GLUCOSE SIGNALING AND REGULATION OF GENE EXPRESSION IN 3T3-L1 ADIPOCYTES

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Summary of work: There is a large body of evidence supporting the concept of glucose toxicity, in which hyperglycemia causes the deterioration of insulin secretion from the pancreas and insulin action in peripheral tissues (e.g., adipose, heart and skeletal muscle). This resistance to insulin action has been linked to an increase in glucose metabolism through the hexosamine biosynthetic pathway. The effect of high concentrations of glucose and glucose metabolites on the expression of genes important for regulation of adipose cell functions remains largely unknown. This study was undertaken to look at the regulation of the transcription factor CCAAT/enhancer-binding protein (C/EBP) alpha by glucose and glucosamine, a product of the hexosamine pathway, in a cultured adipose cell line that is known for its high responsiveness to insulin. This transcription factor has been implicated in the establishment and maintenance of energy homeostasis in adipocytes by controlling the expression of several gene products which includes the insulin-responsive glucose transporter GLUT4, lipid-synthesizing enzymes, and leptin. Leptin is exclusively expressed in adipocytes and is considered as a hormone that acts centrally to regulate body weight through the control of appetite and energy expenditure. Our initial results indicate that the decrease in transcription and mRNA accumulation of C/EBP-alpha gene caused by high concentrations of glucose or glucosamine is accompanied by a proportional reduction in the expression of GLUT4 gene. Stable expression of a portion of the promoter for C/EBP-alpha gene fused to a reporter gene has demonstrated that promoter activity was decreased by these treatments. It is now recognized that the adipose tissue plays major endocrine roles in addition to its function as an energy storage depot. A study aimed at evaluating the effect of hyperglycemia and glucosamine infusion on adipocyte metabolic and endocrine functions in rats is underway. Progress in our understanding of factors regulating adipose cell physiology shall allow the development of effective interventions for obesity-related insulin resistance, a major contributor to morbidity and mortality in the U.S. and other Western societies.