Loss of an estrogen receptor isoform (ER alpha Delta 3) in breast cancer and the consequences of its reexpression: Interference with estrogen-stimulated properties of malignant transformation

Comparison of mRNA ratios of a non-DNA-binding estrogen receptor (ERalpha) isoform, missing exon 3 (ER(alpha)Delta 3), to the full-length ERalpha, in normal breast epithelium to that in primary breast cancers and breast cancer cell lines revealed a 30-fold reduction of this ratio in cancer cells (P < 0.0001). To test what functions may have been affected by the lass of ER(alpha)Delta 3, stable clones of MCF-7 cells expressing ectopic ER(alpha)Delta 3 protein, at the range of physiological ERalpha, were generated. In vector-transfected controls the ER(alpha)Delta 3-mRNA and protein were less than 10% white in the ER(alpha)Delta 3-expressing clones, ER(alpha)Delta 3-mRNA and protein ranged from 36-76% of the total ERalpha. Estrogen (E-2) stimulated the expression of pS2-mRNA in pMV7 vector control cells, but the simulation was reduced by up to 93% in ER(alpha)Delta 3-expressing clones. In addition, several properties associated with the transformed phenotype were also strongly affected when ER(alpha)Delta 3 protein was reexpressed. Compared with vector-transfected control cells, the saturation density of the ER(alpha)Delta 3-expressing clones was reduced by 50-68%, while their exponential growth rate was only slightly (14.5 +/- 5%) lower. The in vivo invasiveness of the ER(alpha)Delta 3-expressing cells was significantly reduced (P = 0.007) by up to 79%. E-2 stimulated anchorage-independent growth of the pMV7 vector control cells, but reduced it to below baseline levels in ER(alpha)Delta 3 clones. The reduction of the pS2 response to E-2 in the ER(alpha)Delta 3-expressing clones and the E-2 block of anchorage-independent growth to below baseline were more pronounced than expected from the dominant negative function of ER(alpha)Delta 3. These observations suggest that E-2 may activate an additional ER(alpha)Delta 3-dependent inhibitory pathway. The drastic reduction of ER(alpha)Delta 3 to ERalpha ratio in breast cancer, and the fact that when present in breast cancer cells this isoform leads to a suppression, rather than enhancement, of the transformed phenotype by E-2 suggests that the regulation of ERalpha-mRNA splicing may need to be altered for the breast carcinogenesis to proceed.