Increased expression of estrogen receptor β 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) β and transforming growth factor β1 was determined in each patient group and quantified against a known reference standard. ER-β 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 β1 was also up-regulated in the tamoxifen-resistant cohort (P = 0.02). The importance of ER-β in tamoxifen resistance was validated using tamoxifen-sensitive and - resistant cell lines, in which it was demonstrated that ER-β mRNA was significantly up-regulated in the resistant cells. These results lend further support to a role for ER-β as a poor prognostic factor in breast cancer.