Caspase cleavage of HER-2 releases a bad-like cell death effector

Human epidermal growth factor receptor-2 (HER-2/ErbB2/neu), a receptor tyrosine kinase that is amplified/overexpressed in poor prognosis breast carcinomas, confers resistance to apoptosis by activating cell survival pathways. Here we demonstrate that the cytoplasmic tail of HER-2 is cleaved by caspases at Asp¹⁰¹⁶/Asp¹⁰¹⁹ to release a ∼47-kDa product, which is subsequently proteolyzed by caspases at Asp¹¹²⁵ into an unstable 22-kDa fragment that is degraded by the proteasome and a predicted 25-kDa product. Both the 47- and 25-kDa products translocate to mitochondria, release cytochrome c by a Bcl-x_L-suppressible mechanism, and induce caspase-dependent apoptosis. The 47- and 25-kDa HER-2 cleavage products share a functional BH3-like domain, which is required for cytochrome c release in cells and isolated mitochondria and for apoptosis induction. Caspase-cleaved HER-2 binds Bcl-x_L and acts synergistically with truncated Bid to induce apoptosis, mimicking the actions of the BH3-only protein Bad. Moreover, the HER-2 cleavage products cooperate with Noxa to induce apoptosis in cells expressing both Bcl-x_L and Mcl-1, confirming their Bad-like function. Collectively, our results indicate that caspases activate a previously unrecognized proapoptotic function of HER-2 by releasing a Bad-like cell death effector.