Abstract 1402: HNRNPA2B1 increases the serine synthesis pathway in endocrine-resistant breast cancer cells

Abstract

Abstract Despite the limited success of new combination therapies with fulvestrant improving survival of breast cancer patients with metastatic disease after initial response to endocrine therapies based on primary estrogen receptor alpha (ER+) tumors, mechanisms for acquired endocrine resistance remain to be fully elucidated. The RNA binding protein HNRNPA2B1, a reader of epitranscriptomic N(6)-methyladenosine (m6A) in transcribed RNA, is upregulated in endocrine-resistant, ER+ LCC9 and LY2 cells compared to parental MCF-7 endocrine-sensitive luminal A breast cancer cells. HNRNPA2B1 regulates pri- to pre- miRNA processing as well as splicing and transcript stability, depending on interacting proteins and cellular context. The miRNA-seq transcriptome of MCF-7 cells transiently overexpressing HNRNPA2B1 identified gene ontology (GO) pathways including cellular response to steroid hormone signaling and serine family amino acid metabolic processes, i.e., the serine synthesis pathway (SSP). Stable, modest (4.5-fold) overexpression of HNRNPA2B1 in MCF-7 cells (MCF-7-A2B1 cells) phenocopies the endocrine-resistance of LCC9 and LY2 cells including ablation of growth inhibition by 4-hydroxytamoxifen (4-OHT) and fulvestrant. This was not the result of decreased ER alpha; rather, ER alpha, and androgen receptor (AR), were increased. Conversely, transient knockdown of HNRNPA2B1 in LCC9 and LY2 cells sensitized the cells to growth inhibition by 4-OHT and fulvestrant. MCF-7-A2B1 cells, like LCC9 and LY2 cells, have higher expression of phosphoglycerate dehydrogenase (PHGDH) and phosphoserine aminotransferase (PSAT1) transcripts and proteins compared to MCF-7 cells. We identified two miRNAs, miR-424-5p and miR-145-5p downregulated in MCF-7-A2B1 cells that directly target the PSAT1 3'UTR in dual luciferase assays. Lower miR-424-5p and miR-145-5p in endocrine-resistant LCC9 and LY2 correlate with increased PSAT1 and higher PSAT1 is associated with reduced overall and metastasis-free survival in breast cancer patients. The correlation of HNRNPA2B1 and PSAT1 expression was examined by IHC in a breast tumor/tissue microarray. Overall, our data suggest a role for increased HNRNPA2B1 in endocrine-resistance and upregulation of the SSP to promote tumor progression in ER+ breast cancer. Citation Format: Carolyn M. Klinge, Belinda J. Petri, Kellianne M. Piell, Brian C. Clem. HNRNPA2B1 increases the serine synthesis pathway in endocrine-resistant breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1402.