Background: Cross-talk between sterol metabolism and inflammatory pathways has been demonstrated to significantly affect the development of atherosclerosis. Cholesterol biosynthetic intermediates and derivatives are increasingly recognized as key immune regulators of macrophages in response to innate immune activation and lipid overloading. 25-Hydroxycholesterol (25-HC) is produced as an oxidation product of cholesterol by the enzyme cholesterol 25-hydroxylase (CH25H) and belongs to a family of bioactive cholesterol derivatives produced by cells in response to fluctuating cholesterol levels and immune activation. Despite the major role of 25-HC as a mediator of innate and adaptive immune responses, its contribution during the progression of atherosclerosis remains unclear. Methods: The levels of 25-HC were analyzed by liquid chromatography-mass spectrometry, and the expression of CH25H in different macrophage populations of human or mouse atherosclerotic plaques, respectively. The effect of CH25H on atherosclerosis progression was analyzed by bone marrow adoptive transfer of cells from wild-type or Ch25h ^–/– mice to lethally irradiated Ldlr ^–/– mice, followed by a Western diet feeding for 12 weeks. Lipidomic, transcriptomic analysis and effects on macrophage function and signaling were analyzed in vitro from lipid-loaded macrophage isolated from Ldlr ^–/– or Ch25h–/–;Ldlr–/– mice . The contribution of secreted 25-HC to fibrous cap formation was analyzed using a smooth muscle cell lineage–tracing mouse model, Myh11 ^ERT2CRE mT/mG;Ldlr ^–/– , adoptively transferred with wild-type or Ch25h ^–/– mice bone marrow followed by 12 weeks of Western diet feeding. Results: We found that 25-HC accumulated in human coronary atherosclerotic lesions and that macrophage-derived 25-HC accelerated atherosclerosis progression, promoting plaque instability through autocrine and paracrine actions. 25-HC amplified the inflammatory response of lipid-loaded macrophages and inhibited the migration of smooth muscle cells within the plaque. 25-HC intensified inflammatory responses of lipid-laden macrophages by modifying the pool of accessible cholesterol in the plasma membrane, which altered Toll-like receptor 4 signaling, promoted nuclear factor-κB–mediated proinflammatory gene expression, and increased apoptosis susceptibility. These effects were independent of 25-HC–mediated modulation of liver X receptor or SREBP (sterol regulatory element–binding protein) transcriptional activity. Conclusions: Production of 25-HC by activated macrophages amplifies their inflammatory phenotype, thus promoting atherogenesis.

Cholesterol biosynthetic intermediates, such as lanosterol and desmosterol, are emergent immune regulators of macrophages in response to inflammatory stimuli or lipid overloading, respectively. However, the participation of these sterols in regulating macrophage functions in the physiological context of atherosclerosis, an inflammatory disease driven by the accumulation of cholesterol-laden macrophages in the artery wall, has remained elusive. Here, we report that desmosterol, the most abundant cholesterol biosynthetic intermediate in human coronary artery lesions, plays an essential role during atherogenesis, serving as a key molecule integrating cholesterol homeostasis and immune responses in macrophages. Depletion of desmosterol in myeloid cells by overexpression of 3β-hydroxysterol Δ²⁴-reductase (DHCR24), the enzyme that catalyzes conversion of desmosterol to cholesterol, promotes the progression of atherosclerosis. Single-cell transcriptomics in isolated CD45⁺CD11b⁺ cells from atherosclerotic plaques demonstrate that depletion of desmosterol increases interferon responses and attenuates the expression of antiinflammatory macrophage markers. Lipidomic and transcriptomic analysis of in vivo macrophage foam cells demonstrate that desmosterol is a major endogenous liver X receptor (LXR) ligand involved in LXR/retinoid X receptor (RXR) activation and thus macrophage foam cell formation. Decreased desmosterol accumulation in mitochondria promotes macrophage mitochondrial reactive oxygen species production and NLR family pyrin domain containing 3 (NLRP3)–dependent inflammasome activation. Deficiency of NLRP3 or apoptosis-associated speck-like protein containing a CARD (ASC) rescues the increased inflammasome activity and atherogenesis observed in desmosterol-depleted macrophages. Altogether, these findings underscore the critical function of desmosterol in the atherosclerotic plaque to dampen inflammation by integrating with macrophage cholesterol metabolism and inflammatory activation and protecting from disease progression. Significance Previous work has shown that foamy macrophages accumulate desmosterol, a cholesterol biosynthetic intermediate. However, the participation of desmosterol in regulating the functions of cholesterol-laden macrophages in the physiological context of atherosclerosis has remained elusive. In this work, we selectively deplete desmosterol by overexpressing DHCR24, the enzyme that catalyzes the conversion of desmosterol into cholesterol, in myeloid cells and demonstrate that desmosterol content in atherosclerotic plaque macrophages controls their function and activation. Molecular analysis of macrophages isolated from lesions revealed that depletion of desmosterol increases the expression of interferon-regulated genes and genes associated with “classical” macrophage activation. Together, our work provides molecular insights into the intimate links between cholesterol metabolism and inflammatory responses of macrophages during atherogenesis.

Hepatic uptake and biosynthesis of fatty acids (FAs), as well as the partitioning of FAs into oxidative, storage, and secretory pathways, are tightly regulated processes. Dysregulation of one or more of these processes can promote excess hepatic lipid accumulation, ultimately leading to systemic metabolic dysfunction. Angiopoietin-like-4 (ANGPTL4) is a secretory protein that inhibits lipoprotein lipase (LPL) and modulates triacylglycerol (TAG) homeostasis. To understand the role of ANGPTL4 in liver lipid metabolism under normal and high-fat–fed conditions, we generated hepatocyte-specific Angptl4 mutant mice (Hmut). Using metabolic turnover studies, we demonstrate that hepatic Angptl4 deficiency facilitates catabolism of TAG-rich lipoprotein (TRL) remnants in the liver via increased hepatic lipase (HL) activity, which results in a significant reduction in circulating TAG and cholesterol levels. Consequently, depletion of hepatocyte Angptl4 protects against diet-induced obesity, glucose intolerance, liver steatosis, and atherogenesis. Mechanistically, we demonstrate that loss of Angptl4 in hepatocytes promotes FA uptake, which results in increased FA oxidation, ROS production, and AMPK activation. Finally, we demonstrate the utility of a targeted pharmacologic therapy that specifically inhibits Angptl4 gene expression in the liver and protects against diet-induced obesity, dyslipidemia, glucose intolerance, and liver damage, which likely occur via increased HL activity. Notably, this inhibition strategy does not cause any of the deleterious effects previously observed with neutralizing antibodies.

Significance: Prior work has suggested that the ability of miR-33, a microRNA involved in regulation of lipid metabolism, to regulate liver function underlies its effects on obesity and/or atherosclerosis. In this work, we selectively remove miR-33 from the liver and demonstrate that, unlike mice globally deficient for miR-33, mice lacking miR-33 in the liver are not predisposed to diet-induced obesity and are actually protected from hepatic insulin resistance and fibrosis. While loss of liver miR-33 increases circulating high-density lipoprotein and increases reverse cholesterol transport in mice on a chow diet, it is not sufficient to decrease atherosclerotic plaque burden under hyperlipidemic conditions where hepatic miR-33 is already repressed. This information will help improve efforts to develop novel therapies against cardiometabolic diseases.

Epigenetic modifications of the genome, including DNA methylation, histone methylation/acetylation, and noncoding RNAs, have been reported to play a fundamental role in regulating immune response during the progression of atherosclerosis. SETDB2 is a member of the KMT1 family of lysine methyltransferases, and members of this family typically methylate histone H3 Lys9 (H3K9), an epigenetic mark associated with gene silencing. Previous studies have shown that SETDB2 is involved in innate and adaptive immunity, the proinflammatory response, and hepatic lipid metabolism. Here, we report that expression of SETDB2 is markedly upregulated in human and murine atherosclerotic lesions. Upregulation of SETDB2 was observed in proinflammatory M1 but not antiinflammatory M2 macrophages. Notably, we found that genetic deletion of SETDB2 in hematopoietic cells promoted vascular inflammation and enhanced the progression of atherosclerosis in BM transfer studies in Ldlr- knockout mice. Single-cell RNA-Seq analysis in isolated CD45 ⁺ cells from atherosclerotic plaques from mice transplanted with SETDB2-deficient BM revealed a significant increase in monocyte population and enhanced expression of genes involved in inflammation and myeloid cell recruitment. Additionally, we found that loss of SETDB2 in hematopoietic cells was associated with macrophage accumulation in atherosclerotic lesions and attenuated efferocytosis. Overall, these studies identify SETDB2 as an important inflammatory cell regulator that controls macrophage activation in atherosclerotic plaques.

Intricate regulatory networks govern the net balance of cholesterol biosynthesis, uptake and efflux; however, the mechanisms surrounding cholesterol homeostasis remain incompletely understood. Here, we develop an integrative genomic strategy to detect regulators of LDLR activity and identify 250 genes whose knockdown affects LDL-cholesterol uptake and whose expression is modulated by intracellular cholesterol levels in human hepatic cells. From these hits, we focus on MMAB, an enzyme which catalyzes the conversion of vitamin B₁₂ to adenosylcobalamin, and whose expression has previously been linked with altered levels of circulating cholesterol in humans. We demonstrate that hepatic levels of MMAB are modulated by dietary and cellular cholesterol levels through SREBP2, the master transcriptional regulator of cholesterol homeostasis. Knockdown of MMAB decreases intracellular cholesterol levels and augments SREBP2-mediated gene expression and LDL-cholesterol uptake in human and mouse hepatic cell lines. Reductions in total sterol content were attributed to increased intracellular levels of propionic and methylmalonic acid and subsequent inhibition of HMGCR activity and cholesterol biosynthesis. Moreover, mice treated with antisense inhibitors of MMAB display a significant reduction in hepatic HMGCR activity, hepatic sterol content and increased expression of SREBP2-mediated genes. Collectively, these findings reveal an unexpected role for the adenosylcobalamin pathway in regulating LDLR expression and identify MMAB as an additional control point by which cholesterol biosynthesis is regulated by its end product.