Effects of the conversion of thyroid hormone on glucose

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Summary: Thyroid hormone plays an important regulatory role in the development and function of virtually all organs and its homeostasis is maintained by a highly regulated, multi-step redundant system. The peripheral metabolism of thyroid hormone, by regulating the circulating and intracellular levels of the active hormone T3, represents an important tissue-specific pre-receptor modulator of the hormonal action. The deiodinases are selenoenzymes which convert the pro-hormone T4 into its active hormone T3 or into the metabolically inactive rT3. Recently, we discovered that a common polymorphism of the type 2 deiodianse gene (Thr92Ala) associates with decreased glucose disposal and, in the presence of a previously described inactivating beta-3 adrenergic receptor polymorphism, with a small but significant increase in body mass index. Interestingly, this polymorphism does not affect indices of insulin resistance in a physically active founder population, the Old Order Amish, suggesting a gene-environment interaction. We speculated that the Thr92Ala polymorphism generates a defective enzyme thus leading to a decrease intracellular conversion of T4 to T3, ultimately leading to reduced energy expenditure and impaired transcription of the insulin-dependent glucose transporter-4, whose gene is thyroid hormone-regulated. During this year we have focused our efforts in defining the role of the peripheral conversion of thyroid hormone in the pathogenesis of insulin resistance and obesity. We further characterized the enzymatic properties of the Thr92Ala variant and discovered that the polymorphism impairs the protesaome-induced degradation of the enzyme. We are now performing in vitro reconstruction experiments to further characterize the protein-protein interaction leading to this phenomenon. During this year we have also devoted efforts toward the characterization of another naturally occurring polymorphism in the 5? UTR region of the human type-II deiodinase gene (258 A/G). Our data indicate that the naturally occurring polymorphism reduces the affinity for a repressor thus increasing the transcription rate of the gene. These data are in keeping with in vivo observations made by another group. Our effort is now devoted to the identification and characterization of the type-2 deiodinase repressor. In order to characterize in vivo the role of the peripheral metabolism of thyroid hormone with respect to glucose and energy metabolism, we have generated a clinical protocol aimed to administer in double blind, cross over fashion either T4 or T3 to thyroidectomized patients (05-DK-0119). Recruitment of patients started in July 05 and currently 8 patients have been randomized, two have competed the study and other thre have completed the first admission. This approach, will allow us to define in vivo the role of the peripheral conversion of thyroid hormone in the maintenance of glucose and energy metabolism. Finally, a clinical protocol aimed to characterize in vivo the action of thyroid hormone and the role of local conversion of thyroid hormone in adipose tissue by means of microdialysis has been approved (06-DK-0133). Normal volunteer will undergo adipose tissue microdialysis and various concentrations of either T4 or T3 will be infused in the subcutaneous adipose tissue. Glycerol concentration in the effluent fluid will be considered as readout for thyroid hormone-induced lipolyisis.