Estrogen Receptor-b Messenger RNA Expression in Human Breast Tumor Biopsies: Relationship to Steroid Receptor Status and Regulation by Progestins

When the level of estrogen receptor (ER)-b mRNA in tumors, determined by reverse transcription-PCR, was assessed according to either ER status or PR status alone, determined by ligand binding assays, the level of ER-b mRNA was significantly lower in PR1 tumors compared with PR2 tumors (P 5 0.036), and no association with ER status was found. Subgroup analysis showed that ER-b mRNA expression in ER1/PR1 breast tumors was significantly less than in ER1/PR2 (P 5 0.009), ER2/PR1 (P 5 0.029), and ER2/PR2 (P 5 0.023) groups. Interestingly, the ER-b mRNA expression was specifically decreased by progestin in T-47D breast cancer cells. The data suggest the possibility that expression of ER-b in human breast tumors is a marker of endocrine therapy responsiveness.