Loss of Estrogen-Regulated <i>MIR135A1</i> at 3p21.1 Promotes Tamoxifen Resistance in Breast Cancer.

Abstract

The dysregulation of miRNAs has been increasingly recognized as a critical mediator of cancer development and progression. Here, we show that frequent deletion of the <i>MIR135A1</i> locus is associated with poor prognosis in primary breast cancer. Forced expression of miR-135a decreased breast cancer progression, while inhibition of miR-135a with a specific miRNA sponge elicited opposing effects, suggestive of a tumor suppressive role of miR-135a in breast cancer. Estrogen receptor alpha (ERα) bound the promoter of <i>MIR135A1</i> for its transcriptional activation, whereas tamoxifen treatment inhibited expression of miR-135a in ERα⁺ breast cancer cells. miR-135a directly targeted <i>ESR1, ESRRA,</i> and <i>NCOA1</i>, forming a negative feedback loop to inhibit ERα signaling. This regulatory feedback between miR-135a and ERα demonstrated that miR-135a regulated the response to tamoxifen. The tamoxifen-mediated decrease in miR-135a expression increased the expression of miR-135a targets to reduce tamoxifen sensitivity. Consistently, miR-135a expression was downregulated in ERα⁺ breast cancer cells with acquired tamoxifen resistance, while forced expression of miR-135a partially resensitized these cells to tamoxifen. Tamoxifen resistance mediated by the loss of miR-135a was shown to be partially dependent on the activation of the ERK1/2 and AKT pathways by miR-135a-targeted genes. Taken together, these results indicate that deletion of the <i>MIR135A1</i> locus and decreased miR-135a expression promote ERα⁺ breast cancer progression and tamoxifen resistance.<b>Significance:</b> Loss of miR-135a in breast cancer disrupts an estrogen receptor-induced negative feedback loop, perpetuating disease progression and resistance to therapy.<b>Graphical Abstract:</b> http://cancerres.aacrjournals.org/content/canres/78/17/4915/F1.large.jpg <i>Cancer Res; 78(17); 4915-28. ©2018 AACR</i>.