DAG accumulation from saturated fatty acids desensitizes insulin stimulation of glucose uptake in muscle cells

Abstract

The increased availability of saturated lipids has been correlated with development of insulin resistance, although the basis for this impairment is not defined. This work examined the interaction of saturated and unsaturated fatty acids (FA) with insulin stimulation of glucose uptake and its relation to the FA incorporation into different lipid pools in cultured human muscle. It is shown that basal or insulin-stimulated 2-deoxyglucose uptake was unaltered in cells preincubated with oleate, whereas basal glucose uptake was increased and insulin response was impaired in palmitate- and stearate-loaded cells. Analysis of the incorporation of FA into different lipid pools showed that palmitate, stearate, and oleate were similarly incorporated into phospholipids (PL) and did not modify the FA profile. In contrast, differences were observed in the total incorporation of FA into triacylglycerides (TAG): unsaturated FA were readily diverted toward TAG, whereas saturated FA could accumulate as diacylglycerol (DAG). Treatment with palmitate increased the activity of membrane-associated protein kinase C, whereas oleate had no effect. Mixture of palmitate with oleate diverted the saturated FA toward TAG and abolished its effect on glucose uptake. In conclusion, our data indicate that saturated FA-promoted changes in basal glucose uptake and insulin response were not correlated to a modification of the FA profile in PL or TAG accumulation. In contrast, these changes were related to saturated FA being accumulated as DAG and activating protein kinase C. Therefore, our results suggest that accumulation of DAG may be a molecular link between an increased availability of saturated FA and the induction of insulin resistance.

Are cultured human myotubes far from home?

“…However, the ratio of GLUT1:GLUT4 is higher in human myotubes compared to adult skeletal muscle (Sarabia, et al, 1992), resulting in a lower insulin responsiveness of glucose transport (Al-Khalili, et al, 2003, Sarabia, et al, 1992. Insulin typically increases glucose uptake by 40-50 % in myotubes (Aas, et al, 2002, Al-Khalili, et al, 2003, McIntyre, et al, 2004, Montell, et al, 2001. Even though the effect of insulin is lower in myotubes than in intact skeletal muscle, the responsible molecular mechanisms of glucose transport remain the same (Al-Khalili, et al, 2003).…”

“…It is also important to take notice of that glucose uptake in vivo is regulated by delivery, transport and metabolism (Rose and Richter, 2005), whereas cultured myotubes only allow studies of transport and metabolism. In these studies, another limitation to the myotube model is that compared to in vivo, the insulin-stimulated glucose uptake in primary human myotubes is relatively low (about 1.5-fold increased) , Al-Khalili, et al, 2003, McIntyre, et al, 2004, Montell, et al, 2001, possibly caused by a low expression of the insulin-responsive glucose transporter GLUT4 (Al-Khalili, et al, 2003, Sarabia, et al, 1992. In vivo, GLUT4 is more expressed in type 1 oxidative muscle fibers than type 2 fibers (Daugaard, et al, 2000, Gaster, et al, 2000, Stuart, et al, 2006, and insulin sensitivity correlates with the proportion of slow twitch oxidative fibers in the muscle (Lillioja, et al, 1987).…”

Adiponectin resistance and vascular dysfunction in the hyperlipidemic state

“…Increased diacylglycerol and ceramide in hyperlipidemia has been shown to activate PKC, decrease IRS-1-associated PI3K activity, and inhibit phosphorylation/activation of Akt [74,75] . These signal transduction effects may decrease eNOS activity and NO production.…”

Skeletal muscle lipid metabolism with obesity

“…It is believed, however, that the accumulation of IMTG is not the direct cause of the development of insulin resistance but that IMTG is an inert marker for the presence of other lipid intermediates (diacylglycerol, fatty acyl-CoAs, or ceramide, etc. ), which have been directly linked to defects in insulin signaling (8,17,25,32,37).To date, the mechanism(s) responsible for the accretion of IMTG and intermediates of lipid metabolism in intact skeletal muscle are not evident. Two possibilities include an increase in lipid synthesis and/or a reduction in fatty acid oxidation, both of which may result in the accumulation of IMTG and other intermediates of lipid metabolism.…”

“…It is believed, however, that the accumulation of IMTG is not the direct cause of the development of insulin resistance but that IMTG is an inert marker for the presence of other lipid intermediates (diacylglycerol, fatty acyl-CoAs, or ceramide, etc. ), which have been directly linked to defects in insulin signaling (8,17,25,32,37).…”