Systematic Synergy of Glucose and GLP-1 to Stimulate Insulin Secretion Revealed by Quantitative Phosphoproteomics

GLP-1 synergizes with glucose in regulating pancreatic β-cell function, including facilitating β-cell survival and insulin secretion. Though it has been widely accepted that phosphorylation is extremely important in regulating β-cell functions, our knowledge to the global mechanism is still limited. Here we performed a quantitative phosphoproteomics study to systematically present the synergistic regulation of INS-1E cell phosphoproteome mediated by glucose and GLP-1. We generated the largest pancreatic β-cell phosphoproteome by identifying 25,327 accurately localized phosphorylation sites on 5,389 proteins. Our results discovered several novel kinases regulated by glucose, GLP-1 or their synergism, and some of these kinases might act as downstream molecules of GLP-1 mediated PKA signaling cascade. A few phosphosites were regulated by both GLP-1 and glucose alone, and these target proteins were highly related to their biological function on pancreatic β-cells. Finally, we found glucose and GLP-1 executed their synergistic effect at multiple levels, especially at pathway level. Both GLP-1 and glucose participated in regulating every single step of the secretion pathway, and systematically synergized their effects in inducing insulin secretion.