CHARACTERIZATION OF 57 KDA STATIN AS A TRUE MARKER FOR GROWTH ARREST IN TISSUE BY ITS DISAPPEARING FROM REGENERATING LIVER

Statin, a 57 kDa nuclear protein, is lost from quiescent fibroblasts i n culture when they are induced to enter the cell cycle by feeding wit h growth factors, or by removal of contact inhibition. In order to inv estigate changes in statin expression during the transition from a qui escent to a cycling state in situ, we performed 70% partial hepatectom y on rats and analyzed the regenerating liver by immunofluorescence mi croscopy with antistatin monoclonal antibodies (S44 mAb), and by immun oblotting of liver proteins in cytoplasmic and enriched nuclear/cytosk eletal fractions. Western blot analysis showed that rat hepatocytes in situ contain a nuclear 57 kDa form of statin, as seen in cultured fib roblasts; however additional S44-immunoreactive polypeptides with mole cular weights of 53 and 110 kDa are also present in both cytoplasmic a nd nuclear/cytoskeletal fractions. Immunofluorescence microscopy indic ates that the proportion of S44-positive hepatocyte nuclei drops to si milar to 60% within 24 hours after hepatectomy, a time period when re- entry of hepatocytes into the cell cycle is first observed. On Western blots of hepatocyte nuclear/cytoskeletal proteins obtained 24 hours a fter hepatectomy, the 57 kDa form of statin is markedly reduced. These results suggest that, although in liver the S44 antibody recognizes t hree proteins (53 kDa, 57 kDa, and 110 kDa), the 57 kDa in intact live r, similar to cultured fibroblasts, is the on ly polypeptide recognize d by the statin antibody that disappears when hepatocytes are induced to re-enter the cell cycle from a quiescent state.