Epidermal growth factor impairs the cytochrome C/caspase-3 apoptotic pathway induced by transforming growth factor beta in rat fetal hepatocytes via a phosphoinositide 3-kinase-dependent pathway

Transforming growth factor beta (TGF-beta)-mediated apoptosis is one of the major death processes in the liver. We have previously shown that epiderma l growth factor (EGF) is an important survival signal for TGF-beta-induced apoptosis in fetal hepatocytes (Fabregat et al., FEES Lett 1996;384:14-18). In this work we have studied the intracellular signaling implicated in the protective effect of EGF, We show here that EGF activates p42 and p44 mito gen-activated protein kinases (MAPK). However, mitogen extracellular kinase (MEK) inhibitors do not block the survival effect of EGF. EGF also activat es phosphoinositide 3-kinase (PI 3-kinase) and protein kinase B (PKB/AKT) i n these cells. The presence of PI 3-kinase inhibitors blocks the protective effect of EGF on cell viability, DNA fragmentation, and caspase-3 activity . We have found that TGF-beta disrupts the mitochondrial transmembrane pote ntial (Delta Psi(m)) and activates the release of cytochrome c, this effect being blocked by EGF, via a PI 3-kinase-dependent pathway. A detailed stud y on bcl-2 superfamily gene expression shows that TGF-beta produces a decre ase in the messenger RNA (mRNA) and protein levels of bcl-x(L), an antiapop totic member of this family, capable of preventing cytochrome c release. EG F is able to maintain bcl-x(L) levels even in the presence of TGF-beta. PI 3-kinase inhibitors completely block the protective effect of EGF on TGF-be ta-induced bcl-xL down-regulation. We conclude that PI 3-kinase mediates th e survival effect of EGF on TGF-beta-induced death by acting upstream from the mitochondrial changes, i.e., preventing bcl-x(L) down-regulation, cytoc hrome c release, and activation of caspase-3.