PROTEIN-KINASE-C ISOZYME EXPRESSION AND DOWN-MODULATION IN GROWING, QUIESCENT, AND TRANSFORMED RENAL PROXIMAL TUBULE EPITHELIAL-CELLS

Renal alpha-protein kinase C (PKC) is rapidly down-modulated in animal s treated with the renal toxin and tumor promoter, folic acid (Dong et al., Cancer Res., 53: 4542-4549, 1993). To further explore the role o f PKC isozymes in renal growth and carcinogenesis, we compared phorbol ester receptor and PKC isozyme content, distribution, and regulation in primary and oncogene-altered rat renal proximal tubule epithelial c ells (RPTE) in culture. Immunoblot analysis and RNase protection assay s indicated that RPTE expressed at least four PKC isozymes, alpha, del ta, epsilon, and zeta. Total phorbol ester receptors were decreased in primary proliferating, E1A-immortalized, and SV40-transformed RPTE co mpared to primary quiescent RPTE. The decrease in PDBu binding was lar gely due to a specific decrease in alpha-PKC protein content to approx imately 50% of the level in quiescent RPTE. Degradation rates and mess age levels were compared to determine the mechanism for the decrease i n alpha-PKC. Whereas alpha-PKC message levels in quiescent and prolife rating primary RPTE were comparable, alpha-PKC degradation was increas ed in proliferating cells. These results indicate that the decreased a lpha-PKC content was due largely to increased turnover. Phorbol ester stimulated the rate of degradation, thus demonstrating a link between degradation rate and PKC activation. These results suggest that the in creased basal degradation rate in proliferating and oncogene-altered c ells reflects an increase in activity of PKC in these cells.