Studies on prolactin regulation of mitochondrial aspartate aminotransferase gene expression and protein kinase C activity and expression in prostate cells

Abstract

Citrate accumulation and secretion are physiological functions of the normal prostate gland in most species. Prolactin (PRL) stimulates citrate accumulation in the rat lateral prostate (LP) and pig prostate by increasing the expression of mitochondrial aspartate aminotransferase (mAAT), the enzyme that catalyzes aspartate transamination. In the present studies, we established the role of PRL and the phorbol ester 12-O-tetra-decanoylphorbol 13-acetate (TPA) in the regulation of mAAT in LNCaP and PC-3 cells. We isolated total RNA from the cells and hybridize it with a mAAT cDNA. Next, we assayed protein kinase C (PKC) activity and expression in PRL and TPA treated prostate cells. We, then determined the effect of PRL and TPA on gene transcription using a mAAT-chloramphenicol acetyltransferase (CAT) reporter gene construct, transiently transfected into PC-3 cells. A fragment of the 5{dollar}\sp\prime{dollar} gene region containing putative TPA response elements (TRE) was cloned into the pCAT-Promoter vector from Promega. The reporter vector was introduced into the cells by the calcium phosphate-DNA co-precipitation method. The cells were then incubated for 48 h with or without PRL or TPA.;The results indicated that PRL and TPA increased the mAAT mRNA level 2-4 fold. Upon PRL and TPA treatment the level of PKC activity in rat LP, and LNCaP and PC-3 cells increased 20-60% and 40-207%, respectively. In addition, long term PRL and TPA treatment resulted in increased PKC expression (10-60% and 10-240%, respectively) in the LP cytosol fraction. Furthermore, the results showed that PRL at a concentration of {dollar}1\times10\sp{lcub}-9{rcub}{dollar} M and TPA (0.1 {dollar}\mu{dollar}g/ml) treatment increased CAT activity in PC-3 cells. These results suggest that PRL regulates mAAT at the transcriptional level. Moreover, these effects of PRL on mAAT mRNA and PKC activity were eliminated by PKC down-regulation. Since both PRL and TPA induced PKC activity and since the effects of PRL and TPA were eliminated by PKC down-regulation, we postulate that the PRL effect on mAAT is mediated via the diacylglycerol (DAG)-PKC signal transduction pathway in rat LP and prostate cancer (CA) cells.