Estrogen-induced proliferation in cultured hepatocytes involves cyclin D1, P21^CIP1 and P27^KIP1

The purpose of this study was to establish if estrogen-induced hepatocyte proliferation in vitro involves the cell cycle regulators cyclin D1, p21^Cip1, and p27^Kip1. Male rat hepatocytes were cultured in presence of 17-β-estradiol (E₂) ± ICI-182780, a pure estrogen antagonist, and [³H]-thymidine, as required. DNA synthesis as well as p21^Cip1, p27^Kip1, and cyclin D1 mRNA and protein levels were evaluated at different times (12, 24, 36, and 48 hours) of incubation. E₂-increased DNA synthesis was correlated with cyclin D1 and p21^Cip1 (mRNA and protein) variations that were reversed by the addition of ICI-182780. p27^Kip1 protein levels progressively increased regardless of the presence of E₂ or ICI-182780. Our data confirm that estrogens' stimulatory effect is related to their ability to increase cyclin D1 levels. The increase of p21^Cip1 is probably related to the reentry of hepatocytes in the quiescent state. p27^Kip1 protein is not able to arrest hepatocyte proliferation.