The inappropriate activation of one or more members of the ErbB family of
receptor tyrosine kinases [ErbB-1 (EGFR), ErbB-2, ErbB-3, ErbB-4] has been linked with
oncogenesis. ErbB-2 is frequently coexpressed with ErbB-3 in
breast cancer cells and in the presence of the
ligand heregulin (
HRG) the ErbB-2/ErbB-3 receptors form a signaling heterodimer that can affect cell proliferation and apoptosis. The major goal of the present study was to determine whether endogenous
HRG causes autocrine/paracrine activation of ErbB-2/ErbB-3 and contributes to the proliferation of mammary epithelial
tumor cells.
Tyrosine-phosphorylated (activated) ErbB-2 and ErbB-3 receptors were detected in the majority of extracts from
tumors that had formed spontaneously or as a result of oncogene expression. HRG-1 transcripts and
protein were found in the epithelial cells of most of these mouse mammary
tumors. Various mouse mammary cell lines also contained activated ErbB-2/ErbB-3 and
HRG transcripts. A approximately 50 kDa C-terminal fragment of pro-
HRG was detected, which indicates that the HRG-1 precursor is readily processed by these cells. It is likely that the secreted mature
HRG activated the ErbB-2/3 receptors. Addition of an antiserum against
HRG to the mammary epithelial tumor cell line TM-6 reduced ErbB-3 Tyr-phosphorylation. Treatment with HRG-1
siRNA oligonucleotides or
infection with a retroviral construct to stably express
HRG siRNA effectively reduced
HRG protein levels, ErbB-2/ErbB-3 activation, and the rate of proliferation, which could be reversed by the addition of
HRG. The cumulative findings from these experiments show that coexpression of the
HRG ligand contributes to activation of ErbB-2/Erb-3 in mouse mammary
tumor cells in an autocrine or paracrine fashion.