Caspase-dependent cleavage of ErbB-2 by geldanamycin and staurosporin

The geldanamycin-induced degradation of ErbB-2 produces a 23-kDa carboxyl-terminal fragment, which has been isolated and subjected to amino-terminal microsequencing. The obtained sequence indicates that the amino terminus of this fragment corresponds to Gly-1126 of ErbB-2. Analysis of the residues immediately before Gly-1126 suggests that cleavage may involve caspase activity. Site-directed mutagenesis of Asp-1125 in ErbB-2 prevents geldanamycin-provoked formation of the 23-kDa fragment, consistent with the requirement of this residue for caspase-dependent cleavage in known substrates. Also, the addition of the pan-caspase inhibitor Z-VAD-FMK blocks formation of the 23-kDa ErbB-2 fragment in cells exposed to geldanamycin. Interestingly, staurosporin and curcumin are also shown to provoke the degradation of ErbB-2 with formation of the 23-kDa carboxyl-terminal fragment. The generation of this fragment by staurosporin or curcumin is likewise blocked by caspase inhibition. Caspase inhibition does not prevent accelerated degradation of the 185-kDa native ErbB-2 in geldanamycin-treated cells but does significantly prevent staurosporin-stimulated metabolic loss of ErbB-2.