RNA Binding Protein FXR1-miR301a-3p axis contributes to p21WAF1 degradation in oral cancer.

RNA-binding proteins (RBPs) associate with the primary, precursor, and mature microRNAs, which in turn control post-transcriptional gene regulation. Here, by small RNAseq, we show that RBP FXR1 controls the expression of a subset of mature miRNAs, including highly expressed miR301a-3p in oral cancer cells. We also confirm that FXR1 controls the stability of miR301a-3p. Exoribonuclease PNPT1 degrades miR301a-3p in the absence of FXR1 in oral cancer cells, and the degradation is rescued in the FXR1 and PNPT1 co-knockdown cells. In vitro, we show that PNPT1 is unable to bind and degrade the miRNA once the FXR1-miRNA complex forms. Both miR301a-3p and FXR1 cooperatively target the 3'-UTR of p21 mRNA to promote its degradation. Thus, our work illustrates the unique role of FXR1 that is critical for the stability of a subset of mature miRNAs or at least miR301a-3p to target p21 in oral cancer. Author summary RNA-binding protein FXR1 is overexpressed in multiple cancers and appears to be involved in poor patient survival. FXR1 has been previously shown to be an oncogene in head and neck and lung squamous cell carcinoma. FXR1 targets tumor suppressor genes, including p21, to promote the growth and proliferation of cancer cells. However, how FXR1 recognizes tumor suppressor genes and block their expression in cancer cells has never been established. Through FXR1-mediated small RNA sequencing, we unexpectedly found that FXR1 stabilizes miR301a-3p, which in turn targets tumor suppressor p21 and blocks its expression in oral cancer cells. miR301a-3p level goes down in the absence of FXR1, increasing p21 to suppress the growth of oral cancer cells. We provide evidence that miR301a-3p expression is rescued by downregulation of both FXR1 and exoribonuclease PNPT1 in oral cancer cells suggesting that FXR1 acts as a stabilizing factor for miR301a-3p against PNPT1. Together, our observations explain why over-expression of FXR1 and miR301a-3p in the oral cancer patient cohort lead to the downregulation of p21 signaling. These findings indicate that FXR1 and miR301a-3p together contribute towards targetting tumor suppressors in oral cancer patients.