RNA binding protein HuD regulates the biosynthesis of glucagon-like peptide 1 in intestinal L-cells.

We investigated HuD (ELAVL4), a neuronal RNA-binding protein with emerging endocrine functions, as a regulator of GLP-1 biogenesis in intestinal L-cells. Using absolute qPCR and immunofluorescence, we found that HuD was significantly expressed in the GLUTag cells derived from mice intestinal L-cell line and was co-localized with GLP-1-positive cells in the small intestine of mice. RNA interference-mediated HuD knockdown reduced both GLP-1 immunoreactivity and levels. Mechanistically, RNA immunoprecipitation and biotinylated 3'UTR pull-down demonstrated that HuD binds to the mRNAs of the Gcg and Pcsk1 (PC1/3) genes. The levels of the proglucagon (∼17 kDa) and PC1/3 proteins decreased without significant changes in their mRNAs, suggesting post-transcriptional control. Metabolic stress converged on this module: palmitate decreased proglucagon/PC1/3 in GLUTag cells, and a high-fat diet reduced GLP-1 and PC1/3 signals in the mouse intestine. Interestingly, zinc sulfate partially restored proglucagon and PC1/3 levels following palmitate treatment. These findings reveal a HuD-Gcg/Pcsk1 axis is crucial for GLP-1 biogenesis and which is susceptible to lipotoxic stress.