Signaling molecules implicated in heregulin induction of growth arrest andapoptosis

Heregulin (HRG) is one of the groups of polypeptide growth factors that act ivate the erbB-2 receptor via induction of heterodimerization with erbB-3 a nd erbB-4 receptors. The biological effects of HRG have been extensively st udied. The vast majority of the reports indicate that HRG induces cell grow th in breast cancer cells expressing normal levels of erbB-2 and growth inh ibition and apoptosis in cells overexpressing erbB-2. However, the mechanis m by which HRG promotes cell growth inhibition and apoptosis is still unkno wn. Previously we reported that constitutive expression of HRG in an erbB-2 -overexpressing cell line (SKBr-3) induced growth arrest and apoptosis. We also demonstrated that constitutive expression of HRG promoted a marked mor phological change, G2/M delay of the cell cycle, and DNA fragmentation. In this study, we demonstrate the mechanism by which HRG induces these cellula r effects. The doubling time of the SK/HRG cells increased in relation to t he level of HRG expression, and the level of HRG expression dictates the mo rphological change of the cells as well as their ability to grow or not gro w in an anchorage-independent manner. We demonstrate that these effects are accompanied by downregulation of both erbB-2 and erbB-3 receptors at the t ranscriptional and translational levels and that downregulation of the erbB -receptors results in reduced receptor tyrosine phosphorylation. The decrea se in erbB-receptor phosphorylation in turn results in a marked reduction o f ERK activity and a significant increase in JNK activity. Consequently, ov erexpression of HRG promoted the expression of PEA3, an Ets nuclear transcr iption factor. Taken together, our data demonstrate that the cellular effec ts induced by constitutive expression of HRG in SKBr-3 cells are correlated with the level of HRG expression. This is a first report demonstrating tha t HRG induction of apoptosis is directly correlated with decreased MAPK act ivity, increased JNK activity resulting in upregulation of PEA3 and downreg ulation of the erbB-2 receptor. Overall, these data provide important clues regarding the mechanism and downstream molecules involved in HRG induction of apoptosis that can be used as targets for therapeutic prevention.