ROLE OF NUTRIENTS IN AGE-RELATED INSULIN RESISTANCE

Nutrient excess and obesity play a central role in the pathogenesis of the insulin resistance syndrome, with important age-related sequelae including accelerated atherosclerosis and type 2 diabetes mellitus (T2DM), and is the subject of this Program Project. This proposal will study the interrelationships of aging, obesity and the hexosamine biosynthetic pathway (HBP) in the induction of nutrient-induced insulin resistance in humans. The HBP provides cellular "satiety" signals with increased circulating levels of such nutrients as glucose and free fatty acids, and increases glycosylation of important intracellular factors including transcription factors, ultimately affecting the expression of many genes. Increased HBP flux in animal models induces insulin resistance, and increases adipocyte gene expression and circulating levels of many cytokines and acute phase reactants. These proteins, whose plasma levels are strongly correlated with insulin resistance in humans, likely contribute to the pathophysiology of many age-related diseases. Importantly, aging itself is associated with increased glycosylation of intracellular proteins, suggesting upregulation of the HBP and therefore increased susceptibility to the effects of nutrients. However, a causal link between the HBP, nutrient availability and aging-induced insulin resistance has not yet been established in humans. We will compare the effects of increased nutrient availability on whole-body insulin action, accumulation of HBP products, and adipocyte gene expression in 4 groups of subjects: young lean, young obese, lean elderly and obese elderly. The experimental design will therefore parallel that of Project 2 in aging ad libitum fed and caloric restricted rodents. We will thereby determine whether increased flux into the HBP mediates nutrient-induced metabolic changes in humans, and whether upregulation of the HBP with aging might exacerbate the metabolic impact of nutrient excess. Furthermore, we will examine subcutaneous and visceral adipose tissue to determine effects of aging and nutrients on depot-specific adipocyte gene expression. Together with Project 2, this project aims to define the biological characteristics ot the visceral fat depot that pose significant risk factors for the metabolic syndrome of aging.