AN IMMUNOLOGICAL APPROACH REVEALS BIOLOGICAL DIFFERENCES BETWEEN THE 2 NDF HEREGULIN RECEPTORS, ERBB-3 AND ERBB4/

The group of subtype I transmembrane tyrosine kinases includes the epi dermal growth factor (EGF) receptor (ErbB-1), an orphan receptor (ErbB -2), and two receptors for the Neu differentiation factor (NDF/heregul in), namely: ErbB-3 and ErbB-4, Here we addressed the distinct functio ns of the two NDF receptors by using an immunological approach. Two se ts of monoclonal antibodies (mAbs) to ErbB-3 and ErbB-4 were generated through immunization with recombinant ectodomains of the correspondin g receptors that were fused to immunoglobulin. We found that the share d ligand binds to highly immunogenic, but immunologically distinct sit es of ErbB-3 and ErbB-4. NDF receptors differed also in their kinase a ctivities; whereas the catalytic activity of ErbB-4 was activable by m Abs, ErbB-3 underwent no activation by mAbs in living cells. Likewise, down-regulation of ErbB-4, but not ErbB-3, was induced by certain mAb s. By using the generated mAbs, we found that the major NDF receptor o n mammary epithelial cells is a heterodimer of ErbB-3 with ErbB-2, whe reas an ErbB-1/ErbB-2 heterodimer, or an ErbB-1 homodimer, is the pred ominant species that binds EGF. Consistent with ErbB-2 being a shared receptor subunit, its tyrosine phosphorylation was increased by both h eterologous ligands and it mediated a trans-inhibitory effect of NDF o n EGF binding. Last, we show that the effect of NDF on differentiation of breast tumor cells can be mimicked by anti-ErbB-4 antibodies, but not by mAbs to ErbB-3. Nevertheless, an ErbB-3-specific mAb partially inhibited the effect of NDF on cellular differentiation. These results suggest that homodimers of ErbB-4 are biologically active, but hetero dimerization of the kinase-defective ErbB-3, probably with ErbB-2, is essential for transmission of NDF signals through ErbB-3.