Characterization of protein kinase C in rat and human prostates

Inhibition of human prostate smooth muscle contraction by the inhibitors of protein kinase C, GF109203X, and Go6983

INTRODUCTION

Prostate smooth muscle contraction is promoted by receptor-induced activation of intracellular signaling pathways. The presumed involvement in etiology and medical treatment of lower urinary tract symptoms (LUTS) suggestive of benign prostatic hyperplasia (BPH) imparts a high clinical relevance to prostate smooth muscle contraction, which is contrasted by incomplete understanding at the molecular level. Involvement of protein kinase C (PKC) has been commonly assumed, but available studies were limited to nonhuman prostate smooth muscle or cell cultures. Here, we examined the effects of the PKC inhibitors Go6983 and GF109203x on contractions of human prostate tissues.

METHODS

Prostate tissues were obtained from radical prostatectomy. Contractions were induced by electric field stimulation (EFS), α₁ -adrenergic agonists (noradrenaline, phenylephrine, methoxamine), thromboxane A₂ analog U46619, endothelin-1, or calcium chloride in an organ bath.

RESULTS

GF109203X (500 nM) and Go6983 (300  nM) reduced EFS-, noradrenaline-, phenylephrine-, methoxamine-, and U46619-induced contractions of human prostate tissues, with maximum inhibitions approaching up to 55%. Using concentrations of 3 µM, GF109203X and Go6983 inhibited EFS- and noradrenaline-induced contractions, with similar effect sizes as 500 and 300 nM, respectively. Endothelin-1-induced contractions were not inhibited by GF109203X, and to neglectable extent by Go6983. After depolarization in calcium-free solution, calcium chloride-induced concentration-dependent contractions, which were inhibited by GF109203X and Go6983.

CONCLUSIONS

GF109203X and Go6983 inhibit neurogenic, α₁ -adrenergic, and thromboxane A₂ -induced smooth muscle contractions in the human prostate, suggesting a role of PKC for human prostate smooth muscle contraction. The inhibition may by be imparted by inhibition of calcium sensitivity.

Regulation of the expression of protein kinase C isoenzymes in rat ventral prostate: effects of age, castration and flutamide treatment

Protein kinase C (PKC) isoenzymes are involved in cell function, growth, apoptosis and neoplastic transformation in the prostate gland. We detected by means of Western blot the expression of the classical alpha and beta1, the novel epsilon and the atypical zeta isoforms of PKC in ventral prostates from rats with different extents of plasma testosterone levels and/or androgen imprinting on the gland. The expression of the four isoforms decreased in 5-day castrated rats showing apoptotical regression of the gland and a drastic reduction of circulating testosterone. However, the expression of PKC isoenzymes (alpha, beta1, epsilon ) increased in prostates from pubertal (35-days old) rats that are characterized by relatively low but extremely bioactive testosterone plasma levels. Treatment of adult rats for 14 days with flutamide (daily s.c. injection of 15 mg/Kg B.W.) resulted in increased expression of the four isoenzymes; it occurred in the presence of increased (normal rats) or drastically reduced (rats castrated after 9 days of flutamide administration) levels of plasma testosterone conceivably through a direct effect of this nonsteroidal antiandrogen on prostate cells. Measurements of PKC(alpha) activity were in agreement with the observations on protein expression and showed that flutamide (that is extensively used in the treatment of advanced prostate cancer) elicits some impairment in the mechanisms of translocation of this isoform from the cytosol to the membrane. Thus, in addition to the possibility of direct effects of flutamide upon the rat prostate, we present evidence that the levels of circulating androgens and/or their bioactivity in the gland regulate the expression of various important PKC isoforms.

Inactivation of melatonin receptors by protein kinase C in human prostate epithelial cells

The pineal hormone melatonin regulates seasonal reproduction and pubertal development in mammals. We recently found melatonin receptors in the human benign prostate tissue, primarily associated with the microsome-enriched fraction of the epithelial cells. In cultured benign prostate epithelial cells, melatonin, at physiological concentrations, suppressed [3H]thymidine incorporation and cGMP levels. The effects of melatonin were transient, suggesting inactivation of the receptors. In the present study, the possibility of inactivation of the prostate melatonin receptors by protein kinase C (PKC) was explored. Treatment of the microsome-enriched fraction with crude rat brain PKC in the presence of phorbol 12-myristate 13-acetate (TPA) or CaCl2 abolished the specific [125I]melatonin binding. This effect was prevented by the PKC inhibitor bisindolylmaleimide (GF-109203). [125I]Melatonin binding could be reinstated by iodoacetamide treatment. In benign prostate epithelial cells in culture, TPA pretreatment markedly reduced the apparent affinity of [125I]melatonin binding. In addition, TPA ablated the cells responses to melatonin, namely the suppression of [3H]thymidine incorporation and cGMP levels. Pretreatment with GF-109203 prevented the TPA effects on [125I]melatonin binding and responses. In addition, GF-109203 slowed down the inactivation of the melatonin-mediated inhibition of [3H]thymidine incorporation. Taken together, these data show that melatonin receptors are desensitized by PKC and imply that the transient response to melatonin may be the outcome of a direct or indirect melatonin-mediated activation of endogenous PKC.