IRS-2 Partially Compensates for the Insulin Signal Defects in IRS-1−/− Mice Mediated by miR-33

Abstract

Insulin signaling is coordinated by insulin receptor substrates (IRSs). Many insulin responses, especially for blood glucose metabolism, are mediated primarily through <i>Irs-1</i> and <i>Irs-2</i>. <i>Irs-1</i> knockout mice show growth retardation and insulin signaling defects, which can be compensated by other IRSs <i>in vivo</i>; however, the underlying mechanism is not clear. Here, we presented an <i>Irs-1</i> truncated mutated mouse (<i>Irs-1</i>^-/-) with growth retardation and subcutaneous adipocyte atrophy. <i>Irs-1</i>^-/- mice exhibited mild insulin resistance, as demonstrated by the insulin tolerance test. Phosphatidylinositol 3-kinase (PI3K) activity and phosphorylated Protein Kinase B (PKB/AKT) expression were elevated in liver, skeletal muscle, and subcutaneous adipocytes in <i>Irs-1</i> deficiency. In addition, the expression of IRS-2 and its phosphorylated version were clearly elevated in liver and skeletal muscle. With miRNA microarray analysis, we found miR-33 was down-regulated in bone marrow stromal cells (BMSCs) of <i>Irs-1</i>^-/- mice, while its target gene <i>Irs-2</i> was up-regulated <i>in vitro</i> studies. In addition, miR-33 was down-regulated in the presence of <i>Irs-1</i> and which was up-regulated in fasting status. What's more, miR-33 restored its expression in re-feeding status. Meanwhile, miR-33 levels decreased and <i>Irs-2</i> levels increased in liver, skeletal muscle, and subcutaneous adipocytes of <i>Irs-1</i>^-/- mice. In primary cultured liver cells transfected with an miR-33 inhibitor, the expression of IRS-2, PI3K, and phosphorylated-AKT (p-AKT) increased while the opposite results were observed in the presence of an miR-33 mimic. Therefore, decreased miR-33 levels can up-regulate IRS-2 expression, which appears to compensate for the defects of the insulin signaling pathway in <i>Irs-1</i> deficient mice.