Plasma C24:0- and C26:0-lysophosphatidylcholines are reliable biomarkers for the diagnosis of peroxisomal β-oxidation disorders

The gold-standard diagnostic test for peroxisomal disorders (PDs) is plasma concentration analysis of very long-chain fatty acids (VLCFAs). However, this method's time-consuming nature and limitations in cases which present normal VLCFA levels necessitates alternative approaches. The analysis of C26:0-lysophosphatydylcholine (C26:0-LPC) in dried blood spot samples by tandem-mass spectrometry (MS/MS) has successfully been implemented in certain newborn screening programs to diagnose X-linked adrenoleukodystrophy (ALD). However, the diagnostic potential of very long-chain LPCs concentrations in plasma remains poorly understood. This study sought to evaluate the diagnostic performance of C26:0-LPC and other very long-chain LPCs, comparing them to VLCFA analysis in plasma. The study, which included 330 individuals affected by a peroxisomal β-oxidation deficiency and 407 control individuals, revealed that C26:0- and C24:0-LPC concentrations demonstrated the highest diagnostic accuracy (98.8% and 98.4%, respectively), outperforming VLCFA when C26:0/C22:0 and C24:0/C22:0 ratios were combined (98.1%). Combining C24:0- and C26:0-LPC gave the highest sensitivity (99.7%), with ALD females exhibiting notably higher sensitivity compared with the VLCFA ratio combination (98.7% vs. 93.5%, respectively). In contrast, C22:0-LPC exhibited suboptimal performance, primarily due to its low sensitivity (75%), but we identified a potential use to help distinguish between ALD and Zellweger spectrum disorders. In summary, MS/MS analysis of plasma C24:0- and C26:0-LPC concentrations represents a rapid and straightforward approach to diagnose PDs, demonstrating superior diagnostic accuracy, particularly in ALD females, compared with conventional VLCFA biomarkers. We strongly recommend integrating very-long chain LPC plasma analysis in the diagnostic evaluation of individuals suspected of having a PD.

Human HDL subclasses modulate energy metabolism in skeletal muscle cells

In addition to its antiatherogenic role, HDL reportedly modulates energy metabolism at the whole-body level. HDL functionality is associated with its structure and composition, and functional activities can differ between HDL subclasses. Therefore, we studied if HDL2 and HDL3, the two major HDL subclasses, are able to modulate energy metabolism of skeletal muscle cells. Differentiated mouse and primary human skeletal muscle myotubes were used to investigate the influences of human HDL2 and HDL3 on glucose and fatty uptake and oxidation. HDL-induced changes in lipid distribution and mRNA expression of genes related to energy substrate metabolism, mitochondrial function, and HDL receptors were studied with human myotubes. Additionally, we examined the effects of apoA-I and discoidal, reconstituted HDL particles on substrate metabolism. In mouse myotubes, HDL subclasses strongly enhanced glycolysis upon high and low glucose concentrations. HDL3 caused a minor increase in ATP-linked respiration upon glucose conditioning but HDL2 improved complex I-mediated mitochondrial respiration upon fatty acid treatment. In human myotubes, glucose metabolism was attenuated but fatty acid uptake and oxidation were markedly increased by both HDL subclasses, which also increased mRNA expression of genes related to fatty acid metabolism and HDL receptors. Finally, both HDL subclasses induced incorporation of oleic acid into different lipid classes. These results, demonstrating that HDL subclasses enhance fatty acid oxidation in human myotubes but improve anaerobic metabolism in mouse myotubes, support the role of HDL as a circulating modulator of energy metabolism. Exact mechanisms and components of HDL causing the change, require further investigation.

A single-day mouse mesenteric lymph surgery in mice: an updated approach to study dietary lipid absorption, chylomicron secretion, and lymphocyte dynamics

The intestine plays a crucial role in regulating whole-body lipid metabolism through its unique function of absorbing dietary fat. In the small intestine, absorptive epithelial cells emulsify hydrophobic dietary triglycerides (TAGs) prior to secreting them into mesenteric lymphatic vessels as chylomicrons. Except for short- and medium-chain fatty acids, which are directly absorbed from the intestinal lumen into portal vasculature, the only way for an animal to absorb dietary TAG is through the chylomicron/mesenteric lymphatic pathway. Isolating intestinal lipoproteins, including chylomicrons, is extremely difficult in vivo because of the dilution of postprandial lymph in the peripheral blood. In addition, once postprandial lymph enters the circulation, chylomicron TAGs are rapidly hydrolyzed. To enhance isolation of large quantities of pure postprandial chylomicrons, we have modified the Tso group's highly reproducible gold-standard double-cannulation technique in rats to enable single-day surgery and lymph collection in mice. Our technique has a significantly higher survival rate than the traditional 2-day surgical model and allows for the collection of greater than 400 μl of chylous lymph with high postprandial TAG concentrations. Using this approach, we show that after an intraduodenal lipid bolus, the mesenteric lymph contains naïve CD4+ T-cell populations that can be quantified by flow cytometry. In conclusion, this experimental approach represents a quantitative tool for determining dietary lipid absorption, intestinal lipoprotein dynamics, and mesenteric immunity. Our model may also be a powerful tool for studies of antigens, the microbiome, pharmacokinetics, and dietary compound absorption.

SSO and other putative inhibitors of FA transport across membranes by CD36 disrupt intracellular metabolism, but do not affect FA translocation

Membrane-bound proteins have been proposed to mediate the transport of long-chain FA (LCFA) transport through the plasma membrane (PM). These proposals are based largely on reports that PM transport of LCFAs can be blocked by a number of enzymes and purported inhibitors of LCFA transport. Here, using the ratiometric pH indicator (2',7'-bis-(2-carboxyethyl)-5-(and-6-)-carboxyfluorescein and acrylodated intestinal FA-binding protein-based dual fluorescence assays, we investigated the effects of nine inhibitors of the putative FA transporter protein CD36 on the binding and transmembrane movement of LCFAs. We particularly focused on sulfosuccinimidyl oleate (SSO), reported to be a competitive inhibitor of CD36-mediated LCFA transport. Using these assays in adipocytes and inhibitor-treated protein-free lipid vesicles, we demonstrate that rapid LCFA transport across model and biological membranes remains unchanged in the presence of these purported inhibitors. We have previously shown in live cells that CD36 does not accelerate the transport of unesterified LCFAs across the PM. Our present experiments indicated disruption of LCFA metabolism inside the cell within minutes upon treatment with many of the "inhibitors" previously assumed to inhibit LCFA transport across the PM. Furthermore, using confocal microscopy and a specific anti-SSO antibody, we found that numerous intracellular and PM-bound proteins are SSO-modified in addition to CD36. Our results support the hypothesis that LCFAs diffuse rapidly across biological membranes and do not require an active protein transporter for their transmembrane movement.

Inflammatory stress promotes the development of obesity-related chronic kidney disease via CD36 in mice

Ectopic fat located in the kidney has emerged as a novel cause of obesity-related chronic kidney disease (CKD). In this study, we aimed to investigate whether inflammatory stress promotes ectopic lipid deposition in the kidney and causes renal injury in obese mice and whether the pathological process is mediated by the fatty acid translocase, CD36. High-fat diet (HFD) feeding alone resulted in obesity, hyperlipidemia, and slight renal lipid accumulation in mice, which nevertheless had normal kidney function. HFD-fed mice with chronic inflammation had severe renal steatosis and obvious glomerular and tubular damage, which was accompanied by increased CD36 expression. Interestingly, CD36 deficiency in HFD-fed mice eliminated renal lipid accumulation and pathological changes induced by chronic inflammation. In both human mesangial cells (HMCs) and human kidney 2 (HK2) cells, inflammatory stress increased the efficiency of CD36 protein incorporation into membrane lipid rafts, promoting FFA uptake and intracellular lipid accumulation. Silencing of CD36 in vitro markedly attenuated FFA uptake, lipid accumulation, and cellular stress induced by inflammatory stress. We conclude that inflammatory stress aggravates renal injury by activation of the CD36 pathway, suggesting that this mechanism may operate in obese individuals with chronic inflammation, making them prone to CKD.

Lxr-driven enterocyte lipid droplet formation delays transport of ingested lipids

Liver X receptors (Lxrs) are master regulators of cholesterol catabolism, driving the elimination of cholesterol from the periphery to the lumen of the intestine. Development of pharmacological agents to activate Lxrs has been hindered by synthetic Lxr agonists' induction of hepatic lipogenesis and hypertriglyceridemia. Elucidating the function of Lxrs in regulating enterocyte lipid handling might identify novel aspects of lipid metabolism that are pharmacologically amenable. We took a genetic approach centered on the single Lxr gene nr1h3 in zebrafish to study the role of Lxr in enterocyte lipid metabolism. Loss of nr1h3 function causes anticipated gene regulatory changes and cholesterol intolerance, collectively reflecting high evolutionary conservation of zebrafish Lxra function. Intestinal nr1h3 activation delays transport of absorbed neutral lipids, with accumulation of neutral lipids in enterocyte cytoplasmic droplets. This delay in transport of ingested neutral lipids protects animals from hypercholesterolemia and hepatic steatosis induced by a high-fat diet. On a gene regulatory level, Lxra induces expression of acsl3a, which encodes acyl-CoA synthetase long-chain family member 3a, a lipid droplet-anchored protein that directs fatty acyl chains into lipids. Forced overexpression of acls3a in enterocytes delays, in part, the appearance of neutral lipids in the vasculature of zebrafish larvae. Activation of Lxr in the intestine cell-autonomously regulates the rate of delivery of absorbed lipids by inducting a temporary lipid intestinal droplet storage depot.