Regulation of MAPK phosphatase expression in the prostate cancer cell line LNCaP : Possible role in apoptosis

Abstract

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, some prostate cancer cells survive in the androgen deprived environment and prostate cancer invariably progresses to an androgen-independent state for poorly understood reasons. Two different agents, 12-O-tetradecanoylphorbol-13-acetate (TPA) and thapsigargin (TG) activate c-Jun N-terminal kinase (JNK) and induce apoptosis in the androgen-responsive prostate cancer cell line LNCaP. Previous results from our laboratory show that androgen treatment of LNCaP cells protects them from TPA- and TG-induced apoptosis due to down-regulation of JNK activation. Ultraviolet light (UV)-induced JNK activity was also inhibited by androgens in LNCaP cells. Gene expression was required for this inhibition and ATP depletion experiments indicated an increase in phosphatase activity. This suggested a possible role for the MAPK phosphatases (MKPs) in inactivating JNK during inhibition of apoptosis in LNCaP cells; this is consistent with other data from our laboratory that showed an up-regulation of MKP-1 in cells treated with apoptosis inducing agent TPA and R1881, synthetic androgen. In this study, we have investigated the possible regulation of MKPs in UV, TPA, or TG treated cells in the presence or absence of R1881 and the data show that some MKPs are up-regulated in the presence of R1881 and TPA or TG. All tested MKPs were significantly down-regulated at the mRNA level in cells exposed to UV irradiation; R1881 did not appreciably affect MKP expression that is inhibited by UV. Analysis of TPA- and TG-induced apoptosis in LNCaP cells ectopically expressing VHR wild-type or a catalytically inactive mutant indicated that VHR interferes with apoptosis. In summary, these data indicate a role for the MKPs in the down-regulation of JNK activation in LNCaP cells and may be at least part of the mechanism as to how androgens inhibit JNK activation.