Epitranscriptomic Reader HNRNPA2B1 Confers Endocrine Resistance to Breast Cancer Cells

Despite new combination therapies improving survival of breast cancer patients with estrogen receptor α (ER+) tumors, the molecular mechanisms for endocrine-resistant metastatic disease remain unresolved. HNRNPA2B1 (Heterogeneous Nuclear Ribonucleoprotein A2/B1), an RNA binding protein that functions as reader of the N(6)-methyladenosine (m6A) mark in transcribed RNA, is upregualted in tamoxifen- and fulvestrant-resistant, estrogen receptor (ERα)+ LCC9 and LY2 cells derived from MCF-7 endocrine-sensitive luminal A breast cancer cells (1). The miRNA-seq transcriptome of MCF-7 cells transiently overexpressing HNRNPA2B1 (A2B1) identified gene ontology (GO) pathways including “cellular response to steroid hormone signaling and estradiol” and “positive regulation of protein ser/thr kinase activity”. Modest (~ 4.5-fold) stable HNRNPA2B1 overexpression in MCF-7 cells (MCF-7-A2B1) results in ablation of growth inhibition by 4-hydroxytamoxifen (4-OHT) and fulvestrant. This was not due to loss or decrease of ERα; in fact, ERα was increased. Conversely, transient knockdown of HNRNPA2B1 in LCC9 and LY2 cells sensitized the cells to growth inhibition by 4-OHT and fulvestrant while reducing ERα. MCF-7-A2B1 cells showed increased migration, invasion, clonogenicity, soft agar colony size, and markers of epithelial-to-mesenchymal transition. Like LCC9 cells, MCF-7-A2B1 cells showed activation of AKT and MAPK and high androgen receptor (AR). Treatment of MCF-7-A2B1 cells with either PI3K inhibitor Wortmannin or MEK inhibitor PD98059 inhibited soft agar colony formation and reduced colony size. Knockdown of HNRNPA2B1 in MCF-7-A2B1 reduces clonogenicity, but had no effect on the clonogenicity of either LCC9 or LY2 cells. These data suggest a role for HNRNPA2B1 in promoting the initiation of acquired endocrine-resistance by activating ser/thr kinase growth factor signaling pathways. Selective inhibition of HNRNPA2B1 may be a target to prevent acquistion of endocrine therapy resistance, but not treat established metastatic disease. Reference: (1) Klinge CM, Piell KM, Tooley CS, Rouchka EC. HNRNPA2/B1 is upregulated in endocrine-resistant LCC9 breast cancer cells and alters the miRNA transcriptome when overexpressed in MCF-7 cells. Scientific reports 2019; 9:9430