AhR ligand aminoflavone suppresses α6-integrin-Src-Akt signaling to attenuate tamoxifen resistance in breast cancer cells

More than 40% of patients with luminal breast cancer treated with endocrine therapy agent tamoxifen demonstrate resistance. Emerging evidence suggests tumor initiating cells (TICs) and aberrant activation of Src and Akt signaling drive tamoxifen resistance and relapse. We previously demonstrated that aryl hydrocarbon receptor ligand aminoflavone (AF) inhibits the expression of TIC gene α6-integrin and disrupts mammospheres derived from tamoxifen-sensitive breast cancer cells. In the current study, we hypothesize that tamoxifen-resistant (TamR) cells exhibit higher levels of α6-integrin than tamoxifen-sensitive cells and that AF inhibits the growth of TamR cells by suppressing α6-integrin-Src-Akt signaling. In support of our hypothesis, TamR cells and associated mammospheres were found to exhibit elevated α6-integrin expression compared with their tamoxifen-sensitive counterparts. Furthermore, tumor sections from patients who relapsed on tamoxifen showed enhanced α6-integrin expression. Gene expression profiling from the TCGA database further revealed that basal-like breast cancer samples, known to be largely unresponsive to tamoxifen, demonstrated higher α6-integrin levels than luminal breast cancer samples. Importantly, AF reduced TamR cell viability and disrupted TamR mammospheres while concomitantly reducing α6-integrin messenger RNA and protein levels. In addition, AF and small interfering RNA against α6-integrin blocked tamoxifen-stimulated proliferation of TamR MCF-7 cells and further sensitized these cells to tamoxifen. Moreover, AF reduced Src and Akt signaling activation in TamR MCF-7 cells. Our findings suggest elevated α6-integrin expression is associated with tamoxifen resistance and AF suppresses α6-integrin-Src-Akt signaling activation to confer activity against TamR breast cancer.

Abstract 4177: Anticancer agent Aminoflavone restores the expression of tumor suppressor miRNA 26a and inhibits putative stemness biomarker α6-integrin in Tamoxifen resistant cells

Despite the efficacy of anti-estrogen agent Tamoxifen, commonly used to treat patients with estrogen receptor positive (ER+) tumors, up to 40% of patients experience recurrence. Breast tumor-initiating cells (TICs), or breast cancer stem cells, exhibit Tamoxifen resistance and contribute substantially to recurrence. We recently demonstrated that investigational anticancer agent Aminoflavone (AF) disrupts mammospheres (in vitro clusters of cells enriched with TICs) by thwarting the expression of α6-integrin. In the current study, we found AF potently inhibited Tamoxifen resistant (TamR) cell growth and blocked Tamoxifen-mediated stimulation of TamR cell proliferation using the Alamar Blue assay. qPCR analyses revealed α6-integrin expression was significantly elevated in ER+ breast cancer cell models of acquired and de novo Tamoxifen resistance relative to Tamoxifen sensitive cells. In particular, AF decreased the expression of both A and B variants of α6-integrin, the B variant being essential for TIC function. Western blotting revealed AF reduced total α6-integrin expression in TamR cells. Furthermore, we found that an anti-α6-integrin blocking antibody sensitized TamR cells to the active Tamoxifen metabolite, 4-hydroxy-Tamoxifen, and enhanced the efficacy of AF in these cells. AF also reduced the protein expression of p-Src, a downstream target of α6-integrin that is linked to Tamoxifen resistance and decreased breast cancer survival. In addition, miRNA sequencing of Tamoxifen sensitive and TamR mammospheres revealed differential expression of several miRNAs. Notably, miR-26a expression was down-regulated 2-fold in TamR mammospheres compared to Tamoxifen sensitive mammospheres and AF restored miR-26a expression 5-fold in TamR mammospheres. Using miRNA target prediction algorithms TargetScan and PicTar, we found miR-26a binding sites on the α6-integrin promoter. Taken together, our data suggest that AF re-expresses tumor suppressor miR-26a and inhibits the α6-integrin/Src signaling axis to reduce TIC capacity and counteract Tamoxifen resistance.

Citation Format: Petreena Campbell, Leah Rowland, Anna Opoku-Agyeman, Nichole Mavingire, Ubaldo Soto, Gayathri Nagaraj, Yonghong Zhang, Sean (Xin) Chen, Charles Wang, Eileen Brantley. Anticancer agent Aminoflavone restores the expression of tumor suppressor miRNA 26a and inhibits putative stemness biomarker α6-integrin in Tamoxifen resistant cells [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 4177. doi:10.1158/1538-7445.AM2017-4177