Cytotoxicity and specificity of directed toxins composed of diphtheria toxin and the EGF-like domain of heregulin β1

As a step in the design of directed toxins, aimed at cells that overexpress HER receptors, particularly breast carcinoma cells, we studied the properties of a chimera of diphtheria toxin (DT) and heregulin beta 1. The EGF-like growth hormone heregulin is a ligand for the HERS and HER4 receptors and their heterodimers with HER2. The 60-residue EGF-like domain (hrg) of heregulin elicits a biological response and binds to these receptors primarily through its N terminus. We tested a fusion protein in which hrg replaces the C-terminal receptor-binding domain of DT (DT(389)hrg) and an alternative design in which this domain is fused to the N terminus of DT(389). Of those two constructs, the N-terminal fusion was not active as a directed toxin but elicited a growth response. The C-terminal fusion of hrg to DT(389) yielded a functional toxin and showed cell line specific cytotoxicity that is consistent with heregulin specificity. The binding of hrg to its cognate receptor is not impaired as shown by receptor activation, direct binding, and competition with free hrg. Cytotoxicity is dependent on high-affinity binding of DT(389)hrg to HERS and HER4 receptors and is not mediated by HER2 overexpression alone. For those cell lines exhibiting high-affinity binding sites, the level of cytotoxicity correlates with the rate of internalization. Thus DT(389)hrg chimeras offer a possible avenue toward directed toxins against cells that overexpress HER receptors.