Tamoxifen resistance by a conformational arrest of the estrogen receptor alpha after PKA activation in breast cancer.

Using a novel approach that detects changes in the conformation of ERalpha, we studied the efficacy of anti-estrogens to inactivate ERalpha under different experimental conditions. We show that phosphorylation of serine-305 in the hinge region of ERalpha by protein kinase A (PKA) induced resistance to tamoxifen. Tamoxifen bound but then failed to induce the inactive conformation, invoking ERalpha-dependent transactivation instead.

Involvement of G1/S cyclins in estrogen-independent proliferation of estrogen receptor-positive breast cancer cells.

Estrogen receptor-mediated transcription is enhanced by overexpression of G1/S cyclins D1, E or A in the presence as well in the absence of estradiol. Excess of G1/S cyclins also prevents the inhibition of transactivation of estrogen receptor (ER) by the pure antiestrogen ICI 182780. Cyclin D1 mediates this transactivation independent of complex formation to its CDK4/6 partner.

Overexpression of cyclin D1 enhances taxol induced mitotic death in MCF7 cells.

Treatment of MCF7 human breast cancer cells with taxol induces G2/M arrest followed by mitotic death. A moderate overexpression of ectopic cyclin D1 accelerated these G2/M associated events and resulted in a reduced clonogenic survival upon taxol treatment. Taxol treatment resulted in elevated expression of p53 and of p21, which was more pronounced and persistent in cyclin D1 overexpressing cells.