HEREGULIN STIMULATES MITOGENESIS AND PHOSPHATIDYLINOSITOL 3-KINASE IN MOUSE FIBROBLASTS TRANSFECTED WITH ERBB2/NEU AND ERBB3

Heregulin (HRG) is a pluripotent growth factor that can stimulate the growth of some human mammary tumor cells and the differentiation of others, Two members of the epidermal growth factor receptor family of receptor/tyrosine kinases, p180(erbB3) and p180(erbB4), serve as receptors for the HRG ligand, While HRG appears to be capable of stimulating the autophospholylation activity of p180(erbB4), the co-expression of p185(erbB2/neu) With p180(erbB3) is necessary for the HRG-stimulated tyrosine phosphorylation of both of these receptors, On the basis of the sequences surrounding their putative tyrosine phosphorylation sites, we predict that the different HRG-responsive receptors couple to different intracellular SH2 domain-containing proteins, Hence, the differ ent receptors may mediate different cellular responses to the HRG ligand, In the present study we show that HRG beta 1 is mitogenic for erbB3-transfected DHFR/G8 cells, an NIH3T3 mouse fibroblast derivative that overexpresses p185(erbB2/neu), HRG stimulated the incorporation of [H-3] thymidine into the DNA of these cells with an EC(50) of 70 +/- 7 pm, HRG was not mitogenic for parental DHFR/G8 cells that do not express the ErbB3 protein, Phosphatidylinositol (PI) 3-kinase, an enzyme believed to be important in cellular growth regulation by growth factors and oncogenes, is predicted to couple to tyrosine-phosphorylated ErbB3, We observed that HRG stimulated the association of PI 3-kinase with both p185(erbB2/neu) and ErbB3 in transfected DHFR/G8 cells, but not in the parental cell line, We conclude that the ErbB3 protein is capable of mediating a proliferative response of fibroblasts to HRG, and that the activation of PI 3-kinase is an integral part of the growth signaling mechanism.