The effects of heregulin on the cell line HB2, derived from immortalised human luminal mammary epithelial cells, have been examined. HB2 cells, which normally form smooth spherical colonies in collagen gels, exhibited a striking heregulin-induced morphological change to colonies projecting a large number of spiky branches. A mitogenic effect of heregulin on HB2 cells was also seen, which was more pronounced on collagen than on plastic, whereas cell motility was unaffected. HB2 cells were found to express the heregulin receptor subunits HER2 and HER3, but not HER4. Treatment of HB2 cells with heregulin also induced tyrosine phosphorylation of a band shown by immunoprecipitation to contain HER3. Using specific receptor-blocking antibodies, it was found that both the morphogenetic and proliferative responses of heregulin in HB2 cells were mediated by HER2 and HER3. To compare the effects of HER2 in heregulin signaling to heregulin-independent HER2 homodimerisation (thought to be a carcinoma-associated event), HB2 cells were transfected with the trk-neu hybrid receptor which could be induced to form homodimers by NGF. Although activated HER2 homodimers induced proliferation in the HB2 transfectants in collagen, a morphological response in collagen was not seen, suggesting that HER3 signaling is important for morphogenesis in this cell type.

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