Reduced glucose uptake precedes insulin signaling defects in adipocytes from heterozygous GLUT4 knockout mice

Decreased GLUT4 expression, impaired insulin receptor (IR), TRS-l, and pp60 /IRS-3 tyrosine phosphorylation are characteristics of adipocytes from insu lin-resistant animal models and obese NIDDM humans. However, the sequence o f events leading to the development of insulin signaling defects and the si gnificance of decreased GLUT4 expression in causing adipocyte insulin resis tance are unknown. The present study used male heterozygous GLUT4 knockout mice (GLUT4(+/-)) as a novel model of diabetes to study the development of insulin signaling defects in adipocytes with the progression of whole body insulin resistance and diabetes. Male GLUT4(+/-) mice with normal fed glyce mia and insulinemia (N/N), normal fed glycemia and hyperinsulinemia (N/H), and fed hyperglycemia with hyperinsulinemia (H/H) exist at all ages. The ex pression of GLUT4 protein and the maximal insulin-stimulated glucose transp ort was 50% decreased in adipocytes from all three groups. Insulin signalin g was normal in N/N adipose cells. From 35 to 70% reductions in insulin-sti mulated tyrosine phosphorylation of IR, IRS-1, and pp60/IRS-3 were noted wi th no changes in the cellular content of IR, IRS-1, and p85 in N/H adipocyt es. Insulin-stimulated protein tyrosine phosphorylation was further decreas ed to 12-23% in H/H adipose cells accompanied by 42% decreased IR and 80% i ncreased p85 expression. Insulin-stimulated, IRS-1-associated PI3 kinase ac tivity was decreased by 20% in N/H and 68% reduced in H/H GLUT4(+/-) adipoc ytes. However, total insulin-stimulated PI3 kinase activity was normal in H /H GLUT4(+/-) adipocytes. Taken together, these results strongly suggest th at hyperinsulinemia triggers a reduction of IR tyrosine kinase activity tha t is further exacerbated by the appearance of hyperglycemia. However, the i nsulin signaling cascade has sufficient plasticity to accommodate significa nt changes in specific components without further reducing glucose uptake. Furthermore, the data indicate that the cellular content of GLUT4 is the ra te-limiting factor in mediating maximal insulin-stimulated glucose uptake i n GLUT4(+/-) adipocytes.-Li, J., Houseknecht, K. L., Stenbit, A. E., Katz, E. B., Charron, M. J. Reduced glucose uptake precedes insulin signaling def ects in adipocytes from heterozygous GLUT4: knockout mice.