Progesterone Receptor A and B Isoforms in the Human Breast and Its Disorders

Two different isoforms of progesterone receptor (PR), PRA and PRB, are expressed in target tissues at comparable levels. In this study, we first examined PRA and PRB immunoreactivity in human breast cancer and various intraductal proliferative epithelial lesions, and correlated these findings with clinicopathologic parameters. We then examined mRNA expression of PRA and PRB in six cases of invasive ductal carcinoma using RT‐PCR. Immunoreactivity for both PRA and PRB was positive in the great majority of proliferative disease without atypia (PDWA) (85% for PRA and 96% for PRB) and atypical ductal hyperplasia (ADH) (100% for PRA and 100% for PRB), but the ratio of immunopositive cases and immunohistochemical (IHC) scores was significantly smaller in ductal carcinoma in situ (DCIS) (65% for PRA and 75% for PRB) and invasive ductal carcinoma (IDC) (66% for PRA and 55% for PRB) than in PDWA and ADH. There was a significant positive correlation between IHC scores for PRA and estrogen receptor α (ERα) in IDC, DCIS and ADH but not between PRB and ERα. In IDC, both PRA and PRB IHC scores were significantly associated with histological grade, but there was no association between PRA or PRB status and lymph node involvement, tumor size, or prognosis of the patients. The expression of mRNAs for both PRA and PRB was detected in all six cases of IDC examined. These results suggest that both PRA and PRB are strongly associated with ERa in human breast and this relation may be disturbed in breast cancer.