Specific inhibition of estrogen receptor alpha function by antisense oligodeoxyribonucleotides

We have tested the effect of a range of antisense oligodeoxyribonucleotides (ODN) directed against the human estrogen receptor alpha (ERalpha) on ERalpha protein expression and function. Antisense ERalpha ODN transfected into the ERalpha-positive human breast carcinoma cell line MCF7-K2 showed variable responses dependent on the oligo used. The most active antisense ODN (oligo 7) decreased the levels of ERa protein by 61% as measured by Western blot analysis. Exogenous 17beta-estradiol (17beta-E2), but not 17alpha-E2, augmented this effect, with a threshold effect at 10(-8) M 17beta-E2. The inhibitory effect of antisense ERa oligo 7 was confirmed by measurement of functional ERalpha protein. 3H-17beta-E2 binding to MCF7 cell extracts was inhibited to approximately 40% of control values in the presence of oligo 7. Antisense-transfected MCF7-K2 cell cultures produced a further 30% binding reduction in the presence of exogenous 17beta-E2. An inhibitory effect on 17beta-E2-dependent cell function was confirmed by the demonstration that ERalpha oligo 7-transfected MCF7-K2 cells failed to exhibit 17beta-E2-stimulated cell proliferation. Exogenous 17beta-E2 enhanced the inhibitory effect of the antisense ODN by increasing ODN transfection efficiency but without ERalpha catabolism via the proteosomal pathway, suggesting an effect of 17beta-E2 on the plasma membrane and the existence of different ERalpha degradation pathways in the MCF7-K2 cell subclone. As 17beta-E2 had no effect on ERalpha protein degradation, we conclude that the observed reduction of ERalpha protein levels is due solely to the presence of the antisense ERalpha ODN. Antisense ERalpha ODN molecules, therefore, may form the basis of effective therapies against ERalpha-dependent malignancies.