Replacement of N‐terminal portions of TGF‐α with corresponding heregulin sequences affects ligand‐induced receptor signaling and intoxication of tumor cells by chimeric growth‐factor toxins

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Abstract
ErbB receptor tyrosine kinases play an important role in developmental processes and tumor formation. Their activity is regulated by a family of structurally related ligands that bind to distinct ErbB receptor subsets, with transforming growth factor (TGF)‐α preferentially interacting with epidermal growth‐factor receptor (EGFR) and heregulin (HRG)‐β1 recognizing ErbB3 and ErbB4. To investigate the contribution of N‐terminal ligand sequences to binding specificity, we have constructed 2 chimeric growth factors termed H181T8 and H194T20, which contain N‐terminal HRG‐β1 sequences linked to complementary fragments of TGF‐α. For bacterial expression and analysis of cell binding, the chimeric ligands were genetically fused to truncated Pseudomonas exotoxin A (ETA). H181T8‐ETA and H194T20‐ETA toxins both were cytotoxic for human tumor cell lines overexpressing EGFR but did not significantly affect the growth of cells that express ErbB receptors other than EGFR. Binding of H181T8, which contains HRG‐β1 residues 177–181, induced rapid autophosphorylation of EGFR, but in contrast to a previously described chimeric ligand based on EGF was unable to activate other ErbB receptors. H194T20, which contains HRG‐β1 residues 177–194, despite specific binding to EGFR was unable to induce autophosphorylation of any of the ErbB family members. However, H194T20 enhanced and modified the activity of parental TGF‐α and HRG‐β1 when these ligands were simultaneously present. Our results show that modification of the N‐terminal TGF‐α sequence can have a significant effect on the signaling properties of the ligand and suggest that different EGF‐like ligands can synergize in the activation of ErbB receptors.