Protein kinase C signal transduction in liver cell proliferation following partial hepatectomy

Abstract

Following partial hepatectomy, a complex series of biochemical and molecular events culminate in hepatocyte proliferation. These events proceed through a number of signal transduction mechanisms, including protein kinase C. The hypothesis of this research project was that the mitogenic signals induced by partial hepatectomy are transduced through protein kinase C activation and that this mechanism is an integral component of the signal transduction pathways culminating in hepatocyte proliferation. After preliminary toxicologic evaluation, we employed nontoxic doses of suramin, a relatively specific protein kinase C inhibitor, to abrogate the proliferative response following partial hepatectomy. Administered as a single intraperitoneal injection fourteen days prior to operation, suramin caused a significant dose-dependent decrease in liver cell proliferation as quantitated by thymidine kinase activity, radiolabelled thymidine incorporation and mitotic indices. Furthermore, suramin inhibited protein kinase C as enzyme activation was nearly abolished in liver particulate fractions of animals which had received 160 mg/kg body weight of the drug; however, diacylglycerol generation was unaffected. This suggests that the observed inhibition was due to a direct effect on protein kinase C rather than an indirect effect mediated by blockade of growth factor ligand-receptor interaction and consequent inhibition of diacylglycerol generation. When administered two hours prior to partial hepatectomy to minimize intracellular accumulation, suramin inhibited the regenerative response in six-week-old animals by nearly 50%, while having no effect on three-week-old rats, suggesting disparate mechanisms of regulation. Interestingly, three-week-old control animals exhibited a 1.6-fold greater degree of thymidine incorporation than 6-week-old control rats. In vitro analysis using a rat brain cytosol enzyme preparation showed that suramin has a biphasic effect on protein kinase C activity; low doses (10 μM) resulted in a nearly six-fold increase in enzyme activity while suramin concentrations greater than 60 μM caused inhibition of activity. Finally, protein kinase M, the constitutively active, catalytic domain of protein kinase C, also exhibited temporal alterations following partial hepatectomy. These results suggest that protein kinase C and, perhaps protein kinase M, are central to the initiation and/or regulation of hepatocyte proliferation following partial hepatectomy.