PCR (qPCR). The determination of HULLK transcription and localization were characterized by strand-specific qPCR and cellular fractionation followed by qPCR, respectively. The correlation between HULLK expression and prostate cancer Gleason score was analyzed by droplet digital PCR. CyQuant assays were conducted to evaluate the effects of knocking down HULLK with shRNAs or overexpressing HULLK on cell growth.

Results

In this study, a previously unannotated lncRNA lying within exon six and 3’UTR of the LCK gene was dramatically upregulated by androgen in a dose-dependent manner, and the anti-androgen enzalutamide completely blocked this hormone-induced increase. Therefore, we labeled this lncRNA “HULLK” for Hormone-Upregulated lncRNA within LCK. Binding sites for two AR coregulators p300 and Brd4 reside near the HULLK transcriptional start site (TSS), and inhibitors of these coregulators downregulated HULLK. HULLK is transcribed from the sense strand of DNA, and predominantly localizes to the cytoplasm. HULLK transcripts are not only expressed in prostate cancer cell lines, but also prostate cancer patient tissue. Remarkably, there was a significant positive correlation between HULLK expression and high-grade PCa in multiple cohorts. shRNAs targeting HULLK significantly decreased PCa cell growth. Moreover, cells overexpressing HULLK were hypersensitive to androgen stimulation.

Conclusions

HULLK is a novel lncRNA situated within the LCK gene that may serve as an oncogene in PCa. Our data enhances our understanding of lncRNA biology and may assist in the development of additional biomarkers or more effective therapeutic targets for advanced PCa.

Electronic supplementary material

The online version of this article (10.1186/s12943-019-1039-6) contains supplementary material, which is available to authorized users.

Development of a multicellular pancreatic tumor microenvironment system using patient-derived tumor cells

The development of drugs to treat cancer is hampered by the inefficiency of translating pre-clinical in vitro monoculture and mouse studies into clinical benefit. There is a critical need to improve the accuracy of evaluating pre-clinical drug efficacy through the development of more physiologically relevant models. In this study, a human triculture 3D in vitro tumor microenvironment system (TMES) was engineered to accurately mimic the tumor microenvironment. The TMES recapitulates tumor hemodynamics and biological transport with co-cultured human microvascular endothelial cells, pancreatic ductal adenocarcinoma, and pancreatic stellate cells. We demonstrate that significant tumor cell transcriptomic changes occur in the TMES that correlate with the in vivo xenograft and patient transcriptome. Treatment with therapeutically relevant doses of chemotherapeutics yields responses paralleling the patients' clinical responses. Thus, this model provides a unique platform to rigorously evaluate novel therapies and is amenable to using patient tumor material directly, with applicability for patient avatars.

Targeting the mesenchymal subtype in glioblastoma and other cancers via inhibition of diacylglycerol kinase alpha

Background

The mesenchymal phenotype in glioblastoma (GBM) and other cancers drives aggressiveness and treatment resistance, leading to therapeutic failure and recurrence of disease. Currently, there is no successful treatment option available against the mesenchymal phenotype.

Methods

We classified patient-derived GBM stem cell lines into 3 subtypes: proneural, mesenchymal, and other/classical. Each subtype's response to the inhibition of diacylglycerol kinase alpha (DGKα) was compared both in vitro and in vivo. RhoA activation, liposome binding, immunoblot, and kinase assays were utilized to elucidate the novel link between DGKα and geranylgeranyltransferase I (GGTase I).

Results

Here we show that inhibition of DGKα with a small-molecule inhibitor, ritanserin, or RNA interference preferentially targets the mesenchymal subtype of GBM. We show that the mesenchymal phenotype creates the sensitivity to DGKα inhibition; shifting GBM cells from the proneural to the mesenchymal subtype increases ritanserin activity, with similar effects in epithelial-mesenchymal transition models of lung and pancreatic carcinoma. This enhanced sensitivity of mesenchymal cancer cells to ritanserin is through inhibition of GGTase I and downstream mediators previously associated with the mesenchymal cancer phenotype, including RhoA and nuclear factor-kappaB. DGKα inhibition is synergistic with both radiation and imatinib, a drug preferentially affecting proneural GBM.

Conclusions

Our findings demonstrate that a DGKα-GGTase I pathway can be targeted to combat the treatment-resistant mesenchymal cancer phenotype. Combining therapies with greater activity against each GBM subtype may represent a viable therapeutic option against GBM.

Abstract 1854: Treatment of head and neck squamous cell carcinoma cells with BMS754807 and Dasatinib induce synergistic cytotoxicity through altered focal adhesion signaling

Head and neck cancer (HNC) is the 6th most common cancer worldwide and the 9th most common cancer in the United States. Singular, targeted cancer therapies rarely produce a robust, complete and durable response and often do not substantially prolong patient survival. Adaptive survival responses often blunt the cytotoxic effects of drugs resulting in resistance. Therefore, targeting compensatory signaling pathways may provide improved therapeutic responses. Even with the current best treatment, 5-year survival for all patients with head and neck squamous cell carcinomas (HNSCC) is only approximately 65%, further highlighting the need for improved combinatorial treatments. Previous work in our laboratory has shown that inhibitors of Insulin Like Growth Factor 1 Receptor (IGF1R) and Src family kinases, when used in combination, synergistically inhibit HNSCC growth in vitro. Therefore, to explore the mechanisms which may contribute to this synergistic cytotoxicity, we utilized reverse phase protein arrays to assess the proteome of HNSCC lines treated with BMS754807 and Dasatinib to inhibit IGF1R and Src, respectively. We identified focal adhesion signaling as a potentially vital node in the cell signaling network of synergistic cytotoxicity upon treatment of HNSCC with BMS754807 and Dasatinib. Focal Adhesion Kinase (FAK) and Paxillin phosphorylation was decreased synergistically with the combination of BMS754807 and Dasatinib. Using western blot analysis, we confirmed a decrease in phosphorylation of FAK Y576/577 and Paxillin Y118 upon combined treatment with BMS754807 and Dasatinib. Interestingly, FAK is upregulated in radioresistant HNSCC patients and, metastases have increased phosphorylation of FAK compared to normal tissues. FAK is amplified in up to 26% of HNSCC and overexpression of Paxillin is associated with unfavorable prognosis, highlighting the importance of focal adhesion signaling in head and neck cancers. Our data suggests that interactions between integrins, receptor tyrosine kinase, and non-receptor kinase signaling regulate cell survival in HNSCC. Disruption of IGFR and Src activity leads to convergent inhibition of focal adhesion signaling and synergistic cytotoxicity in a variety of HNSCC cell lines.

Citation Format: Christine E. Lehman, Rolando MEndez, Mark Axelrod, Julia Wulfkuhle, Emanuel Petricoin, Daniel Gioeli, Mark Jameson. Treatment of head and neck squamous cell carcinoma cells with BMS754807 and Dasatinib induce synergistic cytotoxicity through altered focal adhesion signaling [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 1854.

Abstract 3747: Translating the functional interactions of checkpoint kinase 2 and the androgen receptor into more effective therapies for the treatment of prostate cancer

Prostate cancer remains the most diagnosed cancer among men in the United States behind skin cancer, and advanced prostate cancer is the third leading cause of cancer-related deaths, with a 5-year survival rate of 26%. Radiation is the standard of care for the treatment of prostate cancer at the early and late stages. Checkpoint kinase 2 (CHK2) is a serine/threonine protein kinase whose main function is regulating the DNA damage response (DDR) induced by ionizing radiation. The androgen receptor (AR) is a major driver of prostate cancer, even at the castration-resistant stage of the disease. The development of the second-generation anti-androgen enzalutamide, which is a selective AR antagonist, highlights the enduring importance of the AR. We have previously demonstrated that CHK2 is a critical negative regulator of prostate cancer cell growth, androgen sensitivity, and AR transcriptional activity. We have now uncovered novel molecular interactions between CHK2 and AR that provide mechanistic insight into our observation that CHK2 regulates prostate cancer growth. The AR directly interacts with CHK2, and that interaction increases with radiation. We found that the interaction of CHK2 and AR occurs at sites of DNA damage. The binding of CHK2 with AR can be disrupted with CHK2 kinase inhibitors suggesting that the kinase activity of CHK2 is required. This was verified using kinase-impaired CHK2 variants, including the K373E variant associated with 4.2% of prostate cancer. Furthermore, the radiation-induced increase in CHK2-AR interactions requires AR phosphorylation on both serine 81 and serine 308. Interestingly, CHK2-depletion in LNCaP cells increases ionizing radiation induced AR expression and DNA damage. Together, these data provide the rationale for targeting the CHK2-AR signaling axis to improve the effectiveness of prostate cancer therapies. The combination of CHK2 or CDK1 inhibitors with androgen deprivation therapy (ADT) and radiation shows an additive effect on the repression of tumor cell growth. Nearly every patient with disseminated prostate cancer will relapse following ADT and develop incurable castration-resistant prostate cancer. We have uncovered the molecular details of a signaling axis involving CHK2 and AR that, when perturbed in combination with ADT and/or ionizing radiation, effectively inhibits prostate cancer cell growth. This may enable resensitization of castration-resistant prostate cancer to the currently approved treatment options.

Citation Format: Huy Q. Ta, Natalia Dworak, Rosalie Sleppy, Jeffery A. Allende, Daniel Gioeli. Translating the functional interactions of checkpoint kinase 2 and the androgen receptor into more effective therapies for the treatment of prostate cancer [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 3747.

Inactivation of the CRL4-CDT2-SET8/p21 ubiquitylation and degradation axis underlies the therapeutic efficacy of pevonedistat in melanoma

The cullin-based CRL4-CDT2 ubiquitin ligase is emerging as a master regulator of cell proliferation. CRL4-CDT2 prevents re-initiation of DNA replication during the same cell cycle "rereplication" through targeted degradation of CDT1, SET8 and p21 during S-phase of the cell cycle. We show that CDT2 is overexpressed in cutaneous melanoma and predicts poor overall and disease-free survival. CDT2 ablation inhibited a panel of melanoma cell lines through the induction of SET8- and p21-dependent DNA rereplication and senescence. Pevonedistat (MLN4924), a specific inhibitor of the NEDD8 activating enzyme (NAE), inhibits the activity of cullin E3 ligases, thereby stabilizing a vast number of cullin substrates and resulting in cancer cell inhibition in vitro and tumor suppression in nude mice. We demonstrate that pevonedistat is effective at inhibiting the proliferation of melanoma cell lines in vitro through the induction of rereplication-dependent permanent growth arrest as well as through a transient, non-rereplication-dependent mechanism. CRISPR/Cas9-mediated heterozygous deletion of CDKN1A (encoding p21) or SET8 in melanoma cells demonstrated that the rereplication-mediated cytotoxicity of pevonedistat is mediated through preventing the degradation of p21 and SET8 and is essential for melanoma suppression in nude mice. By contrast, pevonedistat-induced transient growth suppression was independent of p21 or SET8, and insufficient to inhibit tumor growth in vivo. Pevonedistat additionally synergized with the BRAF kinase inhibitor PLX4720 to inhibit BRAF melanoma, and suppressed PLX4720-resistant melanoma cells. These findings demonstrate that the CRL4-CDT2-SET8/p21 degradation axis is the primary target of inhibition by pevonedistat in melanoma and suggest that a broad patient population may benefit from pevonedistat therapy.

RESEARCH IN CONTEXT

The identification of new molecular targets and effective inhibitors is of utmost significance for the clinical management of melanoma. This study identifies CDT2, a substrate receptor for the CRL4 ubiquitin ligase, as a prognostic marker and therapeutic target in melanoma. CDT2 is required for melanoma cell proliferation and inhibition of CRL4(CDT2) by pevonedistat suppresses melanoma in vitro and in vivo through the induction of DNA rereplication and senescence through the stabilization of the CRL4(CDT2) substrates p21 and SET8. Pevonedistat also synergizes with vemurafenib in vivo and suppresses vemurafenib-resistant melanoma cells. These findings show a significant promise for targeting CRL4(CDT2) therapeutically.

αvβ6 Integrin Promotes Castrate-Resistant Prostate Cancer through JNK1-Mediated Activation of Androgen Receptor

Androgen receptor signaling fuels prostate cancer and is a major therapeutic target. However, mechanisms of resistance to therapeutic androgen ablation are not well understood. Here, using a prostate cancer mouse model, Pten(pc-/-), carrying a prostate epithelial-specific Pten deletion, we show that the αvβ6 integrin is required for tumor growth in vivo of castrated as well as of noncastrated mice. We describe a novel signaling pathway that couples the αvβ6 integrin cell surface receptor to androgen receptor via activation of JNK1 and causes increased nuclear localization and activity of androgen receptor. This downstream kinase activation by αvβ6 is specific for JNK1, with no involvement of p38 or ERK kinase. In addition, differential phosphorylation of Akt is not observed under these conditions, nor is cell morphology affected by αvβ6 expression. This pathway, which is specific for αvβ6, because it is not regulated by a different αv-containing integrin, αvβ3, promotes upregulation of survivin, which in turn supports anchorage-independent growth of αvβ6-expressing cells. Consistently, both αvβ6 and survivin are significantly increased in prostatic adenocarcinoma, but are not detected in normal prostatic epithelium. Neither XIAP nor Bcl-2 is affected by αvβ6 expression. In conclusion, we show that αvβ6 expression is required for prostate cancer progression, including castrate-resistant prostate cancer; mechanistically, by promoting activation of JNK1, the αvβ6 integrin causes androgen receptor-increased activity in the absence of androgen and consequent upregulation of survivin. These preclinical results pave the way for further clinical development of αvβ6 antagonists for prostate cancer therapy. Cancer Res; 76(17); 5163-74. ©2016 AACR.

Abstract 979: Identification of a novel long noncoding RNA within the LCK gene locus that regulates prostate cancer cell growth

Prostate cancer remains the second most common type of cancer and frequent cause of cancer-related mortality in American men. Even though many patients with metastatic prostate cancer will initially respond to androgen deprivation therapy, virtually all patients will relapse and develop lethal castration-resistant prostate cancer. Long noncoding RNAs (lncRNAs) are emerging as critical regulatory elements of many cellular biological processes, and there is increasing evidence demonstrating that dysregulation of lncRNAs is associated with many human cancers, including cancers of the prostate, breast, and lung. We have discovered in a high-throughput RNAi screen identifying regulators of prostate cancer cell growth that knockdown of lymphocyte-specific protein tyrosine kinase (LCK) significantly decreases growth of prostate cancer cells in the presence and absence of androgen. Surprisingly, immunoprecipitation and western blot analyses show that LCK is not expressed at the protein level in prostate cancer cells. Rapid amplification of cDNA ends (RACE) and sequencing have revealed that a previously unannotated lncRNA lies within exon six and the 3’UTR of the LCK gene. While short hairpin RNAs (shRNAs) targeting the carboxy-terminus of the LCK gene decreases cell growth, expression of shRNAs specific for the amino-terminus has no effect on growth. Furthermore, only quantitative polymerase chain reaction (qPCR) primers directed toward the 3’ section of the LCK gene yield detectable levels of transcript. These data provide further validation for the existence of a lncRNA within the LCK gene locus. Remarkably, the lncRNA situated within the LCK gene is dramatically upregulated in response to androgen. Therefore, we have labeled this lncRNA “HULLK” for Hormone-upregulated lncRNA within LCK. Cellular fractionation and qPCR show that HULLK predominantly localizes to the cytoplasm. In addition to the effects on prostate cancer cell growth, we have data that alludes to the increase in transcript levels of several Src family members following depletion of HULLK. Thus, these studies indicate that the LCK gene contains a lncRNA involved in the regulation of prostate cancer cell growth and perhaps transcription. While additional analyses will be required to fully characterize HULLK, our data suggest that it may serve as a novel regulator of prostate cancer proliferation.

Citation Format: Huy Q. Ta, Samuel R. Jackson, Hilary Whitworth, Shriti Bhadel, Daniel Gioeli. Identification of a novel long noncoding RNA within the LCK gene locus that regulates prostate cancer cell growth. [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 979.

Combinatorial drug screening and molecular profiling reveal diverse mechanisms of intrinsic and adaptive resistance to BRAF inhibition in V600E BRAF mutant melanomas

Over half of BRAFV600E melanomas display intrinsic resistance to BRAF inhibitors, in part due to adaptive signaling responses. In this communication we ask whether BRAFV600E melanomas share common adaptive responses to BRAF inhibition that can provide clinically relevant targets for drug combinations. We screened a panel of 12 treatment-naïve BRAFV600E melanoma cell lines with MAP Kinase pathway inhibitors in pairwise combination with 58 signaling inhibitors, assaying for synergistic cytotoxicity. We found enormous diversity in the drug combinations that showed synergy, with no two cell lines having an identical profile. Although the 6 lines most resistant to BRAF inhibition showed synergistic benefit from combination with lapatinib, the signaling mechanisms by which this combination generated synergistic cytotoxicity differed between the cell lines. We conclude that adaptive responses to inhibition of the primary oncogenic driver (BRAFV600E) are determined not only by the primary oncogenic driver but also by diverse secondary genetic and epigenetic changes ("back-seat drivers") and hence optimal drug combinations will be variable. Because upregulation of receptor tyrosine kinases is a major source of drug resistance arising from diverse adaptive responses, we propose that inhibitors of these receptors may have substantial clinical utility in combination with inhibitors of the MAP Kinase pathway.

Preventing the Androgen Receptor N/C Interaction Delays Disease Onset in a Mouse Model of SBMA

Spinal and bulbar muscular atrophy (SBMA) is a neurodegenerative disease caused by a polyglutamine expansion in the androgen receptor (AR) and is associated with misfolding and aggregation of the mutant AR. We investigated the role of an interdomain interaction between the amino (N)-terminal FxxLF motif and carboxyl (C)-terminal AF-2 domain in a mouse model of SBMA. Male transgenic mice expressing polyQ-expanded AR with a mutation in the FxxLF motif (F23A) to prevent the N/C interaction displayed substantially improved motor function compared with N/C-intact AR-expressing mice and showed reduced pathological features of SBMA. Serine 16 phosphorylation was substantially enhanced by the F23A mutation; moreover, the protective effect of AR F23A was dependent on this phosphorylation. These results reveal an important role for the N/C interaction on disease onset in mice and suggest that targeting AR conformation could be a therapeutic strategy for patients with SBMA.

Checkpoint Kinase 2 Negatively Regulates Androgen Sensitivity and Prostate Cancer Cell Growth

Prostate cancer is the second leading cause of cancer death in American men, and curing metastatic disease remains a significant challenge. Nearly all patients with disseminated prostate cancer initially respond to androgen deprivation therapy (ADT), but virtually all patients will relapse and develop incurable castration-resistant prostate cancer (CRPC). A high-throughput RNAi screen to identify signaling pathways regulating prostate cancer cell growth led to our discovery that checkpoint kinase 2 (CHK2) knockdown dramatically increased prostate cancer growth and hypersensitized cells to low androgen levels. Mechanistic investigations revealed that the effects of CHK2 were dependent on the downstream signaling proteins CDC25C and CDK1. Moreover, CHK2 depletion increased androgen receptor (AR) transcriptional activity on androgen-regulated genes, substantiating the finding that CHK2 affects prostate cancer proliferation, partly, through the AR. Remarkably, we further show that CHK2 is a novel AR-repressed gene, suggestive of a negative feedback loop between CHK2 and AR. In addition, we provide evidence that CHK2 physically associates with the AR and that cell-cycle inhibition increased this association. Finally, IHC analysis of CHK2 in prostate cancer patient samples demonstrated a decrease in CHK2 expression in high-grade tumors. In conclusion, we propose that CHK2 is a negative regulator of androgen sensitivity and prostate cancer growth, and that CHK2 signaling is lost during prostate cancer progression to castration resistance. Thus, perturbing CHK2 signaling may offer a new therapeutic approach for sensitizing CRPC to ADT and radiation.

Systems Analysis of Adaptive Responses to MAP Kinase Pathway Blockade in BRAF Mutant Melanoma

Fifty percent of cutaneous melanomas are driven by activated BRAF^V600E, but tumors treated with RAF inhibitors, even when they respond dramatically, rapidly adapt and develop resistance. Thus, there is a pressing need to identify the major mechanisms of intrinsic and adaptive resistance and develop drug combinations that target these resistance mechanisms. In a combinatorial drug screen on a panel of 12 treatment-naïve BRAF^V600E mutant melanoma cell lines of varying levels of resistance to mitogen-activated protein kinase (MAPK) pathway inhibition, we identified the combination of PLX4720, a targeted inhibitor of mutated BRaf, and lapatinib, an inhibitor of the ErbB family of receptor tyrosine kinases, as synergistically cytotoxic in the subset of cell lines that displayed the most resistance to PLX4720. To identify potential mechanisms of resistance to PLX4720 treatment and synergy with lapatinib treatment, we performed a multi-platform functional genomics analysis to profile the genome as well as the transcriptional and proteomic responses of these cell lines to treatment with PLX4720. We found modest levels of resistance correlated with the zygosity of the BRAF V600E allele and receptor tyrosine kinase (RTK) mutational status. Layered over base-line resistance was substantial upregulation of many ErbB pathway genes in response to BRaf inhibition, thus generating the vulnerability to combination with lapatinib. The transcriptional responses of ErbB pathway genes are associated with a number of transcription factors, including ETS2 and its associated cofactors that represent a convergent regulatory mechanism conferring synergistic drug susceptibility in the context of diverse mutational landscapes.

Abstract 5049: Checkpoint kinase 2 is a novel regulator of prostate cancer cell growth

Prostate cancer (PCa) is the second leading cause of cancer death in American men, and the cure for metastatic disease remains a significant challenge. While nearly all patients with disseminated PCa initially respond to androgen deprivation therapy (ADT), virtually every patient will relapse and develop incurable castration-resistant prostate cancer (CRPC). Androgen receptor (AR) signaling pathways continue to play a crucial role in CRPC progression. Previous studies have shown that signal transduction pathways can stimulate AR activation, suggesting that the ability of signaling cascades to influence AR function may have a significant role in CRPC progression, and that CRPC may not be effectively treated by ligand-directed therapy alone. A high-throughput RNAi screen identifying signaling pathways that regulate PCa cell growth led to our discovery that knockdown of Checkpoint Kinase 2 (CHK2) dramatically increased PCa proliferation in the presence and absence of androgen. Furthermore, CHK2 depletion hypersensitized cells to castrate androgen levels. These CHK2-mediated effects on growth were dependent on the downstream signaling proteins CDC25C and CDK1 and could be blocked by the AR antagonist MDV3100. Immunohistochemical analysis of CHK2 in patient samples demonstrated that reduced CHK2 expression significantly correlated with increased Gleason score indicating the clinical relevance of CHK2 in PCa. Moreover, CHK2 expression is lower in castration-resistant C4-2 and CWR22Rv1 cells compared to androgen-sensitive LNCaP cells consistent with loss of CHK2 expression during PCa progression. CHK2 depletion increased AR transcriptional activity on both androgen-activated and androgen-repressed genes, substantiating that CHK2 affects PCa growth through the AR. Remarkably, quantitative PCR, chromatin immunoprecipitation, and western blot analyses revealed that CHK2 is a novel AR-repressed gene, suggesting a negative feedback loop between CHK2 and the AR. Furthermore, we show that CHK2 physically associates with the AR, and that cellular stress, such as DNA damage and serum starvation, increases this association. Based on these data, we propose that CHK2 is a negative regulator of androgen sensitivity and PCa growth. Thus, alterations to CHK2 signaling may sensitize CRPC to ADT and radiation.

Citation Format: Huy Q. Ta, Melissa L. Ivey, Henry F. Frierson, Mark R. Conaway, Jaroslaw Dziegielewski, James M. Larner, Daniel Gioeli. Checkpoint kinase 2 is a novel regulator of prostate cancer cell growth. [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 5049. doi:10.1158/1538-7445.AM2015-5049

Cell-cycle-dependent regulation of androgen receptor function

The androgen receptor (AR) is a critical oncogene in prostate cancer (PCa) development and progression. In this study, we demonstrate cell-cycle-dependent regulation of AR activity, localization, and phosphorylation. We show that for three AR-target genes, androgen-stimulated AR transactivation is highest during the G1 phase, decreased during S-phase, and abrogated during G2/M. This change in AR transactivation parallels changes in AR localization and phosphorylation. A combination of imaging techniques and quantitative analysis reveals nuclear AR localization during interphase and the exclusion of the majority, but not all, AR from chromatin during mitosis. Flow cytometry analyses using a phospho-S308 AR-specific antibody in asynchronous and chemically enriched G2/M PCa cells revealed ligand-independent induction of S308 phosphorylation in mitosis when CDK1 is activated. Consistent with our flow cytometry data, IP-western blotting revealed an increase in S308 phosphorylation in G2/M, and the results of an in vitro kinase assay indicated that CDK1 was able to phosphorylate the AR on S308. Pharmacological inhibition of CDK1 activity resulted in decreased S308 phosphorylation in PCa cells. Importantly, using a combination of anti-total AR and phospho-S308-specific antibodies in immunofluorescence experiments, we showed that the AR is excluded from condensed chromatin in mitotic cells when it was phosphorylated on S308. In summary, we show that the phosphorylation of the AR on S308 by CDK1 during mitosis regulates AR localization and correlates with changes in AR transcriptional activity. These findings have important implications for understanding the function of AR as an oncogene.

The convergence of DNA damage checkpoint pathways and androgen receptor signaling in prostate cancer

It is increasingly clear that castration-resistant prostate cancer (PCa) is dependent on the androgen receptor (AR). This has led to the use of anti-androgen therapies that reduce endogenous steroid hormone production as well as the use of AR antagonists. However, the AR does not act in isolation and integrates with a milieu of cell-signaling proteins to affect cell biology. It is well established that cancer is a genetic disease resulting from the accumulation of mutations and chromosomal translocations that enables cancer cells to survive, proliferate, and disseminate. To maintain genomic integrity, there exists conserved checkpoint signaling pathways to facilitate cell cycle delay, DNA repair, and/or apoptosis in response to DNA damage. The AR interacts with, affects, and is affected by these DNA damage-response proteins. This review will focus on the connections between checkpoint signaling and the AR in PCa. We will describe what is known about how components of checkpoint signaling regulate AR activity and what questions still face the field.

Androgen receptor phosphorylation: biological context and functional consequences

The androgen receptor (AR) is a ligand-regulated transcription factor that belongs to the family of nuclear receptors. In addition to regulation by steroid, the AR is also regulated by post-translational modifications generated by signal transduction pathways. Thus, the AR functions not only as a transcription factor but also as a node that integrates multiple extracellular signals. The AR plays an important role in many diseases, including complete androgen insensitivity syndrome, spinal bulbar muscular atrophy, prostate and breast cancer, etc. In the case of prostate cancer, dependence on AR signaling has been exploited for therapeutic intervention for decades. However, the effectiveness of these therapies is limited in advanced disease due to restoration of AR signaling. Greater understanding of the molecular mechanisms involved in AR action will enable the development of improved therapeutics to treat the wide range of AR-dependent diseases. The AR is subject to regulation by a number of kinases through post-translational modifications on serine, threonine, and tyrosine residues. In this paper, we review the AR phosphorylation sites, the kinases responsible for these phosphorylations, as well as the biological context and the functional consequences of these phosphorylations. Finally, what is known about the state of AR phosphorylation in clinical samples is discussed.

Abstract B29: Combinatorial screens with targeted inhibitors reveal diverse compensatory responses and mechanisms of adaptive resistance to therapy

The effectiveness of targeted inhibition of cell signaling can be blunted by compensatory signaling, which generates adaptive resistance mechanisms and reduces therapeutic responses. We have performed high-throughput combinatorial drug screening as a discovery tool to identify compensatory pathways that confer resistance to the cytotoxic effects of targeted therapy. We screened subsets of over 1,000 drug combinations in 14 different epithelial cell lines representing three distinct cancer lineages, and 19 melanoma cell lines, and assessed the ability of each combination to cause synergistic cytotoxicity. We focused on synergistic combinations because they point to mechanistic linkages between the signaling pathways, and also because of the possibility of improved therapeutic index in vivo. Drug substitution studies were used to validate the functionally important drug targets. Of the 84 combinations that caused robust synergy in multiple epithelial cell lines, none were synergistic in more than half of the lines tested, and we observed no pattern with respect to lineage specificity or mutational status of commonly altered oncogenes in the observed synergies. Within the melanoma panel, BRAF mutational status predicted response to single-agent BRAF inhibition, but did not predict synergistic drug combinations, which were different for each cell line. These results reflect the heterogeneity of genetic alterations and the plasticity of cell signaling networks even among cell lines of the same tissue of origin that contain the same predominant driver mutations. We suggest that there is not a sharp dichotomy between “driver” and “passenger” mutations, and that the biological responses to combination therapies are determined by functionally important modifier mutations that we term “back-seat drivers.” This hypothesis is supported by analysis of the transcriptomes and phosphoproteomes of cells treated with drugs singly and in combination, and by exome sequencing. Such analyses also can reveal critical nodes with the potential to function as effective single targets. We found that co-inhibition of EGFR and PI3 Kinase causes synergistic cytotoxicity in some epithelial cancer cell lines, and that phosphoproteomic analysis of signaling pathway responses revealed concordant synergistic inhibition of p70S6 Kinase in KU-7 bladder cancer cells. Using an epistasis paradigm, restoration of p70S6 Kinase signaling by expression of mutationally activated p70S6 Kinase resulted in protection from cytotoxicity, indicating that p70S6 Kinase is a critical node for enhanced cytotoxicity due to combination treatment. AT7867, a potent inhibitor of p70S6 Kinase, was able to inhibit phosphorylation of ribosomal protein S6 and induce cytotoxicity as effectively as the combination drug treatment. We suggest that p70S6 Kinase acts as a functionally important node within the EGFR/PI3 Kinase signaling network and is an attractive target for therapeutic intervention.

Citation Format: Mark J. Axelrod, Devin Roller, Brian Capaldo, Aaron J. Mackey, Mark Conaway, Daniel Gioeli, Michael J. Weber. Combinatorial screens with targeted inhibitors reveal diverse compensatory responses and mechanisms of adaptive resistance to therapy. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Synthetic Lethal Approaches to Cancer Vulnerabilities; May 17-20, 2013; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(5 Suppl):Abstract nr B29.

Combinatorial drug screening identifies compensatory pathway interactions and adaptive resistance mechanisms

Constitutively activated signaling molecules are often the primary drivers of malignancy, and are favored targets for therapeutic intervention. However, the effectiveness of targeted inhibition of cell signaling can be blunted by compensatory signaling which generates adaptive resistance mechanisms and reduces therapeutic responses. Therefore, it is important to identify and target these compensatory pathways with combinations of targeted agents to achieve durable clinical benefit. In this report, we demonstrate the use of high-throughput combinatorial drug screening as a discovery tool to identify compensatory pathways that generate resistance to the cytotoxic effects of targeted therapy. We screened 420 drug combinations in 14 different cell lines representing three cancer lineages, and assessed the ability of each combination to cause synergistic cytotoxicity. Drug substitution studies were used to validate the functionally important drug targets. Of the 84 combinations that caused robust synergy in multiple cell lines, none were synergistic in more than half of the lines tested, and we observed no pattern of lineage specificity in the observed synergies. This reflects the plasticity of cell signaling networks, even among cell lines of the same tissue of origin. Mechanistic analysis of one novel synergistic combination identified in the screen, the multi-kinase inhibitor Ro31-8220 and lapatinib, demonstrated compensatory crosstalk between the p70S6 kinase and EGF receptor pathways. In addition, we identified BAD as a node of convergence between these two pathways that may be playing a role in the enhanced apoptosis observed upon combination treatment.

Abstract 5633: Co-targeting the IGF1R pathway and compensatory signaling enhances cytotoxicity in head and neck cancer

The 5-year survival rate for patients with cancer of the Head and Neck (HNC) has not significantly improved over the past decade. Clearly, new therapeutic targets and strategies for employing existing targeted and cytotoxic therapies are needed. Studies have shown both expression and phosphorylation of the insulin-like growth factor 1 receptor (IGF1R) in both head and neck cancer cell lines and patient tumor samples. We found that small molecule inhibitors targeting IGF1R, when used as a single agent, caused varying degrees of cytotoxicity in HNC cell lines. In order to determine whether the variable biological response to IGF1R inhibition was caused by differences in the effects of the IGF1R inhibitors on the cell signaling networks of the HNC cell lines, we performed reverse phase proteomic array analysis on a panel of HNC cell lines treated with IGF1R inhibitors. Preliminary results suggest that treatment with IGF1R inhibitors caused increased expression and/or phosphorylation of a number of proteins in the array. We hypothesize that these alterations represent compensatory signaling pathways that provide resistance to cytotoxicity upon treatment with IGF1R inhibitors. In order to identify possible mechanisms of compensation for the loss of IGF1R pathway signaling, we screened a panel of small molecule inhibitors of the IGF1R/PI3K/AKT signaling pathway against a panel of inhibitors of proteins important in HNC cell signaling Preliminary results indicate that the inhibition of epigenetic modifying proteins as well as members of canonical cell signaling pathways in combination with inhibition of IGF1R signaling leads to a synergistic increase in cytotoxicity. This suggests that compensatory mechanisms exist in HNC that serve to blunt the cytotoxic effect of inhibition of IGF1R. Co-targeting IGF1R and members of these compensatory pathways may be a viable therapeutic strategy.

Citation Format: Mark J. Axelrod, Daniel Gioeli, Elizabeth R. Sharlow, Mark C. Conaway, Rolando E. Mendez, Ashraf Khalil, Linnea Taniguchi, Emmanual F. Petricoin, Stephanie Leimgruber, Michael J. Weber, Mark J. Jameson. Co-targeting the IGF1R pathway and compensatory signaling enhances cytotoxicity in head and neck cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5633. doi:10.1158/1538-7445.AM2013-5633

Abstract 3577: The role of Checkpoint Kinase 2 in the regulation of androgen receptor signaling and prostate cancer cell growth during progression to castration resistance

As the second leading cause of cancer deaths in men, prostate cancer is a persistent, significant challenge for clinicians and researchers. Almost all patients with disseminated prostate cancer initially respond to androgen deprivation therapy. However, virtually every patient will relapse due to the growth of castration-resistant cancer cells and develop metastatic and lethal disease. Even with the recent development of new androgen ablation treatments such as Abiraterone and Enzalutamide, castration-resistant prostate cancer is still incurable. We hypothesize that compensatory signaling mechanisms that limit the effectiveness of androgen ablation can be overcome with therapeutic strategies targeting kinase cascades. In order to identify signaling pathways that regulate AR activity and prostate cancer cell growth we screened a panel of shRNAs targeting the human kinome in LNCaP prostate cancer cells grown in the presence and absence of androgen. We discovered that knockdown of Checkpoint Kinase 2 (CHK2) dramatically increased LNCaP prostate cancer cell proliferation. This observation is clinically relevant since CHK2 inactivating mutations arise in over 10% of prostate cancer patients and CHK2 expression decreases as prostate cancer progresses to a castration-resistant disease. Consistent with these clinical observations, CHK2 knockdown did not affect cell growth in the castration resistant C4-2 cell line, suggesting that the selection of the castration resistant C4-2 line from LNCaP involved the same effector as observed in clinical specimens. Consistent with this, CHK2 expression is lower in C4-2 cells compared to LNCaP. We determined that CHK2 knockdown increases androgen receptor (AR) transcriptional activity on both androgen activated and androgen repressed genes, providing evidence that CHK2 affects prostate cancer cell proliferation, at least in part, through the AR. These data suggest that CHK2 is a negative regulator of androgen sensitivity and prostate cancer cell growth and that CHK2 is lost during the progression to castration resistance. We are examining the role of CHK2 in regulating androgen-AR signaling, growth, and survival of prostate cancer cells; specifically we are identifying the crucial intracellular proteins that mediate CHK2 signaling and growth regulation of prostate cancer cells. Since CHK2 signaling is activated by DNA damage, such as that triggered by radiation therapy and brachytherapy, understanding how CHK2 signaling regulates the AR will provide critical insights into how we can combine current therapies with androgen blockade for greater clinical effectiveness.

Citation Format: Huy Q. Ta, Melissa L. Ivey, Daniel Gioeli. The role of Checkpoint Kinase 2 in the regulation of androgen receptor signaling and prostate cancer cell growth during progression to castration resistance. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3577. doi:10.1158/1538-7445.AM2013-3577

Synthetic lethal screening with small-molecule inhibitors provides a pathway to rational combination therapies for melanoma

Recent data show that extracellular signals are transmitted through a network of proteins rather than hierarchical signaling pathways, suggesting that the inhibition of a single component of a canonical pathway is insufficient for the treatment of cancer. The biologic outcome of signaling through a network is inherently more robust and resistant to inhibition of a single network component. In this study, we conducted a functional chemical genetic screen to identify novel interactions between signaling inhibitors that would not be predicted on the basis of our current understanding of signaling networks. We screened over 300 drug combinations in nine melanoma cell lines and have identified pairs of compounds that show synergistic cytotoxicity. The synergistic cytotoxicities identified did not correlate with the known RAS and BRAF mutational status of the melanoma cell lines. Among the most robust results was synergy between sorafenib, a multikinase inhibitor with activity against RAF, and diclofenac, a nonsteroidal anti-inflammatory drug (NSAID). Drug substitution experiments using the NSAIDs celecoxib and ibuprofen or the MAP-ERK kinase inhibitor PD325901 and the RAF inhibitor RAF265 suggest that inhibition of COX and mitogen-activated protein kinase signaling are targets for the synergistic cytotoxicity of sorafenib and diclofenac. Cotreatment with sorafenib and diclofenac interrupts a positive feedback signaling loop involving extracellular signal-regulated kinase, cellular phospholipase A2, and COX. Genome-wide expression profiling shows synergy-specific downregulation of survival-related genes. This study has uncovered novel functional drug combinations and suggests that the underlying signaling networks that control responses to targeted agents can vary substantially, depending on unexplored components of the cell genotype.

Post-translational modification of the androgen receptor

Regulation of the androgen receptor (AR) by its cognate ligand is well established, but how post-translational modification modulates AR activity is only emerging. The AR is subject to modification by phosphorylation, acetylation, methylation, SUMOylation, and ubiquitination. As several of the enzymes that modify the AR are altered in prostate cancer, defining the context and physiological effects of these modifications could provide insight into mechanisms that underpin human disease. Here, we review how post-translational modification contributes to AR function as a transcription factor with particular emphasis on phosphorylation and dephosphorylation mechanisms.

Compensatory pathways induced by MEK inhibition are effective drug targets for combination therapy against castration-resistant prostate cancer

Targeted therapies have often given disappointing results when used as single agents in solid tumors, suggesting the importance of devising rational combinations of targeted drugs. We hypothesized that construction of such combinations could be guided by identification of growth and survival pathways whose activity or expression become upregulated in response to single-agent drug treatment. We mapped alterations in signaling pathways assessed by gene array and protein phosphorylation to identify compensatory signal transduction pathways in prostate cancer xenografts treated with a MAP/ERK kinase (MEK) inhibitor PD325901. In addition to numerous components of the extracellular signal-regulated kinase (ERK) signaling pathway, components of the IKK, hedgehog, and phosphoinositide 3-kinase/Akt/mTOR pathways were upregulated following treatment with PD325901. Combinations of PD325901 with inhibitors of any one of these upregulated pathways provided synergistically greater growth inhibition of in vitro cell growth and survival than the individual drugs alone. Thus, the identification of compensatory signal transduction pathways paves the way for rational combinatorial therapies for the effective treatment of prostate cancer.

CDK9 regulates AR promoter selectivity and cell growth through serine 81 phosphorylation

Previously we determined that S81 is the highest stoichiometric phosphorylation on the androgen receptor (AR) in response to hormone. To explore the role of this phosphorylation on growth, we stably expressed wild-type and S81A mutant AR in LHS and LAPC4 cells. The cells with increased wild-type AR expression grow faster compared with parental cells and S81A mutant-expressing cells, indicating that loss of S81 phosphorylation limits cell growth. To explore how S81 regulates cell growth, we tested whether S81 phosphorylation regulates AR transcriptional activity. LHS cells stably expressing wild-type and S81A mutant AR showed differences in the regulation of endogenous AR target genes, suggesting that S81 phosphorylation regulates promoter selectivity. We next sought to identify the S81 kinase using ion trap mass spectrometry to analyze AR-associated proteins in immunoprecipitates from cells. We observed cyclin-dependent kinase (CDK)9 association with the AR. CDK9 phosphorylates the AR on S81 in vitro. Phosphorylation is specific to S81 because CDK9 did not phosphorylate the AR on other serine phosphorylation sites. Overexpression of CDK9 with its cognate cyclin, Cyclin T, increased S81 phosphorylation levels in cells. Small interfering RNA knockdown of CDK9 protein levels decreased hormone-induced S81 phosphorylation. Additionally, treatment of LNCaP cells with the CDK9 inhibitors, 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole and Flavopiridol, reduced S81 phosphorylation further, suggesting that CDK9 regulates S81 phosphorylation. Pharmacological inhibition of CDK9 also resulted in decreased AR transcription in LNCaP cells. Collectively these results suggest that CDK9 phosphorylation of AR S81 is an important step in regulating AR transcriptional activity and prostate cancer cell growth.

Matrix rigidity regulates cancer cell growth and cellular phenotype

Background

The mechanical properties of the extracellular matrix have an important role in cell growth and differentiation. However, it is unclear as to what extent cancer cells respond to changes in the mechanical properties (rigidity/stiffness) of the microenvironment and how this response varies among cancer cell lines.

Methodology/Principal Findings

In this study we used a recently developed 96-well plate system that arrays extracellular matrix-conjugated polyacrylamide gels that increase in stiffness by at least 50-fold across the plate. This plate was used to determine how changes in the rigidity of the extracellular matrix modulate the biological properties of tumor cells. The cell lines tested fall into one of two categories based on their proliferation on substrates of differing stiffness: “rigidity dependent” (those which show an increase in cell growth as extracellular rigidity is increased), and “rigidity independent” (those which grow equally on both soft and stiff substrates). Cells which grew poorly on soft gels also showed decreased spreading and migration under these conditions. More importantly, seeding the cell lines into the lungs of nude mice revealed that the ability of cells to grow on soft gels in vitro correlated with their ability to grow in a soft tissue environment in vivo. The lung carcinoma line A549 responded to culture on soft gels by expressing the differentiated epithelial marker E-cadherin and decreasing the expression of the mesenchymal transcription factor Slug.

Conclusions/Significance

These observations suggest that the mechanical properties of the matrix environment play a significant role in regulating the proliferation and the morphological properties of cancer cells. Further, the multiwell format of the soft-plate assay is a useful and effective adjunct to established 3-dimensional cell culture models.

Karyopherin alpha7 (KPNA7), a divergent member of the importin alpha family of nuclear import receptors

Background

Classical nuclear localization signal (NLS) dependent nuclear import is carried out by a heterodimer of importin α and importin β. NLS cargo is recognized by importin α, which is bound by importin β. Importin β mediates translocation of the complex through the central channel of the nuclear pore, and upon reaching the nucleus, RanGTP binding to importin β triggers disassembly of the complex. To date, six importin α family members, encoded by separate genes, have been described in humans.

Results

We sequenced and characterized a seventh member of the importin α family of transport factors, karyopherin α 7 (KPNA7), which is most closely related to KPNA2. The domain of KPNA7 that binds Importin β (IBB) is divergent, and shows stronger binding to importin β than the IBB domains from of other importin α family members. With regard to NLS recognition, KPNA7 binds to the retinoblastoma (RB) NLS to a similar degree as KPNA2, but it fails to bind the SV40-NLS and the human nucleoplasmin (NPM) NLS. KPNA7 shows a predominantly nuclear distribution under steady state conditions, which contrasts with KPNA2 which is primarily cytoplasmic.

Conclusion

KPNA7 is a novel importin α family member in humans that belongs to the importin α2 subfamily. KPNA7 shows different subcellular localization and NLS binding characteristics compared to other members of the importin α family. These properties suggest that KPNA7 could be specialized for interactions with select NLS-containing proteins, potentially impacting developmental regulation.

Abstract LB-201: Nuclear localization of Src, androgen receptor and PSA in prostate cancer

Src signaling plays an important role in prostate cancer (PrCa) progression. Dasatinib, an oral src family kinase inhibitor, is being tested in patients with PrCa in several Phase II and III clinical trials. It has been shown that src interacts with androgen receptor (AR) and enhances AR transactivation. Although it has been shown that src promotes AR activity, the underlying pathway has not been defined.

To characterize the src-AR pathway, we first analyzed the localization of src, AR, and Prostate Specific Antigen (PSA, an AR target gene). Using sub-cellular fractionation and immunofluorescence, p-src and src were found in the nucleus, apart from their normal cytoplasmic localization, in androgen-dependent LNCaP cells upon androgen stimulation or deprivation conditions. Also, their localization was not affected by androgen stimulation. Similar to src and p-src, AR as well as pSer81-AR (an AR site indirectly phosphorylated by src) were found in the nucleus as well as in the cytoplasm. Unexpectedly, we found PSA localization in the nucleus upon androgen stimulation in LNCaP and C4-2B cells as well as in the nucleus of C4-2B cells upon androgen deprivation.

We further studied the effect of src on AR activity by transfection of dominant negative src (SrcK298M) in LNCaP and androgen-independent C4-2B cells. Transfection with SrcK298M did not affect PSA expression in LNCaP cells whereas in C4-2B cells SrcK298M transfection inhibited PSA expression.

These data show that src is required for AR activity and, consequently, PSA expression in androgen-independent prostate cancer cells, but not in androgen-dependent cells.

In conclusion, these data suggest that the nuclear co-localization of p-src, AR and PSA might allow macromolecular interactions which can further enhance AR transactivation and promote disease progression.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-201.

Abstract 3843: Rational selection of combinatorial therapeutic targets for melanoma identified by synthetic lethal screening with small molecule inhibitors

Cellular signals are not necessarily transmitted via linear pathways, but through a dynamic interconnected network. Effective pharmacologic inhibition of cellular signaling in malignant cells will thus require identifying the key nodes, and intervening at more than one site within the network. To systematically identify functional interactions between signaling proteins within the cell signaling network we have screened nine melanoma cell lines of diverse genetic backgrounds for sensitivity to combinations of drugs targeting the major signaling pathways altered in cancer. Seven drugs representative of standard targets were designated as “primary drugs”. These seven primary targeted therapies were screened against each other and an additional 67 signal transduction inhibitors. The screen was performed robotically with a Biomek NX workstation in a 96-well format. Cell viability was measured with alamarBlue on a microplate reader. Various concentrations of the secondary drug were tested against a partial inhibitory concentration of the primary drug. Twelve percent of a total of 4672 drug dose and cell combinations tested displayed superadditivity in vitro. Nearly 10% of those hits demonstrated greater than 50% superadditivity according to first principles analysis; the actual growth inhibition of the combination was more than 50% greater than the sum of the growth inhibition produced by each drug alone. One intriguing drug combination showing superadditivity in most cell lines tested was sorafenib plus diclofenac. The degree of inhibition and concentration in which superadditivity was observed was cell line dependent, and was independent of known mutational status. The MEK inhibitor, PD325901, was able to substitute qualitatively for sorafenib indicating that Raf may be the major target for sorafenib when used in combination with diclofenac. Consistent with this, sorafenib inhibited MEK and ERK activity at doses that achieved superadditivity with diclofenac. However, the combination of PD325901 with diclofenac was less robust at inhibiting growth than sorafenib plus diclofenac suggesting that the alternate sorafenib targets may play a role in the observed superadditivity. Celecoxib, a COX2 inhibitor, or ibuprofen, a preferential COX1 inhibitor, substituted for diclofenac indicating that diclofenac was acting as a COX inhibitor, although the inhibition was considerably less robust, suggesting a role for a balanced inhibition of COX1 and COX2, or a role for off-target effects. Collectively, these results indicate an unexpected functional interaction between Raf and COX signaling in melanoma. Since both sorafenib and diclofenac are in clinical use, the therapeutic potential of these observations is striking and is being further explored in pre-clinical models.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3843.

Dasatinib inhibits site-specific tyrosine phosphorylation of androgen receptor by Ack1 and Src kinases

Activation of androgen receptor (AR) may play a role in the development of castration resistant prostate cancer. Two intracellular tyrosine kinases, Ack1 (activated cdc42-associated kinase) and Src, phosphorylate and enhance AR activity and promote prostate xenograft tumor growth in castrated animals. However, the upstream signals that activate these kinases and lead to AR activation are incompletely characterized. In this study, we investigated AR phosphorylation in response to non-androgen ligand stimulation using phospho-specific antibodies. Treatment of LNCaP and LAPC-4 cells with epidermal growth factor (EGF), heregulin, Gas6 (ligand binding to Mer receptor tyrosine kinase and activating Ack1 downstream), interleukin (IL)-6 or bombesin stimulated cell proliferation in the absence of androgen. Treatment of LNCaP and LAPC-4 cells with EGF, heregulin, or Gas6 induced AR phosphorylation at Tyr-267; IL-6 or bombesin treatment did not. AR phosphorylation at Tyr-534 was induced by treatment with EGF, IL-6 or bombesin, but not by heregulin or Gas6. siRNA-mediated knockdown of Ack1 or Src showed that Ack1 mediates heregulin- and Gas6-induced AR Tyr-267 phosphorylation whereas Src mediates Tyr-534 phosphorylation induced by EGF, IL-6, and bombesin. Dasatinib, a Src inhibitor, blocked EGF-induced Tyr-534 phosphorylation. In addition, we show dasatinib also inhibited Ack1 kinase. Dasatinib inhibited heregulin-induced Ack1 kinase activity and AR Tyr-267 phosphorylation. Dasatinib inhibited heregulin-induced AR-dependent reporter activity. Dasatinib also inhibited heregulin-induced expression of endogenous AR target genes. Dasatinib inhibited Ack1-dependent colony formation and prostate xenograft tumor growth in castrated mice. Interestingly, Ack1 or Src knockdown or dasatinib did not inhibit EGF-induced AR Tyr-267 phosphorylation or EGF-stimulated AR activity, suggesting the existence of an additional tyrosine kinase that phosphorylates AR at Tyr-267. These data suggest that specific tyrosine kinases phosphorylate AR at distinct sites and that dasatinib may exert anti-tumor activity in prostate cancer through inhibition of Ack1.

The neuroendocrine-derived peptide parathyroid hormone-related protein promotes prostate cancer cell growth by stabilizing the androgen receptor

During progression to an androgen-independent state following androgen ablation therapy, prostate cancer cells continue to express the androgen receptor (AR) and androgen-regulated genes, indicating that AR is critical for the proliferation of hormone-refractory prostate cancer cells. Multiple mechanisms have been proposed for the development of AR-dependent hormone-refractory disease, including changes in expression of AR coregulatory proteins, AR mutation, growth factor-mediated activation of AR, and AR protein up-regulation. The most prominent of these progressive changes is the up-regulation of AR that occurs in >90% of prostate cancers. A common feature of the most aggressive hormone-refractory prostate cancers is the accumulation of cells with neuroendocrine characteristics that produce paracrine factors and may provide a novel mechanism for the regulation of AR during advanced stages of the disease. In this study, we show that neuroendocrine-derived parathyroid hormone-related protein (PTHrP)-mediated signaling through the epidermal growth factor receptor (EGFR) and Src pathways contributes to the phenotype of advanced prostate cancer by reducing AR protein turnover. PTHrP-induced accumulation of AR depended on the activity of Src and EGFR and consequent phosphorylation of the AR on Tyr(534). PTHrP-induced tyrosine phosphorylation of AR resulted in reduced AR ubiquitination and interaction with the ubiquitin ligase COOH terminus of Hsp70-interacting protein. These events result in increased accumulation of AR and thus enhanced growth of prostate cancer cells at low levels of androgen.

Restoration of PTEN expression alters the sensitivity of prostate cancer cells to EGFR inhibitors

INTRODUCTION

Prostate cancer (CaP) progression from an androgen-dependent to an androgen-independent state is associated with overexpression of EGFR family members or activation of their downstream signaling pathways, such as PI3K-Akt and MAPK. Although there are data implicating PI3K-Akt or MAPK pathway activation with resistance to EGFR inhibitors in CaP, the potential cross-talk between these pathways in response to EGFR or MAPK inhibitors remains to be examined.

METHODS

Cross-talk between PTEN and MAPK signaling and its effects on CaP cell sensitivity to EGFR or MAPK inhibitors were examined in a PTEN-null C4-2 CaP cell, pTetOn PTEN C4-2, where PTEN expression was restored conditionally.

RESULTS

Expression of PTEN in C4-2 cells exposed to EGF or serum was associated with increased phospho-ERK levels compared to cells without PTEN expression. Similar hypersensitivity of MAPK signaling was observed when cells were treated with a PI3K inhibitor LY294002. This enhanced sensitivity of MAPK signaling in PTEN-expressing cells was associated with a growth stimulatory effect in response to EGF. Furthermore, EGFR inhibitors gefitinib and lapatinib abrogated hypersensitivity of MAPK signaling and cooperated with PTEN expression to inhibit cell growth in both monolayer and anchorage-independent conditions. Similar cooperative growth inhibition was observed when cells were treated with the MEK inhibitor, CI1040, in combination with PTEN expression suggesting that inhibition of MAPK signaling could mediate the cooperation of EGFR inhibitors with PTEN expression.

CONCLUSIONS

Our results suggest that signaling cross-talk between the PI3K-Akt and MAPK pathways occurs in CaP cells, highlighting the potential benefit of targeting both the PI3K-Akt and MAPK pathways in CaP treatment.

Activation of the DNA-dependent protein kinase stimulates nuclear export of the androgen receptor in vitro

The androgen receptor undergoes nuclear import in response to ligand, but the mechanism by which it undergoes nuclear export is poorly understood. We developed a permeabilized cell assay to characterize nuclear export of the androgen receptor in LNCaP prostate cancer cells. We found that nuclear export of endogenous androgen receptor can be stimulated by short double-stranded DNA oligonucleotides. This androgen receptor export pathway is dependent on ATP hydrolysis and is enhanced by phosphatase inhibition with okadaic acid. Fluorescence recovery after photobleaching in permeabilized cells, under the conditions that stimulate androgen receptor export, suggested that double-stranded DNA-dependent export does not simply reflect the relief of a nuclear retention mechanism. A radiolabeled androgen was used to show that the androgen receptor remains ligand-bound during translocation through the nuclear pore complex. A specific inhibitor to the DNA-dependent protein kinase, NU7026, inhibits androgen receptor export and phosphorylation. In living cells, NU7026 treatment increases androgen-dependent transcription from endogenous genes that are regulated by androgen receptor. We suggest that DNA-dependent protein kinase phosphorylation of the androgen receptor, or an interacting component, helps target the androgen receptor for export from the nucleus.

Rap2 regulates androgen sensitivity in human prostate cancer cells

BACKGROUND

Progression of prostate cancer to a fatal androgen-independent disease is associated with activation of MAP kinase, consistent with chronic stimulation of the Ras-signaling pathway. We have previously shown that Ras activation is sufficient to induce androgen-independent growth of prostate cancer cells. One mechanism of MAP kinase regulation is modulation of Ras signaling by other Ras family members, the Rap gene paralogs Rap1a/b and Rap2a/b. Here we ask if Rap proteins play a role in determining androgen sensitivity of human prostate cancer cells either alone or in the context of an activated Ras.

METHODS

To evaluate the role of Rap proteins in androgen responsiveness we use Rap over-expression with or without mutated Ras co-transfection and Rap siRNA knockdown to evaluate androgen-dependent prostate-specific antigen (PSA) promoter reporter expression and cell growth in androgen-dependent LNCaP and independent C4-2 human prostate cancer cells.

RESULTS

Rap1 is equally expressed between LNCaP and C4-2 cells and thus we focused on Rap2 which is minimally expressed in C4-2. Rap2a affects androgen-dependent PSA reporter expression in a dose-dependent manner in LNCaP and C4-2 cells. Low levels of Rap2a enhance PSA reporter expression, whereas higher concentrations inhibit expression. We show that Rap2a antagonizes the enhanced PSA reporter expression conferred by an active RasV12 gene in prostate cancer cells. siRNA knockdown data indicate that Rap2 has a greater effect on androgen-stimulated growth in LNCaP than in C4-2 cells.

CONCLUSIONS

We show that Rap2 is involved in androgen-mediated transcriptional and growth responses of human prostate cancer cells.

Subcellular Localization Modulates Activation Function 1 Domain Phosphorylation in the Androgen Receptor

Although the steady-state distribution of the androgen receptor (AR) is predominantly nuclear in androgen-treated cells, androgen-bound AR shuttles between the nucleus and the cytoplasm. In the present study we have addressed how nucleocytoplasmic shuttling contributes to the regulation of AR. Nuclear transport signal fusions were used to force AR localization to the nucleus or cytoplasm of prostate cancer cells, and the effect of localization on shuttling, transcription, androgen binding, and phosphorylation was determined. Fusing the simian virus 40 nuclear localization signal or c-Abl nuclear export signal to AR resulted in androgen-independent localization to the nucleus or cytoplasm, respectively. AR forced to the nucleus was transcriptionally active on prostate-specific antigen and mouse mammary tumor virus promoters driving reporter genes. AR forced to the cytoplasm was largely inactive on the prostate-specific antigen promoter, but, surprisingly, AR was active on the mouse mammary tumor virus promoter and on two endogenous genes examined. Thus, highly transient nuclear localization of AR is sufficient to activate transcription. Androgen dissociation rates and the dissociation constant (KD) of AR for androgen were similar whether AR was localized to the cytoplasm or the nucleus, suggesting the ligand-binding cycle of AR is not strictly linked to its compartmentalization. Using phosphosite antibodies, we found that compartmentalization influences the phosphorylation state of AR. We show there is a bias for androgen-dependent phosphorylation of Ser81, Ser256, and Ser308 in the nucleus and androgen-independent phosphorylation of Ser94 in the cytoplasm. We propose that one function of nucleocytoplasmic shuttling is to integrate the signaling environment in the cytoplasm with AR activity in the nucleus.

Integration of Rapid Signaling Events with Steroid Hormone Receptor Action in Breast and Prostate Cancer

Steroid hormone receptors (SRs) are ligand-activated transcription factors and sensors for growth factor-initiated signaling pathways in hormonally regulated tissues, such as the breast or prostate. Recent discoveries suggest that several protein kinases are rapidly activated in response to steroid hormone binding to cytoplasmic SRs. Induction of rapid signaling upon SR ligand binding ensures that receptors and coregulators are appropriately phosphorylated as part of optimal transcription complexes. Alternatively, SR-activated kinase cascades provide additional avenues for SR-regulated gene expression independent of SR nuclear action. We provide an overview of SR and signaling cross talk in breast and prostate cancers, using the human progesterone receptor (PR) and androgen receptor (AR) as models. Kinases are emerging as key mediators of SR action. Cross talk between SR and membrane-initiated signaling events suggests a mechanism for coordinate regulation of gene subsets by mitogenic stimuli in hormonally responsive normal tissues; such cross talk is suspected to contribute to cancer biology.

Receptor for activated C kinase 1 (RACK1) and Src regulate the tyrosine phosphorylation and function of the androgen receptor

The androgen receptor (AR) remains functionally important in the development and progression of prostate cancer even when the disease seems androgen "independent." Because signal transduction by growth factor receptors increases in advanced prostate cancer and is capable of sensitizing the AR to androgen, there is considerable interest in determining the mechanisms by which signaling systems can modulate AR function. We show herein that the adaptor/scaffolding protein receptor for activated C kinase 1 (RACK1), which was previously reported to interact with the AR, modulates the tyrosine phosphorylation of AR and its interaction with the Src tyrosine kinase. We also show that down-regulation of RACK1 by short interfering RNA inhibits growth and stimulates prostate-specific antigen transcription in androgen-treated prostate cancer cells. Our results suggest that RACK1 mediates the cross-talk of AR with additional binding partners, such as Src, and facilitates the tyrosine phosphorylation and transcriptional activity of the AR.

Conditional expression of PTEN alters the androgen responsiveness of prostate cancer cells

BACKGROUND

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is lost as a function of prostate tumor androgen dependence. While the transcriptional activity of the androgen receptor (AR) is inhibited by PTEN in androgen sensitive prostate cancer (CaP), the role of PTEN in androgen disease is unclear.

METHODS

We developed a system where PTEN can be conditionally re-expressed at physiologic levels into a PTEN null metastatic human CaP cell line, C4-2, and androgen responsiveness examined.

RESULTS

PTEN induction reduces cell growth and blocks the growth effect of synthetic androgen R1881. The anti-androgen Casodex enhances the growth-inhibitory action of PTEN and this effect is independent of Akt phosphorylation. Combined PTEN induction and Casodex, result in a further decrease in prostate specific antigen promoter activity compared to PTEN but not Casodex alone.

CONCLUSIONS

PTEN induction confers androgen independent CaP cells enhanced responsiveness to the anti-proliferative effects of anti-androgens and this action may involve non-AR mediated effects.

Stress kinase signaling regulates androgen receptor phosphorylation, transcription, and localization

Activation of signal transduction kinase cascades is known to alter androgen receptor (AR) activity, but the molecular mechanisms are still poorly defined. Here we show that stress kinase signaling regulates Ser 650 phosphorylation and AR nuclear export. In LNCaP prostate cancer cells, activation of either MAPK kinase (MKK) 4:c-Jun N-terminal kinase (JNK) or MKK6:p38 signaling pathways increased Ser 650 phosphorylation, whereas pharmacologic inhibition of JNK or p38 signaling led to a reduction of AR Ser 650 phosphorylation. Both p38alpha and JNK1 phosphorylated Ser 650 in vitro. Small interfering RNA-mediated knockdown of either MKK4 or MKK6 increased endogenous prostate-specific antigen (PSA) transcript levels, and this increase was blocked by either bicalutamide or AR small interfering RNA. Stress kinase inhibition of PSA transcription is, therefore, dependent on the AR. Similar experiments involving either activation or inhibition of MAPK/ERK kinase:ERK signaling had little effect on Ser 650 phosphorylation or PSA mRNA levels. Ser 650 is proximal to the DNA binding domain that contains a nuclear export signal. Mutation of Ser 650 to alanine reduced nuclear export of the AR, whereas mutation of Ser 650 to the phosphomimetic amino acid aspartate restored AR nuclear export. Pharmacologic inhibition of stress kinase signaling reduced wild-type AR nuclear export equivalent to the S650A mutant without affecting nuclear export of the S650D mutant. Our data suggest that stress kinase signaling and nuclear export regulate AR transcriptional activity.