Serum dihydroceramides correlate with insulin sensitivity in humans and decrease insulin sensitivity in vitro

Combined diet and exercise training decreases serum lipids associated with insulin resistance

OBJECTIVE

Circulating lipids are linked with insulin resistance and increased cardiovascular disease risk. We previously reported that dihydroceramides, a specific type of sphingolipid, are elevated in insulin-resistant individuals; however, little is known regarding whether insulin-sensitizing lifestyle interventions can improve profiles of sphingolipids and other lipid species.

METHODS

A total of 21 individuals with obesity participated in a 3-month lifestyle intervention of combined weight loss and exercise training. Insulin sensitivity was assessed by hyperinsulinemic-euglycemic clamps, and serum lipidomics was conducted.

RESULTS

Following the intervention, BMI was significantly reduced by 10%; VO2peak and insulin sensitivity increased by 12% and 57%, respectively; and total serum triacylglycerol (TAG), diacylglycerol, dihydroceramides, sphingosine-1-phosphate, and sphinganine-1-phosphate were significantly reduced, as were specific species of dihydroceramides (C18:0 and C24:1). Individuals with higher preintervention TAG concentrations had significant decreases in serum lipids, which were not significantly changed in individuals with lower preintervention TAG.

CONCLUSIONS

These data show that serum sphingolipid species previously linked to insulin resistance in humans can be reduced with insulin-sensitizing lifestyle interventions. Furthermore, individuals with elevated serum TAG may significantly benefit from lifestyle interventions that increase insulin sensitivity due to a greater decrease in serum lipids related to insulin resistance.

Oxidised phosphatidylcholine induces sarcolemmal ceramide accumulation and insulin resistance in skeletal muscle

AIMS/HYPOTHESIS

Intracellular ceramide accumulation in specific cellular compartments is a potential mechanism explaining muscle insulin resistance in the pathogenesis of type 2 diabetes. Muscle sarcolemmal ceramide accumulation negatively impacts insulin sensitivity in humans, but the mechanism explaining this localised accumulation is unknown. Previous reports revealed that circulating oxidised LDL is elevated in serum of individuals with obesity and type 2 diabetes. Oxidised phosphatidylcholine, which is present in oxidised LDL, has previously been linked to ceramide pathway activation, and could contribute to localised ceramide accumulation in skeletal muscle. We hypothesised that oxidised phosphatidylcholine inversely correlates with insulin sensitivity in serum, and induces sarcolemmal ceramide accumulation and decreases insulin sensitivity in muscle.

METHODS

We used LC-MS/MS to quantify specific oxidised phosphatidylcholine species in serum from a cross-sectional study of 58 well-characterised individuals spanning the physiological range of insulin sensitivity. We also performed in vitro experiments in rat L6 myotubes interrogating the role of specific oxidised phosphatidylcholine species in promoting sarcolemmal ceramide accumulation, inflammation and insulin resistance in skeletal muscle cells.

RESULTS

Human serum oxidised phosphatidylcholine levels are elevated in individuals with obesity and type 2 diabetes, inversely correlated with insulin sensitivity, and positively correlated with sarcolemmal C18:0 ceramide levels in skeletal muscle. Specific oxidised phosphatidylcholine species, particularly 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (POVPC), increase total ceramide and dihydroceramide and decrease total sphingomyelin in the sarcolemma of L6 myotubes by de novo ceramide synthesis and sphingomyelinase activation. POVPC also increases inflammatory signalling and causes insulin resistance in L6 myotubes.

CONCLUSIONS/INTERPRETATION

These data suggest that circulating oxidised phosphatidylcholine species promote ceramide accumulation and decrease insulin sensitivity in muscle, help explain localised sphingolipid accumulation and muscle inflammatory response, and highlight oxidised phosphatidylcholine species as potential targets to combat insulin resistance.

Effect of CFTR Modulators on Oxidative Stress and Autophagy in Non-CFTR-Expressing Cells

The triple combination therapy for cystic fibrosis (CF), including elexacaftor, tezacaftor and ivacaftor (ETI or Trikafta), has been shown to improve lung function and reduce pulmonary exacerbations, thereby enhancing the quality of life for most CF patients. Recent findings suggest that both the individual components and ETI may have potential off-target effects, highlighting the need to understand how these modulators impact cellular physiology, particularly in cells that do not express CF transmembrane conductance regulator (CFTR). We used HEK293 cells, as a cell model not expressing the CFTR protein, to evaluate the effect of ETI and each of its components on autophagic machinery and on the Rab5/7 components of the Rab pathway. We firstly demonstrate that the single modulators Teza and Iva, and the combinations ET and ETI, increased ROS production in the absence of their target while decreasing it in cells expressing the CFTR ∆F508del. This increase in cellular stress was followed by an increase in the total level of polyubiquitinated proteins as well as the p62 level and LC3II/LC3I ratio. Furthermore, we found that ETI had the opposite effect on Rabs by increasing Rab5 levels while decreasing Rab7. Interestingly, these changes were abolished by the expression of mutated CFTR. Overall, our data suggest that in the absence of their target, both the individual modulators and ETI increased ROS production and halted both autophagic flux and plasma membrane protein recycling.

Analysis of Inflammatory Markers in Response to Induction of Reprometabolic Syndrome by a Eucaloric High Fat Diet in Normal Weight Women

Obesity is associated with chronic low-level inflammation and is known to contribute to metabolic dysfunction and hypogonadotropic hypogonadism, which we have previously termed the 'Reprometabolic Syndrome.' To investigate potential factors involved in obesity-related reproductive endocrine dysfunction, we conducted a secondary analysis of inflammatory markers in a sample of normal weight women exposed to a one-month eucaloric high-fat diet (HFD), which, as reported earlier, induced the relative hypogonadotropic hypogonadism characteristic of Reprometabolic Syndrome. Eighteen healthy women with a BMI between 18.0-24.9 kg/m2 and regular menstrual cycles participated in the study. Frequent blood sampling was performed during the early follicular phase before and after the one-month eucaloric HFD intervention (48% of calories from fat). Serum samples pooled from each participant were analyzed using immunoassay to measure levels of 30 cytokines, interleukins, and chemokines. Differences between pre- and post-HFD intervention measures were examined by one-sample t-tests. Exposure to the eucaloric HFD did not result in changes in body weight. HFD-induction of Reprometabolic Syndrome in normal weight women was associated with a significant elevation only in the anti- inflammatory cytokine IL-10 (p = 0.04). Eotaxin, IL-6 and MIP-1β also increased in response to the HFD, but not statistically significantly (p = 0.07). Results suggest that the increase in multiple inflammatory markers, typically associated with obesity, are not primary mediators of the relative hypogonadotropic hypogonadism of Reprometabolic Syndrome. Clinical Trials Registration Number: NCT02653092; Date of Registration: January 6, 2016.

Intramuscular diacylglycerol accumulates with acute hyperinsulinemia in insulin-resistant phenotypes

Elevated skeletal muscle diacylglycerols (DAGs) and ceramides can impair insulin signaling, and acylcarnitines (acylCNs) reflect impaired mitochondrial fatty acid oxidation, thus, the intramuscular lipid profile is indicative of insulin resistance. Acute (i.e., postprandial) hyperinsulinemia has been shown to elevate lipid concentrations in healthy muscle and is an independent risk factor for type 2 diabetes (T2D). However, it is unclear how the relationship between acute hyperinsulinemia and the muscle lipidome interacts across metabolic phenotypes, thus contributing to or exacerbating insulin resistance. We therefore investigated the impact of acute hyperinsulinemia on the skeletal muscle lipid profile to help characterize the physiological basis in which hyperinsulinemia elevates T2D risk. In a cross-sectional comparison, endurance athletes (n = 12), sedentary lean adults (n = 12), and individuals with obesity (n = 13) and T2D (n = 7) underwent a hyperinsulinemic-euglycemic clamp with muscle biopsies. Although there were no significant differences in total 1,2-DAG fluctuations, there was a 2% decrease in athletes versus a 53% increase in T2D during acute hyperinsulinemia (P = 0.087). Moreover, C18 1,2-DAG species increased during the clamp with T2D only, which negatively correlated with insulin sensitivity (P < 0.050). Basal muscle C18:0 total ceramides were elevated with T2D (P = 0.029), but not altered by clamp. Acylcarnitines were universally lowered during hyperinsulinemia, with more robust reductions of 80% in athletes compared with only 46% with T2D (albeit not statistically significant, main effect of group, P = 0.624). Similar fluctuations with acute hyperinsulinemia increasing 1,2 DAGs in insulin-resistant phenotypes and universally lowering acylcarnitines were observed in male mice. In conclusion, acute hyperinsulinemia elevates muscle 1,2-DAG levels with insulin-resistant phenotypes. This suggests a possible dysregulation of intramuscular lipid metabolism in the fed state in individuals with low insulin sensitivity, which may exacerbate insulin resistance.NEW & NOTEWORTHY Postprandial hyperinsulinemia is a risk factor for type 2 diabetes and may increase muscle lipids. However, it is unclear how the relationship between acute hyperinsulinemia and the muscle lipidome interacts across metabolic phenotypes, thus contributing to insulin resistance. We observed that acute hyperinsulinemia elevates muscle 1,2-DAGs in insulin-resistant phenotypes, whereas ceramides were unaltered. Insulin-mediated acylcarnitine reductions are also hindered with high-fat feeding. The postprandial period may exacerbate insulin resistance in metabolically unhealthy phenotypes.

Ceramides are fuel gauges on the drive to cardiometabolic disease

Ceramides are signals of fatty acid excess that accumulate when a cell's energetic needs have been met and its nutrient storage has reached capacity. As these sphingolipids accrue, they alter the metabolism and survival of cells throughout the body including in the heart, liver, blood vessels, skeletal muscle, brain, and kidney. These ceramide actions elicit the tissue dysfunction that underlies cardiometabolic diseases such as diabetes, coronary artery disease, metabolic-associated steatohepatitis, and heart failure. Here, we review the biosynthesis and degradation pathways that maintain ceramide levels in normal physiology and discuss how the loss of ceramide homeostasis drives cardiometabolic pathologies. We highlight signaling nodes that sense small changes in ceramides and in turn reprogram cellular metabolism and stimulate apoptosis. Finally, we evaluate the emerging therapeutic utility of these unique lipids as biomarkers that forecast disease risk and as targets of ceramide-lowering interventions that ameliorate disease.

Adipose Tissue Dysfunction Determines Lipotoxicity and Triggers the Metabolic Syndrome: Current Challenges and Clinical Perspectives

The adipose tissue organ is organised as distinct anatomical depots located all along the body axis, and it is constituted of three different types of adipocytes: white, beige and brown, which are integrated with vascular, immune, neural, and extracellular stroma cells. These distinct adipocytes serve different specialised functions. The main function of white adipocytes is to ensure healthy storage of excess nutrients/energy and its rapid mobilisation to supply the demand of energy imposed by physiological cues in other organs, whereas brown and beige adipocytes are designed for heat production through uncoupling lipid oxidation from energy production. The concerted action of the three types of adipocytes/tissues ensures an optimal metabolic status. However, when one or several of these adipose depots become dysfunctional because of sustained lipid/nutrient overload, then insulin resistance and associated metabolic complications ensue. These metabolic alterations close a vicious cycle that negatively affects the adipose tissue functionality and compromises global metabolic homeostasis. Optimising white adipose tissue expandability and ensuring its functional metabolic flexibility and/or promoting brown/beige mediated thermogenic activity are complementary strategies that counteract obesity and its associated lipotoxic metabolic effects. However, the development of these therapeutic approaches requires a deep understanding of adipose tissue in all broad aspects. In this chapter, we will discuss the characteristics of the different adipose tissue depots with respect to origins and precursors recruitment, plasticity, cellular composition, and expandability capacity potential as well as molecular and metabolic characteristic signatures in both physiological and pathophysiological conditions. Current antilipotoxic strategies for future clinical application are also discussed in this chapter.

Inter-organ crosstalk during development and progression of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is characterized by tissue-specific insulin resistance and pancreatic β-cell dysfunction, which result from the interplay of local abnormalities within different tissues and systemic dysregulation of tissue crosstalk. The main local mechanisms comprise metabolic (lipid) signalling, altered mitochondrial metabolism with oxidative stress, endoplasmic reticulum stress and local inflammation. While the role of endocrine dysregulation in T2DM pathogenesis is well established, other forms of inter-organ crosstalk deserve closer investigation to better understand the multifactorial transition from normoglycaemia to hyperglycaemia. This narrative Review addresses the impact of certain tissue-specific messenger systems, such as metabolites, peptides and proteins and microRNAs, their secretion patterns and possible alternative transport mechanisms, such as extracellular vesicles (exosomes). The focus is on the effects of these messengers on distant organs during the development of T2DM and progression to its complications. Starting from the adipose tissue as a major organ relevant to T2DM pathophysiology, the discussion is expanded to other key tissues, such as skeletal muscle, liver, the endocrine pancreas and the intestine. Subsequently, this Review also sheds light on the potential of multimarker panels derived from these biomarkers and related multi-omics for the prediction of risk and progression of T2DM, novel diabetes mellitus subtypes and/or endotypes and T2DM-related complications.

Mitochondrial electron transport chain, ceramide, and coenzyme Q are linked in a pathway that drives insulin resistance in skeletal muscle

Insulin resistance (IR) is a complex metabolic disorder that underlies several human diseases, including type 2 diabetes and cardiovascular disease. Despite extensive research, the precise mechanisms underlying IR development remain poorly understood. Previously we showed that deficiency of coenzyme Q (CoQ) is necessary and sufficient for IR in adipocytes and skeletal muscle (Fazakerley et al., 2018). Here, we provide new insights into the mechanistic connections between cellular alterations associated with IR, including increased ceramides, CoQ deficiency, mitochondrial dysfunction, and oxidative stress. We demonstrate that elevated levels of ceramide in the mitochondria of skeletal muscle cells result in CoQ depletion and loss of mitochondrial respiratory chain components, leading to mitochondrial dysfunction and IR. Further, decreasing mitochondrial ceramide levels in vitro and in animal models (mice, C57BL/6J) (under chow and high-fat diet) increased CoQ levels and was protective against IR. CoQ supplementation also rescued ceramide-associated IR. Examination of the mitochondrial proteome from human muscle biopsies revealed a strong correlation between the respirasome system and mitochondrial ceramide as key determinants of insulin sensitivity. Our findings highlight the mitochondrial ceramide-CoQ-respiratory chain nexus as a potential foundation of an IR pathway that may also play a critical role in other conditions associated with ceramide accumulation and mitochondrial dysfunction, such as heart failure, cancer, and aging. These insights may have important clinical implications for the development of novel therapeutic strategies for the treatment of IR and related metabolic disorders.

Serum ceramides in early pregnancy as predictors of gestational diabetes

Ceramides contribute to the development of type 2 diabetes but it is uncertain whether they predict gestational diabetes (GDM). In this multicentre case-control study including 1040 women with GDM and 958 non-diabetic controls, early pregnancy (mean 10.7 gestational weeks) concentrations of four ceramides-Cer(d18:1/16:0), Cer(d18:1/18:0), Cer(d18:1/24:0) and Cer(d18:1/24:1)-were determined by a validated mass-spectrometric method from biobanked serum samples. Traditional lipids including total cholesterol, LDL, HDL and triglycerides were measured. Logistic and linear regression and the LASSO logistic regression were used to analyse lipids and clinical risk factors in the prediction of GDM. The concentrations of four targeted ceramides and total cholesterol, LDL and triglycerides were higher and HDL was lower among women with subsequent GDM than among controls. After adjustments, Cer(d18:1/24:0), triglycerides and LDL were independent predictors of GDM, women in their highest quartile had 1.44-fold (95% CI 1.07-1.95), 2.17-fold (95% CI 1.57-3.00) and 1.63-fold (95% CI 1.19-2.24) odds for GDM when compared to their lowest quartiles, respectively. In the LASSO regression modelling ceramides did not appear to markedly improve the predictive performance for GDM alongside with clinical risk factors and triglycerides. However, their adverse alterations highlight the extent of metabolic disturbances involved in GDM.

Liraglutide reduces plasma dihydroceramide levels in patients with type 2 diabetes

BACKGROUND

Emerging evidence supports that dihydroceramides (DhCer) and ceramides (Cer) contribute to the pathophysiology of insulin resistance and liver steatosis, and that their circulating concentrations are independently associated with cardiovascular outcomes. Circulating DhCer levels are increased in patients with type 2 diabetes (T2D). On the other hand, the GLP-1 receptor agonist liraglutide reduces major adverse cardiac events, insulin resistance and liver steatosis in T2D patients. The main purpose of the present study was therefore to investigate whether liraglutide decreases circulating levels of DhCer and Cer in T2D patients, which could be a mechanism involved in its cardiometabolic benefits. The secondary purpose was to assess the relationship between liraglutide-induced changes in DhCer/Cer levels and insulin resistance and liver steatosis.

METHODS

Plasma concentrations of 11 DhCer and 15 Cer species were measured by a highly-sensitive mass spectrometry system in 35 controls and 86 T2D patients before and after 6 months of liraglutide (1.2 mg/day). Insulin resistance was estimated by the triglyceride-glucose (TyG) index. Liver fat content (LFC) was assessed in 53 patients by proton magnetic resonance spectroscopy.

RESULTS

Plasma levels of total DhCer, 7 DhCer and 7 Cer species were increased in T2D patients compared to controls. Liraglutide decreased total DhCer by 15.1% (p = 0.005), affecting 16:0 (p = 0.037), 18:0 (p < 0.0001), 18:1 (p = 0.0005), 20:0 (p = 0.0003), 23:0 (p = 0.005) and 24:1 (p = 0.04) species. Total plasma Cer did not significantly change after liraglutide (p = 0.18), but 5 Cer species decreased significantly, i.e. 18:0 and 18:1 (both p < 0.0001), 19:0 and 24:1 (both p < 0.01) and 26:1 (p = 0.04). In multivariate analysis, the reduction in DhCer after liraglutide was independently associated with the reduction in LFC (p = 0.0005) and in TyG index (p = 0.05).

CONCLUSIONS

Liraglutide reduces plasma levels of numerous DhCer and Cer species in T2D patients, which may contribute to the cardiovascular benefit observed in the LEADER trial. The independent association between the decrease in plasma DhCer level with the reduction in LFC and TyG index adds new insights regarding the relationship between DhCer, liver steatosis and insulin resistance. Trial registration ClinicalTrials.gov identifier: NCT02721888.

Chewing the fat: How lipidomics is changing our understanding of human health and disease in 2022

Lipids are biological molecules that play vital roles in all living organisms. They perform many cellular functions, such as 1) forming cellular and subcellular membranes, 2) storing and using energy, and 3) serving as chemical messengers during intra- and inter-cellular signal transduction. The large-scale study of the pathways and networks of cellular lipids in biological systems is called "lipidomics" and is one of the fastest-growing omics technologies of the last two decades. With state-of-the-art mass spectrometry instrumentation and sophisticated data handling, clinical studies show how human lipid composition changes in health and disease, thereby making it a valuable medium to collect for clinical applications, such as disease diagnostics, therapeutic decision-making, and drug development. This review gives a comprehensive overview of current workflows used in clinical research, from sample collection and preparation to data and clinical interpretations. This is followed by an appraisal of applications in 2022 and a perspective on the exciting future of clinical lipidomics.