| The discovery of specific coactivators that modulate the transcriptional activity of the estrogen receptor (ER) and the identification of ER cofactors that are amplified and overexpressed in breast tumors led to the hypothesis that some of these factors contribute directly to development of breast cancer. Some of the best-characterized nuclear receptor (NR) coactivators include SRC-family, CBP/p300, DRIP/TRAP/SMCC, p68, and SRA-2. Cofactors that are amplified and overexpressed in breast tumors include PBP, ACS2, SRA-2, and AIB1. Although many of the cofactors bind the same NRs and enhance the transcriptional activity of the same receptors in vitro, their in vivo effects have not been determined. Differential expression patterns of these coactivators in breast tumors have not been extensively reported and the functional role for these coactivators in development of the normal mammary gland and breast tumors is unclear.; In these studies, we have demonstrated that ERAP 140, SRC-1, and SMRT physically and functionally interact with AhR and ER signaling pathways. Coactivation of ER by SRCs, p68, and p300 was observed in COS-1 and CHO-1 cells; however, only SRC-2 and p68 enhanced transactivation of ER/ERE-dependent promoter in MCF-7 cells and SRC-2 and p300 enhanced activity in MDA-MB-231 cells. Coactivation of ER by NR coactivators is cell context-dependent and high expression of NR coactivators in breast cancer cells does not correlate with their coactivation of ER in transient transfection experiment as previous reported in other cancer cells. Cotransfection with pSp13 and several SRCs, p300, SRA-2, or p68 did not enhance ER/Sp1 action in MCF-7 cells but tended to exhibit inhibitory interactions. In contrast, DRIP 130 enhanced ER/Sp1 action in MCF-7 and ZR-75 breast cancer cells and physically interacted with ER/Sp1 complex using Sp1 antibodies in coimmunoprecipitation assays. In addition, the effects of growth factors on ER/Sp1 action in breast cancer cells may be dependent on the AF-1 domain of ER and regulated by AF-1-interacting proteins. We demonstrated that SRCs inhibited (≥50%) growth factor-induced transactivation indicating the p160 family coactivators may not be involved in growth factor activation of ER/Sp1 in breast cancer cells. |
