Decrease of microRNA-122 Causes Hepatic Insulin Resistance by Inducing Protein Tyrosine Phosphatase 1B, Which Is Reversed by Licorice Flavonoid

Abstract

Protein tyrosine phosphatase 1B (PTP1B) inhibits hepatic insulin signaling by dephosphorylating tyrosine residues in insulin receptor (IR) and insulin receptor substrate (IRS). Micro-RNAs may modulate metabolic functions. In view of the lack of understanding of the regulatory mechanism of PTP1B and its chemical inhibitors, this study investigated whether dysregulation of specific microRNA causes PTP1B-mediated hepatic insulin resistance, and if so, what the underlying basis is. In high-fat-diet-fedmice or hepatocyte models with insulin resistance, the expression of microRNA-122 (miR-122), the most abundant microRNA in the liver, was substantially down-regulated among those predicted to interact with the 3'-untranslated region of PTP1B messenger RNA (mRNA). Experiments using miR-122 mimic and its inhibitor indicated that miR-122 repression caused PTP1B induction. Overexpression of c-Jun N-terminal kinase 1 (JNK1) resulted in miR-122 down-regulation with the induction of PTP1B. A dominant-negative mutant of JNK1 had the opposite effect. JNK1 facilitated inactivating phosphorylation of hepatocyte nuclear factor 4 alpha (HNF4 alpha) responsible for miR-122 expression, as verified by the lack of HNF4 alpha binding to the gene promoter. The regulatory role of JNK1 in PTP1B induction by a decrease in miR-122 level was