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 cyclindependent kinases (cdks). Specific cyclin-cdk complexes are sequentially activated during the cell cycle. Cyclin D-cdk4 and cyclin E-cdk2 are activated during the G 1 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 G 1 /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. (HEPATOL-OGY 1999;29:385-395.)In pluricellular organisms, the cell cycle is regulated by growth factors, anchorage to the extracellular matrix, and contact inhibition. These extracellular factors generate intracellular signals that finally converge into a family of serinethreonine kinases, named cyclin-dependent kinases (cdks), which act as molecular integrators of the signals that regulate cell-cycle progression.The liver contains hepatocytes (85%-95% of the hepatic mass) and nonhepatocytes (5%-15% of hepatic mass). In normal livers from adult animals, all these cells are quiescent.

Blunted DNA synthesis and delayed S-phase entry following inhibition of Cdk2 activity in the regenerating rat liver

“…[3][4][5] Several cyclins and Cdks are activated in the regenerating rat hepatocyte with the peak of DNA synthesis (S phase) occurring around the 24th hour after PH. 6,8,9 Although the activation of some cyclin/Cdk complexes, including cyclin E/Cdk2, might be more specifically associated with the regenerative process itself, 10 there is no direct evidence to date that activation of cyclin E/Cdk2 plays a determinant role in orchestrating cell cycle progression in the regenerating liver. We therefore investigated the impact of targeted inhibition of Cdk2 kinase activity on proliferation of hepatocytes after partial hepatectomy in the rat.…”

“…[3][4][5] Several studies found an induction of cyclin E and Cdk2 at time points near the G1/S boundary in the regenerating rat hepatocytes. [6][7][8][9] Furthermore, a subcellular rearrangement of cyclins and Cdks is observed in the remnant liver with cyclin E and Cdk2 moving from the cytosol into the nucleus. 6 The cyclin E/Cdk2 complex acts as a histone H1 kinase and the peak of its activity closely correlates with the peak of DNA synthesis in the regenerating hepatocytes.…”

“…[6][7][8][9] Furthermore, a subcellular rearrangement of cyclins and Cdks is observed in the remnant liver with cyclin E and Cdk2 moving from the cytosol into the nucleus. 6 The cyclin E/Cdk2 complex acts as a histone H1 kinase and the peak of its activity closely correlates with the peak of DNA synthesis in the regenerating hepatocytes. 6,10 Cyclin E and Cdk2 induction, redistribution and activity have been shown to be disturbed in several models of impaired liver regeneration.…”

“…6 The cyclin E/Cdk2 complex acts as a histone H1 kinase and the peak of its activity closely correlates with the peak of DNA synthesis in the regenerating hepatocytes. 6,10 Cyclin E and Cdk2 induction, redistribution and activity have been shown to be disturbed in several models of impaired liver regeneration. [11][12][13] In addition, hepatocyte proliferation can be stimulated by targeted modulation of cyclin E. 14 Although, these data suggest that cyclin E and Cdk2 might be important in orchestrating the cell cycle in the regenerating liver, the exact role of the active cyclinE/Cdk2 complex remains to be defined.…”

Transforming growth factor‐β in liver cancer stem cells and regeneration

“…Cdk1, in association with cyclins A and B, is then essential for entry and exit from mitosis. Cyclin D1 has been shown to be activated by 6 hours and maximal levels of Cdk4 are present at 24 hours after PHx in rats 110. Meanwhile, Cdk1 is sharply induced between 18 and 24 hours, followed by a transient decrease, before another increase at 30 hours post‐PHx in rats 111.…”