Competitive Regulation of Nucleolin Expression by HuR and miR-494

Abstract

ABSTRACTThe RNA-binding protein (RBP) nucleolin promotes the expression of several proliferative proteins. Nucleolin levels are high in cancer cells, but the mechanisms that control nucleolin expression are unknown. Here, we show that nucleolin abundance is controlled posttranscriptionally via factors that associate with its 3′ untranslated region (3′UTR). The RBP HuR was found to interact with the nucleolin (NCL) 3′UTR and specifically promoted nucleolin translation without affecting nucleolin mRNA levels. In human cervical carcinoma HeLa cells, analysis of a traceableNCL3′UTR bearingMS2RNA hairpins revealed thatNCLRNA was mobilized to processing bodies (PBs) after silencing HuR, suggesting that the repression of nucleolin translation may occur in PBs. Immunoprecipitation of MS2-taggedNCL3′UTR was used to screen for endogenous repressors of nucleolin synthesis. This search identified miR-494 as a microRNA that potently inhibited nucleolin expression, enhancedNCLmRNA association with argonaute-containing complexes, and inducedNCLRNA transport to PBs. Importantly, miR-494 and HuR functionally competed for modulation of nucleolin expression. Moreover, the promotion of cell growth previously attributed to HuR was due in part to the HuR-elicited increase in nucleolin expression. Our collective findings indicate that nucleolin expression is positively regulated by HuR and negatively regulated via competition with miR-494.