Using a multiplex reverse transcription-PCR assay, we compared the relative expression of estrogen receptor (ER) α and ER-β mRNA between adjacent samples of normal breast tissue and matched primary breast tumors obtained from 18 different patients. Within this cohort, 7 tumors were ER negative, and 11 tumors were ER positive, as determined by the ligand binding assay. No differences in the ratio of ER-α:ER-β expression were observed in the ER-negative cohort. However, in the ER-positive cohort, a significantly (P < 0.02) higher ER-α:ER-β ratio was observed in the tumor compared with that of the normal tissue component. Our data revealed that the increase in the ER-α:ER-β ratio was due primarily to a significant (P < 0.05) increase in ER-α mRNA expression in conjunction with a lower ER-β mRNA expression in the tumor compared with that of the normal compartment in some, but not all, ER-positive cases. These results suggest that the role of ER-α- and ER-β-driven pathways and/or their interaction change during breast tumorigenesis.

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Supported by grants from the Canadian Breast Cancer Research Initiative and the U. S. Army Medical Research and Material Command. The Manitoba Breast Tumor Bank is supported by funding from the National Cancer Institute of Canada. P. H. W. is a Medical Research Council of Canada clinician-scientist. L. C. M. is a Medical Research Council of Canada scientist. E. L. is a recipient of a U. S. Army Medical Research and Material Command Postdoctoral Fellowship.

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