The Role of Preadipocyte 11β Hydroxysteroid Dehydrogenase type 1 in Regional Fat Distribution

Abstract

Glucocorticoid excess promotes visceral obesity, which is closely associated with morbidity and cardiometabolic disease. Similar features are found in the Metabolic Syndrome in the absence of elevated plasma cortisol. Whilst elevated activity of the intracellular glucocorticoid amplifying enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) within adipocytes might explain this paradox, the potential role of preadipocyte 11β-HSD1 is less clear. Data from murine cell lines suggested 11β-HSD1 was expressed at late stages of preadipocyte differentiation, where it exhibited keto-reductase activity, converting inactive into active glucocorticoids. In contrast, data from human preadipocytes suggested expression of 11β-HSD1 inactivated glucocorticoids (dehydrogenase action) in a depot-specific manner. In this study 11β-HSD1 mRNA levels and activity in homogenates were measured in preadipocyte-enriched stromal/vascular fraction (SVF), from metabolically “disadvantageous” visceral (mesenteric) and “safer” peripheral (subcutaneous) adipose tissue in mice. The results suggest that levels of 11β- HSD1 mRNA and enzyme are similar, within a given adipose tissue depot, in freshly isolated SVF and adipocytes. Crucially, 11β-HSD1 exclusively functioned as a keto-reductase in intact SVF, whatever the adipose depot of origin. Consistent with this, hexose-6-phosphate dehydrogenase (H6PDH), which drives 11β-HSD1 keto-reduction, is expressed in SVF. Unexpectedly, glucocorticoid reactivation was higher in freshly isolated intact mesenteric SVF cells than those from subcutaneous adipose, despite lower levels of 11β- HSD1 mRNA and enzyme in mesenteric SVF, suggesting a novel posttranscriptional control over enzyme activity. Mice with a targeted deletion of 11β-HSD1 (11β-HSD1-/-) showed no difference in expression of pref-1 (a preadipocyte marker) in adipose tissue, compared to control C57BL/6J mice, suggesting that 11β-HSD1 has no influence on preadipocyte proliferation. Upon high fat feeding, higher preadipocyte differentiation, as inferred from pref-1 mRNA levels, was higher within mesenteric than subcutaneous fat in 11β-HSD1+/+ mice. This differed from 11β-HSD1-/- mice where preadipocyte differentiation was greater in subcutaneous than mesenteric fat. These observations corroborate the literature in which mesenteric fat accumulation is more pronounced in 11β- HSD1+/+ mice than in 11β-HSD1-/-. Further, following HF diet, 11β-HSD1 and GR mRNA expression in SVF were decreased more markedly in mesenteric than in subcutaneous fat in 11β-HSD1+/+. This suggests an adaptive mechanism to counteract detrimental effects of high GC levels occurring at both pre-receptor and receptor level, mainly in the mesenteric adipose. Preliminary data from human preadipocytes suggested lower levels of 11β-HSD1 activity in preadipocytes compared to mice. Importantly, cortisone was being metabolised into an as yet unknown compound. Taken together, these results allow a greater understanding of specific regulation of 11β-HSD1 between preadipocytes from different depots. As well, the results in this thesis suggest that in vivo, in addition to effects on adipocyte hypertrophy, 11β-HSD1 expression in preadipocytes influences preadipocyte differentiation and this may be important in determining regional fat distribution.