Osteoblastic glucocorticoid signaling exacerbates high-fat-diet- induced bone loss and obesity.

Chronic high-fat diet (HFD) consumption not only promotes obesity and insulin resistance, but also causes bone loss through mechanisms that are not well understood. Here, we fed wild-type CD-1 mice either chow or a HFD (43% of energy from fat) for 18 weeks; HFD-fed mice exhibited decreased trabecular volume (-28%) and cortical thickness (-14%) compared to chow-fed mice. In HFD-fed mice, bone loss was due to reduced bone formation and mineral apposition, without obvious effects on bone resorption. HFD feeding also increased skeletal expression of sclerostin and caused deterioration of the osteocyte lacunocanalicular network (LCN). In mice fed HFD, skeletal glucocorticoid signaling was activated relative to chow-fed mice, independent of serum corticosterone concentrations. We therefore examined whether skeletal glucocorticoid signaling was necessary for HFD-induced bone loss, using transgenic mice lacking glucocorticoid signaling in osteoblasts and osteocytes (HSD2OB/OCY-tg mice). In HSD2OB/OCY-tg mice, bone formation and mineral apposition rates were not suppressed by HFD, and bone loss was significantly attenuated. Interestingly, in HSD2OB/OCY-tg mice fed HFD, both Wnt signaling (less sclerostin induction, increased ß-catenin expression) and glucose uptake were significantly increased, relative to diet- and genotype-matched controls. The osteocyte LCN remained intact in HFD-fed HSD2OB/OCY-tg mice. When fed a HFD, HSD2OB/OCY-tg mice also increased their energy expenditure and were protected against obesity, insulin resistance, and dyslipidemia. Therefore, glucocorticoid signaling in osteoblasts and osteocytes contributes to the suppression of bone formation in HFD-fed mice. Skeletal glucocorticoid signaling is also an important determinant of glucose uptake in bone, which influences the whole-body metabolic response to HFD.

Skeletal glucocorticoid signalling determines leptin resistance and obesity in aging mice.

OBJECTIVE: Aging and chronic glucocorticoid excess share a number of critical features, including the development of central obesity, insulin resistance and osteoporosis. Previous studies have shown that skeletal glucocorticoid signalling increases with aging and that osteoblasts mediate the detrimental skeletal and metabolic effects of chronic glucocorticoid excess. Here, we investigated whether endogenous glucocorticoid action in the skeleton contributes to metabolic dysfunction during normal aging. METHODS: Mice lacking glucocorticoid signalling in osteoblasts and osteocytes (HSD2OB/OCY-tg mice) and their wild-type littermates were studied until 3, 6, 12 and 18 months of age. Body composition, adipose tissue morphology, skeletal gene expression and glucose/insulin tolerance were assessed at each timepoint. Leptin sensitivity was assessed by arcuate nucleus STAT3 phosphorylation and inhibition of feeding following leptin administration. Tissue-specific glucose uptake and adipose tissue oxygen consumption rate were also measured. RESULTS: As they aged, wild-type mice became obese and insulin-resistant. In contrast, HSD2OB/OCY-tg mice remained lean and insulin-sensitive during aging. Obesity in wild-type mice was due to leptin resistance, evidenced by an impaired ability of exogenous leptin to suppress food intake and phosphorylate hypothalamic STAT3, from 6 months of age onwards. In contrast, HSD2OB/OCY-tg mice remained leptin-sensitive throughout the study. Compared to HSD2OB/OCY-tg mice, leptin-resistant wild-type mice displayed attenuated sympathetic outflow, with reduced tyrosine hydroxylase expression in both the hypothalamus and thermogenic adipose tissues. Adipose tissue oxygen consumption rate declined progressively in aging wild-type mice but was maintained in HSD2OB/OCY-tg mice. At 18 months of age, adipose tissue glucose uptake was increased 3.7-fold in HSD2OB/OCY-tg mice, compared to wild-type mice. CONCLUSIONS: Skeletal glucocorticoid signalling is critical for the development of leptin resistance, obesity and insulin resistance during aging. These findings underscore the skeleton's importance in the regulation of body weight and implicate osteoblastic/osteocytic glucocorticoid signalling in the aetiology of aging-related obesity and metabolic disease.

Androgens sensitise mice to glucocorticoid-induced insulin resistance and fat accumulation.

AIMS/HYPOTHESIS: Chronic glucocorticoid therapy causes insulin resistance, dyslipidaemia, abnormal fat accumulation, loss of muscle mass and osteoporosis. Here we describe a hitherto unknown sexual dimorphism in the metabolic response to chronic glucocorticoid exposure in mice. This led us to investigate whether glucocorticoid-induced insulin resistance and obesity were dependent on sex hormones. METHODS: Male and female CD1 mice were treated for 4 weeks with supraphysiological doses (~250 µg/day) of corticosterone, the main glucocorticoid in rodents, or equivalent volume of vehicle (drinking water without corticosterone). To investigate the effects of sex hormones, a separate group of mice were either orchidectomised or ovariectomised prior to corticosterone treatment, with or without dihydrotestosterone replacement. Body composition was determined before and after corticosterone treatment, and insulin tolerance was assessed after 7 and 28 days of treatment. Adipocyte morphology was assessed in white and brown adipose tissues by immunohistochemistry, and fasting serum concentrations of NEFA, triacylglycerols, total cholesterol and free glycerol were measured using colorimetric assays. Obesity- and diabetes-related hormones were measured using multiplex assays, and RNA and protein expression in adipose tissues were measured by RT-PCR and immunoblotting, respectively. RESULTS: Chronic corticosterone treatment led to insulin resistance, fasting hyperinsulinaemia, increased adiposity and dyslipidaemia in male, but not female mice. In males, orchidectomy improved baseline insulin sensitivity and attenuated corticosterone-induced insulin resistance, but did not prevent fat accumulation. In androgen-deficient mice (orchidectomised males, and intact and ovariectomised females) treated with dihydrotestosterone, corticosterone treatment led to insulin resistance and dyslipidaemia. In brown adipose tissue, androgens were required for corticosterone-induced intracellular lipid accumulation ('whitening'), and dihydrotestosterone specifically exacerbated corticosterone-induced accumulation of white adipose tissue by increasing adipocyte hypertrophy. Androgens also suppressed circulating adiponectin concentrations, but corticosterone-induced insulin resistance did not involve additional suppression of adiponectin levels. In white adipose tissue, androgens were required for induction of the glucocorticoid target gene Gilz (also known as Tsc22d3) by corticosterone. CONCLUSIONS/INTERPRETATION: In mice, androgens potentiate the development of insulin resistance, fat accumulation and brown adipose tissue whitening following chronic glucocorticoid treatment.