We have previously found that loss of C/EBPα in hepatocytes of newborn livers leads to increased proliferation, to a reduction in p21 protein levels and to an induction of S phase-specific E2F/p107 complexes. In this paper, we investigated C/EBPα-dependent regulation of E2F complexes in a well-characterized cell line, 3T3-L1, and in stable transformants that conditionally express C/EBPα, C/EBPα and C/EBPβ proteins are induced in 3T3-L1 preadipocytes during differentiation with different kinetics and potentially may regulate E2F/Rb family complexes. In pre-differentiated cells, three E2F complexes are observed: cdk2/E2F/p107, E2F/p130 and E2F4. cdk2/E2F/p107 complexes are induced in nuclear extracts of 3T3-L1 cells during mitotic expansion, but are not detectable in nuclear extracts at later stages of 3T3-L1 differentiation. The reduction in E2F/p107 complexes is associated with elevation of C/EBPα, but is independent of C/EBPβ expression. Bacterially expressed, purified His-C/EBPα is able to disrupt E2F/p107 complexes that are observed at earlier stages of 3T3-L1 differentiation. C/EBPβ, however, does not disrupt E2F/p107 complexes. A short C/EBPα peptide with homology to E2F is sufficient to bring about the disruption of E2F/p107 complexes from 3T3-L1 cells in vitro. Induction of C/EBPα in stable 3T3-L1 clones revealed that C/EBPα causes disruption of p107/E2F complexes in these cells. In contrast, E2F/p130 complexes are induced in cells expressing C/EBPα. Our data suggest that induction of p130/E2F complexes by C/EBPα occurs via up-regulation of p21, which, in turn, leads to association with and inhibition of, cdk2 kinase activity. The reduction in cdk2 kinase activity correlates with alterations of p130 phosphorylation and with induction of p130/E2F complexes in 3T3-L1 stable clones. Our data suggest two pathways of C/EBPα-dependent regulation of E2F/Rb family complexes: disruption of S phase-specific E2F/p107 complexes and induction of E2F/p130 complexes.
Bibliographical noteFunding Information:
The authors thank Boris Sarcevic for the gift of cyclin E and cdk2-expressing baculoviruses. We also thank Brenda Rieland for isolation of cyclinE/cdk2 and K. Faraj for excellent assistance in the preparation of the manuscript. This work was supported by NIH grants GM55188 (N.A.T.), AG00766 (N.A.T.), DK45285 (G.J.D.), AG13663 (G.J.D.) and DK-54921 (J.H.A.).