We developed a competitive PCR assay based on coamplification of the two ERs in target nucleotide sequences displaying a high homology (exons 3 and 4). Coamplification experiments with varying amounts of plasmids containing ER-alpha and -beta cDNAs showed that this assay was reliable for discriminating as Little as a 2-fold difference in the initial ER-alpha:ER-beta cDNA ratio. The relative expression of ER-alpha compared with ER-beta mRNAs was studied in human ovarian cancer cell lines (n = 5) and in normal ovaries (n = 6), then in human benign and malignant tumor samples including ovarian cysts (n = 24), borderline tumors (n = 3), and cancers (n = 10), In normal ovaries, ER-beta mRNA was the predominant ER form, whereas in ovarian cancer cell lines ER-alpha mRNA was markedly increased as compared with ER-beta. In benign and borderline tumors, ER-beta mRNA was detected in 78% of tumors, whereas ER-alpha mRNA was detected in 29%, In ovarian carcinomas, both ER-alpha and -beta mRNAs were expressed in 80% of tumors. The ER-alpha:ER-beta mRNA ratio was >1 in only one cyst sample (4%), In contrast, the ER-alpha:ER-beta mRNA ratio was markedly increased in ovarian cancers because 60% showed an ER-alpha: ER-beta mRNA >1, In situ hybridization experiments showed overlapping tissular distribution of ER-beta and -alpha expression in cancers and cysts, with a main localization in the epithelium and only a low level of expression in stromal cells.

In summary, we found an increase in the ER-alpha:ER-beta mRNA ratio in ovarian carcinomas as compared with normal ovaries and cysts. These data suggest that overexpression of ER-alpha relative to ER-beta mRNA may be a marker of ovarian carcinogenesis.