Previous data disagree as to the role of extracellular signal-regulated kinase (ERK) in the EGF receptor-mediated stimulation of proliferation in hepatocytes. Using cultured rat hepatocytes, we here show that EGF receptor stimulation in mid/late G(1) phase caused a sustained ERK activation that lasted for at least 48 h. Inhibition of the early part of this activity by a single addition of the MEK inhibitor PD98059 did not affect the DNA synthesis. However, inhibition of both the early and the sustained phase of ERK activation by wash-out and repeated administrations of PD98059 abolished the DNA synthesis induced by EGF and TGFalpha. EGF receptor stimulation also transiently activated p38, and inhibition of p38 by SB203580 markedly decreased the DNA synthesis. Furthermore, EGF and TGFalpha stimulated phosphorylation of Akt, a downstream target of the PI3-kinase pathway, and the PI3-kinase inhibitors wortmannin and LY294002 blocked the EGF-induced DNA synthesis. These results support a mechanism for EGF receptor-mediated mitogenic signalling in hepatocytes where ERK has an obligatory role, acting in concert with PI3-kinase, and augmented by p38. Furthermore, the data suggest that to perform this role ERK has to be activated for a prolonged period.
Copyright 2003 S. Karger AG, Basel