Quantification of estrogen receptor α and β expression in sporadic breast cancer

The recent cloning of a second estrogen receptor (ER), designated ER beta, has prompted a reevaluation of the role of ERs in breast cancer. We have developed and validated a real-time RT - PCR assay to quantify ER alpha and ER beta gene expression at the mRNA level in a series of 131 patients with unilateral invasive primary breast cancer. Although ER beta expression showed wide variations in tumor tissues, its range (nearly three orders of magnitude) was smaller than that of ER alpha (nearly four orders of magnitude), suggesting that ER beta is more tightly controlled than ER alpha. We observed a negative correlation between ER alpha and ER beta expression. 'ER alpha -negative' tumors (containing very low ER alpha mRNA levels) were associated with SBR histopathological grade III, RB1 underexpression and ERBB2 overexpression, confirming that ER alpha negativity delineates poorly differentiated tumors. The amount of ER alpha mRNA (but not that of ER beta mRNA) increased with age and was consequently higher in postmenopausal patients' tumors. Expression of ER alpha (but not that of ER beta) also correlated strongly with progesterone receptor (PR) and PS2 expression, suggesting that ER alpha has stronger transcriptional activity than ER beta towards genes containing an ERE (estrogen response element) in their promoters. Interestingly, we found a negative correlation between the expression of ER beta (but not ER alpha) and CCND1, which contains an AP1 element but not an ERE in its promoter. Taken together, these data confirm that ER alpha and ER beta play different roles in breast cancer, partly by mediating the transcription of various genes via different types of DNA enhancer. PR and PS2 seem to be mainly ER alpha -responsive genes, whereas CCND1 may be mainly ER beta -responsive. Our findings also underline the need for a reliable method, providing full range of quantitative values, to determine ER alpha and ER beta status in the clinical setting.