The heregulins (HRGs) are a family of growth factors that bind directly to erbB3 and erbB4 and induce tyrosine phosphorylation of erbB2 via receptor heterodimerization. Since erbB2, erbB3, and erbB4 (erbB2-4) are often overexpressed in human breast cancer cells, we produced recombinant HRGs and a HRG-based ligand toxin to investigate the signaling events triggered by HRGs and the ability of these ligands to specifically target such cells. Recombinant HRGβ2 stimulated the tyrosine phosphorylation of erbB2-4 in ZR- 75-1 human breast cancer cells. This was accompanied by the tyrosine phosphorylation of Shc and the formation of complexes between Shc and the adapter protein Grb2. Complexes were also detected between Shc and erbB2-4. However, Grb2 was detected in erbB2 and erbB4 but not erbB3 immunoprecipitates. Thus, these receptors exhibit mechanistic differences in their coupling to Ras signaling, and HRGβ2 administration triggers multiple inputs into the Ras signaling pathway, involving receptor-Grb2, receptor- Shc, and Shc-Grb2 complexes. HRGβ2 addition also stimulated the association of erbB3 with phosphatidylinositol-3-kinase. In accordance with the activation of key mitogenic signaling pathways, HRGβ2 stimulated the proliferation of MCF-7 and T-47D human breast cancer cells. Moreover, when tested for the ability to stimulate cell cycle re-entry of T-47D cells arrested under serum-free conditions, HRGβ2 was more effective than insulin, previously the most potent mitogen identified using this system. Finally, a HRGβ2/PE40 ligand toxin was constructed and found to exhibit cytotoxic activity against human breast cancer cells overexpressing erbB3 alone or in combination with erbB4 and/or erbB2.
|Number of pages||11|
|Journal||Cell Growth and Differentiation|
|Publication status||Published - 1 Jan 1995|