PLIN5 deletion remodels intracellular lipid composition and causes insulin resistance in muscle

Defective control of lipid metabolism leading to lipotoxicity causes insulin resistance in skeletal muscle, a major factor leading to diabetes. Here, we demonstrate that perilipin (PLIN) 5 is required to couple intramyocellular triacylglycerol lipolysis with the metabolic demand for fatty acids. PLIN5 ablation depleted triacylglycerol stores but increased sphingolipids including ceramide, hydroxylceramides and sphingomyelin. We generated perilipin 5 (Plin5)-/- mice to determine the functional significance of PLIN5 in metabolic control and insulin action. Loss of PLIN5 had no effect on body weight, feeding or adiposity but increased whole-body carbohydrate oxidation. Plin5-/- mice developed skeletal muscle insulin resistance, which was associated with ceramide accumulation. Liver insulin sensitivity was improved in Plin5-/- mice, indicating tissue-specific effects of PLIN5 on insulin action. We conclude that PLIN5 plays a critical role in coordinating skeletal muscle triacylglycerol metabolism, which impacts sphingolipid metabolism, and is requisite for the maintenance of skeletal muscle insulin action. © 2014 The Authors

Plasma sphingosine-1-phosphate is elevated in obesity

Background: Dysfunctional lipid metabolism is a hallmark of obesity and insulin resistance and a risk factor for various cardiovascular and metabolic complications. In addition to the well known increase in plasma triglycerides and free fatty acids, recent work in humans and rodents has shown that obesity is associated with elevations in the bioactive class of sphingolipids known as ceramides. However, in obesity little is known about the plasma concentrations of sphinogsine-1-phosphate (S1P), the breakdown product of ceramide, which is an important signaling molecule in mammalian biology. Therefore, the purpose of this study was to examine the impact of obesity on circulating S1P concentration and its relationship with markers of glucose metabolism and insulin sensitivity. Methodology/Principal Findings: Plasma S1P levels were determined in high-fat diet (HFD)-induced and genetically obese (ob/ob) mice along with obese humans. Circulating S1P was elevated in both obese mouse models and in obese humans compared with lean healthy controls. Furthermore, in humans, plasma S1P positively correlated with total body fat percentage, body mass index (BMI), waist circumference, fasting insulin, HOMA-IR, HbA1c (%), total and LDL cholesterol. In addition, fasting increased plasma S1P levels in lean healthy mice. Conclusion: We show that elevations in plasma S1P are a feature of both human and rodent obesity and correlate with metabolic abnormalities such as adiposity and insulin resistance

Plasma S1P concentrations are elevated in obesity.

<p>Plasma S1P concentrations in chow (N = 13) and HFD (N = 14) fed mice (A); plasma S1P levels in lean (N = 6) and <i>ob/ob</i> (N = 8) mice (B) and circulating S1P in lean (N = 15) and obese (N = 10) humans (C). Data are mean ± SEM. **P&lt;0.01.</p

Reversing diet-induced metabolic dysregulation by diet switching leads to altered hepatic de novo lipogenesis and glycerolipid synthesis

In humans, low-energy diets rapidly reduce hepatic fat and improve/normalise glycemic control. Due to difficulties in obtaining human liver, little is known about changes to the lipid species and pathway fluxes that occur under these conditions. Using a combination of stable isotope, and targeted metabolomic approaches we investigated the acute (7-9 days) hepatic effects of switching high-fat high-sucrose diet (HFD) fed obese mice back to a chow diet. Upon the switch, energy intake was reduced, resulting in reductions of fat mass and hepatic triacyl- and diacylglycerol. However, these parameters were still elevated compared to chow fed mice, thus representing an intermediate phenotype. Nonetheless, glucose intolerance and hyperinsulinemia were completely normalized. The diet reversal resulted in marked reductions in hepatic de novo lipogenesis when compared to the chow and HFD groups. Compared with HFD, glycerolipid synthesis was reduced in the reversal animals, however it remained elevated above that of chow controls, indicating that despite experiencing a net loss in lipid stores, the liver was still actively esterifying available fatty acids at rates higher than that in chow control mice. This effect likely promotes the re-esterification of excess free fatty acids released from the breakdown of adipose depots during the weight loss period

Plasma S1P concentrations in mice are elevated in response to fasting.

<p>Changes in body mass (A), blood glucose (B), plasma NEFA (C) and plasma S1P concentrations (D) in response to fasting. Data are mean ± SEM. *P&lt;0.05; **P&lt;0.01, N = 11–12.</p

Plasma S1P concentrations correlate with clinical indices of metabolic dysfunction in humans.

<p>Plasma S1P concentrations correlate with percent body fat (A), BMI (B), waist circumference (C), fasting plasma insulin (D), HOMA-IR (E), HbA1c (F), total (G) and LDL (H) cholesterol.</p

Overexpression of sphingosine kinase 1 in liver reduces triglyceride content in mice fed a low but not high-fat diet

&nbsp;Hepatic insulin resistance is a major risk factor for the development of type 2 diabetes and is associated with the accumulation of lipids, including diacylglycerol (DAG), triacylglycerols (TAG) and ceramide. There is evidence that enzymes involved in ceramide or sphingolipid metabolism may have a role in regulating concentrations of glycerolipids such as DAG and TAG. Here we have investigated the role of sphingosine kinase (SphK) in regulating hepatic lipid levels. We show that mice on a high-fat high-sucrose diet (HFHS) displayed glucose intolerance, elevated liver TAG and DAG, and a reduction in total hepatic SphK activity. Reduced SphK activity correlated with downregulation of SphK1, but not SphK2 expression, and was not associated with altered ceramide levels. The role of SphK1 was further investigated by overexpressing this isoform in the liver of mice in vivo. On a low-fat diet (LFD) mice overexpressing liver SphK1, displayed reduced hepatic TAG synthesis and total TAG levels, but with no change to DAG or ceramide. These mice also exhibited no change in gluconeogenesis, glycogenolysis or glucose tolerance. Similarly, overexpression of SphK1 had no effect on the pattern of endogenous glucose production determined during a glucose tolerance test. Under HFHS conditions, normalization of liver SphK activity to levels observed in LFD controls did not alter hepatic TAG concentrations. Furthermore, DAG, ceramide and glucose tolerance were also unaffected. In conclusion, our data suggest that SphK1 plays an important role in regulating TAG metabolism under LFD conditions

The CDP-Ethanolamine Pathway Regulates Skeletal Muscle Diacylglycerol Content and Mitochondrial Biogenesis without Altering Insulin Sensitivity.

Accumulation of diacylglycerol (DG) in muscle is thought to cause insulin resistance. DG is a precursor for phospholipids, thus phospholipid synthesis could be involved in regulating muscle DG. Little is known about the interaction between phospholipid and DG in muscle; therefore, we examined whether disrupting muscle phospholipid synthesis, specifically phosphatidylethanolamine (PtdEtn), would influence muscle DG content and insulin sensitivity. Muscle PtdEtn synthesis was disrupted by deleting CTP:phosphoethanolamine cytidylyltransferase (ECT), the rate-limiting enzyme in the CDP-ethanolamine pathway, a major route for PtdEtn production. While&nbsp;PtdEtn was reduced in muscle-specific ECT knockout mice, intramyocellular and membrane-associated DG was markedly increased. Importantly, however, this was not associated with insulin resistance. Unexpectedly, mitochondrial biogenesis and muscle oxidative capacity were increased in muscle-specific ECT knockout mice and were accompanied by enhanced exercise performance. These findings highlight the importance of the CDP-ethanolamine pathway in regulating muscle DG content and challenge the DG-induced insulin resistance hypothesis