Epidermal growth factor and insulin-induced deoxyribonucleic acid synthesis in primary rat hepatocytes is phosphatidylinositol 3-kinase dependent anddissociated from protooncogene induction

The mitogenic response to insulin and epidermal growth factor (EGF) was stu died in subconfluent and confluent cultures of primary rat hepatocytes. In subconfluent cultures, wortmannin, LY294002, and rapamycin reversed insulin - and EGF-induced [H-3]thymidine incorporation into DNA. The mitogen-activa ted protein kinase (MAPK) kinase 1 (MEK1) inhibitor PD98059 was without sig nificant effect on either insulin- or EGF-induced [H-3] thymidine incorpora tion. Insulin treatment did not alter levels of messenger RNAs (mRNAs) for c-fos, c-jun, and c-myc. EGF induced an increase in c-myc, but not c-fos or c-jun, mRNA levels in subconfluent hepatocyte cultures. This increase in c -myc mRNA was abolished by PD98059. In confluent cells that could not be in duced to synthesize DNA, EGF treatment also promoted an increase in c-myc m RNA to levels seen in subconfluent cultures. This increase was also abrogat ed by PD98059. These data indicate that in primary rat hepatocyte cultures, 1) the phosphoinositol S-kinase pathway, perhaps through p70(s6k) activati on, regulates DNA synthesis in response to insulin and EGF; 2) the MAPK pat hway is not involved in insulin- and EGF-induced DNA synthesis; and 3) p44/ 42 MAPKs are involved the induction of c-myc mRNA levels, although this ind uction is not required for DNA synthesis. These studies define two distinct signal transduction pathways that independently mediate growth-related res ponses in a physiologically relevant, normal cell system.