Calcitriol concomitantly enhances insulin sensitivity and alters myocellular lipid partitioning in high fat-treated skeletal muscle cells

Abstract

Vitamin D reduces myocellular insulin resistance, but the effects of vitamin D on intramyocellular lipid (IMCL) partitioning are unknown. The purpose of this study was to understand how calcitriol, the active vitamin D metabolite, affects insulin sensitivity and lipid partitioning in skeletal muscle cells. C2C12 myotubes were treated with calcitriol (100 nM) or vehicle control for 96 h. Insulin-stimulated Akt phosphorylation (Thr 308) was determined by western blot. Intramyocellular triacylglycerol (IMTG), diacylglycerol (DAG), and ceramide content were measured by LC/MS. IMTG partitioning and lipid droplet accumulation were assessed by oil red O. Expression of genes involved in lipid droplet packaging and lipolysis were measured by RT-PCR. Compared to vehicle-treated myotubes, calcitriol augmented insulin-stimulated pAkt. Calcitriol increased total ceramides and DAG in a subspecies-specific manner. Specifically, calcitriol preferentially increased ceramide 24:1 (1.78 fold) and di-18:0 DAG (46.89 fold). Calcitriol increased total IMTG area as assessed by oil red O, but decreased the proportion of lipid within myotubes. Calcitriol increased mRNA content of genes involved in lipid droplet packaging (perilipin 2; PLIN 2, 2.07 fold) and lipolysis (comparative gene identification-58; CGI-58 and adipose triglyceride lipase; ATGL, ~ 1.80 fold). Calcitriol alters myocellular lipid partitioning and lipid droplet packaging which may favor lipid turnover and partially explain improvements in insulin sensitivity.

Defining lipid mediators of insulin resistance: controversies and challenges

“…These findings are consistent with the glucose intolerance/IR observed in livers of CerS2-knockout mice and obese HFD-fed CerS2 haploinsufficient mice (Park et al 2013, Raichur et al 2014. In addition, elevations in C24-ceramide content have recently been associated with increased insulin sensitivity in skeletal and in cardiac muscle, both in vitro and in vivo (Xie et al 2015, Chung et al 2017b, Jefferson et al 2017. Role differentiation mandates that, rather than total accumulation or deficit per se, cell health is underlain by ceramide equilibrium.…”

“…As a determinant in dissecting conflicting results, however, DAG acyl species may not wholly apply in isolation: di-C18:0 DAG accumulation in muscle has been identified as the greatest contributor to DAGmediated IR (Van Hees et al 2011, Bergman et al 2012, Holloway et al 2014, but also as preferentially correlated with the enhanced insulin sensitivity displayed by athletes (Amati et al 2011) and, more recently, via calcitriol supplementation (Jefferson et al 2017). There is some indication that chain length, too, can depend on subcellular location for its relevance.…”

Vitamin D deficiency in the aetiology of obesity‐related insulin resistance

“…In line, C2C12 mouse myotubes treated with 10 nM 1,25(OH) 2 D 3 for 48 hours ameliorated lipid‐induced insulin resistance, through improved insulin receptor substrate‐1 (IRS‐1) tyrosine phosphorylation and increased serine phosphorylation of protein kinase B ( PKB ), also known as Akt . Additionally, 1,25(OH) 2 D 3 increased mRNA content of perilipin 2 (PLIN 2) as well as adipose triglyceride lipase (ATGL) in C2C12 myotubes, indicating an involvement of vitamin D in muscle lipolysis and intramuscular lipid catabolism.…”

“…Obesity, insulin resistance, and T2D are often characterized by a relative circulating vitamin D deficiency (hypovitaminosis D), suggesting a role for vitamin D in the pathophysiology of obesity‐related metabolic disorders . There is accumulating mechanistic evidence for a role of vitamin D in glucose and lipid metabolism in insulin‐sensitive tissues like the adipose tissue, skeletal muscle, and liver . In this review, we discuss the latest knowledge on vitamin D metabolism as well as its role in glycaemic control and lipid metabolism.…”

New Insights on Low Vitamin D Plasma Concentration as a Potential Cardiovascular Risk Factor.

“…There are further data, obtained in animal models, which support the hypothesis that the positive effect of vitamin D on muscle cells can be therapeutically exploited. For example, in mice affected by dietinduced obesity and insulin resistance, the administration of vitamin D significantly improves the response to an oral glucose load and ameliorates the HOMA index; this is related to a direct effect on muscle cells which is paralleled by a reduction of lipid storage and myosteatosis [165,166]. Similar data have been obtained on diabetic rats [167].…”