Trastuzumab (Herceptin), a humanized anti-HER2 receptor monoclonal antibody, inhibits basal and activated HER2 ectodomain cleavage in breast cancer cells

HER2 is a ligand-less tyrosine kinase receptor of the ErbB family that is f requently overexpressed in breast cancer, It undergoes proteolytic cleavage that results in the release of the extracellular domain and the production of a truncated membrane-bound fragment, p95, We show that HER2 shedding is activated by 4-aminophenylmercuric acetate (APMA), a well-known matrix met alloprotease activator, in HER2-overexpressing breast cancer cells. The HER 2 p95 fragment, which appears after APMA-induced cleavage, is phosphorylate d, We analyzed 21 human breast cancer specimens, and a phosphorylated M, 95 ,000 HER2 band could be detected in some of them, which indicated that the truncated receptor is also present in vivo. The activation of HER2 shedding by APMA in cells was blocked with batimastat, a broad-spectrum metalloprot ease inhibitor. Trastuzumab (Herceptin; Genentech, San Francisco, CA), a hu manized monoclonal antibody directed at the HERS ectodomain, which has been shown to be active in patients with HER2-overexpressing breast cancer, inh ibited basal and induced HERS cleavage and, as a consequence, the generatio n of phosphorylated p95, This inhibitory effect of trastuzumab was not shar ed by 2C4, an antibody against a different epitope of the HERS ectodomain, The inhibition of basal and APMA-induced cleavage of HER2 by trastuzumab pr eceded antibody-induced receptor down-modulation, which indicated that the effect of trastuzumab an cleavage was not attributable to a decrease in cel l-surface HER2 induced by trastuzumab, We propose that the inhibition of HE R2 cleavage and prevention of the production of an active truncated HER2 fr agment represent a novel mechanism of action of trastuzumab.