Decreased Chicken Ovalbumin Upstream Promoter Transcription Factor II Expression in Tamoxifen-Resistant Breast Cancer Cells

Abstract

Tamoxifen (TAM) is successfully used for the treatment and prevention of breast cancer. However, many patients that are initially TAM responsive develop tumors that are antiestrogen/TAM resistant (TAM-R). The mechanism behind TAM resistance in estrogen receptor A (ERA)-positive tumors is not understood. The orphan nuclear receptor chicken ovalbumin upstream promoter transcription factor (COUP-TF)-I interacts directly with 4-hydroxytamoxifen (4-OHT)-and estradiol (E 2 )-occupied ERA, corepressors NCoR and SMRT, and inhibit E 2 -induced gene transcription in breast cancer cells. Here we tested the hypothesis that reduced COUP-TFI and COUP-TFII correlate with TAM resistance. We report for the first time that COUP-TFII, but not COUP-TFI, is reduced in three antiestrogen/TAM-R cell lines derived from TAMsensitive (TAM-S) MCF-7 human breast cancer cells and in MDA-MB-231 cells compared with MCF-7. ERA and ERB protein expression was not different between TAM-S and TAM-R cells, but progesterone receptor (PR) was decreased in TAM-R cells. Further, E 2 increased COUP-TFII transcription in MCF-7, but not TAM-R, cells. Importantly, reexpression of COUP-TFII in TAM-S cells to levels comparable to those in MCF-7 was shown to increase 4-OHT-mediated growth inhibition and increased apoptosis. Conversely, knockdown of COUP-TFII in TAM-S MCF-7 cells blocked growth inhibitory activity and increased 4-OHT agonist activity. 4-OHT increased COUP-TFII-ERA interaction f2-fold in MCF-7 cells. COUP-TFII expression in TAM-R cells also inhibited 4-OHT-induced endogenous PR and pS2 mRNA expression. These data indicate that reduced COUP-TFII expression correlates with acquired TAM resistance in human breast cancer cell lines and that COUP-TFII plays a role in regulating the growth inhibitory activity of TAM in breast cancer cells.

COUP-TFII and its interacting proteins in breast cancer and endocrine resistance.

“…These findings indicate a function for COUP-TFII in inhibiting tumor progression. A positive correlation with ERa is consistent with a previous report that siRNA knockdown of ERa in MCF-7 breast cancer cells decreased COUP-TFII expression and treatment with estradiol increased the expression of COUP-TFII [116]. ERa is a positive prognostic factor in breast tumors and is the target of endocrine-targeted cancer therapeutics such as the selective estrogen receptor modulators (SERMs) tamoxifen and raloxifene [48].…”

“…In contrast, COUP-TFII decreased cell motility when transfected into L Y2 tamoxifen-resistant breast cancer cells, while having no significant effect on invasion [116].…”

“…COUP-TFII, but not COUP-TFI, is reduced in tamoxifen-resistant human breast cancer cells, and re-expression of COUP-TFII can restore tamoxifen-sensitivity [116]. As ERa expression is important in keeping breast cancer cells responsive to treatment, the correlation of COUP-TFII and ERa further demonstrates a beneficial role for COUP-TFII, highlighting its potential importance in maintaining differentiation and endocrine sensitivity.…”

“…However, this has not been empirically tested and it is worth noting that the N-terminus is divergent ( Figure 2) and immunoprecipitation studies indicate differences in proteins interacting with COUP-TFI [114] and COUP-TFII [115], although, again, this has not been [116]. This review will focus specifically on COUP-TFII.…”

Dysregulation of microRNA expression in acquired endocrine-resistant breast cancer.

“…Whole cell lysates were prepared and western blots were performed as described [277]. Nuclear extracts (NE) were prepared using the NE-PER kit from Chemiluminescent bands on the PVDF membranes were visualized on a Kodak…”