Knockdown of PTEN decreases expression of estrogen receptor β and tamoxifen sensitivity of human breast cancer cells

Estrogen receptors (ERs) and the PTEN-Akt-mTor pathway are important growth regulators in human breast cancer cells, which both are known to affect response to tamoxifen therapy. Recently it was reported that ER beta activates PTEN expression and tamoxifen sensitivity of human breast cancer cells. In this study we examined whether expression of ER beta in turn might be affected by tumor suppressor PTEN, analyzed the effect of this interaction on tamoxifen response and the co-expression of both genes in human breast cancer samples. After siRNA-mediated PTEN knockdown, Western blot analysis revealed a reduction of ER beta protein expression by 67.2% in MCF-7 cells and by 73.6% in T-47D cells (both p < 0.01), results which could be verified on the mRNA level. In cells with normal PTEN and ER beta status, after 6 days of treatment with 1 mu M 4-OH tamoxifen, E2-driven proliferation was decreased by 64.5% in MCF-7 and by 57.7% in T-47D cells (both p < 0.01). After knockdown of PTEN expression, the same concentration of 4-OH TAM reduced E2-triggered growth only by 34.9% (MCF-7) and by 41.8% (T-47D) (both p < 0.01 vs control siRNA). Importantly, treatment with ER beta agonist DPN (5 nM) significantly decreased the inhibitory effect of a PTEN knockdown on tamoxifen response of both cell lines (p < 0.05). Additionally, Spearman rank association analysis of PTEN and ER beta 1 mRNA levels in 115 normal and malignant breast tissue samples revealed a strong positive correlation of both genes (rho = 0.6085, p < 0.0001). The data of previous studies reporting an important role of ER beta in tamoxifen sensitivity and our findings suggest down-regulation of ER beta triggered by PTEN knockdown contributed to the decreased response of breast cancer cells to tamoxifen observed in this study. Our data also suggest expression of ER beta might be maintained by tumor suppressor PTEN in human breast cancer cells.