Hepatocytes from old rats retain responsiveness of c-myc expression to EGF in primary culture but do not enter S phase.

Responsiveness of rat hepatocytes from 22- to 24-month-old animals to growth stimulation was examined in primary culture by assay of replicative DNA synthesis and c-myc expression as parameters. Old rat hepatocytes showed very low DNA synthesis in response to EGF, rat sera, or hepatectomized rat sera, as compared to 2- to 3-month-old rat hepatocytes. However, c-myc expression was induced by EGF to a similar extent in cells from both old and young rats. These results suggest that the old cells can traverse from G0 to G1 phase of the cell cycle but they somehow are blocked from entering the S phase.