Abstract
11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) catalyses intracellular regeneration of active glucocorticoids, notably in liver and adipose tissue. 11β-HSD1 is increased selectively in adipose tissue in human obesity, a change implicated in the pathogenesis of metabolic syndrome. With high fat (HF)-feeding, adipose tissue 11β-HSD1 is down-regulated in mice, plausibly to counteract metabolic disease. Transcription of 11β-HSD1 is directly regulated by members of the CCAAT/enhancer binding protein (C/EBP) family. Here we show that while total C/EBPβ in adipose tissue is unaltered by HF diet, the ratio of the C/EBPβ isoforms liver-enriched inhibitor protein (LIP) and liver-enriched activator protein (LAP) (C/EBPβ-LIP:LAP) is increased in subcutaneous adipose. This may cause changes in 11β-HSD1 expression since genetically modified C/EBPβ(+/L) mice, with increased C/EBPβ-LIP:LAP ratio, have decreased subcutaneous adipose 11β-HSD1 mRNA levels, whereas C/EBPβΔuORF mice, with decreased C/EBPβ-LIP:LAP ratio, show increased subcutaneous adipose 11β-HSD1. C/EBPβ-LIP:LAP ratio is regulated by endoplasmic reticulum (ER) stress and mTOR signalling, both of which are altered in obesity. In 3T3-L1 adipocytes, 11β-HSD1 mRNA levels were down-regulated following induction of ER stress by tunicamycin but were up-regulated following inhibition of mTOR by rapamycin. These data point to a central role for C/EBPβ and its processing to LIP and LAP in transcriptional regulation of 11β-HSD1 in adipose tissue. Down-regulation of 11β-HSD1 by increased C/EBPβ-LIP:LAP in adipocytes may be part of a nutrient-sensing mechanism counteracting nutritional stress generated by HF diet.
Highlights
To test whether altered CCAAT/enhancer binding protein (C/EBP) expression may underlie the dietary regulation of 11b-HSD1, we examined C/EBPa, b, d and CHOP expression in adipose tissue
Consistent with mRNA levels, western blot analysis showed an increase in CHOP protein levels but no alteration in total C/EBPa (p42+ p30 isoforms), total C/EBPb (LAP* + liver-enriched activator protein (LAP) + liver-enriched inhibitor protein (LIP)) or C/EBPd (Fig. 1B–E) protein levels with high fat (HF) diet
Knock-down of C/EBPb in fully differentiated mature 3T3-L1 adipocytes showed that C/EBPb is required for normal 11b-HSD1 expression in differentiated adipocytes, consistent with a previous report of reduced 11bHSD1 expression in adipose tissue of C/EBPb-deficient mice [29]
Summary
11b-hydroxysteroid dehydrogenase type 1 (11b-HSD1) is highly expressed in liver and adipose tissue where it catalyses the regeneration of active glucocorticoids (corticosterone, cortisol) from inert 11keto- forms (11-dehydrocorticosterone, cortisone) increasing intracellular glucocorticoid action [1]. 11b-HSD1 expression is elevated selectively in adipose tissue of obese humans and in monogenic rodent genetic obesity, whereas levels in liver are unaffected or even decreased [2,3,4]. High fat (HF) diet downregulated 11b-HSD1 selectively in adipose tissue in mice and rats [9,10,11]. This down-regulation is greatest in obesity-resistant strains [9] suggesting it may be a mechanism to minimise metabolic disease with adiposity. C/ EBPa, b, d and CHOP are essential for adipocyte differentiation and function in vitro and in vivo [18,19,20,21]. LIP lacks the transcription activation domain of C/EBPb and is typically a dominant-negative regulator of C/EBP function [24]. C/EBPa is the major known inducer of 11b-HSD1 transcription in liver, where C/EBPb acts as a relative repressor. We have tested the hypothesis that C/EBPs are modulated by diet in mice and mediate the regulation of 11bHSD1 in adipose
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