Role of nuclear receptors in the regulation of human adipose tissue metabolism

Nuclear receptors modulate expression of genes involved in adipose tissue (AT) metabolism. Their improved understanding may provide new treatment options for metabolic disorders such as obesity, insulin resistance (IR) and type 2 diabetes (T2D). This thesis explored the role of nuclear receptors, mainly, glucocorticoid and estrogen receptors (GR and ER, respectively) and peroxisome proliferator-activated receptor gamma (PPARγ), and their interplay in the regulation of metabolic function and dysfunction in human AT. In Paper I, the regulation of adipokine lipocalin 2 (LCN2) expression by synthetic glucocorticoid, dexamethasone and effect of LCN2 on glucose and lipid metabolism in AT were studied. In pre-menopausal but not post-menopausal women or men, dexamethasone upregulated LCN2 gene expression, which also correlated with markers of obesity and IR. LCN2 inhibited adipocyte glucose uptake. In Paper II, the effect of estrogen (E2) and its interaction with GR in LCN2 regulation in AT from post-menopausal women were examined. E2 increased LCN2 expression, what seems to be mediated by ERβ. E2 and dexamethasone co-treatment increased LCN2 gene expression in presence of ERα but not ERβ antagonist. Dexamethasone decreased ERα, while increased ERβ gene expression. In Paper III and IV, the feasibility of genotype-based recall (GBR), a participant recruitment approach, was tested by undertaking clinical and AT phenotyping of different PPARγ Pro12Ala carriers. The baseline characteristics were comparable between genotypes. Compared to fasting, a decreased hormone-sensitive lipase gene expression in Pro/Pro group also accompanied with a higher antilipolytic effect of insulin after oral glucose. Adipocyte glucose uptake and adipogenesis remained unchanged between genotypes. Overall, LCN2 can induce IR in human AT and may mediate metabolic defects by excess glucocorticoids in pre-menopausal women. GR selectively interacts with ERα and ERβ, the latter two acts oppositely to control LCN2 expression in AT. PPARγ Pro12Ala had no major effect on clinical and adipose phenotype, likely due to a small sample size in relation to the modest effect the Ala variant or tissues other than adipose could be critical in conferring protection by Pro12Ala against T2D risk. Further, the GBR approach deemed feasible, however, would be more suitable in the characterization of rare genetic variants