Protective role of 11β-HSD1 inhibition in the metabolic syndrome and atherosclerosis
Obesity is associated with an increased risk of diabetes type 2, dyslipidaemia and atherosclerosis. These cardiovascular and metabolic abnormalities are exacerbated by dietary fats such as cholesterol and its metabolites. High adipose tissue glucocorticoid levels, generated by the intracellular enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) are also implicated in the pathogenesis of obesity, metabolic syndrome and atherosclerosis. Transgenic mice over-expressing 11β-HSD1 selectively in adipose tissue develop the metabolic syndrome whereas 11β-HSD1-/- mice have a ‘cardioprotective’ phenotype, deriving in part from improved adipose tissue function. Consistent with this, prototypical therapeutic 11β-HSD1 inhibitors ameliorate metabolic disturbances associated with obesity. 11β-HSD1 also inter-converts the atherogenic oxysterols 7-ketocholesterol (7KC) and 7β-hydroxycholesterol (7β-HC). Work presented in the first part of the thesis defines the impact of these alternative substrates on the metabolism of glucocorticoids in adipocyte cell lines (3T3-L1 and 3T3-F442A). 11β-HSD1 catalyses the reduction of 7KC in mature adipocytes leading to accumulation of 7β-HC. Oxysterol and glucocorticoid conversion by 11β-HSD1 was competitive and occurred within a physiologically-relevant IC50 range of 450nM for 7KC inhibition of glucocorticoid metabolism. Working as an inhibitor of 11β-HSD1 activity, 7KC decreased the regeneration of active glucocorticoid and limited the process of preadipocyte differentiation. 7-oxysterols did not display intrinsic activation of the glucocorticoid receptor (GR). However, when co-incubated with glucocorticoid, 7KC repressed, and 7β-HC enhanced GR transcriptional activity. The effect of 7-oxysterols resulted from the modulation of 11β-HSD1 reaction direction, at least in transfected HEK293 cells, and could be abrogated by over-expression of hexose 6-phosphate dehydrogenase, which supplies NADPH to drive the reductase activity of 11β-HSD1. 11β-HSD1 inhibition protects from atherosclerosis, yet it is unknown whether it is an effect of alterations in the metabolism of 7-oxysterols. 7KC and 7β-HC did not activate the potential cognate receptor LXRα and FXR/RXR in transactivation assays. No differential regulation of key gene targets of LXRα, FXR and RORα in the liver and fat depots of high fat fed 11β-HSD1-/- and wild type mice was observed. To further determine the molecular basis for the metabolically beneficial phenotype of 11β-HSD1-/- mice I analysed global gene expression in subcutaneous and mesenteric adipose tissues of high fat-fed (4 weeks) 11β-HSD1-/- and congenic C57BL/6J mice by microarrays, followed by pathway analysis, gene clustering and realtime-PCR validation of transcripts with >1.5-fold difference between genotypes. 11β-HSD1-/- mice gained less weight and distributed adipose tissue to subcutaneous rather than visceral depots. Broadly, high fat-fed 11β-HSD1-/- mice showed up-regulation of transcripts in subcutaneous fat (70% of 1622 differentially-expressed transcripts), but down-regulation in mesenteric adipose tissue (73% of 849 transcripts). Genes up-regulated in 11β-HSD1-/- subcutaneous adipose were associated with β-adrenergic signaling, glucose metabolism, lipid oxidation, oxidative phosphorylation, MAPK, Wnt/β-catenin, EGF, and PI3K/AKT insulin signaling pathways. Increased subcutaneous fat insulin signaling was confirmed by increased IRS-1 and Akt phosphorylation in vivo. Down-regulated genes in 11β-HSD1-/- mesenteric fat were associated with immune cells, NK-kappaB, Jak/Stat, SAPK/JNK, chemokine, toll-like-receptor and Wnt signaling pathways suggesting reduced immune cell infiltration in mesenteric adipose in high fat-fed 11β-HSD1-/- mice. 11β-HSD1 deficiency protects against metabolic disease by increasing peripheral fat insulin sensitivity and through a novel mechanism involving reduction in visceral fat immune/inflammatory cell function. Data presented in this thesis contribute to the understanding of the role of 11β-HSD1 in adipose tissues in obesity and, potentially, atherosclerosis.