Triglyceride alters lysosomal cholesterol ester metabolism in cholesteryl ester-laden macrophage foam cells

Low-density lipoproteins cause atherosclerotic cardiovascular disease: pathophysiological, genetic, and therapeutic insights: a consensus statement from the European Atherosclerosis Society Consensus Panel

Collapse

Monocytes from men living with HIV exhibit heightened atherogenic potential despite long-term viral suppression with antiretroviral therapy

OBJECTIVE

People living with HIV have an increased risk of cardiovascular disease (CVD) despite effective antiretroviral therapy (ART). Monocytes play a key role in the early stages of atherosclerosis-driven CVD by forming lipid-laden foam cells within artery walls. HIV infection potentiates foam cell formation ex vivo, but the mechanisms contributing to this are not known.

METHODS

We investigated the atherosclerosis-promoting potential of monocytes from 39 virologically suppressed men living with HIV (MLHIV) on ART and no evidence of CVD, and 25 HIV-uninfected controls of comparable age, sex, smoking status and CVD risk.

RESULTS

Despite absence of clinical atherosclerosis in both MLHIV and uninfected cohorts (evidenced by a carotid intima-media thickness of 0.6 mm for both groups; P = 0.254), monocytes from MLHIV showed increased potential to form atherosclerosis-promoting foam cells compared with controls in an ex-vivo assay (36.6% vs. 27.6%, respectively, P = 0.003). Consistent with observations of persistent inflammation and immune/endothelial activation in ART-treated HIV infection, levels of soluble tumour necrosis factor receptor II, CXCL10 and soluble VCAM-1 were elevated in MLHIV (P ≤ 0.005 for all), but were not significantly associated with foam cell formation. Foam cell formation was associated with an impaired ability of monocytes to undergo reverse transmigration, and a reduced ability to efflux cholesterol ex vivo (P < 0.05 for both). Importantly, foam cell formation declined significantly with duration of viral suppression (P = 0.004).

CONCLUSION

These findings highlight the persistence of HIV-related changes to the atherogenic potential of monocytes despite long-term viral suppression, and provide insights into mechanisms potentially driving increased CVD in ART-treated HIV infection.

The physiology of foamy phagocytes in multiple sclerosis

Multiple sclerosis (MS) is a chronic disease of the central nervous system characterized by massive infiltration of immune cells, demyelination, and axonal loss. Active MS lesions mainly consist of macrophages and microglia containing abundant intracellular myelin remnants. Initial studies showed that these foamy phagocytes primarily promote MS disease progression by internalizing myelin debris, presenting brain-derived autoantigens, and adopting an inflammatory phenotype. However, more recent studies indicate that phagocytes can also adopt a beneficial phenotype upon myelin internalization. In this review, we summarize and discuss the current knowledge on the spatiotemporal physiology of foamy phagocytes in MS lesions, and elaborate on extrinsic and intrinsic factors regulating their behavior. In addition, we discuss and link the physiology of myelin-containing phagocytes to that of foamy macrophages in other disorders such atherosclerosis.

Blood-derived macrophages prone to accumulate lysosomal lipids trigger oxLDL-dependent murine hepatic inflammation

Despite the consistent rise of non-alcoholic steatohepatitis (NASH) worldwide, the mechanisms that govern the inflammatory aspect of this disease remain unknown. Previous research showed an association between hepatic inflammation and lysosomal lipid accumulation in blood-derived hepatic macrophages. Additionally, in vitro findings indicated that lipids, specifically derived from the oxidized low-density lipoprotein (oxLDL) particle, are resistant to removal from lysosomes. On this basis, we investigated whether lysosomal lipid accumulation in blood-derived hepatic macrophages is causally linked to hepatic inflammation and assessed to what extent increasing anti-oxLDL IgM autoantibodies can affect this mechanism. By creating a proof-of-concept mouse model, we demonstrate a causal role for lysosomal lipids in blood-derived hepatic macrophages in mediating hepatic inflammation and initiation of fibrosis. Furthermore, our findings show that increasing anti-oxLDL IgM autoantibody levels reduces inflammation. Hence, therapies aimed at improving lipid-induced lysosomal dysfunction and blocking oxLDL-formation deserve further investigation in the context of NASH.

Salusin-β induces foam cell formation and monocyte adhesion in human vascular smooth muscle cells via miR155/NOX2/NFκB pathway

Vascular smooth muscle cells (VSMCs) are indispensible components in foam cell formation. Salusin-β is a stimulator in the progression of atherosclerosis. Here, we showed that salusin-β increased foam cell formation evidenced by accumulation of lipid droplets and intracellular cholesterol content, and promoted monocyte adhesion in human VSMCs. Salusin-β increased the expressions and activity of acyl coenzyme A:cholesterol acyltransferase-1 (ACAT-1) and vascular cell adhesion molecule-1 (VCAM-1) in VSMCs. Silencing of ACAT-1 abolished the salusin-β-induced lipid accumulation, and silencing of VCAM-1 prevented the salusin-β-induced monocyte adhesion in VSMCs. Salusin-β caused p65-NFκB nuclear translocation and increased p65 occupancy at the ACAT-1 and VCAM-1 promoter. Inhibition of NFκB with Bay 11-7082 prevented the salusin-β-induced ACAT-1 and VCAM-1 upregulation, foam cell formation and monocyte adhesion in VSMCs. Scavenging ROS, inhibiting NADPH oxidase or knockdown of NOX2 abolished the effects of salusin-β on ACAT-1 and VCAM-1 expressions, p65-NFκB nuclear translocation, lipid accumulation and monocyte adhesion in VSMCs. Salusin-β increased miR155 expression, and knockdown of miR155 prevented the effects of salusin-β on ACAT-1 and VCAM-1 expressions, p65-NFκB nuclear translocation, lipid accumulation, monocyte adhesion and ROS production in VSMCs. These results indicate that salusin-β induces foam formation and monocyte adhesion via miR155/NOX2/NFκB-mediated ACAT-1 and VCAM-1 expressions in VSMCs.

Lysosomal acid lipase: at the crossroads of normal and atherogenic cholesterol metabolism

Unregulated cellular uptake of apolipoprotein B-containing lipoproteins in the arterial intima leads to the formation of foam cells in atherosclerosis. Lysosomal acid lipase (LAL) plays a crucial role in both lipoprotein lipid catabolism and excess lipid accumulation as it is the primary enzyme that hydrolyzes cholesteryl esters derived from both low density lipoprotein (LDL) and modified forms of LDL. Evidence suggests that as atherosclerosis progresses, accumulation of excess free cholesterol in lysosomes leads to impairment of LAL activity, resulting in accumulation of cholesteryl esters in the lysosome as well as the cytosol in foam cells. Impaired metabolism and release of cholesterol from lysosomes can lead to downstream defects in ATP-binding cassette transporter A1 regulation, needed to offload excess cholesterol from plaque foam cells. This review focuses on the role LAL plays in normal cholesterol metabolism and how the associated changes in its enzymatic activity may ultimately contribute to atherosclerosis progression.

Lysosomal cholesterol accumulation: driver on the road to inflammation during atherosclerosis and non-alcoholic steatohepatitis

Many studies show an association between the accumulation of cholesterol inside lysosomes and the progression towards inflammatory disease states that are closely related to obesity. While in the past, the knowledge regarding lysosomal cholesterol accumulation was limited to its association with plaque severity during atherosclerosis, recently, a growing body of evidence indicates a causal link between lysosomal cholesterol accumulation and inflammation. These findings make lysosomal cholesterol accumulation an important target for intervention in metabolic diseases that are characterized by the presence of an inflammatory response. In this review, we aim to show the importance of cholesterol trapping inside lysosomes to the development of inflammation by focusing upon cardiovascular disease and non-alcoholic steatohepatitis (NASH) in particular. We summarize current data supporting the hypothesis that lysosomal cholesterol accumulation plays a key role in the development of inflammation during atherosclerosis and NASH. In addition, potential mechanisms by which disturbed lysosomal function can trigger the inflammatory response, the challenges in improving cholesterol trafficking in macrophages and recent successful research directions will be discussed.

Adipose triglyceride lipase in immune response, inflammation, and atherosclerosis

Consistent with its central importance in lipid and energy homeostasis, lipolysis occurs in essentially all tissues and cell types, including macrophages. The hydrolytic cleavage of triacylglycerol by adipose triglyceride lipase (ATGL) generates non-esterified fatty acids, which are subsequently used as essential precursors for lipid and membrane synthesis, mediators in cell signaling processes or as energy substrate in mitochondria. This review summarizes the current knowledge concerning the consequences of ATGL deficiency in macrophages with particular emphasis on macrophage (dys)-function, apoptosis, and atherosclerosis.

Triacylglycerol accumulation activates the mitochondrial apoptosis pathway in macrophages

Programmed cell death of lipid-laden macrophages is a prominent feature of atherosclerotic lesions and mostly ascribed to accumulation of excess intracellular cholesterol. The present in vitro study investigated whether intracellular triacylglycerol (TG) accumulation could activate a similar apoptotic response in macrophages. To address this question, we utilized peritoneal macrophages isolated from mice lacking adipose triglyceride lipase (ATGL), the major enzyme responsible for TG hydrolysis in multiple tissues. In Atgl(-/-) macrophages, we observed elevated levels of cytosolic Ca(2+) and reactive oxygen species, stimulated cytochrome c release, and nuclear localization of apoptosis-inducing factor. Fragmented mitochondria prior to cell death were indicative of the mitochondrial apoptosis pathway being triggered as a consequence of defective lipolysis. Other typical markers of apoptosis, such as externalization of phosphatidylserine in the plasma membrane, caspase 3 and poly(ADP-ribose) polymerase cleavage, were increased in Atgl(-/-) macrophages. An artificial increase of cellular TG levels by incubating wild-type macrophages with very low density lipoprotein closely mimicked the apoptotic phenotype observed in Atgl(-/-) macrophages. Results obtained during the present study define a novel pathway linking intracellular TG accumulation to mitochondrial dysfunction and programmed cell death in macrophages.

Lysosomes, cholesterol and atherosclerosis

Cholesterol-engorged macrophage foam cells are a critical component of the atherosclerotic lesion. Reducing the sterol deposits in lesions reduces clinical events. Sterol accumulations within lysosomes have proven to be particularly hard to mobilize out of foam cells. Moreover, excess sterol accumulation in lysosomes has untoward effects, including a complete disruption of lysosome function. Recently, we demonstrated that treatment of sterol-engorged macrophages in culture with triglyceride-containing particles can reverse many of the effects of cholesterol on lysosomes and dramatically reduce the sterol burden in these cells. This article describes what is known about lysosomal sterol engorgement, discusses the possible mechanisms by which triglyceride could produce its effects, and evaluates the possible positive and negative effects of reducing the lysosomal cholesterol levels in foam cells.