Prostaglandin E2 upregulates EGF-stimulated signaling in mitogenic pathways involving Akt and ERK in hepatocytes

Prostaglandins (PGs) such as PGE(2) enhance proliferation in many cells, apparently through several distinct mechanisms, including transactivation of the epidermal growth factor (EGF) receptor (EGFR) as well as EGFR-independent pathways. In this study we found that in primary cultures of rat hepatocytes PGE(2) did not induce phosphorylation of the EGFR, and the EGFR tyrosine kinase blockers gefitinib and AG1478 did not affect PGE(2)-Stimulated phosphorylation of ERK 1/2. In contrast, PGE(2) elicited EGFR phosphorylation and EGFR tyrosine kinase inhibitor-sensitive ERK phosphorylation in MH1C1 hepatoma cells. These findings suggest that PGE(2) elicits EGFR transactivation in MH1C1 cells but not in hepatocytes. Treatment of the hepatocytes with PGE(2) at 3 h after plating amplified the stimulatory effect on DNA synthesis of EGF administered at 24 h and advanced and augmented the cyclin D1 expression in response to EGF in hepatocytes. The pretreatment of the hepatocytes with PGE(2) resulted in an increase in the magnitude of EGF-stimulated Akt phosphorylation and ERK 1/2 phosphorylation and kinase activity, including an extended duration of the responses, particularly of ERK, to EGF in PGE(2)-treated cells. Pertussis toxin abolished the ability of PGE(2) to enhance the Akt and ERK responses to EGF. The results suggest that in hepatocytes, unlike MH1C1 hepatoma cells, PGE(2) does not transactivate the EGFR, but instead acts in synergism with EGF by modulating mitogenic mechanisms downstream of the EGFR. These effects seem to be at least in part G(i) protein-mediated and include upregulation of signaling in the PI3K/Akt and the Ras/ERK pathways.