Serotonin as a Novel Biomarker for Progression to Type 2 Diabetes in Obese Adolescents

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PROJECT SUMMARY: Type 2 diabetes (T2D) is a worldwide pandemic affecting nearly 300 million adults and a rapidly increasing number of children, with profound individual and community health consequences. Estimated diabetes costs in the United States now exceed $200 billion. It is the major cause of blindness, leg amputation, and end-stage kidney disease and a principal cause of myocardial infarction and stroke. The two major determinants of T2D are obesity and insulin resistance (IR), yet the pathogenesis of T2D remains poorly understood. Only one-half of obese children are IR, and a far lower percentage progress to T2D. Why some obese children avoid metabolic dysfunction while others develop T2D is a critical gap in in the understanding of T2D pathogenesis and precludes the design and implementation of programs for diabetes prevention and treatment. My long-term career goal is to establish an independent research program dedicated to identifying and treating obese children at highest risk for IR and T2D. Using state-of-the-art metabolomic profiling of plasma samples, we previously found that childhood IR is associated with increased levels of the branch-chain amino acids (BCAA). Recently we have employed metabolic profiling of 24-hour urine samples, which has the advantage of integrating differences in metabolic status over time non-invasively. We identified a decrease in urine 5-Hydroxyindoleacetic acid (5- HIAA), the major metabolite of serotonin, as a novel biomarker associated with T2D in obese youth. This finding is particularly interesting given that serotonin increases pancreatic β-cell replication, β-cell mass, and glucose- stimulated insulin secretion. The reduction in 5HIAA in T2D likely reflects a decrease in cellular serotonin availability, since: (a) elevated BCAA compete with tryptophan, the precursor of serotonin, for uptake into β-cells and other tissues; and (b) in obesity, tryptophan metabolism is dysregulated and shifted towards production of kynurenine rather than serotonin. These findings suggest that reductions in tryptophan availability and serotonin production contribute to diabetes pathogenesis through inhibition of insulin secretion, promoting progression from obesity to glucose intolerance and T2D. We hypothesize that serotonin and serotonin metabolites including 5-HIAA constitute useful urine biomarkers that predict development of T2D in obese youth. This hypothesis will be tested in three Specific Aims. Aim 1 will identify urinary metabolic signatures that differ among non-obese and obese children with and without T2D. Aim 2 will determine if these urinary metabolic signatures correlate with parameters of glucose tolerance and glycemic control. Aim 3 will determine if urinary 5-HIAA measured at baseline or during follow-up predicts changes in disposition index, a composite measure of β-cell function in a longitudinal study of lean and obese subjects without T2D. My mentoring team, which includes Drs. Christopher Newgard (scientific mentor) and Michael Freemark (academic mentor), will provide guidance and quantitative and methodological expertise in clinical research. With an outstanding Duke University research environment, this award will facilitate my transition to independence as an academic pediatric endocrinologist.