Insulin increases macrophage triglyceride accumulation under diabetic conditions through the down regulation of hormone sensitive lipase and adipose triglyceride lipase

Cytosolic lipolysis in non-adipose tissues: energy provision and beyond

Cytosolic lipolysis is a well-defined biochemical process that plays important roles in the mobilization of stored neutral lipids. Lipid turnover, regulated by cytosolic lipolysis, has been extensively studied in adipose tissue, liver, and muscle. The storage and utilization of neutral lipids is a basic function of most, if not all, tissues and cells. In this review, we focus on the functions of cytosolic lipolysis mainly in non-adipose tissues and in several physiological processes, including cancer, longevity, and pathogen infection. The mechanisms underlying the impact of cytosolic lipolysis on these events will be discussed. Detailed understanding of cytosolic lipolysis in both adipose and non-adipose tissues will have implications for future clinical translation.

Leucine supplementation attenuates macrophage foam-cell formation: Studies in humans, mice, and cultured macrophages

Whereas atherogenicity of dietary lipids has been largely studied, relatively little is known about the possible contribution of dietary amino acids to macrophage foam-cell formation, a hallmark of early atherogenesis. Recently, we showed that leucine has antiatherogenic properties in the macrophage model system. In this study, an in-depth investigation of the role of leucine in macrophage lipid metabolism was conducted by supplementing humans, mice, or cultured macrophages with leucine. Macrophage incubation with serum obtained from healthy adults supplemented with leucine (5 g/d, 3 weeks) significantly decreased cellular cholesterol mass by inhibiting the rate of cholesterol biosynthesis and increasing cholesterol efflux from macrophages. Similarly, leucine supplementation to C57BL/6 mice (8 weeks) resulted in decreased cholesterol content in their harvested peritoneal macrophages (MPM) in relation with reduced cholesterol biosynthesis rate. Studies in J774A.1 murine macrophages revealed that leucine dose-dependently decreased cellular cholesterol and triglyceride mass. Macrophages treated with leucine (0.2 mM) showed attenuated uptake of very low-density lipoproteins and triglyceride biosynthesis rate, with a concurrent down-regulation of diacylglycerol acyltransferase-1, a key enzyme catalyzing triglyceride biosynthesis in macrophages. Similar effects were observed when macrophages were treated with α-ketoisocaproate, a key leucine metabolite. Finally, both in vivo and in vitro leucine supplementation significantly improved macrophage mitochondrial respiration and ATP production. The above studies, conducted in human, mice, and cultured macrophages, highlight a protective role for leucine attenuating macrophage foam-cell formation by mechanisms related to the metabolism of cholesterol, triglycerides, and energy production. © 2018 BioFactors, 44(3):245-262, 2018.

Silicon dioxide nanoparticles increase macrophage atherogenicity: Stimulation of cellular cytotoxicity, oxidative stress, and triglycerides accumulation

Nanoparticle research has focused on their toxicity in general, while increasing evidence points to additional specific adverse effects on atherosclerosis development. Arterial macrophage cholesterol and triglyceride (TG) accumulation and foam cell formation are the hallmark of early atherogenesis, leading to cardiovascular events. To investigate the in vitro atherogenic effects of silicon dioxide (SiO2 ), J774.1 cultured macrophages (murine cell line) were incubated with SiO2 nanoparticle (SP, d = 12 nm, 0-20 µg/mL), followed by cellular cytotoxicity, oxidative stress, TG and cholesterol metabolism analyses. A significant dose-dependent increase in oxidative stress (up to 164%), in cytotoxicity (up to 390% measured by lactate dehydrogenase (LDH) release), and in TG content (up to 63%) was observed in SiO2 exposed macrophages compared with control cells. A smaller increase in macrophage cholesterol mass (up to 22%) was noted. TG accumulation in macrophages was not due to a decrease in TG cell secretion or to an increased TG biosynthesis rate, but was the result of attenuated TG hydrolysis secondary to decreased lipase activity and both adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) protein expression (by 42 and 25%, respectively). Overall, SPs showed pro-atherogenic effects on macrophages as observed by cytotoxicity, increased oxidative stress and TG accumulation. © 2014 Wiley Periodicals, Inc. Environ Toxicol 31: 713-723, 2016.

Insulin induces human acyl-coenzyme A: cholesterol acyltransferase1 gene expression via MAP kinases and CCAAT/enhancer-binding protein α

Insulin resistance characterized by hyperinsulinemia is associated with increased risk of atherosclerosis. Acyl-coenzyme A: cholesterol acyltransferase (ACAT) is an intracellular enzyme involved in cellular cholesterol homeostasis and in atherosclerotic foam cell formation. To investigate the relationship between hyperinsulinemia and atherosclerosis, we investigated whether insulin induced ACAT1 gene expression and found that insulin up-regulated ACAT1 mRNA, protein and enzyme activity in human THP-1 cells and THP-1-derived macrophages. Moreover, luciferase assays revealed that insulin enhanced the ACAT1 gene P1 promoter activity but not the P7 promoter. To explore the molecular mechanisms involved, deletion analysis of the human ACAT1 P1 promoter revealed an insulin response element (IRE) upstream of the P1 promoter (from -603 to -580), EMSA experiments demonstrated that CCAAT/enhancer binding protein α(C/EBPα) bound to the P1 promoter IRE. Insulin-induced ACAT1 upregulation was blocked by the presence of PD98059 (an inhibitor of extracellular signal-regulated kinase, ERK) and SB203580 (an inhibitor of p38 mitogen-activated protein kinase, p38MAPK) but not by Wortmannin (an inhibitor of phosphatidylinositol 3-kinase, PI3K) or U73122 (an inhibitor of phospholipase C-γ, PLCγ). These studies demonstrate that insulin promotes ACAT1 gene expression at the transcriptional level. The molecular mechanism of insulin action is mediated via interaction of the functional IRE upstream of the ACAT1 P1 promoter with C/EBPα and is MAPK-dependent.

Hypoxia is present in murine atherosclerotic plaques and has multiple adverse effects on macrophage lipid metabolism

RATIONALE

Human atherosclerotic plaques contain large numbers of cells deprived of O(2). In murine atherosclerosis, because the plaques are small, it is controversial whether hypoxia can occur.

OBJECTIVE

To examine if murine plaques contain hypoxic cells, and whether hypoxia regulates changes in cellular lipid metabolism and gene expression in macrophages.

METHODS AND RESULTS

Aortic plaques from apolipoprotein-E-deficient mice were immunopositive for hypoxia-inducible transcription factor (HIF-1α) and some of its downstream targets. Murine J774 macrophages rendered hypoxic demonstrated significant increases in cellular sterol and triglycerides. The increase in sterol content in hypoxic macrophages correlated with elevated 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase activity and mRNA levels. In addition, when macrophages were incubated with cholesterol complexes, hypoxic cells accumulated 120% more cholesterol, predominately in the free form. Cholesterol-efflux assays showed that hypoxia significantly decreased efflux mediated by ATP-binding cassette subfamily A member 1 (ABCA1), whose sub cellular localization was altered in both J774 and primary macrophages. Furthermore, in vivo expression patterns of selected genes from cells in hypoxic regions of murine plaques were similar to those from J774 and primary macrophages incubated in hypoxia. The hypoxia-induced accumulation of sterol and decreased cholesterol efflux was substantially reversed in vitro by reducing the expression of the hypoxia-inducible transcription factor, HIF-1α.

CONCLUSION

Hypoxic regions are present in murine plaques. Hypoxic macrophages have increased sterol content due to the induction of sterol synthesis and the suppression of cholesterol efflux, effects that are in part mediated by HIF-1α.