Activation of the canonical ER Stress IRE1-XBP1 Pathway by Insulin Regulates Glucose and Lipid Metabolism.

Abstract

Knockout of the transcription factor X-box binding protein (XBP1) is known to decrease liver glucose production and lipogenesis. However, whether insulin can regulate gluconeogenesis and lipogenesis through XBP1 and how insulin activates the inositol-requiring enzyme (IRE1)-XBP1 ER stress pathway remain unexplored. Here we report that in the fed state, insulin-activated kinase AKT directly phosphorylates IRE1 at S724, which in turn mediates the splicing of XBP1u mRNA, thus favoring the generation of the spliced form, XBP1s, in the liver of mice. Subsequently, XBP1s stimulates the expression of lipogenic genes and upregulates liver lipogenesis as previously reported. Intriguingly, we find that fasting leads to an increase in XBP1u along with a drastic decrease in XBP1s in the liver of mice, and XBP1u, not XBP1s, significantly increases PKA-stimulated CRE reporter activity in cultured hepatocytes. Furthermore, we demonstrate overexpression of XBP1u significantly increases cAMP-stimulated expression of rate-limiting gluconeogenic genes, G6pc and Pck1, and glucose production in primary hepatocytes. Re-expression of XBP1u in the liver of mice with XBP1 depletion significantly increases fasted blood glucose levels and gluconeogenic gene expression. These data support an important role of XBP1u in upregulating gluconeogenesis in the fasted state. Taken together, we reveal that insulin signaling via AKT controls the expression of XBP1 isoforms, and that XBP1u and XBP1s function in different nutritional states to regulate liver gluconeogenesis and lipogenesis, respectively.