Abstract P3-09-05: Let-7 microRNAs Regulate a Novel Variant of Estrogen Receptor Alpha, ER-≥36, Nongenomic Estrogen Signal Pathway, and Tamoxifen Resistance in Breast Cancer

Abstract Background: MicroRNAs (miRNAs) have important regulatory functions in breast cancer tumorigenesis. We previously found that let-7 miRNAs were significantly downregulated in formalin-fixed paraffin-embedded (FFPE) breast cancer tissues. Material and Methods: QRT-PCR was used to check let-7 family miRNAs in FFPE tissues and breast cancer cell lines as well as expression of estrogen receptor (ER)-≥36, a variant of ER-≥66, after let-7 miRNA transfecton. Immunoblot analysis was employed to check protein expression in FFPE tissue and breast cancer cell lines. Luciferase reporter assay was used to detect direct regulation of let-7 miRNA on ER-α expression. MTT assay was applied for cell proliferation after transfection of let-7 miRNAs. Results: We found that there was an inverse correlation between the expression of ER-≥36 and several members of let-7 family miRNAs in the FFPE tissue set. Let-7 miRNA sequences match sequence in the 3’ untranslated region (3’ UTR) of ER-≥36, indicating ER-≥36 may be a target of let-7. Cotransfection of let-7 mimics with ER-≥36 3’ UTR luciferase construct decreased the activity of reporter gene. Conversely, let-7 inhibitors enhanced the reporter gene activity. Transfection of let-7 mimics inhibited both the mRNA and protein levels of ER-≥36 and further inhibited the non-genomic estrogen pathway mediated by ER-≥36 in MDA-MB-231 cells. On the contrary, transfection of let-7 inhibitors enhanced the ER-≥36 expression at both mRNA and protein levels in 184A1 cells. The high expression of ER-≥36 in tamoxifen resistant MCF7 cells can be inhibited by transfection of let-7 mimics and sensitivity toward tamoxifen is enhanced. Conclusion: Let-7 miRNAs regulate the expression of ER-≥36, which is involved in non-genomic estrogen pathway and tamoxifen resistance. Let-7 could be therapeutic target for breast cancer treatment. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P3-09-05.