Distinct expressions of microRNAs that directly target estrogen receptor α in human breast cancer

Estrogen receptor (ER) α is essential for estrogen-dependent growth, and its level of expression is a crucial determinant of response to endocrine therapy and prognosis in ERα-positive breast cancer. Breast cancer patients show a wide range of ERα expression levels and the levels of expression in individual patients change during disease progression and in response to systemic therapies. However, little is known about how the expression of ERα in human breast cancer is regulated. Recently, several microRNAs (miRNAs) that directly target ERα have been identified, and we previously demonstrated that miR-206 expression was downregulated in ERα-positive human breast cancer. In this study, expression levels of miRNAs that directly target ERα, including miR-18a, miR-18b, miR-22, miR-193b, miR-221/222 and miR-302c, were analyzed in human breast cancer samples by quantitative reverse transcription-PCR analysis. Correlations between the expression levels of these miRNAs and clinicopathological factors, including prognosis, were analyzed. miR-18a expression was much higher in ERα-negative than in ERα-positive tumors ( P