Timing of protooncogene expression varies in toxin-induced liver regeneration

Hepatic expression of the protooncogenes c-fos and c-myc occurs within 2 h after partial hepatectomy, and these immediate early genes are thought to prime the hepatocytes for subsequent proliferation. To examine whether such gene activation occurred in the setting of hepatocyte proliferation after toxic liver injury, protooncogene expression was examined during the regenerative response following liver injury from carbon tetrachloride (CCl4) or galactosamine (GalN). The pattern of protooncogene expression after CCl4 mirrored that seen after partial hepatectomy, with rises in c-fos and c-myc mRNA content within 2 h, and then a rapid return to baseline levels. In contrast, early c-fos and c-myc expression did not occur after GalN injury. Instead GalN-induced regeneration led to a delayed, and prolonged c-fos and c-myc activation which peaked 24-48 h after injury. Increases in c-jun, jun-B, and jun-D mRNA levels also occurred in both models at times similar to the rises of c-fos and c-myc expression. Although the timing of DNA synthesis was identical after GalN or CCl4 treatment, the proliferative response after GalN injury was significantly less than that of CCl4, and marked by the histologic appearance of oval cells. The coadministration of 2-acetylaminofluorene, an inhibitor of differentiated hepatocyte proliferation, together with CCl4 altered the usual pattern of post-CCl4 protooncogene expression to one resembling that seen after GalN injury. Thus, the timing of protooncogene expression during liver regeneration may vary considerably. These variations may influence the nature of the proliferative response in terms of which cell type(s) proliferates, and the amount of regeneration that ensues.