Activation of Cdk4 and Cdk2 During Rat Liver Regeneration Is Associated With Intranuclear Rearrangements of Cyclin–Cdk Complexes

Abstract

Partial hepatectomy (PH) triggers the entry of rat liver cells into the cell cycle. The signals leading to cell–cycle activation converge into a family of kinases named cyclin–dependent kinases (cdks). Specific cyclin–cdk complexes are sequentially activated during the cell cycle. Cyclin D–cdk4 and cyclin E–cdk2 are activated during the G₁ phase, cyclin A–cdk2 is activated during the S phase, and cyclin B–cdk1 during mitosis. In the present study, we have examined the timing of the activation of cdk4 and cdk2, the intracellular location of G₁/S cyclins and cdks, and the relationship between location and cdk4 and cdk2 activities during rat liver regeneration after a PH. Results showed that the activity of both kinases started at 13 hours and showed maximal levels at 24 hours after hepatectomy. In quiescent cells, cyclin D3 and cdk4 were cytoplasmatic, whereas cyclin D1 was nuclear. At 5 hours after hepatectomy, cyclin D3 and cdk4 began to move into the nucleus, and at 13 hours, they were mostly nuclear. During the first 13 hours after hepatectomy, significant amounts of cyclin D1–cdk4 and cyclin D3–cdk4 complexes were formed, but they were mostly inactive. At 24 hours, these complexes were maximally activated. This activation was associated with the accumulation of cyclin D1, cyclin D3, and cdk4 in a nuclear subfraction extractable with nucleases. At 28 hours, the activity of cdk4 in this nuclear subfraction decreased when cyclin D1 moved from this fraction to the nuclear matrix (NM) and the levels of cyclin D3 diminished. The maximal activation of cdk2 at 24 hours was also associated with the accumulation of cyclin E, cyclin A, and cdk2 in this nuclease–sensitive fraction. The inactivation of cdk2 at 28 hours was associated with a strong decrease in cdk2 in this nuclear subfraction. Thus, results reported here indicate that the activation of cdk4 and cdk2 observed in rat liver cells after a PH is associated with a specific intranuclear location of these cdks and their associated cyclins