Previously it has been reported that caveolin-1 (cav-1) has antiapoptotic activities in prostate cancer cells and functions downstream of androgenic stimulation. In this study, we demonstrate that cav-1 overexpression significantly reduced thapsigargin (Tg)-stimulated apoptosis. Examination of the phosphatidylinositol 3-kinase (PI3-K)/Akt signaling cascade revealed higher activities of PDK1 and Akt but not PI3-K in cav-1-stimulated cells compared to control cells. We subsequently found that cav-1 interacts with and inhibits serine/threonine protein phosphatases PP1 and PP2A through scaffolding domain binding site interactions. Deletion of the cav-1 scaffolding domain significantly reduces phosphorylated Akt and cell viability compared with wild-type cav-1. Analysis of potential substrates for PP1 and PP2A revealed that cav-1-mediated inhibition of PP1 and PP2A leads to increased PDK1, Akt, and ERK1/2 activities. We demonstrate that increased Akt activities are largely responsible for cav-1-mediated cell survival using dominant-negative Akt mutants and specific inhibitors to MEK1/MEK and show that cav-1 increases the half-life of phosphorylated PDK1 and Akt after inhibition of PI3-K by LY294002. We further demonstrate that cav-1-stimulated Akt activities lead to increased phosphorylation of multiple Akt substrates, including GSK3, FKHR, and MDM2. In addition, overexpression of cav-1 significantly increases translocation of phosphorylated androgen receptor to nucleus. Our studies therefore reveal a novel mechanism of Akt activation in prostate cancer and potentially other malignancies.Prostate cancer remains the second leading cause of cancer mortality among American males. The predominant reason for such high and persistent mortality is the lack of curative therapies for androgen-resistant metastatic disease. It is critical to elucidate the molecular mechanisms that underlie the ultimate androgen-resistant state of prostate cancer and to develop effective therapies for this condition.Previously, Yang et al. reported that caveolin-1 (cav-1) levels were elevated in metastatic mouse and human prostate cancer (85). cav-1 is a major component of caveolae, flask-shaped membrane invaginations which are involved in multiple cellular processes, including the regulation and transportation of cellular cholesterol and lipids, clathrin-independent endocytosis, and signal transduction (24,27,60,62,66). The participation of cav-1 in these critical pathways involves the interaction of cav-1 with a relatively large number of molecules in either a scaffolding binding-dependent or -independent manner (41, 63). The wide spectrum of molecular interactions involving cav-1 is consistent with important, context-dependent roles for cav-1 in signal transduction, molecular transport, and other regulatory activities.The biological functions of cav-1 in cancer are complex, multifaceted, and somewhat controversial (42,55,72,73). Numerous experimental results indicate that cav-1 is a growth suppressor (14,17,35). Some investigators have assert...

Cancers with dysfunctional mutations in BRCA1 or BRCA2, most commonly associated with some breast cancers, are deficient in the DNA damage repair pathway called homologous recombination (HR), which makes them exquisitely vulnerable to poly(ADP-ribose) polymerase (PARP) inhibitors, such as olaparib. This functional state and therapeutic sensitivity is referred to as “BRCAness”. Pharmaceutical induction of BRCAness could expand the use of PARP inhibitors to other tumor types. For example, BRCA mutations are present in only a small proportion of prostate cancer (PCa) patients. We found that castration-resistant PCa (CRPC) cells increased expression of a set of HR-associated genes, including BRCA1, RAD54L and RMI2. Androgen-targeted therapy is typically not effective in CRPC patients. However, the androgen receptor (AR) inhibitor enzalutamide suppressed the expression of those HR genes, thus creating HR deficiency and BRCAness in CRPC cells. In a manner dependent on these gene expression effects, a “lead-in” treatment strategy, in which enzalutamide was followed by the combination of enzalutamide and olaparib, promoted DNA damage-induced cell death and inhibited clonal proliferation of PCa cells in culture and suppressed the growth of PCa xenografts in mice. Thus, our study suggests that anti-androgen and PARP inhibitor combination therapy may be effective for patients with CRPC, and that pharmaceutically-induced BRCAness may expand the clinical use of PARP inhibitors.

Although prostate cancer cells are often initially sensitive to androgen ablation, they eventually lose this response and continue to survive, grow and spread in the absence of androgenic steroids. The mechanism(s) that underlie resistance to androgen ablation therapy remain mostly unknown. We have demonstrated that elevated caveolin protein levels are associated with human prostate cancer progression in pathological specimens. Here we show that suppression of caveolin expression by a stably transfected antisense caveolin-1 cDNA vector converted androgen-insensitive metastatic mouse prostate cancer cells to an androgen-sensitive phenotype. Orthotopically grown tumors and low-density cell cultures derived from antisense caveolin clones had increased apoptosis in the absence of androgenic steroids, whereas similarly grown tumors and cells from vector (control) clones and parental cells were not sensitive to androgens. Studies using a representative antisense caveolin clone showed that selection for androgen resistance in vivo correlated with increased caveolin levels, and that adenovirus-mediated caveolin expression blocked androgen sensitivity. Our results identify a new candidate gene for hormone-resistant prostate cancer in man and indicate that androgen insensitivity can be an inherent property of metastatic prostate cancer.

We identified a novel mouse gene, mRTVP-1, as a p53 target gene using differential display PCR and extensive promoter analysis. The mRTVP-1 protein has 255 amino acids and differs from the human RTVP-1 (hRTVP-1) protein by two short in-frame deletions of two and nine amino acids. RTVP-1 mRNA was induced in multiple cancer cell lines by adenovirus-mediated delivery of p53 and by gamma irradiation or doxorubicin both in the presence and in the absence of endogenous p53. Analysis of RTVP-1 expression in nontransformed and transformed cells further supported p53-independent gene regulation. Using luciferase reporter and electrophoretic mobility shift assays we identified a p53 binding site within intron 1 of the mRTVP-1 gene. Overexpression of mRTVP-1 or hRTVP-1 induced apoptosis in multiple cancer cell lines including prostate cancer cell lines 148-1PA, 178-2BMA, PC-3, TSU-Pr1, and LNCaP, a human lung cancer cell line, H1299, and two isogenic human colon cancer cell lines, HCT116 p53 ؉/؉ and HCT116 p53 ؊/؊ , as demonstrated by annexin V positivity, phase-contrast microscopy, and in selected cases 4,6-diamidino-2-phenylindole staining and DNA fragmentation. Deletion of the signal peptide from the N terminus of RTVP-1 reduced its apoptotic activities, suggesting that a secreted and soluble form of RTVP-1 may mediate, in part, its proapoptotic activities.The p53 tumor suppressor protein is induced by specific stress-related cell stimuli and in turn mediates important regulatory pathways involving cell cycle progression, apoptosis, senescence, DNA repair, cell differentiation, and angiogenesis (reviewed in references 5, 9, 11, 13, 16, and 27). Mutations in the p53 gene can lead to critical cellular malfunctions that contribute to malignant growth. Many human cancers, including colon, lung, breast, and prostate cancers have been shown to harbor p53 mutations (reviewed in reference 42). Numerous studies also support a specific metastasis suppressor role for p53 in prostate cancer (reviewed in reference 39). Initial experimental results using an in vivo mouse model of prostate cancer metastasis demonstrated that loss of p53 function can lead to the development of metastases that seed from relatively small numbers of cells within the primary tumor (38). Subsequent studies demonstrated that although p53 mutations in human primary prostate cancer tissues are heterogeneous and relatively infrequent, they occur at significant levels in metastatic disease, with mutation frequencies ranging from 21 to 30% in lymph node metastasis to Ͼ90% in androgen-insensitive disseminated disease (7,19,23,28,35,43). This pattern of mutations suggests that only a few cells harboring p53 mutations in the primary tumor can seed metastases that clonally expand at distant sites. Indeed, studies have demonstrated that specific p53 mutations in primary tumors are clonally expanded in metastatic prostate cancer (35) and that a pattern of aberrant p53 expression in primary tumors termed clustered p53 staining has significant prognostic value in predict...

Androgen deprivation is the standard systemic treatment for advanced prostate cancer (PCa), but most patients ultimately develop castration-resistance. We show here that MYB is transcriptionally activated by androgen deprivation or impairment of androgen receptor (AR) signaling. MYB gene silencing significantly inhibited PCa growth in vitro and in vivo. Microarray data revealed that c-Myb shares a substantial subset of DNA damage response (DDR) target genes with AR, suggesting that c-Myb may replace AR for the dominant role in the regulation of their common DDR target genes in AR inhibition-resistant or AR-negative PCa. Gene signatures comprising AR, MYB, and their common DDR target genes are significantly correlated with metastasis, castration-resistance, recurrence, and shorter overall survival in PCa patients. We demonstrated in vitro that silencing of MYB, BRCA1 or TOPBP1 synergized with poly (ADP-ribose) polymerase (PARP) inhibitor olaparib (OLA) to increase cytotoxicity to PCa cells. We further demonstrated that targeting the c-Myb-TopBP1-ATR-Chk1 pathway by using the Chk1 inhibitor AZD7762 synergizes with OLA to increase PCa cytotoxicity. Our results reveal new mechanism-based therapeutic approaches for PCa by targeting PARP and the c-Myb-TopBP1-ATR-Chk1 pathway.

Recent data indicating that overexpression of caveolin-1 as well as c-myc are relatively common features of advanced prostate cancer prompted us to test for potential cooperative interactions between caveolin-1 and c-myc that would be consistent with malignant progression. We used the well-characterized Rat1Amy-cER TM cells to show that the caveolin-1 gene is downregulated at the level of transcription by c-myc. By maintaining relatively high levels of caveolin-1 with an adenoviral vector or in stably transfected clones we show that caveolin-1 can suppress c-myc-induced apoptosis. Further we established human prostate cancer cell lines with the mycER TM construct and show that clones with increased caveolin-1 are more resistant to myc-induced apoptosis and have increased capacity for growth in soft agar when c-myc is activated. Oncogene (2000) 19, 3256 ± 3265.

Glioma pathogenesis-related protein 1 (GLIPR1), a novel p53 target gene, is down-regulated by methylation in prostate cancer and has p53-dependent and -independent proapoptotic activities in tumor cells. These properties suggest an important tumor suppressor role for GLIPR1, yet direct genetic evidence of a tumor suppressor function for GLIPR1 is lacking and the molecular mechanism(s), through which GLIPR1 exerts its tumor suppressor functions, has not been shown. Here, we report that the expression of GLIPR1 is significantly reduced in human prostate tumor tissues compared with adjacent normal prostate tissues and in multiple human cancer cell lines. Overexpression of GLIPR1 in cancer cells leads to suppression of colony growth and induction of apoptosis. Mice with an inactivated Glipr1 gene had significantly shorter tumor-free survival times than either Glipr1 +/+ or Glipr1 +/À mice in both p53 +/+ and p53 +/À genetic backgrounds, owing to their development of a unique array of malignant tumors. Mechanistic analysis indicated that GLIPR1 up-regulation increases the production of reactive oxygen species (ROS) leading to apoptosis through activation of the c-Jun-NH 2 kinase (JNK) signaling cascade. Thus, our results identify GLIPR1 as a proapoptotic tumor suppressor acting through the ROS-JNK pathway and support the therapeutic potential for this protein. [Cancer Res 2008;68(2):434-43]

We previously identified and characterized a novel p53-regulated gene in mouse prostate cancer cells that was homologous to a human gene that had been identified in brain cancers and termed RTVP-1 or GLIPR. In this report, we document that the human RTVP-1 gene is also regulated by p53 and induces apoptosis in human prostate cancer cell lines. We show that the expression of the human RTVP-1 gene is down-regulated in human prostate cancer specimens compared with normal human prostate tissue at the mRNA and protein levels. We further document epigenetic changes consistent with RTVP-1 being a tumor suppressor in human prostate cancer.