Acquired endocrine-resistance occurs in ~ 40% of estrogen receptor α (ERα)+ breast cancer patients after tamoxifen (TAM) or aromatase inhibitor therapy. MicroRNAs (miRs) are dysregulated in breast cancer, but their contribution to endocrine-resistance is not yet completely understood. Microarray analysis of miRNAs in TAM-sensitive MCF-7 versus TAM-resistant LY2 breast cancer cells (derived from MCF-7 cells) identified differential regulation of miR-29b-1 and miR-29a. miR-29a and miR-29b-1, which are co-transcribed as a single primary-miR, were repressed by TAM in MCF-7 and stimulated by TAM in LY2 cells. We confirmed these observations by real time qPCR in MCF-7 and LY2 cells and observed that 4-OH-TAM (4-OHT) also downregulated miR-29b-1/a in LCC2 and LCC9 endocrine-resistant cells. These changes were at the primary transcriptional level. The role of these two miRs in endocrine-resistant breast cancer is unknown. We tested the hypothesis that the TAM-stimulated increase in miR-29b-1/a promotes endocrine-resistance by downregulating targets that contribute to TAM’s antiproliferative/pro-apoptotic activity. Our goals were to: 1) Determine if ERα was responsible for TAM regulation of miR-29b-1/a expression; 2) Determine the functional role(s) of miR-29b-1/a; 3) Identify specific miR-29b-1/a target genes that are differentially regulated by 4-OHT in MCF-7 versus LCC9 and LY2 cells. Knockdown of ERα blocked 4-OHT inhibition of miR-29b-1/a in MCF-7 and 4-OHT simulation of miR-29-b-1/a transcription in LCC9 and LY2 cells. Transient over-expression of miR-29b-1/a inhibited proliferation of MCF-7, LCC9, and LY2 cells and decreased migration and colony formation of LY2 cells. Repression of miR-29b-1/a has no significant effect on MCF-7, LCC9, or LY2 cell proliferation. Repression of miR-29-b-1/a did not sensitize the TAM-resistant cells to growth inhibition by 4-OHT. We observed that the bona fide miR-29 target gene, DICER1, was inhibited by 4-OHT in LY2 cells and anti-miR29b-1 or anti-miR-29a decreased the suppression of DICER1 by 4-OHT. These data suggest that 4-OHT-ERα increases miR-29b-1/a which target and downregulate DICER 1 in endocrine resistant breast cancer cells. Future studies will identify other targets of miR-29b-1/a to better understand the pathways leading to acquired endocrine resistance.

Citation Format: Penn Muluhngwi, Carolyn M. Klinge. Tamoxifen differentially regulates miR-29b-1 and miR-29a depending on endocrine sensitivity in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3428. doi:10.1158/1538-7445.AM2017-3428