Stephen J. Libertini

Evidence for Calpain-Mediated Androgen Receptor Cleavage as a Mechanism for Androgen Independence

Prostate carcinoma is the most commonly diagnosed cancer in men and the second leading cause of death due to cancer in Western civilization. Androgen ablation therapy is effective in treating androgen-dependent tumors, but eventually, androgen-independent tumors recur and are refractory to conventional chemotherapeutics. Hence, the emergence of androgen independence is the most challenging problem in managing prostate tumors. We report a novel mechanism of androgen independence: calpain cleaves the androgen receptor (AR) into an androgen-independent isoform. In vitro and in vivo analyses show that calpain removes the COOH-terminal ligand binding domain generating a constitutively active molecule. Analysis of human prostate tumors indicates that several tumors express higher levels of this truncated AR than noncancerous prostate tissue. In transient transfection studies, the truncated AR is three to five times more potent than the full-length receptor in transactivating transcription. The androgen-independent Rv1 cells express high levels of the truncated AR, and treatment of these cells with a calpain inhibitor reduces truncated AR expression. In the absence of androgen, inhibition of calpain activity induces apoptosis. The HIV protease inhibitor amprenavir inhibits calpain activity and is also effective in inducing apoptosis in the Rv1 cell line. The cell culture studies were reproduced in a mouse xenograft model, where, in the absence of androgens, amprenavir significantly reduces tumor growth. Together, these studies indicate that calpain-dependent proteolysis of the AR may be a mechanism of androgen independence. The calpain inhibition studies suggest that inhibiting this activity may be a potential treatment for some androgen-independent prostate tumors.

Cyclin E Both Regulates and Is Regulated by Calpain 2, a Protease Associated with Metastatic Breast Cancer Phenotype

Overexpression of cyclin E in breast tumors is associated with a poor response to tamoxifen therapy, greater genomic instability, more aggressive behavior, and a poor clinical prognosis. These tumors also express low molecular weight isoforms of cyclin E that are associated with higher kinase activity and increased metastatic potential. In the current study, we show that cyclin E overexpression in MCF7 cells transactivates the expression of calpain 2, leading to proteolysis of cyclin E as well as several known calpain substrates including focal adhesion kinase (FAK), calpastatin, pp60src, and p53. In vivo inhibition of calpain activity in MCF7-cyclin E cells impedes cyclin E proteolysis, whereas in vivo induction of calpain activity promotes cyclin E proteolysis. An analysis of human breast tumors shows that high levels of cyclin E are coincident with the expression of the low molecular weight isoforms, high levels of calpain 2 protein, and proteolysis of FAK. Lastly, studies using a mouse model of metastasis reveal that highly metastatic tumors express proteolyzed cyclin E and FAK when compared to tumors with a low metastatic potential. Our results suggest that cyclin E-dependent deregulation of calpain may be pivotal in modifying multiple cellular processes that are instrumental in the etiology and progression of breast cancer.

ERK Regulates Calpain 2-induced Androgen Receptor Proteolysis in CWR22 Relapsed Prostate Tumor Cell Lines

Androgen ablation therapy is effective in treating androgen-dependent prostate tumors; however, tumors that can proliferate in castrate levels of androgen eventually arise. We previously reported that in CWR22Rv1 (Rv1) cells, the protease calpain 2 can cleave the androgen receptor (AR) into a constitutively active ∼80,000 low molecular weight (LMW) form. In this study, we further dissect the mechanisms that produce the AR LMW forms using Rv1 cells and the related CWR22-R1 (R1) cells. The 39-amino acid insertional mutation in the Rv1-AR (E3DM-AR) sensitizes this AR to calpain 2 proteolysis. R1 cells encode the same AR molecule as the parental CWR22 xenograft. Using calpain 2 small interfering RNA and calpeptin, we find that calpain 2 plays a role in the generation of the LMW-AR in R1 cells. Furthermore, LMW-AR expression is regulated by the activation of calpain 2 by ERK 1 and 2. Inhibition of ERK phosphorylation or small interfering RNA-mediated decrease of ERK expression reduces LMW-AR levels in R1 cells. Conversely, activation of the MAPK pathway results in increased ERK phosphorylation and increased levels of LMW-AR. Finally, analyses of human tumor samples found that LMW-AR levels are higher in tumors that have an increased calpain/calpastatin ratio and/or increased levels of phospho-ERK (pERK). This suggests that a higher calpain/calpastatin ratio collaborates with activated ERK to promote the generation of the LMW-AR.

Genome-wide analysis of androgen receptor binding and gene regulation in two CWR22-derived prostate cancer cell lines

Prostate carcinoma (CaP) is a heterogeneous multifocal disease where gene expression and regulation are altered not only with disease progression but also between metastatic lesions. The androgen receptor (AR) regulates the growth of metastatic CaPs; however, sensitivity to androgen ablation is short lived, yielding to emergence of castrate-resistant CaP (CRCaP). CRCaP prostate cancers continue to express the AR, a pivotal prostate regulator, but it is not known whether the AR targets similar or different genes in different castrate-resistant cells. In this study, we investigated AR binding and AR-dependent transcription in two related castrate-resistant cell lines derived from androgen-dependent CWR22-relapsed tumors: CWR22Rv1 (Rv1) and CWR-R1 (R1). Expression microarray analysis revealed that R1 and Rv1 cells had significantly different gene expression profiles individually and in response to androgen. In contrast, AR chromatin immunoprecipitation (ChIP) combined with promoter DNA microarrays (ChIP-on-chip) studies showed that they have a similar AR-binding profile. Coupling of the microarray study with ChIP-on-chip analysis identified direct AR targets. The most prominent function of transcripts that were direct AR targets was transcriptional regulation, although only one transcriptional regulator, CCAAT/enhancer binding protein δ, was commonly regulated in both lines. Our results indicate that the AR regulates the expression of different transcripts in the two lines, and demonstrate the versatility of the AR-regulated gene expression program in prostate tumors.

miR-148a dependent apoptosis of bladder cancer cells is mediated in part by the epigenetic modifier DNMT1

Urothelial cell carcinoma of the bladder (UCCB) is the most common form of bladder cancer and it is estimated that ∼15,000 people in the United States succumbed to this disease in 2013. Bladder cancer treatment options are limited and research to understand the molecular mechanisms of this disease is needed to design novel therapeutic strategies. Recent studies have shown that microRNAs play pivotal roles in the progression of cancer. miR‐148a has been shown to serve as a tumor suppressor in cancers of the prostate, colon, and liver, but its role in bladder cancer has never been elucidated. Here we show that miR‐148a is down‐regulated in UCCB cell lines. We demonstrate that overexpression of miR‐148a leads to reduced cell viability through an increase in apoptosis rather than an inhibition of proliferation. We additionally show that miR‐148a exerts this effect partially by attenuating expression of DNA methyltransferase 1 (DNMT1). Finally, our studies demonstrate that treating cells with both miR‐148a and either cisplatin or doxorubicin is either additive or synergistic in causing apoptosis. These data taken together suggest that miR‐148a is a tumor suppressor in UCCB and could potentially serve as a novel therapeutic for this malignancy.

The interleukin 6 receptor is a direct transcriptional target of E2F3 in prostate tumor derived cells

The E2F/RB pathway is frequently disrupted in multiple human cancers. E2F3 levels are elevated in prostate tumors and E2F3 overexpression independently predicts clinical outcome. The goals of this study were to identify direct transcriptional targets of E2F3 in prostate tumor derived cells.

Dicer ablation promotes a mesenchymal and invasive phenotype in bladder cancer cells

Dicer expression is frequently altered in cancer and affects a wide array of cellular functions acting as an oncogene or tumor suppressor in varying contexts. It has been shown that Dicer expression is also deregulated in urothelial cell carcinoma of the bladder (UCCB) but the nature of this deregulation differs between reports. The aim of the present study was to gain a better understanding of the role of Dicer in bladder cancer to help determine its contribution to the disease. The results showed that Dicer transcript levels were decreased in UCCB tumor tissues as compared to normal tissues, suggesting that Dicer is a tumor suppressor. However, consistent with previous results, we demonstrated that knockdown of Dicer decreases cell viability and increases the induction of apoptosis, suggesting that Dicer is an oncogene. To resolve this discrepancy, we assessed the effects of decreased Dicer expression on epithelial-to‑mesenchymal transition, migration and invasion. We showed that decreased Dicer levels promoted a mesenchymal phenotype and increased migration. Additionally, the results showed that Dicer protein ablation leads to increased cell invasion, higher levels of matrix metalloproteinase-2, and decreased levels of key miRNAs shown to inhibit invasion. The results of this study suggest that decreased Dicer levels may portend a more malignant phenotype.

Abstract 3574: Studies of castrate resistant 22Rv1 cells identifies AR regulated interrelated networks of transcription factors, co-regulators, chromatin, and nuclear scaffolding proteins.

Prostate cancer development, progression and acquisition of castrate resistance are reliant on the activity of the androgen receptor (AR), a transcription factor that governs the proliferation of prostate cancer cells. Multiple studies have focused on the principle components that are critical for cell cycle transversal and these studies have identified members of the E2F/RB family, c-Myc, cyclins and cyclin dependent kinase inhibitors as key proteins that are instrumental in prostate cell proliferation and development of castrate resistance. But the mechanism by which the AR dominates this process is unclear. To identify the link between the AR and cell cycle control we used 22Rv1 cells which are castrate resistant due to the expression of low molecular weight AR forms that are missing the ligand binding domain. However, a siRNA mediated decrease in AR expression induces a growth arrest confirming that proliferation is AR dependent. RNA-seq analysis of gene expression at castrate levels of androgen and following AR ablation identified transcripts that were AR dependent. This methodology reduced the number of AR regulated transcripts, by excluding transcripts regulated by super-physiological levels of androgens. The AR regulated transcripts included previously identified genes (PMEPA1, C1orf116, PCDH7, APP) including NFAT a gene shown to be regulated by a low molecular weight AR isoform. The analysis also revealed that interrelated networks of transcription factors and co-factors were AR transactivated or repressed. These included proteins such as the LBR, LMNB, TMPO and SATB1 that do not directly promote transcription, but rather link chromatin to the nuclear membrane or scaffolding, suggesting that the AR may modulate gene expression through epigenetics mechanisms. Network analysis showed connections between these proteins and cell cycle components including E2Fs and Myc where a number of the regulated transcripts are known E2F or Myc targets. In addition, this analysis identified a number of non-coding RNAs; some that have been previously described and some that have yet to be annotated. qPCR studies found that a number of the transcripts regulated in 22Rv1 cells are also AR regulated in LNCaP cells. Coupled with ChIP-seq data, the study characterizes the link between the AR and the cell cycle machinery and chromatin modulating proteins. Citation Format: Maria Mudryj, Stephen J. Libertini, Alan P. Lombard. Studies of castrate resistant 22Rv1 cells identifies AR regulated interrelated networks of transcription factors, co-regulators, chromatin, and nuclear scaffolding proteins. [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 3574. doi:10.1158/1538-7445.AM2013-3574

XPO1 inhibition by selinexor induces potent cytotoxicity against high grade bladder malignancies

Treatment options for high grade urothelial cancers are limited and have remained largely unchanged for several decades. Selinexor (KPT-330), a first in class small molecule that inhibits the nuclear export protein XPO1, has shown efficacy as a single agent treatment for numerous different malignancies, but its efficacy in limiting bladder malignancies has not been tested. In this study we assessed selinexor-dependent cytotoxicity in several bladder tumor cells and report that selinexor effectively reduced XPO1 expression and limited cell viability in a dose dependent manner. The decrease in cell viability was due to an induction of apoptosis and cell cycle arrest. These results were recapitulated in in vivo studies where selinexor decreased tumor growth. Tumors treated with selinexor expressed lower levels of XPO1, cyclin A, cyclin B, and CDK2 and increased levels of RB and CDK inhibitor p27, a result that is consistent with growth arrest. Cells expressing wildtype RB, a potent tumor suppressor that promotes growth arrest and apoptosis, were most susceptible to selinexor. Cell fractionation and immunofluorescence studies showed that selinexor treatment increased nuclear RB levels and mechanistic studies revealed that RB ablation curtailed the response to the drug. Conversely, limiting CDK4/6 dependent RB phosphorylation by palbociclib was additive with selinexor in reducing bladder tumor cell viability, confirming that RB activity has a role in the response to XPO1 inhibition. These results provide a rationale for XPO1 inhibition as a novel strategy for the treatment of bladder malignancies.

Abstract 5051: Androgen receptor-mediated regulation of p14ARF transcription in prostate tumor cells

The p14ARF tumor suppressor is often deleted or silenced in malignancies. Prostate tumors are an exception, where p14 expression is elevated. To understand this phenomenon, we assessed the expression of p53 pathway members, which are most effected by p14ARF. The expression of androgen receptor (AR), a pivotal prostate cancer regulator, which is also affected by p14arf and MDM2 was analyzed as well. The studies used archival prostate tumor tissues obtained from prostatectomies performed at the Veterans Affairs-Northern California Health Care System in Mather California between 1996 and 2002 to better define the relationship between these interrelated networks. A prostate tumor tissue array consisting of 78 tumors of differing stages and grades was constructed to evaluate correlations between multiple parameters. Immunohistochemical studies assessed expression of the proliferation marker Ki67, p53, MDM2, MDM4, p14ARF, and the AR in the nuclear and cytoplasmic compartments of tumor and adjacent cells. p53, MDM4, p14ARF and AR were detected in nuclear and cytoplasmic compartments of tumor and non-tumor cells, but were predominantly nuclear. MDM2 expression was primarily cytoplasmic in tumor cells. Multivariate analysis of the immunohistochemical markers identified a strong correlation between expression of p14ARF and AR. Studies utilizing the prostate CWR22 xenograft and LNCaP cell line models revealed that castration or androgen deprivation resulted in reduced p14arf levels and that this effect correlated with a precipitous decline in E2F1-3a levels. In a reciprocal analysis, RB ablation enhanced p14ARF transcription, arguing that the E2F/RB pathway mediates AR-dependent p14ARF expression. However, we also identified an AR binding site located ∼40 KB upstream of the p14ARF gene. Chromatin immunoprecipitation (ChIP) studies showed that in prostate cells this site was bound by AR. ChIP studies also revealed E2F1 and E2F3 were present at the p14ARF promoter. Together, the studies argue p14ARF is a direct transcriptional target of AR and that AR and E2F collaborate to promote p14ARF expression. . Citation Format: Maria Mudryj, Salma Siddiqui, Stephen J. Libertini, Alan P. Lombard, Benjamin Mooso, Leandro D9Abronzo, Frank Melgoza, Alexander Borowsky, Christiana Drake, LiHong Qi, Paramita M. Ghosh. Androgen receptor-mediated regulation of p14ARF transcription in prostate tumor cells. [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 5051. doi:10.1158/1538-7445.AM2015-5051