High concentrations of glucose induce insulin resistance, impair insulin secretion, and affect hepatic glucose production in a manner that mirrors type 2 diabetes, and hexosamines mimic many of these effects. This has led to the hypothesis that cells use hexosamine flux as a glucose- and satiety-sensing pathway. The hexosamine hypothesis for glucose sensing has been validated by overexpressing the rate-limiting enzyme for hexosamine synthesis, GFA in several tissues including muscle, liver, fat, and beta cells. Excess hexosamine flux leads to a coordinated response whereby fuel is shunted toward long term storage, mirroring the """"""""thrifty phenotype"""""""" and resulting ultimately in obesity, hyperlipidemia, and insulin resistance. The results suggest a mechanism by which chronic overnutrition leads to the phenotype of type 2 diabetes. Studies are proposed to investigate (a) the full extent of consequences of increased hexosamine flux for the organism; and (b) the mechanisms underlying these changes.
The specific aims are: 1. Based on the previous demonstration that overexpression of GFA in striated muscle plus fat causes insulin resistance, we propose to overexpress GFA separately in these tissues. Using the aP2 promoter to target to fat, glucose and fat metabolism, insulin sensitivity, and leptin levels will be assessed in the whole animal and in isolated fat cells. Targets for gene regulation by hexosamines will be identified. 2. Parallel experiments will be performed in mice overexpressing GFA only in striated muscle to determine if muscle becomes insulin resistant secondary to signaling from fat. 3. The investigators will study why GFA overexpression in liver results in increased liver glycogen, increased free fatty acid (FFA) and triglyceride levels and eventually obesity and insulin resistance. 4. The investigators will test the hypothesis that hexosamines exert their effects through O-linked glycosylation.
