ErbB-3 mediates differential mitogenic effects of NDF/heregulin isoforms on mouse keratinocytes

The family of Neu differentiation factors (NDFs, or heregulins) includes a dozen secreted glycoproteins, whose receptor binding domain displays two variants, α and β, and they bind to two receptor tyrosine kinases, ErbB-3 and ErbB-4. Certain isoforms were reported to induce growth-arrest and differentiation of mammary tumor cells, while other breast cancer cell lines responded mitogenically. The present study addressed the biologic effects of various NDF isoforms on normal EGF-dependent epithelial cells, Balb/MK keratinocytes, that can undergo either proliferation or differentiation. We found that β isoforms of NDP induced a mitogenic effect, that was significantly smaller than the maximal response to EGF. By contrast with NDF-β, NDF-α isoforms exerted almost no mitogenic effect, but they were sufficient to maintain keratinocytes in culture. Consistent with their higher mitogenic potency, NDP-β isoforms bound to Balb/MK cells with higher affinity (K(d) = 2.2 nM) than α isoforms, however both groups shared their receptor, that we identified as ErbB-3. No transcript of ErbB-4 was detectable in the keratinocytes, but these cells express multiple NDF mRNAs and also ErbB-2. We conclude that different isoforms of NDF induce distinct growth regulatory effects on cultured keratinocytes, through direct interaction with ErbB-3.