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We have previously reported that induction of EGFR and erbB2 in response to antihormones may provide an early mechanism allowing breast cancer cells to evade the growth inhibitory action of such therapies and ultimately drive resistant growth. More recently, another member of the erbB receptor family, erbB3, has been implicated in antihormone resistance in breast cancer. In the present study we have investigated whether induction of erbB3, and related family member erbB4, may provide an alternative resistance mechanism to antihormonal action in a panel of four ER-positive breast cancer cell lines. MCF-7, T47D, BT474 and MDAMB361 cell lines were exposed to fulvestrant (100 nM) for 7 days, and effects on erbB3/4 signalling and growth were assessed. Effects of the erbB3/4 ligand heregulin-β1 were also examined in the absence and presence of fulvestrant. Fulvestrant potently reduced ER expression and transcriptional activity and significantly inhibited growth in all four cell lines. However, alongside this inhibitory activity, fulvestrant also consistently induced protein expression and activity of erbB4 in the four cell lines and also promoted erbB3, erbB2 and EGFR protein expression and activity in MCF-7 and T47D cells. Consequently, fulvestrant treatment sensitised each cell line to the actions of heregulin-β1 with enhanced erbB3/4-driven signalling activity and significant increases in cell proliferation being observed when compared with untreated cells. Indeed, in T47D and MDAMB361, heregulin-β1 was converted from a ligand having negligible or suppressive growth activity into one that potently promoted cell proliferation. Consequently, fulvestrant-induced growth inhibition was completely overridden by heregulin-β1 in all four cell lines. In conclusion, these findings would suggest that although antihormones, such as fulvestrant, may have potent acute growth inhibitory activity in ER-positive breast cancer cells, their ability to induce and sensitize cells to growth factors, such as heregulins, may serve to reduce and ultimately limit their inhibitory activity.