Significance Lipid droplets (LDs) are ubiquitous organelles that play important roles in cellular energy homeostasis, tightly regulating the accumulation and release of lipids. In macrophages, lipids accumulate in LDs during inflammation. However, it is unclear how inflammatory activation promotes the accumulation of lipids in LDs, and how the dynamic between lipid accumulation and breakdown could drive or inhibit inflammation. Elucidating the role of lipid accumulation during inflammation may provide important knowledge to influence inflammatory processes during health and disease. We identify the importance of the hypoxia-inducible lipid droplet–associated protein and the intracellular adipose triglyceride lipase in the regulation of lipid accumulation and breakdown in inflammatory macrophages. Furthermore, we determine the regulatory effect of lipid breakdown from LDs in supporting inflammation.

Elevated plasma triglyceride levels are increasingly considered as an independent risk factor for cardiovascular diseases (1-3). Triglycerides circulate in the blood in two major forms: as chylomicrons carrying the dietary triglycerides and as very low density lipoproteins carrying endogenously produced triglycerides (4). The clearance of plasma triglycerides is primarily mediated by the action of LPL. This secretory enzyme is produced by parenchymal cells of fat tissue, skeletal muscle, and heart, as well as by macrophages. With the help of the endothelial protein, glycosylphosphatidylinositol-anchored HDL binding protein 1 (GPIHBP1), LPL is transferred from the surface of the sub-endothelial myocytes and adipocytes to the luminal side of the capillary endothelium. There, LPL hydrolyzes the triglycerides contained in the triglyceride-rich lipoproteins to release fatty acids for uptake by the underlying tissues (5-8). The activity of LPL is regulated posttranslationally by numerous factors, many of which are produced in the liver, including several apolipoproteins. In addition, LPL activity is governed by several members of the family of angiopoietin-like proteins (ANGPTLs): ANGPTL3 (9), ANGPTL4 (10-12), and ANGPTL8 (13-15). ANGPTL3 is produced in the liver and cooperates with ANGPTL8 to inhibit LPL activity in peripheral tissues Abstract Angiopoietin-like protein (ANGPTL)4 regulates plasma lipids, making it an attractive target for correcting dyslipidemia. However, ANGPTL4 inactivation in mice fed a high fat diet causes chylous ascites, an acute-phase response, and mesenteric lymphadenopathy. Here, we studied the role of ANGPTL4 in lipid uptake in macrophages and in the above-mentioned pathologies using Angptl4-hypomorphic and Angptl4 / mice. Angptl4 expression in peritoneal and bone marrow-derived macrophages was highly induced by lipids. Recombinant ANGPTL4 decreased lipid uptake in macrophages, whereas deficiency of ANGPTL4 increased lipid uptake, upregulated lipid-induced genes, and increased respiration. ANGPTL4 deficiency did not alter LPL protein levels in macrophages. Angptl4-hypomorphic mice with partial expression of a truncated N-terminal ANGPTL4 exhibited reduced fasting plasma triglyceride, cholesterol, and NEFAs, strongly resembling Angptl4 / mice. However, during high fat feeding, Angptl4-hypomorphic mice showed markedly delayed and attenuated elevation in plasma serum amyloid A and much milder chylous ascites than Angptl4 / mice, despite similar abundance of lipid-laden giant cells in mesenteric lymph nodes. In conclusion, ANGPTL4 deficiency increases lipid uptake and respiration in macrophages without affecting LPL protein levels. Compared with the absence of ANGPTL4, low levels of N-terminal ANGPTL4 mitigate the development of chylous ascites and an acutephase response in mice.-Oteng, A-B.

Highlights d HILPDA is a lipid-induced, physiological inhibitor of ATGLmediated lipolysis d HILPDA-deficient macrophages show enhanced breakdown of triglycerides through ATGL d Reduced lipid storage in macrophages does not affect adipose tissue inflammation

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Highlights d HILPDA is a lipid-induced, physiological inhibitor of ATGLmediated lipolysis d HILPDA-deficient macrophages show enhanced breakdown of triglycerides through ATGL d Reduced lipid storage in macrophages does not affect adipose tissue inflammation

The development of a chronic, low-grade inflammation originating from adipose tissue in obese subjects is widely recognized to induce insulin resistance, leading to the development of type 2 diabetes. The adipose tissue microenvironment drives specific metabolic reprogramming of adipose tissue macrophages, contributing to the induction of tissue inflammation. Uncoupling protein 2 (UCP2), a mitochondrial anion carrier, is thought to separately modulate inflammatory and metabolic processes in macrophages, and is upregulated in macrophages in the context of obesity and diabetes. Here, we investigate the role of UCP2 in macrophage activation in the context of obesity-induced adipose tissue inflammation and insulin resistance. Using a myeloid-specific knockout of UCP2 (Ucp2ΔLysM), we found that Ucp2 deficiency significantly increases glycolysis and oxidative respiration, both unstimulated and after inflammatory conditions. Strikingly, fatty acid loading abolished the metabolic differences between Ucp2ΔLysM macrophages and their floxed controls. Furthermore, Ucp2ΔLysM macrophages show attenuated pro-inflammatory responses towards Toll-like receptor-2 and -4 stimulation. To test the relevance of macrophage-specific Ucp2 deletion in vivo, Ucp2ΔLysM and Ucp2fl/fl mice were rendered obese and insulin resistant through high-fat feeding. Although no differences in adipose tissue inflammation or insulin resistance was found between the two genotypes, adipose tissue macrophages isolated from diet-induced obese Ucp2ΔLysM mice showed decreased TNFα secretion after ex vivo LPS stimulation compared to their Ucp2fl/fl littermates. Together, these results demonstrate that while UCP2 regulates both metabolism and the inflammatory response of macrophages, its activity is not crucial in shaping macrophage activation in the adipose tissue during obesity-induced insulin resistance.

Diabetes is associated with increased cardiovascular risk and higher occurrence of infections. These complications suggest altered responses of the innate immune system. Recent studies have shown that energy metabolism of monocytes is crucial in determining their functionality. Here we investigate whether monocyte metabolism and function are changed in patients with diabetes and aim to identify diabetes-associated factors driving these alterations. Patients with type 1 diabetes (T1D) (n = 41) and healthy age-, sex-, and BMI-matched control subjects (n = 20) were recruited. Monocytes were isolated from peripheral blood to determine immune functionality, metabolic responses, and transcriptome profiles. Upon ex vivo stimulation with Toll-like receptor (TLR) 4 or TLR-2 agonists, monocytes of patients with T1D secreted lower levels of various cytokines and showed lower glycolytic rates compared with monocytes isolated from matched control subjects. Stratification based on HbA1c levels revealed that lower cytokine secretion was coupled to higher glycolytic rate of monocytes in patients with a higher glycemic burden. Circulating monocytes displayed an enhanced inflammatory gene expression profile associated with high glycemic burden. These results suggest that a high glycemic burden in patients with T1D is related to expression of inflammatory genes of monocytes and is associated with an impaired relationship between metabolism and inflammatory function upon activation.