Hypoxia in murine atherosclerotic plaques and its adverse effects on macrophages

Role of glycolysis in diabetic atherosclerosis

Diabetes mellitus is a kind of typical metabolic disorder characterized by elevated blood sugar levels. Atherosclerosis (AS) is one of the most common complications of diabetes. Modern lifestyles and trends that promote overconsumption and unhealthy practices have contributed to an increase in the annual incidence of diabetic AS worldwide, which has created a heavy burden on society. Several studies have shown the significant effects of glycolysis-related changes on the occurrence and development of diabetic AS, which may serve as novel thera-peutic targets for diabetic AS in the future. Glycolysis is an important metabolic pathway that generates energy in various cells of the blood vessel wall. In particular, it plays a vital role in the physiological and pathological activities of the three important cells, Endothelial cells, macrophages and vascular smooth muscle cells. There are lots of similar mechanisms underlying diabetic and common AS, the former is more complex. In this article, we describe the role and mechanism underlying glycolysis in diabetic AS, as well as the therapeutic targets, such as trained immunity, microRNAs, gut microbiota, and associated drugs, with the aim to provide some new perspectives and potentially feasible programs for the treatment of diabetic AS in the foreseeable future.

Aryl hydrocarbon receptor: A bridge linking immuno-inflammation and metabolism in atherosclerosis

Cardiovascular disease is the leading cause of death worldwide, and atherosclerosis is a major contributor to this etiology. The ligand-activated transcription factor, known as the aryl hydrocarbon receptor (AhR), plays an essential role in the interactions between genes and the environment. In a number of human diseases, including atherosclerosis, the AhR signaling pathway has recently been shown to be aberrantly expressed and activated. It's reported that AhR can regulate the immuno-inflammatory response and metabolism pathways in atherosclerosis, potentially serving as a bridge that links these processes. In this review, we highlight the involvement of AhR in atherosclerosis. From the literature, we conclude that AhR is a potential target for controlling atherosclerosis through precise interventions.

Hypoxia and panvascular diseases: exploring the role of hypoxia-inducible factors in vascular smooth muscle cells under panvascular pathologies

As an emerging discipline, panvascular diseases are a set of vascular diseases with atherosclerosis as the common pathogenic hallmark, which mostly affect vital organs like the heart, brain, kidney, and limbs. As the major responser to the most common stressor in the vasculature (hypoxia)-hypoxia-inducible factors (HIFs), and the primary regulator of pressure and oxygen delivery in the vasculature-vascular smooth muscle cells (VSMCs), their own multifaceted nature and their interactions with each other are fascinating. Abnormally active VSMCs (e.g., atherosclerosis, pulmonary hypertension) or abnormally dysfunctional VSMCs (e.g., aneurysms, vascular calcification) are associated with HIFs. These widespread systemic diseases also reflect the interdisciplinary nature of panvascular medicine. Moreover, given the comparable proliferative characteristics exhibited by VSMCs and cancer cells, and the delicate equilibrium between angiogenesis and cancer progression, there is a pressing need for more accurate modulation targets or combination approaches to bolster the effectiveness of HIF targeting therapies. Based on the aforementioned content, this review primarily focused on the significance of integrating the overall and local perspectives, as well as temporal and spatial balance, in the context of the HIF signaling pathway in VSMC-related panvascular diseases. Furthermore, the review discussed the implications of HIF-targeting drugs on panvascular disorders, while considering the trade-offs involved.

Atherosclerosis treatment with nanoagent: potential targets, stimulus signals and drug delivery mechanisms

Cardiovascular disease (CVDs) is the first killer of human health, and it caused up at least 31% of global deaths. Atherosclerosis is one of the main reasons caused CVDs. Oral drug therapy with statins and other lipid-regulating drugs is the conventional treatment strategies for atherosclerosis. However, conventional therapeutic strategies are constrained by low drug utilization and non-target organ injury problems. Micro-nano materials, including particles, liposomes, micelles and bubbles, have been developed as the revolutionized tools for CVDs detection and drug delivery, specifically atherosclerotic targeting treatment. Furthermore, the micro-nano materials also could be designed to intelligently and responsive targeting drug delivering, and then become a promising tool to achieve atherosclerosis precision treatment. This work reviewed the advances in atherosclerosis nanotherapy, including the materials carriers, target sites, responsive model and treatment results. These nanoagents precisely delivery the therapeutic agents to the target atherosclerosis sites, and intelligent and precise release of drugs, which could minimize the potential adverse effects and be more effective in atherosclerosis lesion.

Vascular Inflammatory Diseases and Endothelial Phenotypes

The physiological functions of endothelial cells control vascular tone, permeability, inflammation, and angiogenesis, which significantly help to maintain a healthy vascular system. Several cardiovascular diseases are characterized by endothelial cell activation or dysfunction triggered by external stimuli such as disturbed flow, hypoxia, growth factors, and cytokines in response to high levels of low-density lipoprotein and cholesterol, hypertension, diabetes, aging, drugs, and smoking. Increasing evidence suggests that uncontrolled proinflammatory signaling and further alteration in endothelial cell phenotypes such as barrier disruption, increased permeability, endothelial to mesenchymal transition (EndMT), and metabolic reprogramming further induce vascular diseases, and multiple studies are focusing on finding the pathways and mechanisms involved in it. This review highlights the main proinflammatory stimuli and their effects on endothelial cell function. In order to provide a rational direction for future research, we also compiled the most recent data regarding the impact of endothelial cell dysfunction on vascular diseases and potential targets that impede the pathogenic process.

Role of miR-199a-5p in the post-transcriptional regulation of ABCA1 in response to hypoxia in peritoneal macrophages

Hypoxia is a crucial factor contributing to maintenance of atherosclerotic lesions. The ability of ABCA1 to stimulate the efflux of cholesterol from cells in the periphery, particularly foam cells in atherosclerotic plaques, is an important anti-atherosclerotic mechanism. The posttranscriptional regulation by miRNAs represents a key regulatory mechanism of a number of signaling pathways involved in atherosclerosis. Previously, miR-199a-5p has been shown to be implicated in the endocytic and retrograde intracellular transport. Although the regulation of miR-199a-5p and ABCA1 by hypoxia has been already reported independently, the role of miR-199a-5p in macrophages and its possible role in atherogenic processes such us regulation of lipid homeostasis through ABCA1 has not been yet investigated. Here, we demonstrate that both ABCA1 and miR-199a-5p show an inverse regulation by hypoxia and Ac-LDL in primary macrophages. Moreover, we demonstrated that miR-199a-5p regulates ABCA1 mRNA and protein levels by directly binding to its 3’UTR. As a result, manipulation of cellular miR-199a-5p levels alters ABCA1 expression and cholesterol efflux in primary mouse macrophages. Taken together, these results indicate that the correlation between ABCA1-miR-199a-5p could be exploited to control macrophage cholesterol efflux during the onset of atherosclerosis, where cholesterol alterations and hypoxia play a pathogenic role.

Mechanistic insights on the effect of crocin, an active ingredient of saffron, on atherosclerosis in apolipoprotein E knockout mice

BACKGROUND

We investigated the effect of crocin treatment on atherosclerosis and serum lipids in apolipoprotein E knockout (ApoE-/-) mice, focusing on the expression of endothelial nitric oxide synthase (eNOS) and hypoxia-induced factor-1 alpha (HIF-1α).

METHODS

Sixty-two animals were divided into two groups and randomly allocated to crocin (100 mg/kg/day) in drinking water or no crocin. All mice were maintained on standard chow diet containing 5% fat. Crocin was initiated at the 16th week of age and continued for 16 additional weeks. At 32 weeks of age, after blood sampling for plasma lipid determination and euthanasia, proximal aorta was removed and 3 μm sections were used to measure the atherosclerotic area and determine the expression of eNOS and HIF-1α by immunohistochemistry.

RESULTS

Each group consisted of 31 animals (17 males and 14 females in each group). Crocin significantly reduced the atherosclerotic area (mm2 ± SEM) in treated mice compared to controls, both in males (0.0798 ± 0.017 vs. 0.1918 ± 0.028, P < 0.002, respectively) and females (0.0986 ± 0.023 vs. 0.1765 ± 0.025, P < 0.03, respectively). eNOS expression was significantly increased in crocin-treated mice compared to controls, both in males (2.77 ± 0.24 vs. 1.50 ± 0.34, P=0.004, respectively) and females (3.41 ± 0.37 vs. 1.16 ± 0.44, P=0.003, respectively). HIF-1α expression was significantly decreased in crocin-treated mice compared to controls, both in males (21.25 ± 2.14 vs. 156.5 ± 6.67, P < 0.001, respectively) and females (35.3 ± 7.20 vs. 113.3 ± 9.0, P < 0.01, respectively). No difference was noticed in total, low- and high-density lipoprotein cholesterol between treated and control mice.

CONCLUSION

Crocin reduces atherosclerosis possibly by modulation of eNOS and HIF-1α expression in ApoE-/- mice without affecting plasma cholesterol.

PM2.5 induce the defective efferocytosis and promote atherosclerosis via HIF-1α activation in macrophage

Epidemiological studies demonstrate that fine particulate matter (PM_2.5) promotes the development of atherosclerosis. However, the mechanism insight of PM_2.5-induced atherosclerosis is still lacking. The aim of this study was to explore the biological effects of hypoxia-inducible factor 1α (HIF-1α) on PM_2.5-triggered atherosclerosis. The vascular stiffness, carotid intima-media thickness (CIMT), lipid and atherosclerotic lesion were increased when von Hippel-Lindau (VHL)-null mice were exposed to PM_2.5. Yet, knockout of HIF-1α markedly decreased the PM_2.5-triggered atherosclerotic lesion. We firstly performed microarray analysis in PM_2.5-treated bone morrow-derived macrophages (BMDMs), which showed that PM_2.5 significantly changed the genes expression patterns and affected biological processes such as phagocytosis, apoptotic cell clearance, cellular response to hypoxia, apoptotic process and inflammatory response. Moreover, the data showed knockout of HIF-1α remarkably relieved PM_2.5-induced defective efferocytosis. Mechanistically, PM_2.5 inhibited the level of genes and proteins of efferocytosis receptor c-Mer tyrosine kinase (MerTK), especially in VHL-null BMDMs. In addition, PM_2.5 increased the genes and proteins of a disintegrin and metallopeptidase domain 17 (ADAM17), which caused the MerTK cleavage to form soluble MerTK (sMer) in plasma and cellular supernatant. The sMer was significantly up-regulated in plasma of VHL-null PM_2.5-exposed mice. Moreover, PM_2.5 could induce defective efferocytosis and activate inflammatory response through MerTK/IFNAR1/STAT1 signaling pathway in macrophages. Our results demonstrate that PM_2.5 could induce defective efferocytosis and inflammation by activating HIF-1α in macrophages, ultimately resulting in accelerating atherosclerotic lesion formation and development. Our data suggest HIF-1α in macrophages might be a potential target for PM_2.5-related atherosclerosis.

Cholesterol and HIF-1α: Dangerous Liaisons in Atherosclerosis

HIF-1α exerts both detrimental and beneficial actions in atherosclerosis. While there is evidence that HIF-1α could be pro-atherogenic within the atheromatous plaque, experimental models of atherosclerosis suggest a more complex role that depends on the cell type expressing HIF-1α. In atheroma plaques, HIF-1α is stabilized by local hypoxic conditions and by the lipid microenvironment. Macrophage exposure to oxidized LDLs (oxLDLs) or to necrotic plaque debris enriched with oxysterols induces HIF-1α -dependent pathways. Moreover, HIF-1α is involved in many oxLDL-induced effects in macrophages including inflammatory response, angiogenesis and metabolic reprogramming. OxLDLs activate toll-like receptor signaling pathways to promote HIF-1α stabilization. OxLDLs and oxysterols also induce NADPH oxidases and reactive oxygen species production, which subsequently leads to HIF-1α stabilization. Finally, recent investigations revealed that the activation of liver X receptor, an oxysterol nuclear receptor, results in an increase in HIF-1α transcriptional activity. Reciprocally, HIF-1α signaling promotes triglycerides and cholesterol accumulation in macrophages. Hypoxia and HIF-1α increase the uptake of oxLDLs, promote cholesterol and triglyceride synthesis and decrease cholesterol efflux. In conclusion, the impact of HIF-1α on cholesterol homeostasis within macrophages and the feedback activation of the inflammatory response by oxysterols via HIF-1α could play a deleterious role in atherosclerosis. In this context, studies aimed at understanding the specific mechanisms leading to HIF-1α activation within the plaque represents a promising field for research investigations and a path toward development of novel therapies.

Macrophage Metabolic Signaling during Ischemic Injury and Cardiac Repair

Macrophages are instrumental for the repair of organs that become injured due to ischemia, yet their potential for healing is sensitive to the availability of metabolites from the surrounding milieu. This sensitivity extends beyond anabolic and catabolic reactions, as metabolites are also leveraged to control production of secreted factors that direct intercellular crosstalk. In response to limiting extracellular oxygen, acute-phase macrophages activate hypoxia-inducible transcription factors that repurpose cellular metabolism. Subsequent repair-phase macrophages secrete cytokines to activate stromal cells, the latter which contribute to matrix deposition and scarring. As we now appreciate, these distinct functions are calibrated by directing flux of carbons and cofactors into specific metabolic shunts. This occurs through glycolysis, the pentose phosphate shunt, the tricarboxylic acid cycle, oxidative phosphorylation, nicotinamide adenine dinucleotides, lipids, amino acids, and through lesser understood pathways. The integration of metabolism with macrophage function is particularly important during injury to the ischemic heart, as glucose and lipid imbalance lead to inefficient repair and permanent loss of non-regenerative muscle. Here we review macrophage metabolic signaling under ischemic stress with implications for cardiac repair.

Significant difference between sirolimus and paclitaxel nanoparticles in anti-proliferation effect in normoxia and hypoxia: The basis of better selection of atherosclerosis treatment

Compared with paclitaxel, sirolimus has been more used in the treatment of vascular restenosis gradually as an anti-proliferative drug, but few basic studies have elucidated its mechanism. The anti-proliferative effects of sirolimus or paclitaxel have been demonstrated by numerous studies under normoxia, but few studies have been achieved focusing hypoxia. In this study, porcine carotid artery injury model and classical cobalt chloride hypoxia cell model were established. Sirolimus nanoparticles (SRM-NPs), paclitaxel nanoparticles (PTX-NPs) and blank nanoparticles (Blank-NPs) were prepared respectively. The effect of RPM-NPs on the degree of stenosis, proliferative index and the expression of PCNA after 28 days of porcine carotid artery injury model was evaluated. Compared with saline group and SRM groups, SRM-NPs group suppressed vascular stenosis, proliferative index and the expression of PCNA (P < 0.01 and P < 0.05). Endothelial cell (EC) and smooth muscle cell (SMC) were pre-treated with cobaltous chloride, followed by SRM-NPs, PTX-NPs, Blank-NPs or PBS control treating, the effects on cell proliferation, HIF-1 expression and glycolysis were detected. SRM-NPs could inhibit EC and SMC proliferation under hypoxia, while PTX-NPs couldn't (P < 0.001). Significant differences between sirolimus and paclitaxel NPs in anti-proliferation effect under normoxia and hypoxia may due to the different inhibitory effects on HIF-1α expression and glycolysis. In conclusion, these results suggest that sirolimus can inhibit the proliferation of hypoxic cells more effectively than paclitaxel. These observations may provide a basis for understanding clinical vascular stenosis therapeutic differences between rapamycin and paclitaxel.

Mitochondria orchestrate macrophage effector functions in atherosclerosis

Macrophages are pivotal in the initiation and development of atherosclerotic cardiovascular diseases. Recent studies have reinforced the importance of mitochondria in metabolic and signaling pathways to maintain macrophage effector functions. In this review, we discuss the past and emerging roles of macrophage mitochondria metabolic diversity in atherosclerosis and the potential avenue as biomarker. Beyond metabolic functions, mitochondria are also a signaling platform integrating epigenetic, redox, efferocytic and apoptotic regulations, which are exquisitely linked to their dynamics. Indeed, mitochondria functions depend on their density and shape perpetually controlled by mitochondria fusion/fission and biogenesis/mitophagy balances. Mitochondria can also communicate with other organelles such as the endoplasmic reticulum through mitochondria-associated membrane (MAM) or be secreted for paracrine actions. All these functions are perturbed in macrophages from mouse or human atherosclerotic plaques. A better understanding and integration of how these metabolic and signaling processes are integrated and dictate macrophage effector functions in atherosclerosis may ultimately help the development of novel therapeutic approaches.

[18F]ZCDD083: A PFKFB3-Targeted PET Tracer for Atherosclerotic Plaque Imaging

PFKFB3, a glycolysis-related enzyme upregulated in inflammatory conditions and angiogenesis, is an emerging target for diagnosis and therapy of atherosclerosis. The fluorinated phenoxindazole [¹⁸F]ZCDD083 was synthesized, radiolabeled in 17 ± 5% radiochemical yield and >99% radiochemical purity, and formulated for preclinical PET/CT imaging in mice. In vivo stability analysis showed no significant metabolite formation. Biodistribution studies showed high blood pool activity and slow hepatobiliary clearance. Significant activity was detected in the lung 2 h postinjection (pi) (11.0 ± 1.5%ID/g), while at 6 h pi no pulmonary background was observed. Ex vivo autoradiography at 6 h pi showed significant high uptake of [¹⁸F]ZCDD083 in the arch region and brachiocephalic artery of atherosclerotic mice, and no uptake in control mice, matching plaques distribution seen by lipid staining along with PFKFB3 expression seen by immunofluorescent staining. In vivo PET scans showed higher aortic region uptake of [¹⁸F]ZCDD083 in atherosclerotic ApoE^-/-Fbn1^C1039G+/- than in control mice (0.78 ± 0.05 vs 0.44 ± 0.09%ID/g). [¹⁸F]ZCDD083 was detected in aortic arch and brachiocephalic artery of ApoE^-/- (with moderate atherosclerosis) and ApoE^-/-Fbn1^C1039G+/- (with severe, advanced atherosclerosis) mice, suggesting this tracer may be useful for the noninvasive detection of atherosclerotic plaques in vivo.

HDL and Reverse Cholesterol Transport

Cardiovascular disease, with atherosclerosis as the major underlying factor, remains the leading cause of death worldwide. It is well established that cholesterol ester-enriched foam cells are the hallmark of atherosclerotic plaques. Multiple lines of evidence support that enhancing foam cell cholesterol efflux by HDL (high-density lipoprotein) particles, the first step of reverse cholesterol transport (RCT), is a promising antiatherogenic strategy. Yet, excitement towards the therapeutic potential of manipulating RCT for the treatment of cardiovascular disease has faded because of the lack of the association between cardiovascular disease risk and what was typically measured in intervention trials, namely HDL cholesterol, which has an inconsistent relationship to HDL function and RCT. In this review, we will summarize some of the potential reasons for this inconsistency, update the mechanisms of RCT, and highlight conditions in which impaired HDL function or RCT contributes to vascular disease. On balance, the evidence still argues for further research to better understand how HDL functionality contributes to RCT to develop prevention and treatment strategies to reduce the risk of cardiovascular disease.

Acid-sensing ion channels: Linking extracellular acidification with atherosclerosis

Extracellular acidification in atherosclerosis-prone regions of arterial walls is considered pro-atherosclerotic by exerting detrimental effect on macrophages, endothelial cells (ECs) and vascular smooth muscle cells (VSMCs). Acid-sensing ion channels (ASICs), a family of extracellular H⁺ (proton)-gated cation channels, are present extensively in the nervous system and other tissues, implying physiologic as well as pathophysiologic importance. Aberrant activation of ASICs is thought to be associated in EC dysfunction, macrophage phenotypic switch, and VSMC migration and proliferation. Although in vitro evidence acknowledges the contribution of ASIC activation in atherosclerosis, no direct evidence confirms their pro-atherosclerotic roles in vivo. In this review, the effect of extracellular acidity on three major contributors, ECs, macrophages, and VSMCs, is discussed focusing on the potential roles of ASICs in atherosclerotic development and underlying pathology. A more comprehensive understanding of ASICs in these processes may provide promising new therapeutic targets for treatment and prevention of atherosclerotic diseases.

Role of the COP9 Signalosome (CSN) in Cardiovascular Diseases

The constitutive photomorphogenesis 9 (COP9) signalosome (CSN) is an evolutionarily conserved multi-protein complex, consisting of eight subunits termed CSN1-CSN8. The main biochemical function of the CSN is the control of protein degradation via the ubiquitin-proteasome-system through regulation of cullin-RING E3-ligase (CRL) activity by deNEDDylation of cullins, but the CSN also serves as a docking platform for signaling proteins. The catalytic deNEDDylase (isopeptidase) activity of the complex is executed by CSN5, but only efficiently occurs in the three-dimensional architectural context of the complex. Due to its positioning in a central cellular pathway connected to cell responses such as cell-cycle, proliferation, and signaling, the CSN has been implicated in several human diseases, with most evidence available for a role in cancer. However, emerging evidence also suggests that the CSN is involved in inflammation and cardiovascular diseases. This is both due to its role in controlling CRLs, regulating components of key inflammatory pathways such as nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), and complex-independent interactions of subunits such as CSN5 with inflammatory proteins. In this case, we summarize and discuss studies suggesting that the CSN may have a key role in cardiovascular diseases such as atherosclerosis and heart failure. We discuss the implicated molecular mechanisms ranging from inflammatory NF-κB signaling to proteotoxicity and necrosis, covering disease-relevant cell types such as myeloid and endothelial cells or cardiomyocytes. While the CSN is considered to be disease-exacerbating in most cancer entities, the cardiovascular studies suggest potent protective activities in the vasculature and heart. The underlying mechanisms and potential therapeutic avenues will be critically discussed.

Atherosclerosis Treatment with Stimuli-Responsive Nanoagents: Recent Advances and Future Perspectives

Atherosclerosis is the root of approximately one-third of global mortalities. Nanotechnology exhibits splendid prospects to combat atherosclerosis at the molecular level by engineering smart nanoagents with versatile functionalizations. Significant advances in nanoengineering enable nanoagents to autonomously navigate in the bloodstream, escape from biological barriers, and assemble with their nanocohort at the targeted lesion. The assembly of nanoagents with endogenous and exogenous stimuli breaks down their shells, facilitates intracellular delivery, releases their cargo to kill the corrupt cells, and gives imaging reports. All these improvements pave the way toward personalized medicine for atherosclerosis. This review systematically summarizes the recent advances in stimuli-responsive nanoagents for atherosclerosis management and its progress in clinical trials.

Hypoxia-inducible protein 2 Hig2/Hilpda mediates neutral lipid accumulation in macrophages and contributes to atherosclerosis in apolipoprotein E-deficient mice

Recently we identified hypoxia-inducible protein 2 (HIG2)/hypoxia-inducible lipid droplet-associated (HILPDA) as lipid droplet (LD) protein. Because HILPDA is highly expressed in atherosclerotic plaques, we examined its regulation and function in murine macrophages, compared it to the LD adipose differentiation-related protein (Adrp)/perilipin 2 (Plin2), and investigated its effects on atherogenesis in apolipoprotein E-deficient (ApoE^-/-) mice. Tie2-Cre-driven Hilpda conditional knockout (cKO) did not affect viability, proliferation, and ATP levels in macrophages. Hilpda proved to be a target of hypoxia-inducible factor 1 (Hif-1) and peroxisome proliferator-activated receptors. In contrast, Adrp/Plin2 was not induced by Hif-1. Hilpda localized to the endoplasmic reticulum-LD interface, the site of LD formation. Hypoxic lipid accumulation and storage of oxidized LDL, cholesteryl esters and triglycerides were abolished in Hilpda cKO macrophages, independent of the glycolytic switch, fatty acid or lipoprotein uptake. Hilpda depletion reduced resistance against lipid overload and increased production of reactive oxygen species after reoxygenation. LPS-stimulated prostaglandin-E2 production was dysregulated in macrophages, demonstrating the substrate buffer and reservoir function of LDs for eicosanoid production. In ApoE^-/- Hilpda cKO mice, total aortic plaque area, plaque macrophages and vascular Vegf expression were reduced. Thus, macrophage Hilpda is crucial to foam-cell formation and lipid deposition, and to controlled prostaglandin-E2 production. By these means Hilpda promotes lesion formation and progression of atherosclerosis.-Maier, A., Wu, H., Cordasic, N., Oefner, P., Dietel, B., Thiele, C., Weidemann, A., Eckardt, K.-U., Warnecke, C. Hypoxia-inducible protein 2 Hig2/Hilpda mediates neutral lipid accumulation in macrophages and contributes to atherosclerosis in apolipoprotein E-deficient mice.

Hypoxia in Atherogenesis

The anoxemia theory proposes that an imbalance between the demand for and supply of oxygen in the arterial wall is a key factor in the development of atherosclerosis. There is now substantial evidence that there are regions within the atherosclerotic plaque in which profound hypoxia exists; this may fundamentally change the function, metabolism, and responses of many of the cell types found within the developing plaque and whether the plaque will evolve into a stable or unstable phenotype. Hypoxia is characterized in molecular terms by the stabilization of hypoxia-inducible factor (HIF) 1α, a subunit of the heterodimeric nuclear transcriptional factor HIF-1 and a master regulator of oxygen homeostasis. The expression of HIF-1 is localized to perivascular tissues, inflammatory macrophages, and smooth muscle cells adjacent to the necrotic core of atherosclerotic lesions and regulates several genes that are important to vascular function including vascular endothelial growth factor, nitric oxide synthase, endothelin-1, and erythropoietin. This review summarizes the effects of hypoxia on the functions of cells involved in atherogenesis and the evidence for its potential importance from experimental models and clinical studies.

Disruption of Glut1 in Hematopoietic Stem Cells Prevents Myelopoiesis and Enhanced Glucose Flux in Atheromatous Plaques of ApoE(-/-) Mice

RATIONALE

Inflamed atherosclerotic plaques can be visualized by noninvasive positron emission and computed tomographic imaging with (18)F-fluorodeoxyglucose, a glucose analog, but the underlying mechanisms are poorly understood.

OBJECTIVE

Here, we directly investigated the role of Glut1-mediated glucose uptake in apolipoprotein E-deficient (ApoE(-/-)) mouse model of atherosclerosis.

METHODS AND RESULTS

We first showed that the enhanced glycolytic flux in atheromatous plaques of ApoE(-/-) mice was associated with the enhanced metabolic activity of hematopoietic stem and multipotential progenitor cells and higher Glut1 expression in these cells. Mechanistically, the regulation of Glut1 in ApoE(-/-) hematopoietic stem and multipotential progenitor cells was not because of alterations in hypoxia-inducible factor 1α signaling or the oxygenation status of the bone marrow but was the consequence of the activation of the common β subunit of the granulocyte-macrophage colony-stimulating factor/interleukin-3 receptor driving glycolytic substrate utilization by mitochondria. By transplanting bone marrow from WT, Glut1(+/-), ApoE(-/-), and ApoE(-/-)Glut1(+/-) mice into hypercholesterolemic ApoE-deficient mice, we found that Glut1 deficiency reversed ApoE(-/-) hematopoietic stem and multipotential progenitor cell proliferation and expansion, which prevented the myelopoiesis and accelerated atherosclerosis of ApoE(-/-) mice transplanted with ApoE(-/-) bone marrow and resulted in reduced glucose uptake in the spleen and aortic arch of these mice.

CONCLUSIONS

We identified that Glut1 connects the enhanced glucose uptake in atheromatous plaques of ApoE(-/-) mice with their myelopoiesis through regulation of hematopoietic stem and multipotential progenitor cell maintenance and myelomonocytic fate and suggests Glut1 as potential drug target for atherosclerosis.

Potential contributions of intimal and plaque hypoxia to atherosclerosis

Injury of arterial endothelium by abnormal shear stress and other insults induces migration and proliferation of vascular smooth muscle cells (VSMCs), which in turn leads to intimal thickening, hypoxia, and vasa vasorum angiogenesis. The resultant new blood vessels extend from the tunica media into the outer intima, allowing blood-borne oxidized low-density lipoprotein (oxLDL) particles to accumulate in outer intimal tissues by extravasation through local capillaries. In response to oxLDL accumulation, monocytes infiltrate into arterial wall tissues, where they differentiate into macrophages and subsequently evolve into foam cells by uptaking large quantities of oxLDL particles, the latter process being stimulated by hypoxia. Increased oxygen demand due to expanding macrophage and foam cell populations contributes to persistent hypoxia in plaque lesions, whereas hypoxia further promotes plaque growth by stimulating angiogenesis, monocyte infiltration, and oxLDL uptake into macrophages. Molecularly, the accumulation of hypoxia-inducible factor (HIF)-1α and the expression of its target genes mediate many of the hypoxia-induced processes during plaque initiation and growth. It is hoped that further understanding of the underlying mechanisms may lead to novel therapies for effective intervention of atherosclerosis.

HIF-1α and PFKFB3 Mediate a Tight Relationship Between Proinflammatory Activation and Anerobic Metabolism in Atherosclerotic Macrophages

OBJECTIVE

Although it is accepted that macrophage glycolysis is upregulated under hypoxic conditions, it is not known whether this is linked to a similar increase in macrophage proinflammatory activation and whether specific energy demands regulate cell viability in the atheromatous plaque.

APPROACH AND RESULTS

We studied the interplay between macrophage energy metabolism, polarization, and viability in the context of atherosclerosis. Cultured human and murine macrophages and an in vivo murine model of atherosclerosis were used to evaluate the mechanisms underlying metabolic and inflammatory activity of macrophages in the different atherosclerotic conditions analyzed. We observed that macrophage energetics and inflammatory activation are closely and linearly related, resulting in dynamic calibration of glycolysis to keep pace with inflammatory activity. In addition, we show that macrophage glycolysis and proinflammatory activation mainly depend on hypoxia-inducible factor and on its impact on glucose uptake, and on the expression of hexokinase II and ubiquitous 6-phosphofructo-2-kinase. As a consequence, hypoxia potentiates inflammation and glycolysis mainly via these pathways. Moreover, when macrophages' ability to increase glycolysis through 6-phosphofructo-2-kinase is experimentally attenuated, cell viability is reduced if subjected to proinflammatory or hypoxic conditions, but unaffected under control conditions. In addition to this, granulocyte-macrophage colony-stimulating factor enhances anerobic glycolysis while exerting a mild proinflammatory activation.

CONCLUSIONS

These findings, in human and murine cells and in an animal model, show that hypoxia potentiates macrophage glycolytic flux in concert with a proportional upregulation of proinflammatory activity, in a manner that is dependent on both hypoxia-inducible factor -1α and 6-phosphofructo-2-kinase.

A transcriptional perspective on human macrophage biology

Macrophages are a major cell type in tissue homeostasis and contribute to both pathology and resolution in all acute and chronic inflammatory diseases ranging from infections, cancer, obesity, atherosclerosis, autoimmune disorders to neurodegenerative diseases such as Alzheimer's disease. The cellular and functional diversity of macrophages depends upon tightly regulated transcription. The innate immune system is under profound evolutionary selection. There is increasing recognition that human macrophage biology differs very significantly from that of commonly studied animal models, which therefore can have a limited predictive value. Here we report on the newest findings on transcriptional control of macrophage activation, and how we envision integrating studies on transcriptional and epigenetic regulation, and more classical approaches in murine models. Moreover, we provide new insights into how we can learn about transcriptional regulation in the human system from larger efforts such as the FANTOM (Functional Annotation of the Mammalian Genome) consortium.

Acidification of the intimal fluid: the perfect storm for atherogenesis

Atherosclerotic lesions are often hypoxic and exhibit elevated lactate concentrations and local acidification of the extracellular fluids. The acidification may be a consequence of the abundant accumulation of lipid-scavenging macrophages in the lesions. Activated macrophages have a very high energy demand and they preferentially use glycolysis for ATP synthesis even under normoxic conditions, resulting in enhanced local generation and secretion of lactate and protons. In this review, we summarize our current understanding of the effects of acidic extracellular pH on three key players in atherogenesis: macrophages, apoB-containing lipoproteins, and HDL particles. Acidic extracellular pH enhances receptor-mediated phagocytosis and antigen presentation by macrophages and, importantly, triggers the secretion of proinflammatory cytokines from macrophages through activation of the inflammasome pathway. Acidity enhances the proteolytic, lipolytic, and oxidative modifications of LDL and other apoB-containing lipoproteins, and strongly increases their affinity for proteoglycans, and may thus have major effects on their retention and the ensuing cellular responses in the arterial intima. Finally, the decrease in the expression of ABCA1 at acidic pH may compromise cholesterol clearance from atherosclerotic lesions. Taken together, acidic extracellular pH amplifies the proatherogenic and proinflammatory processes involved in atherogenesis.

Macrophage-mediated response to hypoxia in disease

Hypoxia plays a critical role in the pathobiology of various inflamed, diseased tissues, including malignant tumors, atherosclerotic plaques, myocardial infarcts, the synovia of rheumatoid arthritic joints, healing wounds, and sites of bacterial infection. These areas of hypoxia form when the blood supply is occluded and/or the oxygen supply is unable to keep pace with cell growth and/or infiltration of inflammatory cells. Macrophages are ubiquitous in all tissues of the body and exhibit great plasticity, allowing them to perform divergent functions, including, among others, patrolling tissue, combating invading pathogens and tumor cells, orchestrating wound healing, and restoring homeostasis after an inflammatory response. The number of tissue macrophages increases markedly with the onset and progression of many pathological states, with many macrophages accumulating in avascular and necrotic areas, where they are exposed to hypoxia. Recent studies show that these highly versatile cells then respond rapidly to the hypoxia present by altering their expression of a wide array of genes. Here we review the evidence for hypoxia-driven macrophage inflammatory responses in various disease states, and how this influences disease progression and treatment.

Effects of copper sulfate-oxidized or myeloperoxidase-modified LDL on lipid loading and programmed cell death in macrophages under hypoxia

Atheromatous plaques contain heavily lipid-loaded macrophages that die, hence generating the necrotic core of these plaques. Since plaque instability and rupture is often correlated with a large necrotic core, it is important to understand the mechanisms underlying foam cell death. Furthermore, macrophages within the plaque are associated with hypoxic areas but little is known about the effect of low oxygen partial pressure on macrophage death. The aim of this work was to unravel macrophage death mechanisms induced by oxidized low-density lipoproteins (LDL) both under normoxia and hypoxia. Differentiated macrophages were incubated in the presence of native, copper sulfate-oxidized, or myeloperoxidase-modified LDL. The unfolded protein response, apoptosis, and autophagy were then investigated. The unfolded protein response and autophagy were triggered by myeloperoxidase-modified LDL and, to a larger extent, by copper sulfate-oxidized LDL. Electron microscopy observations showed that oxidized LDL induced excessive autophagy and apoptosis under normoxia, which were less marked under hypoxia. Myeloperoxidase-modified LDL were more toxic and induced a higher level of apoptosis. Hypoxia markedly decreased apoptosis and cell death, as marked by caspase activation. In conclusion, the cell death pathways induced by copper sulfate-oxidized and myeloperoxidase-modified LDL are different and are differentially modulated by hypoxia.

Hypoxia in atherosclerosis and inflammation

PURPOSE OF REVIEW

Hypoxia triggers various cellular processes, both in physiological and pathological conditions, and has recently also been implicated in atherosclerosis. This review summarizes the recent evidence for the presence and the role of hypoxia in atherosclerosis. Additionally, it will elucidate on hypoxic signaling, which is interlinked with inflammatory signaling, and discuss recent advances in imaging of hypoxia in atherosclerosis.

RECENT FINDINGS

Hypoxia is present in atherosclerotic plaques in humans and animal models, and systemic hypoxia promotes atherosclerosis. Hypoxia stimulates proatherosclerotic processes, like deficient lipid efflux, inflammation, interference with macrophage polarization and glucose metabolism. However, the molecular mechanism of hypoxia-mediated atherogenesis remains unclear. Noninvasive imaging directly targeting plaque hypoxia has been applied in animal models of atherosclerosis, but remains to be validated in humans. Meanwhile, the metabolic marker ¹⁸F-fluorodeoxyglucose, used to detect human atherosclerosis in vivo, may serve as an indirect marker of plaque hypoxia due to enhanced glucose uptake in anaerobic metabolism.

SUMMARY

Recent studies underscore the proatherogenic role of hypoxia in macrophage lipid and glucose metabolism, inflammation and polarization. These studies provide new insights into the pathogenesis of atherosclerosis and unravel novel therapeutic targets and new options for noninvasive imaging of human atherosclerotic plaques.