Heregulin beta1 (
HRG), a combinatorial
ligand for human
growth factor receptors 3 and 4, is a regulatory
polypeptide that promotes the differentiation of mammary epithelial cells into secretory lobuloalveoli. Emerging evidence suggests that the processes of secretory pathways, such as biogenesis and trafficking of vesicles in neurons and adipose cells, are regulated by the Rab family of low-molecular-weight
GTPases. In this study, we identified Rab3A as a gene product induced by
HRG. Full-length Rab3A was cloned from a mammary gland cDNA library. We demonstrated that
HRG stimulation of human
breast cancer cells and normal breast epithelial cells induces the expression of
Rab3A protein and
mRNA in a
cycloheximide-independent manner.
HRG-mediated induction of Rab3A expression was blocked by an inhibitor of
phosphatidylinositol 3-kinase but not by inhibitors of
mitogen-activated protein kinases p38(MAPK) and p42/44(MAPK). Human breast epithelial cells also express other components of regulated vesicular traffic, such as rabphilin 3A, Doc2, and
syntaxin. Rab3A was predominantly localized in the cytosol, and
HRG stimulation of the epithelial cells also raised the level of membrane-bound Rab3A.
HRG treatment induced a profound alteration in the cell morphology in which cells displayed neuron-like membrane extensions that contained Rab3A-coated, vesicle-like structures. In addition,
HRG also promoted the secretion of cellular
proteins from the mammary epithelial cells. The ability of
HRG to modify exocytosis was verified by using a
growth hormone transient-transfection system. Analysis of mouse mammary gland development revealed the expression of Rab3A in mammary epithelial cells. Furthermore, expression of the
HRG transgene in Harderian
tumors in mice also enhanced the expression of Rab3A. These observations provide new evidence of the existence of a Rab3A pathway in mammary epithelial cells and suggest that it may play a role in vesicle trafficking and secretion of
proteins from epithelial cells in response to stimulation by the
HRG expressed within the mammary mesenchyma.