Acquired resistance to oestrogen deprivation: role for growth factor signalling kinases/oestrogen receptor cross-talk revealed in new MCF-7X model

In vitro models of long-term oestrogen deprivation utilise increased oestrogen receptor (ER) and are oestrogen hypersensitive, with emerging evidence that growth factor signalling contributes and interacts with ER. However, such models are commonly derived in the presence of serum growth factors that may force the resistance mechanism. Our new in vitro model, MCF-7X, has thus been developed under conditions of both oestrogen and growth factor depletion. ER expression, serine 118 phosphorylation on this receptor and its transcriptional activity were modestly increased compared to the parental MCF-7 cells, although MCF-7X cells were not oestrogen hypersensitive. Faslodex (0.1 mu M) partially decreased ER and its transcriptional activity, with associated decreases in serine 118 phosphorylation. Faslodex inhibited MCF-7X growth by 50% for 10 weeks. Classical growth factor receptors did not impact on MCF-7X growth and only a modest contribution for MAP kinase was revealed using PD98059 (25 mu M; 35% inhibition for 3 weeks). However, the phosphatidylinositol-3-OH (P13)-kinase inhibitor LY294002 (5 mu M) inhibited MCF-7X growth by 65% for 10 weeks. In contrast to PD98059, LY294002 also partially-inhibited ER transcriptional activity and decreased serine 167 ER phosphorylation. Co-treatment with faslodex plus LY294002 to decrease activity of both serine 118 and 167 proved superior vs the single agents in decreasing ER transcriptional activity and MCF-7X growth (90% inhibition for 25 weeks). However, triple treatment including PD98059 was required to prevent resistance in MCF-7X, an event dependent on maximal depletion of serine 118 phosphorylation and ER transcriptional activity. Kinases clearly contribute in resistance to oestrogen deprivation, cross-talking with ER signalling via AF-1 phosphorylation. While inhibiting each pathway has potential to treat this state, combined therapy targeting all regulators of ER phosphorylation may be required to block subsequent emergence of resistance.