The interaction of the ER with ERBB2 and PI3K results in elevated levels of AKT and p90^RSK in tamoxifen-resistant MCF-7 cells

Excerpt

Despite advances in endocrine therapies, the majority of patients receiving tamoxifen will eventually relapse while retaining functional estrogen receptors. We have previously shown, using an MCF-7 cell line (Tam^R) resistant to the anti-proliferative effects of tamoxifen, that elevated levels of phosphorylated AKT and p90^RSK lead to an apparent ligand-independent phosphorylation of ERα ser¹⁶⁷. Analysis of the growth factor receptors in these cells indicated elevated levels of both phosphorylated ERBB2 and total ERBB3, which we postulated formed heterodimers and activated the PI3 kinase pathway leading to elevated AKT. However, our recent data suggest that elevated AKT results from an interaction between ERα and ERBB2. This association is knocked out by treatment with the pure anti-estrogen ICI 182,780 and is absent in the WT parental cell line. Similarly, we have demonstrated an association between the p85 subunit of PI3K and ERα in Tam^R but not in WT cells. Treatment of the cell lines with the specific AKT inhibitor SH6 and the MEK1/2 inhibitor U0126 caused greater decrease in cell proliferation and concomitant ERα-directed transactivation in the Tam^R cells versus the WT, confirming that these pathways are integral to the Tam^R phenotype. To establish whether p90^RSK or AKT was responsible for the phosphorylation of the estrogen receptor at ser¹⁶⁷, Tam^R and WT cells were treated with SH6, U0126 or a combination of the two. Blocking either pathway individually had little effect on ERα ser¹⁶⁷phosphorylation. However, a combination of the two inhibitors resulted in almost complete loss of phosphorylation. These data were confirmed using siRNA technology to suppress MAPK and AKT expression. Taken together these data suggest that, in this setting, the ER functions via a non-genomic mechanism, associating with ERBB2 and PI3K at the cell membrane leading to activation of both p90^RSK and AKT. This in turn leads to phosphorylation of ERα ser¹⁶⁷, ultimately regulating cell growth via genomic mechanisms. Although several of these complexes have previously been postulated, to our knowledge this is the first demonstration of this phenomenon in a tamoxifen-resistant cell line. …