Glucocorticoids and 11β-HSD1 are major regulators of intramyocellular protein metabolism

Abstract

The adverse metabolic effects of prescribed and endogenous glucocorticoid excess, ‘Cushing’s syndrome’, create a significant health burden. While skeletal muscle atrophy and resultant myopathy is a clinical feature, the molecular mechanisms underpinning these changes are not fully defined. We have characterized the impact of glucocorticoids upon key metabolic pathways and processes regulating muscle size and mass including: protein synthesis, protein degradation, and myoblast proliferation in both murine C2C12 and human primary myotube cultures. Furthermore, we have investigated the role of pre-receptor modulation of glucocorticoid availability by 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) in these processes. Corticosterone (CORT) decreased myotube area, decreased protein synthesis, and increased protein degradation in murine myotubes. This was supported by decreased mRNA expression of insulin-like growth factor (IGF1), decreased activating phosphorylation of mammalian target of rapamycin (mTOR), decreased phosphorylation of 4E binding protein 1 (4E-BP1), and increased mRNA expression of key atrophy markers including: atrogin-1, forkhead box O3a (FOXO3a), myostatin (MSTN), and muscle-ring finger protein-1 (MuRF1). These findings were endorsed in human primary myotubes, where cortisol also decreased protein synthesis and increased protein degradation. The effects of 11-dehydrocorticosterone (11DHC) (in murine myotubes) and cortisone (in human myotubes) on protein metabolism were indistinguishable from that of CORT/cortisol treatments. Selective 11β-HSD1 inhibition blocked the decrease in protein synthesis, increase in protein degradation, and reduction in myotube area induced by 11DHC/cortisone. Furthermore, CORT/cortisol, but not 11DHC/cortisone, decreased murine and human myoblast proliferative capacity. Glucocorticoids are potent regulators of skeletal muscle protein homeostasis and myoblast proliferation. Our data underscores the potential use of selective 11β-HSD1 inhibitors to ameliorate muscle-wasting effects associated with glucocorticoid excess.

Cellular and genetic models of H6PDH and 11β‐HSD1 function in skeletal muscle

“…However, 11β‐HSD1KO mice are protected from the adverse metabolic effects of glucocorticoid excess, with muscle being particularly resistant . Thus, we have established that regulation of 11β‐HSD1 expression, more so than H6PDH or G6PT, appears a robust mechanism of altering intracellular capacity to regenerate glucocorticoid and mechanistically linked to propagating the deleterious effect of glucocorticoid excess in muscle …”

“…We show here that glucocorticoid signalling increases the expression of 11β‐HSD1 and H6PDH, but not G6PT in muscle. In the context of endogenous or exogenous glucocorticoid excess, upregulation of 11β‐HSD1 in muscle may represent an unwanted “side‐effect” exacerbating myopathy . In support of this, 11β‐HSD1KO mice do not display detectable defects in muscle physiology indicating that 11β‐HSD1 is dispensable for differentiation and more critical to modulating muscle tissue responses to glucocorticoid.…”

“…Glucocorticoids fulfil important permissive and adaptive roles in the regulation of skeletal muscle energy metabolism, impacting glucose utilization, insulin sensitivity, amino acid, and lipid metabolism . Glucocorticoids can also influence muscle mass by regulating the balance between pathways affecting protein synthesis and protein degradation, via modulation of amino acid transport, the anabolic actions of insulin and IGF1, and through the regulation of catabolic pathways such as the ubiquitin proteasome system and E3 ubiquitin ligases atrogin‐1 and muscle ring finger 1 . Indeed, the importance of glucocorticoids to muscle is exemplified in patients with glucocorticoid excess (Cushing's syndrome) who develop insulin resistance, muscle weakness, and profound proximal myopathy …”

“…However, not all 11β‐HSD1 knockout (HSD1KO) loss‐of‐function models have demonstrated protection from metabolic disease and suggest that use of inhibitors will need to be tailored to specific pathologies . Indeed, it has been shown that 11β‐HSD1KO mice are almost completely protected from the deleterious metabolic effects of glucocorticoid excess, and inhibition in this scenario may be truly beneficial …”

Learning pharmacokinetic models for in vivo glucocorticoid activation

“…11 β-HSD1 is widely expressed in metabolically active tissues including liver, adipose, muscle, bone, skin and the central nervous system and the enzyme has been implicated in the pathogenesis of associated diseases ( Cooper et al, 1993;Tiganescu et al, 2013 ). Cell culture and animal models have suggested that 11 β-HSD1 is a major regulator of obesity and of the features of glucocorticoid excess as seen in Cushing's Syndrome ( Clayton et al, 2016;Markey et al, 2016;Morgan et al, 2016a;2016b;. Pharmaceutical companies have developed a number of selective inhibitors of 11 β-HSD1 and are assessing their therapeutic potential.…”