The unfolded protein response (UPR) is a homeostatic mechanism to maintain endoplasmic reticulum (ER) function. The UPR is activated by various physiological conditions as well as in disease states, such as cancer. As androgens regulate secretion and development of the normal prostate and drive prostate cancer (PCa) growth, they may affect UPR pathways. Here, we show that the canonical UPR pathways are directly and divergently regulated by androgens in PCa cells, through the androgen receptor (AR), which is critical for PCa survival. AR bound to gene regulatory sites and activated the IRE1α branch, but simultaneously inhibited PERK signaling. Inhibition of the IRE1α arm profoundly reduced PCa cell growth in vitro as well as tumor formation in preclinical models of PCa in vivo. Consistently, AR and UPR gene expression were correlated in human PCa, and spliced XBP-1 expression was significantly upregulated in cancer compared with normal prostate. These data establish a genetic switch orchestrated by AR that divergently regulates the UPR pathways and suggest that targeting IRE1α signaling may have therapeutic utility in PCa.

STAMP1 is predicted to encode a six-transmembrane protein whose expression is highly prostate enriched and is deregulated in prostate cancer. However, the biological role of STAMP1 in prostate cancer cells, or its expression profile at the protein level, is unknown. Here, we find that ectopic expression of STAMP1 significantly increased proliferation of DU145 prostate cancer cells as well as COS-7 cells in vitro; conversely, small interfering RNA-mediated knockdown of STAMP1 expression in LNCaP cells inhibited cell growth and, at least partially, induced cell cycle arrest. In parallel, there were alterations in cell cycle-regulatory gene expression. Knockdown of STAMP1 expression in LNCaP cells also induced significant apoptosis under basal conditions as well as in response to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) alone, or TRAIL + AKT inhibitor LY294002, previously established apoptotic agents in LNCaP cells. Consistently, LNCaP cells with short hairpin RNA-mediated knockdown of STAMP1 were dramatically retarded in their ability to grow as xenografts in nude mice. Interestingly, activation of extracellular signal-regulated kinase, which has previously been implicated in prostate cancer progression, was significantly increased on ectopic expression of STAMP1 in DU145 cells and, conversely, was strongly downregulated on STAMP1 knockdown in LNCaP cells. In the normal prostate, STAMP1 protein is localized to the cytosol and the cell membrane of the prostate epithelial cells; furthermore, its expression is increased in prostate cancer compared with normal prostate. Taken together, these data suggest that STAMP1 is required for prostate cancer growth, which may be a useful target in prostate cancer treatment. Cancer Res; 70(14); 5818-28. ©2010 AACR.

The endoplasmic reticulum (ER) is an essential organelle that contributes to several key cellular functions, including lipogenesis, gluconeogenesis, calcium storage, and organelle biogenesis. The ER also serves as the major site for protein folding and trafficking, especially in specialized secretory cells. Accumulation of misfolded proteins and failure of ER adaptive capacity activates the unfolded protein response (UPR) which has been implicated in several chronic diseases, including cancer. A number of recent studies have implicated UPR in prostate cancer (PCa) and greatly expanded our understanding of this key stress signaling pathway and its regulation in PCa. Here we summarize these developments and discuss their potential therapeutic implications.

Androgen ablation during the initial stages of prostate cancer causes regression of the tumor due to an increase in apoptosis and reduced cellular proliferation. However, prostate cancer invariably progresses to an androgen-independent state for poorly understood reasons. Previous studies showed that c-Jun NH 2 terminal kinase (JNK) is required for 12-O-tetradecanoylphorbol-13-acetate (TPA)-and thapsigargin (TG)-induced apoptosis in the androgen-responsive prostate cancer cell line LNCaP. Androgens protect LNCaP cells from TPA-induced or TG-induced apoptosis via down-regulation of JNK activation. However, the molecular mechanisms of this inhibition are not clear. Here, we systematically investigated the possible regulation of mitogen-activated protein kinase phosphatases/ dual-specificity phosphatases during apoptosis of LNCaP cells and found that Vaccinia H1-related protein (VHR/DUSP3) is up-regulated by androgens during inhibition of apoptosis in LNCaP cells, but not in androgen-independent DU145 cells. Ectopic expression of wild-type VHR, but not a catalytically inactive mutant, interfered with TPA-and TG-induced apoptosis. Consistently, small interfering RNAmediated knockdown of endogenous VHR increased apoptosis in response to TPA or TG in the presence of androgens. Furthermore, COS7 cells stably expressing wild-type VHR, but not a mutant, had a decrease in JNK phosphorylation. In vivo, VHR expression decreased in the androgen-dependent human prostate cancer xenograft CWR22 upon androgen withdrawal and was inversely correlated to JNK phosphorylation. Expression analysis in human prostate cancer specimens showed that VHR is increased in prostate cancer compared with normal prostate. These data show that VHR has a direct role in the inhibition of JNK-dependent apoptosis in LNCaP cells and may therefore have a role in prostate cancer progression. [Cancer Res 2008;68(22):9255-64]