Endocrine Abstracts

Mineralocorticoid Receptor (MR) is able to regulate the transcription of a number of genes in the myotube, nevertheless the role of this steroid hormone receptor in skeletal muscle (SM) metabolism needs to be clarified. SM represents a major site for glucose uptake and local metabolic derangements play a pivotal role in the development of insulin resistance (IR). The aim of this study was to investigate the contribution of MR in mediating SM metabolic alterations in a mouse model of diet-induced obesity. We observed that mice fed a high fat diet (HFD mice), showed impaired glucose tolerance compared to mice fed a normal diet (ND mice). Mice fed a HFD treated with the MRA spironolactone (HFD+spiro mice) revealed an improvement in glucose tolerance, compared with HFD mice. In addition, MRA-treated obese mice showed brown adipose tissue activation, which was expected to contribute to the improved glucose metabolism. To investigate if MR blockade in SM could contribute to the observed MRA-mediated metabolic effects, we analyzed MR expression in gastrocnemius, observing that MR protein abundance was downregulated by HFD compared to ND mice, whereas administration of spiro was able to partially revert this effect in HFD+spiro mice. Interestingly, SM MR expression profile was opposite to that observed in adipose tissue (AT), showing increased protein abundance in HFD group, thus suggesting a different metabolic function of MR in AT and SM. Upregulated levels of mineralocorticoids, as well as up-regulated MR activation, are associated with impaired glucose metabolism. In our experimental model, downregulated expression and function of MR in SM of HFD mice with altered glucose tolerance, suggests that SM MR has a completely different role in regulation of glucose metabolism. To confirm this hypothesis, we investigated the effect of MR blockade on insulin signalling in a cellular model of myocytes (C2C12) with IR, which was obtained by treatment with palmitate, in the presence or absence of spiro. We confirmed MR protein downregulation in insulin resistant myotubes. We also analysed Akt phosphorylation, upon insulin stimulation, and we did not observe any difference between palmitate- and palmitate+spiro-treated cells. These results were confirmed by in vitro glucose uptake analysis. Taken together our data indicate that MR does not regulate insulin signaling in mouse myocytes, excluding any protective effect of SM MR pharmacological blockade upon local and systemic insulin sensitivity.