Omics Techniques Applied to Diabetes Research - Focus on HSL-Null Mice and Clonal β-Cells
Abstract: Diabetes mellitus is a disease characterized by increased blood glucose levels. For overt type 2 diabetes to develop, both insulin action and insulin secretion must be perturbed. In the work presented in this thesis, two models were analyzed: hormone-sensitive lipase (HSL) null mice and a pancreatic clonal β-cell line to study the mechanisms underlying insulin resistance and insulin secretion perturbations, respectively. To achieve this, techniques allowing a global analysis of the transcriptome, proteome and metabolome were used. HSL is best known as the enzyme hydrolyzing acylglycerides stored in adipose tissue. But HSL has broad substrate specificity and is widely expressed elsewhere than in adipose tissue. Its role in non-adipose tissues is not completely understood, but the phenotype revealed by the characterization of several independently generated HSL-null mouse lines during the recent years suggests that HSL has several functions in addition to its role in adipocyte lipolysis. The role of HSL in the liver was studied in this thesis. More specifically, the liver phenotype of HSL-null mice was investigated at the transcriptome (Paper I and IV) and proteome (Paper IV) levels. The obtained results didn’t allow the obvious identification of possible mechanisms behind the hepatic insulin resistance observed in our HSL-null mouse strain. However, our results show that HSL plays an important role as a cholesteryl ester hydrolase in the liver and that HSL influences overall cholesterol homeostasis by indirectly controlling hepatic HDL-cholesterol clearance. We also demonstrated the importance of a cross-talk between white adipose tissue and liver that regulates cholesterol homeostasis via the type of non-esterified fatty acids (NEFA) released. Moreover, changes in expression of proteins involved in polyamine metabolism were observed in the liver of HSL-null mice, which could be responsible for the increased liver weight characterizing HSL-null mice. The physiological response of HSL-null mice to aerobic treadmill exercise was also investigated (Paper II). HSL was shown to play an important role during aerobic exercise in controlling the mobilization of lipid stores from white adipose tissue, a function which cannot be fully compensated by any other acylglyceride lipases. Proteome and metabolome analyses were performed in Paper III to study glucose-stimulated insulin secretion in a β-cell line cultured in presence of normal or toxic glucose concentrations. A metabolite fingerprint which is characteristic for β-cells cultured at high glucose concentrations was obtained.
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