Abstract #4723: MicroRNA-221/222 confer breast cancer resistance to fulvestrant by targeting multiple oncogenic activities

Breast cancer is the leading cause of female death worldwide and will afflict 1 of 8 U.S. women in their lifetime. One commonly used breast cancer therapy, fulvestrant, has potent activity against hormone-sensitive breast cancers. However, almost all patients eventually develop resistance to fulvestrant, and the underlying mechanism(s) of loss of drug response remains unknown. We had previously generated a fulvestrant-resistant cell line (MCF7-F) highly characteristic of this clinical phenomenon (Cancer Res, 66, 2006). MicroRNAs, 19-22-nucleotide non-coding regulatory RNAs, have recently been implicated in breast cancer, and we subjected MCF7-F cells to microRNA expression analysis using a 612-feature custom microarray. From those analyses, we determined that two specific microRNAs, miR-221 and miR-222, were significantly upregulated by 9-fold and 6-fold respectively in MCF7-F cells (as compared to the parental MCF7 cell line). To further assess the role of miR-221/222 in MCF7-F cells, we downregulated miR-221/222 by using chemically stable, antisense oligoribonucleotides (\#8220;antagomirs\#8221;) against those specific microRNA sequences, demonstrating that miR-221/222 antagomir treatment significantly decreased cell proliferation by 40%. In addition, the cyclin-dependent kinase inhibitor (and tumor suppressor) p27(Kip1) (CDKN1B, a known target protein for miR-221/222), which is downregulated in MCF7-F, was upregulated after antagomir treatment, with cell cycle analyses demonstrating G1 cell cycle arrest. Based on this evidence, we hypothesize that miR-221/222 upregulation contributes to the development of fulvestrant resistance, leading to loss of p27(Kip1), which subsequently facilitates estrogen-independent cell cycle progression. However, as any single microRNA likely targets numerous genes to influence the overall malignant phenotype, we sought to identify additional miR-221/222 targets in MCF7-F cells. MCF7-F cells were treated with miR-221/222 antagomirs, and then subjected to Affymetrix gene expression profiling and pathway enrichment analysis. Several well known signaling pathways appeared to be regulated by miR-221/222, including p53, TGF-beta, Notch and MAPK. We are currently validating the regulatory role of miR-221/222 in these pathways and investigating their role in the development of fulvestrant resistance. Overall, the findings indicate that miR-221 and miR-222 may represent promising therapeutic targets for fulvestrant resensitization in patients with advanced breast cancer. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 4723.