¹Department of Discovery Medicine, GlaxoSmithKline, Five Moore Drive, Research Triangle Park, North Carolina, NC 27709-3398, USA

Correspondence: NL Spector, E-mail: Neil.L.Spector@gsk.com

Received 5 July 2003; Revised 26 August 2003; Accepted 27 August 2003.

Abstract

The expression of the NH₂ terminally truncated ErbB2 receptor (p95^ErbB2) in breast cancer correlates with metastatic disease progression compared with the expression of full-length p185^ErbB2. We now show that heregulin (HRG), but not EGF, stimulates p95^ErbB2 phosphorylation in BT474 breast cancer cells. Furthermore, phospho-p95^ErbB2 forms heterodimers with ErbB3, but not EGFR, while p185^ErbB2 heterodimerizes with both EGFR and ErbB3. The predilection of p95^ErbB2 to heterodimerize with ErbB3 provides an explanation for its regulation by HRG, an ErbB3 ligand. GW572016, a reversible small molecule inhibitor of EGFR and ErbB2 tyrosine kinases, inhibits baseline p95^ErbB2 phosphorylation in BT474 cells and tumor xenografts. Inhibition of p95^ErbB2, p185^ErbB2, and EGFR phosphorylation by GW572016 resulted in the inhibition of downstream phospho-Erk1/2, phospho-AKT, and cyclin D steady-state protein levels. Increased phosphorylation of p95^ErbB2 and AKT in response to HRG was abrogated to varying degrees by GW572016. In contrast, trastuzumab did not inhibit p95^ErbB2 phosphorylation or the expression of downstream phospho-Erk1/2, phospho-AKT, or cyclin D. It is tempting to speculate that trastuzumab resistance may be mediated in part by the selection of p95^ErbB2-expressing breast cancer cells capable of exerting potent growth and prosurvival signals through p95^ErbB2-ErbB3 heterodimers. Thus, p95^ErbB2 represents a target for therapeutic intervention, and one that is sensitive to GW572016 therapy.