Treatment of human skeletal muscle cells with inhibitors of diacylglycerol acyltransferases 1 and 2 to explore isozyme-specific roles on lipid metabolism

Abstract

Diacylglycerol acyltransferases (DGAT) 1 and 2 catalyse the final step in triacylglycerol (TAG) synthesis, the esterification of fatty acyl-CoA to diacylglycerol. Despite catalysing the same reaction and being present in the same cell types, they exhibit different functions on lipid metabolism in various tissues. Yet, their roles in skeletal muscle remain poorly defined. In this study, we investigated how selective inhibitors of DGAT1 and DGAT2 affected lipid metabolism in human primary skeletal muscle cells. The results showed that DGAT1 was dominant in human skeletal muscle cells utilizing fatty acids (FAs) derived from various sources, both exogenously supplied FA, de novo synthesised fA, or fA derived from lipolysis, to generate TAG, as well as being involved in de novo synthesis of tAG. on the other hand, DGAT2 seemed to be specialised for de novo synthesis of TAG from glycerol-3-posphate only. Interestingly, DGAT activities were also important for regulating FA oxidation, indicating a key role in balancing FAs between storage in TAG and efficient utilization through oxidation. Finally, we observed that inhibition of DGAT enzymes could potentially alter glucose-FA interactions in skeletal muscle. In summary, treatment with DGAT1 or DGAT2 specific inhibitors resulted in different responses on lipid metabolism in human myotubes, indicating that the two enzymes play distinct roles in TAG metabolism in skeletal muscle.Skeletal muscle utilizes both carbohydrates and fat as energy sources. Approximately 50-60% of the free fatty acids (FFAs) taken up by skeletal muscle are stored as triacylglycerol (TAG) in lipid droplets (LDs) 1 . TAG, which is a neutral lipid, consists of a glycerol backbone and three FAs attached by ester bonds. The terminal and only committed step of TAG synthesis, the esterification of fatty acyl-CoA to diacylglycerol (DAG), is catalysed by the enzymes diacylglycerol acyltransferase (DGAT) 1 and 2 2-4 . Both DGAT enzymes reside at the endoplasmic reticulum 5,6 , though DGAT2 is also found to co-localize with LDs and mitochondria in cultured fibroblasts and adipocytes, in contrast to DGAT1 5,6 . Although the two isozymes catalyse the same reaction, there are several differences between them. They share no sequence homology with each other, belong to unrelated families of proteins 4 and overexpression of the two isozymes in rat hepatoma cells give rise to LDs with markedly different morphology (size) and intracellular distribution 7 . In addition, they are non-redundant in some functions, which are reflected by the phenotype of mice lacking DGAT1 or DGAT2. Whereas Dgat1 −/− mice are viable with a favourable metabolic phenotype showing an increased insulin and leptin sensitivity and resistance to diet-induced obesity, Dgat2 −/− mice die shortly after birth; they are lipopenic, have a defect in the skin barrier leading to rapid dehydration 8-10 , and are possibly unable to utilize glucose in brown adipocytes for thermoregulation 11 . TAG formation can occur in two ways, namely from re-esterificati...

Free Fatty Acid Species Differentially Modulate the Inflammatory Gene Response in Primary Human Skeletal Myoblasts

“…It has been previously suggested that FFAs may affect lipid metabolism in differentiated human myotubes [ 14 , 15 ]. In human myoblasts, C18[2]c exclusively led to lipid accumulation within 48 h. Although the long-term effects of C18[2]c were not studied here, we assume that unceasing lipid accumulation may affect signaling pathways and gene expression.…”

“…Immortalized rodent muscle cell lines have been employed to study the molecular mechanisms of FFA actions in the regulation of metabolism, apoptosis and insulin resistance [ 9 , 10 , 11 , 12 ]. To extend the relevance of the outcomes to humans, some studies have utilized in vitro differentiated human myotubes to study the effects of FFAs on lipid metabolism, mitochondrial function and the expression of particular genes of interest [ 13 , 14 , 15 , 16 ]. In organisms, FFAs commonly exhibit diverse primary or secondary structures.…”

“…The immortalized murine cell line C2C12 or transformed rat cell line L6 is commonly utilized to study the regulation of metabolism and myogenesis in skeletal muscle cells [ 9 , 10 , 11 , 12 , 25 , 26 , 27 ]. Few studies have utilized in vitro differentiated human myotubes obtained by long-term maintenance of human myoblasts from multiple donors in the presence of recombinant growth hormones [ 13 , 14 , 15 , 16 ]. A comparative study revealed that in vitro differentiated human myotubes deviate significantly from their precursor myocytes [ 28 ].…”

Skeletal muscle energy metabolism in obesity

“…DGAT1 and DGAT2 are enzymes that catalyze the final step of TAG synthesis. Our recent study has shown that acute inhibition of DGAT1 and DGAT2 using selective enzymatic inhibitors had distinct effects on lipid metabolism in myotubes (127). Inhibition of DGAT1, the most abundant isoform in skeletal muscle, resulted in an almost totally abolished incorporation of exogenous FAs into TAG and LDs.…”

DGAT2 Inhibition Potentiates Lipid Droplet Formation To Reduce Cytotoxicity in APOL1 Kidney Risk Variants

“…DGAT1 and DGAT2 are key enzymes important for the final step in LD formation. DGAT1 inhibition is known to inhibit LD formation thus leading to a reduction in the number of LDs in cells, but some studies have shown that DGAT2 inhibition can lead to a paradoxical increase of LDs 40 – 42 . Therefore, we hypothesized that DGAT2 inhibition could increase both LD size and number, leading to both recruitment of APOL1 G2/G2 and improved cell survival.…”