Epidermal growth factor induction of apolipoprotein A-I is mediated by theRas-MAP kinase cascade and Sp1

Insulin induces apolipoprotein A-I, apoA-I gene transcription via a membran e receptor with intrinsic tyrosine kinase activity. This finding prompted u s to ask whether the gene is stimulated by epidermal growth factor (EGF), E GF a peptide hormone that binds to another member of the receptor superfami ly with tyrosine kinase activity. Our data showed that like insulin, EGF in creased abundance of apoA-I protein and transcription of the gene in human hepatoma, Hep G2 cells. The effects of both hormones appeared direct becaus e their induction of apoA-I gene transcription was not affected by the prot ein synthesis inhibitor, cycloheximide. Although both insulin and EGF stimu late apoA-I expression, each hormone binds to a distinct membrane receptor thus suggesting differential intracellular signaling. Therefore, we used a panel of inhibitors to define the pathway(s) that mediate the actions of th ese hormones. Whereas, the actions of EGF required only the Ras-mitogen-act ivated protein, MAP kinase, those of insulin were mediated by equal partici pation of both the Ras-MAP kinase and protein kinase:C, PKC cascades, Despi te differences in signaling pathways triggered by each hormone receptor, th e activation of apoA-I transcription required the participation of a single transcription factor, Sp1, Furthermore, EGF induction of transcription was attenuated by mutating the MAP kinase site at amino acid, Thr(266) renderi ng Sp1 phosphorylation deficient, In summary, EGF stimulation of apoA-I exp ression is mediated solely by the Ras-MAP kinase cascade and enhanced activ ity of this pathway requires Sp1 with an intact phosphorylation site at Thr (266). However, insulin induction of this gene is different and requires bo th Ras-MAP kinase and PKC pathways but their actions are also mediated by S p1.