In tissue culture systems, p21 and p27 inhibit cyclin-dependent kinase (CDK) activity and cell cycle progression in response to numerous stimuli, but little is known about their involvement in cell growth in vivo. We examined the modulation of CDK activity by these proteins after 70% partial hepatectomy (PH), an in vivo model of synchronous hepatocyte cell cycle progression. After PH in BALB/c mice, p21 was induced during 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 CDK2, CDK4, and cyclin D1 in the regenerating liver. The activity of CDK2-, CDK4- and cyclin D1-associated kinases was upregulated after PH, and maximal activity of these enzyme complexes corresponded to peak DNA synthesis. Immunodepletion experiments suggested that p27 plays a role in downregulatlng 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 cyclin D1- and CDK2-associated kinase activity, and appearance of a phosphorylated retinoblastoma protein (Rb) species occurred earlier in the p21 -/- mice. These results suggest that p21 and p27 modulate CDK activity in the regenerating liver, and that p21 regulates the rate of progression through G1 phase of the cell cycle in vivo.
Bibliographical noteFunding Information:
Departments of 1Medicine and 2Pathology, Hennepin County Medical Center, Minneapolis, Minnesota 55415; 3Minneapolis Medical Research Foundation, Minneapolis, Minnesota 55404, USA; 4Department of Biochemistry, Hong Kong University of Science and Technology, Kowloon, Hong Kong; 5Laboratory of Biochemistry and Metabolism, National Institute of Diabetes, Digestive, and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA
- Cyclin-dependent kinases
- G1 phase
- Liver regeneration