INVOLVEMENT OF P21 AND P27 IN THE REGULATION OF CDK ACTIVITY AND CELL-CYCLE PROGRESSION IN THE REGENERATING LIVER

In tissue culture systems, p21 and p27 inhibit cyclin-dependent kinase (CDK) activity and cell cycle progression in response to numerous sti muli, but little is known about their involvement in cell growth in vi vo. We examined the modulation of CDK activity by these proteins after 70% partial hepatectomy (PH), an in vivo model of synchronous hepatoc yte cell cycle progression, After PH In BALB/c mice, p21 was induced d uring the prereplicative (G1) phase and was maximally expressed after peak hepatocyte DNA synthesis. p27 was present in quiescent liver and was minimally induced after PH. p21 and p27 immunoprecipitated with CD K2, CDK4, and cyclin D1 in the regenerating liver. The activity of CDK 2-, CDK4- and cyclin D1-associated kinases was upregulated after PH, a nd maximal activity of these enzyme complexes corresponded to peak DNA synthesis. Immunodepletion experiments suggested that p27 plays a rol e in downregulating CDK2 activity before and after peak DNA synthesis. Compared to cogenic wild-type mice, p21-/-mice demonstrated evidence of markedly accelerated hepatocyte progression through G1 phase after PH: DNA synthesis, upregulation of cyclin A and PCNA, induction of cyc lin D1- and CDK2-associated kinase activity, and appearance of a phosp horylated retinoblastoma protein (Rh) species occurred earlier in the p21-/-mice. These results suggest that p21 and p27 modulate CDK activi ty in the regenerating liver, and that p21 regulates the rate of progr ession through G1 phase of the cell cycle in vivo.