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SummaryThe relationship between oestrogen (E 2 ) and insulin-like growth factor-one (IGF-1) was examined in both tamoxifen-sensitive (MCF 7/5-21) and tamoxifen-resistant (MCF 7/5-23) subclones of the MCF 7 cell line. Both subclones were grown in defined, serum-free (SF) medium over a period of 7 days with the addition of E 2 or IGF-1 or a combination of both agents. Growth of both MCF 7/5-21 and 7/5-23 cells was stimulated (245% and 350%, respectively) by E 2 . However, only the growth of MCF 7/5-23 cells was stimulated (266%) by IGF-1. A combination of E 2 and IGF-1 significantly enhanced MCF 7/5-21 and 7/5-23 cell growth (581% and 695%, respectively). E 2 -induced IGF-1 receptor (IGF-1R) levels (as measured by 125 I-IGF-1 binding and Northern analyses) in only MCF 7/5-23 cells. This effect was partially inhibited by tamoxifen. In medium containing serum, the growth of only the MCF 7/5-23 cells was significantly inhibited by the IGF-1R monoclonal antibody, αIR-3. The detection of E 2 -induced expression of IGF-2 using RT-PCR was demonstrated in the MCF 7/5-23 cells. These experiments indicate that E 2 may sensitize tamoxifen-resistant MCF 7/5-23 cells to the growth stimulatory actions of IGF-2 via upregulation of the IGF-1R and describes a cell-survival mechanism that may manifest itself as tamoxifen resistance.

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