Increased expression of estrogen receptor beta mRNA in tamoxifen-resistant breast cancer patients.

Tamoxifen is currently the first-line therapy for treatment of hormone-dependent breast cancer. However, despite initial benefits, most patients eventually relapse. Two groups of patients were identified: (a) a tamoxifen-sensitive group (n = 8); and (b) a tamoxifen-resistant group (n = 9). Using reverse transcription-PCR, the relative expression of mRNA for both estrogen receptor (ER) beta and transforming growth factor beta1 was determined in each patient group and quantified against a known reference standard. ER-beta mRNA was significantly up-regulated in the tamoxifen-resistant group as compared with the tamoxifen-sensitive group (P = 0.001 by Fisher's exact test), and, consistent with previous findings, transforming growth factor beta1 was also up-regulated in the tamoxifen-resistant cohort (P = 0.02). The importance of ER-beta in tamoxifen resistance was validated using tamoxifen-sensitive and -resistant cell lines, in which it was demonstrated that ER-beta mRNA was significantly up-regulated in the resistant cells. These results lend further support to a role for ER-beta as a poor prognostic factor in breast cancer.