Role of GALNT2 in the modulation of ENPP1 expression, and insulin signaling and action: GALNT2: a novel modulator of insulin signaling.

Ectonucleotide pyrophosphatase phosphodiesterase 1 (ENPP1) inhibits insulin signaling and action. Understanding the mechanisms underlying ENPP1 expression may help unravel molecular mechanisms of insulin resistance. Recent data suggest a role of ENPP1-3'untraslated region (UTR), in controlling ENPP1 expression. We sought to identify trans-acting ENPP1-3'UTR binding proteins, and investigate their role on insulin signaling. By RNA pull-down, 49 proteins bound to ENPP1-3'UTR RNA were identified by mass spectrometry (MS). Among these, in silico analysis of genome wide association studies and expression profile datasets pointed to N-acetylgalactosaminyltransferase 2 gene (GALNT2) for subsequent investigations. Gene expression levels were evaluated by RT-PCR. Protein expression levels, IRS-1 and Akt phosphorylation were evaluated by Western blot. Insulin receptor (IR) autophosphorylation was evaluated by ELISA. GALNT2 down-regulation increased while GALNT2 over-expression reduced ENPP1 expression levels. In addition, GALNT2 down-regulation reduced insulin stimulation of IR, IRS-1 and Akt phosphorylation and insulin inhibition of phosphoenolpyruvate carboxykinase (PEPCK) expression, a key neoglucogenetic enzyme. Our data point to GALNT2 as a novel factor involved in the modulation of ENPP1 expression as well as insulin signaling and action in human liver HepG2 cells.