Inhibitory and anti-inhibitory factors of rat serum active on the G1-S transition of hepatocyte cell cycle.

Rat hepatocytes are responsive to a serum factor inhibiting their progression through the cell cycle from the late G1 phase to the S phase. After fractionation of normal adult rat serum by two chromatographic steps on DEAE cellulose and sephadex gel filtration, the inhibitory activity was linked to proteins having a high electronegative charge and of apparent high molecular weight. Polyacrylamide gel electrophoretic analysis of active fraction showed that the alpha1 macroglobulin was its main component. Male and female baby rats were sensitive to the inhibitory factor from normal rats. Contrary to the normal adult rat serum the whole hepatectomized adult rat serum did not exhibit any ingibitory activity on the G1-S transition. However, two components having antagonist activities: an alpha1 globulin and a gamma globulin, were separated by chromatographic procedures from hepatectomized rat serum. (a) The alpha1 globulin showed an inhibitory activity. It had an apparent molecular weight lower than that found in normal rats. Its activity was sex related: only male baby rats were responsive. (b) The factor present in the gamma globulin fraction was found to be antagonistic to the alpha1 globulin factor. Its occurrence after hepatectomy explains the absence of inhibitory activity in the serum of hepatectomized rats.