Selective Clearance of Macrophages in Atherosclerotic Plaques by the Protein Synthesis Inhibitor Cycloheximide

KCTD5 regulates Ikaros degradation induced by chemotherapeutic drug etoposide in hematological cells

Therapy-related leukemia carries a poor prognosis, and leukemia after chemotherapy is a growing risk in clinic, whose mechanism is still not well understood. Ikaros transcription factor is an important regulator in hematopoietic cells development and differentiation. In the absence of Ikaros, lymphoid cell differentiation is blocked at an extremely early stage, and myeloid cell differentiation is also significantly affected. In this work, we showed that chemotherapeutic drug etoposide reduced the protein levels of several isoforms of Ikaros including IK1, IK2 and IK4, but not IK6 or IK7, by accelerating protein degradation, in leukemic cells. To investigate the molecular mechanism of Ikaros degradation induced by etoposide, immunoprecipitation coupled with LC-MS/MS analysis was conducted to identify changes in protein interaction with Ikaros before and after etoposide treatment, which uncovered KCTD5 protein. Our further study demonstrates that KCTD5 is the key stabilizing factor of Ikaros and chemotherapeutic drug etoposide induces Ikaros protein degradation through decreasing the interaction of Ikaros with KCTD5. These results suggest that etoposide may induce leukemic transformation by downregulating Ikaros via KCTD5, and our work may provide insights to attenuate the negative impact of chemotherapy on hematopoiesis.

Programmed death of macrophages in atherosclerosis: mechanisms and therapeutic targets

Atherosclerosis is a progressive inflammatory disorder of the arterial vessel wall characterized by substantial infiltration of macrophages, which exert both favourable and detrimental functions. Early in atherogenesis, macrophages can clear cytotoxic lipoproteins and dead cells, preventing cytotoxicity. Efferocytosis - the efficient clearance of dead cells by macrophages - is crucial for preventing secondary necrosis and stimulating the release of anti-inflammatory cytokines. In addition, macrophages can promote tissue repair and proliferation of vascular smooth muscle cells, thereby increasing plaque stability. However, advanced atherosclerotic plaques contain large numbers of pro-inflammatory macrophages that secrete matrix-degrading enzymes, induce death in surrounding cells and contribute to plaque destabilization and rupture. Importantly, macrophages in the plaque can undergo apoptosis and several forms of regulated necrosis, including necroptosis, pyroptosis and ferroptosis. Regulated necrosis has an important role in the formation and expansion of the necrotic core during plaque progression, and several triggers for necrosis are present within atherosclerotic plaques. This Review focuses on the various forms of programmed macrophage death in atherosclerosis and the pharmacological interventions that target them as a potential means of stabilizing vulnerable plaques and improving the efficacy of currently available anti-atherosclerotic therapies.

Caffeoylquinic acids isolated from Lonicera japonica Thunb. as TAK1 inhibitors protects against LPS plus IFN-γ-stimulated inflammation by interacting with KEAP1-regulated NRF2 activation

The transforming growth factor-β-activated kinase 1 (TAK1) phosphorylation promotes inflammation occurrence. Meanwhile, TAK1 directly interacts with KEAP1 and strenghtenes NRF2/HO-1 pathway downregulated-inflammation. Recently, we found that caffeoylquinic acids not only possessed powderful anti-inflammation function, but also attenuated oxidative damage through KEAP1/NRF2 pathway. Whereas it's rarely understood whether the anti-inflammatory activity were regulated by their interaction between TAK1 and NRF2. Herein, 34 caffeoylquinic acids including five new (2, 4-7) were systematically isolated and identified on the basis of spectroscopic evidence from Lonicera japonica Thunb. flower buds. Their inhibitory effects on inflammation induced by LPS plus IFN-γ were exerted substantial NO scavenging activity, and inhibited massive production of inflammatory cytokines and related proteins. Compound 3 (4F5C-QAME) exhibited the best anti-inflammation activity. 4F5C-QAME down-regulated the phosphorylation of TAK1, JNK, and c-JUN, thereby alleviated inflammation stimulated by LPS plus IFN-γ. Meanwhile, 4F5C-QAME could alleviate the interaction between TAK1 and KEAP1, inhibit the ubiquitination degradation of NRF2, activate NRF2/HO-1 signaling pathway, result in the increase in ROS elimination. Furthermore, 4F5C-QAME effectively protected against inflammation through direct inhibition of TAK1 phosphorylation. Based on these findings, 4F5C-QAME directly targeting TAK1 could be represented as a potential drug candidate for preventing/treating inflammatory diseases that regulated NRF2 activation through alleviating the interaction between TAK1 and KEAP1. Moreover, the regulatory mechanism of TAK1 on NRF2 activation under exogenous oxidative stress was revealed for the first time.

Macrophage Death as a Pharmacological Target in Atherosclerosis

Atherosclerosis is a chronic inflammatory disorder characterized by the gradual build-up of plaques within the vessel wall of middle-sized and large arteries. Over the past decades, treatment of atherosclerosis mainly focused on lowering lipid levels, which can be accomplished by the use of statins. However, some patients do not respond sufficiently to statin therapy and therefore still have a residual cardiovascular risk. This issue highlights the need for novel therapeutic strategies. As macrophages are implicated in all stages of atherosclerotic lesion development, they represent an important alternative drug target. A variety of anti-inflammatory strategies have recently emerged to treat or prevent atherosclerosis. Here, we review the canonical mechanisms of macrophage death and their impact on atherogenesis and plaque stability. Macrophage death is a prominent feature of advanced plaques and is a major contributor to necrotic core formation and plaque destabilization. Mechanisms of macrophage death in atherosclerosis include apoptosis, passive or accidental necrosis as well as secondary necrosis, a type of death that typically occurs when apoptotic cells are insufficiently cleared by neighboring cells via a phagocytic process termed efferocytosis. In addition, less-well characterized types of regulated necrosis in macrophages such as necroptosis, pyroptosis, ferroptosis, and parthanatos may occur in advanced plaques and are also discussed. Autophagy in plaque macrophages is an important survival pathway that protects against cell death, yet massive stimulation of autophagy promotes another type of death, usually referred to as autosis. Multiple lines of evidence indicate that a better insight into the different mechanisms of macrophage death, and how they mutually interact, will provide novel pharmacological strategies to resolve atherosclerosis and stabilize vulnerable, rupture-prone plaques.

Cimetidine down-regulates stability of Foxp3 protein via Stub1 in Treg cells

Foxp3-expressing Treg cells have been well documented to provide immune regulation by promoting immune tolerance and suppressing immune over-reaction. Cimetidine (CIM), used to inhibit stomach acid secretion, has been reported to promote immune responses and suppress Treg cell function in several studies. However, the underlying mechanism is unknown. To investigate CIM effects on the suppressive function of Treg and Foxp3, here we used CIM to stimulate human CD4⁺CD25⁺ Treg cells and Jurkat T cells and evaluated changes of Foxp3 expression and stability. Our data showed that CIM leads to a reduction of Foxp3 via E3 ligase Stub1-mediated proteosomal degradation, which is dependent on an activated PI3K-AKT-mTOR pathway. Thus, CIM affects the suppressive function of Treg cells by destabilizing their Foxp3 expression.

Caspase-3 Deletion Promotes Necrosis in Atherosclerotic Plaques of ApoE Knockout Mice

Apoptosis of macrophages and vascular smooth muscle cells (VSMCs) in advanced atherosclerotic plaques contributes to plaque progression and instability. Caspase-3, a key executioner protease in the apoptotic pathway, has been identified in human and mouse atherosclerotic plaques but its role in atherogenesis is not fully explored. We therefore investigated the impact of caspase-3 deletion on atherosclerosis by crossbreeding caspase-3 knockout (Casp3^−/−) mice with apolipoprotein E knockout (ApoE^−/−) mice. Bone marrow-derived macrophages and VSMCs isolated from Casp3^−/−ApoE^−/− mice were resistant to apoptosis but showed increased susceptibility to necrosis. However, caspase-3 deficiency did not sensitize cells to undergo RIP1-dependent necroptosis. To study the effect on atherosclerotic plaque development, Casp3^+/+ApoE^−/− and Casp3^−/−ApoE^−/− mice were fed a western-type diet for 16 weeks. Though total plasma cholesterol, triglycerides, and LDL cholesterol levels were not altered, both the plaque size and percentage necrosis were significantly increased in the aortic root of Casp3^−/−ApoE^−/− mice as compared to Casp3^+/+ApoE^−/− mice. Macrophage content was significantly decreased in plaques of Casp3^−/−ApoE^−/− mice as compared to controls, while collagen content and VSMC content were not changed. To conclude, deletion of caspase-3 promotes plaque growth and plaque necrosis in ApoE^−/− mice, indicating that this antiapoptotic strategy is unfavorable to improve atherosclerotic plaque stability.

Impact of the Everolimus-eluting Bioresorbable Scaffold in Coronary Atherosclerosis

INTRODUCTION AND OBJECTIVES

The Absorb bioresorbable vascular scaffold has been shown to decrease total plaque areas in the treated segment. However, it is unknown whether plaque size is modified in scaffolded segments only or whether the modification extends to other coronary segments.

METHODS

Absorb Cohort A is a single-arm, prospective study, with safety and imaging endpoints, in which 30 patients underwent percutaneous coronary intervention with the first generation Absorb bioresorbable vascular scaffold. Noninvasive multislice computed tomography imaging was performed in 18 patients at 18 months and 5 years of follow-up. The present study was an intrapatient comparison of matched segments (normalized by the segment length) of the scaffolded region with nonintervened segments for lumen volume, vessel volume, plaque volume, plaque burden, and percent change in plaque atheroma volume.

RESULTS

All 18 scaffolded segments could be analyzed. In the nonintervened segments, 1 of 72 segments had a motion artifact and was excluded. Serial comparison showed that the scaffolded segments showed no significant change in the mean plaque burden, total atheroma volume, total lumen volume, or vessel volume between 18 months and 5 years. Conversely, the untreated segments showed a significant increase in plaque burden (2.7 ± 6.5%; P < .01) and normalized plaque volumes (8.0 ± 22.8mm(3); P < .01). This resulted in a significant difference in plaque burden between scaffolded and nonintervened segments (P = .03).

CONCLUSIONS

In this small series, the Absorb bioresorbable vascular scaffold showed the potential to provide an additional benefit to pharmacological therapy in locally reducing progression of percent plaque burden. These findings need to be confirmed in larger studies.

Autophagy in Vascular Disease

Autophagy is a reparative, life-sustaining process by which cytoplasmic components are sequestered in double-membrane vesicles and degraded on fusion with lysosomal compartments. Growing evidence reveals that basal autophagy is an essential in vivo process mediating proper vascular function. Moreover, autophagy is stimulated by many stress-related stimuli in the arterial wall to protect endothelial cells and smooth muscle cells against cell death and the initiation of vascular disease, in particular atherosclerosis. Basal autophagy is atheroprotective during early atherosclerosis but becomes dysfunctional in advanced atherosclerotic plaques. Little is known about autophagy in other vascular disorders, such as aneurysm formation, arterial aging, vascular stiffness, and chronic venous disease, even though autophagy is often impaired. This finding highlights the need for pharmacological interventions with compounds that stimulate the prosurvival effects of autophagy in the vasculature. A large number of animal studies and clinical trials have indicated that oral or stent-based delivery of the autophagy inducer rapamycin or derivatives thereof, collectively known as rapalogs, effectively inhibit the basic mechanisms that control growth and destabilization of atherosclerotic plaques. Other autophagy-inducing drugs, such as spermidine or add-on therapy with widely used antiatherogenic compounds, including statins and metformin, are potentially useful to prevent vascular disease with minimal adverse effects.

Overexpression of smooth muscle myosin heavy chain leads to activation of the unfolded protein response and autophagic turnover of thick filament-associated proteins in vascular smooth muscle cells

Duplications spanning nine genes at the genomic locus 16p13.1 predispose individuals to acute aortic dissections. The most likely candidate gene in this region leading to the predisposition for dissection is MYH11, which encodes smooth muscle myosin heavy chain (SM-MHC). The effects of increased expression of MYH11 on smooth muscle cell (SMC) phenotypes were explored using mouse aortic SMCs with transgenic overexpression of one isoform of SM-MHC. We found that these cells show increased expression of Myh11 and myosin filament-associated contractile genes at the message level when compared with control SMCs, but not at the protein level due to increased protein degradation. Increased expression of Myh11 resulted in endoplasmic reticulum (ER) stress in SMCs, which led to a paradoxical decrease of protein levels through increased autophagic degradation. An additional consequence of ER stress in SMCs was increased intracellular calcium ion concentration, resulting in increased contractile signaling and contraction. The increased signals for contraction further promote transcription of contractile genes, leading to a feedback loop of metabolic abnormalities in these SMCs. We suggest that overexpression of MYH11 can lead to increased ER stress and autophagy, findings that may be globally implicated in disease processes associated with genomic duplications.

mTOR inhibition: a promising strategy for stabilization of atherosclerotic plaques

Statins are currently able to stabilize atherosclerotic plaques by lowering plasma cholesterol and pleiotropic effects, but a residual risk for atherosclerotic disease remains. Therefore, effective prevention of atherosclerosis and treatment of its complications is still a major clinical challenge. A large body of evidence indicates that mammalian target of rapamycin (mTOR) inhibitors such as rapamycin or everolimus have pleiotropic anti-atherosclerotic effects so that these drugs can be used as add-on therapy to prevent or delay the pathogenesis of atherosclerosis. Moreover, bioresorbable scaffolds eluting everolimus trigger a healing process in the vessel wall, both in pigs and humans, that results in late lumen enlargement and plaque regression. At present, this phenomenon of atheroregression is poorly understood. However, given that mTOR inhibitors suppress cell proliferation and trigger autophagy, a cellular survival pathway and a process linked to cholesterol efflux, we hypothesize that these compounds can inhibit (or reverse) the basic mechanisms that control plaque growth and destabilization. Unfortunately, adverse effects associated with mTOR inhibitors such as dyslipidemia and hyperglycemia have recently been identified. Dyslipidemia is manageable via statin treatment, while the anti-diabetic drug metformin would prevent hyperglycemia. Because metformin has beneficial macrovascular effects, this drug in combination with an mTOR inhibitor might have significant promise to treat patients with unstable plaques. Moreover, both statins and metformin are known to inhibit mTOR via AMPK activation so that they would fully exploit the beneficial effects of mTOR inhibition in atherosclerosis.

Cycloheximide and lipopolysaccharide downregulate αENaC mRNA via different mechanisms in alveolar epithelial cells

Active Na(+) transport mediated by epithelial Na(+) channel (ENaC) is vital for fetal lung fluid reabsorption at birth and pulmonary edema resolution. Previously, we demonstrated that αENaC expression and activity are downregulated in alveolar epithelial cells by cycloheximide (Chx) and Pseudomonas aeruginosa. The regulatory mechanisms of αENaC mRNA expression by Chx and lipopolysaccharide (LPS) from P. aeruginosa were further studied in the present work. Both agents decreased αENaC mRNA expression to 50% of control values after 4 h. Chx repressed αENaC expression in a dose-dependent manner independently of protein synthesis. Although extracellular signal-regulated kinases 1 and 2 (ERK1/2) and p38 mitogen-activated protein kinase (MAPK) pathways were activated by the two treatments, their mechanisms of ENaC mRNA modulation were different. First, activation of the signaling pathways was sustained by Chx but only transiently by LPS. Second, ERK1/2 or p38 MAPK inhibition attenuated the effects of Chx on αENaC mRNA, whereas suppression of both signaling pathways was necessary to alleviate the outcome of LPS on αENaC mRNA. The molecular mechanisms involved in the decrease of αENaC expression were investigated in both conditions. LPS, but not Chx, significantly reduced αENaC promoter activity via the ERK1/2 and p38 MAPK pathways. These results suggest that LPS attenuates αENaC mRNA expression via diminution of transcription, whereas Chx could trigger some posttranscriptional mechanisms. Although LPS and Chx downregulate αENaC mRNA expression similarly and with similar signaling pathways, the mechanisms modulating ENaC expression are different depending on the nature of the cellular stress.

Knockdown of mTOR by lentivirus‑mediated RNA interference suppresses atherosclerosis and stabilizes plaques via a decrease of macrophages by autophagy in apolipoprotein E‑deficient mice

Atherosclerotic plaque destabilization and rupture leads to acute coronary syndromes which cause serious damage to human health worldwide. However, there is currently a lack of efficient therapeutic methods. Mammalian target of rapamycin (mTOR) has been suggested to be involved in the development of atherosclerotic plaques and serves as a therapeutic target. The present study was performed to determine whether RNA interference (RNAi) of mTOR in vivo by LV‑mediated small hairpin RNA (shRNA) was capable of inhibiting the progression of atherosclerotic plaques. LV‑mediated shRNA against mTOR (LV‑shmTOR) was designed and obtained. Male apolipoprotein E‑deficient mice were fed a high‑fat diet and a constrictive collar was placed around the right carotid arteries of these mice to induce plaque formation. Eight weeks after surgery, mice were randomly divided into the mTOR RNA interference (LV‑shmTOR) group, receiving treatment with LV‑mTOR‑shRNA; the LV‑shCON group, receiving treatment with LV‑non‑specific‑shRNA; and the control group, receiving treatment with phosphate‑buffered saline. Following transfection, the mice were sacrificed to evaluate the effects of mTOR expression silencing on atherosclerosis. Transfection of LV‑mTOR‑shRNA markedly inhibited the mRNA and protein expression levels. Knockdown of mTOR ameliorated dysregulated blood lipid metabolism and stabilized aortic atherosclerotic plaques by decreasing the plaque area and increasing the fibrous cap and cap‑to‑core ratio. Furthermore, macrophages were decreased by silencing mTOR in atherosclerotic plaques. In addition, western blot analysis revealed that the knockdown of mTOR increased autophagy‑related protein 13 (Atg13) dephosphorylation and light chain 3‑I/light chain 3‑II (LC3‑I/LC3‑II) ratios, both of which were associated with a high activity of autophagy, suggesting an increase of autophagy in atherosclerotic plaques. Moreover, genes including matrix metalloproteinase 2, monocyte chemoattractant protein 1 and tissue factor, which promote plaque instability, were downregulated by silencing mTOR. These results demonstrate that LV‑mediated mTOR silencing by RNAi treatment induces macrophage autophagy and is a potential strategy for the treatment of atherosclerotic plaques.

Drug-induced macrophage autophagy in atherosclerosis: for better or worse?

Autophagy is a reparative, life-sustaining process by which cytoplasmic components are sequestered in double membrane vesicles and degraded upon fusion with lysosomal compartments. Mice with a macrophage-specific deletion of the essential autophagy gene Atg5 develop plaques with increased apoptosis and oxidative stress as well as enhanced plaque necrosis. This finding indicates that basal autophagy in macrophages is anti-apoptotic and present in atherosclerotic plaques to protect macrophages against various atherogenic stressors. However, autophagy is impaired in advanced stages of atherosclerosis and its deficiency promotes atherosclerosis in part through activation of the inflammasome. Because basal autophagy can be intensified selectively in macrophages by specific drugs such as mammalian target of rapamycin (mTOR) inhibitors or Toll-like receptor 7 (TLR7) ligands, these drugs were recently tested as potential plaque stabilizing compounds. Stent-based delivery of the mTOR inhibitor everolimus promotes a stable plaque phenotype, whereas local administration of the TLR7 ligand imiquimod stimulates inflammation and plaque progression. Therefore, more drugs capable of inducing autophagy should be tested in plaque macrophages to evaluate the feasibility of this approach. Given that drug-induced macrophage autophagy is associated with pro-inflammatory responses due to cytokine release, induction of postautophagic necrosis or activation of phagocytes after clearance of the autophagic corpse, cotreatment with anti-inflammatory compounds may be required. Overall, this review highlights the pros and cons of macrophage autophagy as a drug target for plaque stabilization.

Therapeutic strategies to deplete macrophages in atherosclerotic plaques

Macrophages can be found in all stages of atherosclerosis and are major contributors of atherosclerotic plaque development, progression and destabilization. Continuous recruitment of monocytes drives this chronic inflammatory disease, which can be intervened by several strategies: reducing the inflammatory stimulus by lowering circulating lipids and promoting cholesterol efflux from plaque, direct and indirect targeting of adhesion molecules and chemokines involved in monocyte adhesion and transmigration and inducing macrophage death in atherosclerotic plaques in combination with anti-inflammatory drugs. This review discusses the outlined strategies to deplete macrophages from atherosclerotic plaques to promote plaque stabilization.

Everolimus Triggers Cytokine Release by Macrophages

Objective—

Stent-based delivery of the mammalian target of rapamycin (mTOR) inhibitor everolimus is a promising strategy for the treatment of coronary artery disease. We studied potential adverse effects associated with mTOR inhibition.

Methods and Results—

Macrophages in culture were either treated with everolimus or starved to inhibit mTOR. Everolimus led to inhibition of protein translation, activation of p38 MAPK, and the release of proinflammatory cytokines (eg, IL-6, TNFα) and chemokines (eg, MCP1, Rantes) before induction of autophagic death. These effects were also observed with rapamycin, but not after starvation. Everolimus-induced cytokine release was similar in macrophages lacking the essential autophagy gene Atg7 but was inhibited when macrophages were cotreated with p38 MAPK inhibitor SB202190 or the glucocorticoid clobetasol. Combined stent-based delivery of clobetasol and everolimus in rabbit plaques downregulated TNFα expression as compared with everolimus-treated plaques but did not affect the ability of everolimus to induce macrophage clearance.

Conclusion—

mTOR inhibition by everolimus triggers cytokine release in macrophages through inhibition of protein translation and p38 activation. These findings provide a rationale for combined local treatment of atherosclerotic plaques with everolimus and an anti-inflammatory agent.

Pharmacological modulation of cell death in atherosclerosis: a promising approach towards plaque stabilization?

Despite tremendous advances over the last 15 years in identifying vulnerable atherosclerotic plaques, the incidence of death and disability caused by such lesions still remains the number one health threat in developed countries. Therefore, new systemic or focal therapies aimed at decreasing the overall burden of disease, and a change to a more benign phenotype, are needed. Because cell death is a prominent feature of advanced atherosclerotic plaques with a major impact on plaque destabilization, an increasing number of compounds targeting the apoptotic or autophagic machinery in atherosclerosis are being explored, predominantly at the preclinical level. This review will provide an overview of these compounds, with a focus on both inhibition and stimulation of cell death, to prevent acute coronary syndromes and sudden cardiac death.

Protein phosphatase 2A promotes endothelial survival via stabilization of translational inhibitor 4E-BP1 following exposure to tumor necrosis factor-α

OBJECTIVE

Tumor necrosis factor-α (TNFα) may change from a stimulator of reversible activation of endothelial cells (ECs) to a killer when combined with cycloheximide (CHX). The means by which endothelial cells are destined to either the survival pathway or the apoptotic pathway are not fully understood. We investigated the role of p38 mitogen-activated protein kinase (MAPK) and protein phosphatase 2A (PP2A) activation and their regulation of 4E-BP1 stability in ECs to determine whether this pathway contributes to apoptosis induced by TNFα and CHX.

METHODS AND RESULTS

Apoptosis was induced in human umbilical vein ECs (HUVECs) by treating them with a combination of TNFα and CHX (TNFα/CHX). Activation of p38 MAPK was increased in HUVECs undergoing apoptosis, which was associated with degradation of eukaryotic initiation factor 4A regulator 4E-BP1 in a p38 MAPK-dependent manner. CHX attenuated a TNFα-stimulated increase in the expression and activity of PP2A. Silencing PP2A expression with small interfering RNA transfection mimicked CHX sensitization, increasing HUVEC apoptosis with TNFα stimulation and suggesting a protective role for PP2A in the apoptotic process.

CONCLUSION

Our data suggest that (1) TNFα stimulates PP2A and HUVECs elude apoptosis by PP2A-dependent dephosphorylation of p38 MAPK, and (2) CHX-induced inhibition of PP2A leads to maintenance of p38 activity and degradation of 4E-BP1, resulting in enhanced TNFα-induced apoptosis.

Translation inhibitors and their unique biological properties

In eukaryotes, many translation inhibitors have been widely used as bioprobes to evaluate the contribution of translation to signaling pathways and cellular functions. Several types of translation inhibitors are also known to trigger the activation of the mitogen-activated protein kinase superfamily in an intracellular mechanism called ribotoxic stress response. This perspective focuses on the biological properties of recently identified translation inhibitors that trigger ribotoxic stress response, particularly glutarimides as well as triene-ansamycins.

Inhibition of inositol monophosphatase by lithium chloride induces selective macrophage apoptosis in atherosclerotic plaques

BACKGROUND AND PURPOSE

Lithium chloride (LiCl) inhibits inositol monophosphatase (IMPase) at therapeutic concentrations. Given that LiCl induces death in cultured macrophages and that macrophages play an active role in atherosclerotic plaque destabilization, we investigated whether LiCl would induce selective macrophage death to stabilize the structure of the plaque.

EXPERIMENTAL APPROACH

The effect of LiCl was assessed on macrophages and smooth muscle cells (SMCs) in culture, in isolated atherosclerotic carotid arteries from rabbits and after local in vivo treatment via osmotic minipumps to rabbits with collared atherosclerotic carotid arteries. In addition, in vitro experiments were performed to elucidate the mechanism of LiCl-induced macrophage death.

KEY RESULTS

In vitro, whereas SMCs were highly resistant, LiCl induced macrophage death characterized by externalization of phosphatidylserine, caspase-3 cleavage and DNA fragmentation, all indicative of apoptosis. LiCl reduced inositol-1,4,5-trisphosphate levels in macrophages. Moreover, the IMPase inhibitor L-690 330 as well as IMPase gene silencing induced macrophage apoptosis. Both in vitro treatment of rabbit atherosclerotic carotid arteries with LiCl and local in vivo administration of LiCl to the plaques decreased plaque macrophages through apoptosis, as shown by terminal deoxynucleotidyl transferase deoxyuridine triphosphate (dUTP) nick-end labelling (TUNEL), without affecting SMCs. Vasomotor studies in vitro showed that LiCl did not affect the functionality of SMCs and endothelial cells.

CONCLUSIONS AND IMPLICATIONS

LiCl selectively decreased the macrophage load in rabbit atherosclerotic plaques via IMPase inhibition without affecting the viability or functionality of SMCs and endothelial cells. These data provide evidence for local administration of an IMPase inhibitor to stabilize atherosclerotic plaques.

Apoptosis does not mediate macrophage depletion in rabbit atherosclerotic plaques after dietary lipid lowering

Unstable atherosclerotic plaques are characterized by a thin fibrous cap that contains few smooth muscle cells (SMCs) and numerous foam cells of macrophage origin. Previously we and others demonstrated that macrophages disappear from atherosclerotic plaques after dietary lipid lowering. However, it remains unclear whether loss of macrophages after lipid lowering occurs via increased apoptosis, decreased macrophage replication and/or recruitment, or via a combination of both. Rabbits were fed a diet supplemented with cholesterol (0.3%) for 24 weeks followed by a normal diet for 4, 12, or 24 weeks. After 24 weeks of cholesterol supplement, plaques showed apoptosis in both macrophages and SMCs, as determined by terminal deoxynucleotidyl transferase dUTP nick-end labeling. Cell replication (Ki-67 immunolabeling) was predominantly present in macrophages. After 24 weeks of cholesterol withdrawal, the thickness and areas of the plaques were unchanged. Nevertheless, plaques showed a considerable loss of macrophages. This event was associated with a reduced immunoreactivity for vascular cell adhesion molecule-1 (VCAM-1) in the endothelial cells starting 4 weeks after cholesterol withdrawal. Apoptosis did not increase after lipid lowering but showed a steady decline. Apart from decreased VCAM-1 expression, a strong decrease in Ki-67 immunolabeling was observed after 12 weeks of cholesterol withdrawal. Our findings suggest that loss of macrophages in atherosclerotic plaques after dietary lipid lowering is not related to induction of macrophage apoptosis but mainly a consequence of impaired monocyte recruitment followed by decreased macrophage replication. This information is essential for understanding the effects of aggressive lipid lowering on plaque stability.

Involvement of mTOR kinase in cytokine-dependent microglial activation and cell proliferation

Neuroinflammation plays a prominent role in the pathophysiology of several neurodegenerative disorders, including Multiple Sclerosis. Reactive microglial cells are always found in areas of active demyelination as well as in normal-appearing white matter. Microglia contribute to initiating and maintaining brain inflammation, and once activated release pro-inflammatory mediators potentially cytotoxic, like nitric oxide (NO). It is now evident that the mTOR signaling pathway regulates different functions in the innate immune system, contributing to macrophage activation. More recently, mTOR has been found to enhance the survival of EOC2 microglia during oxygen-glucose deprivation and increase NO synthase 2 (NOS2) expression during hypoxia in BV2 microglial cell line, thus suggesting an involvement in microglial pro-inflammatory activation. In the present study, we detected mTOR activation in response to two different stimuli, namely LPS and a mixture of cytokines, in primary cultures of rat cortical microglia. Moreover, mTOR inhibitors reduced NOS activity and NOS2 expression induced by cytokines, but not those induced by LPS. The mTOR inhibitor RAD001, in combination with cytokines, also reduced microglial proliferation and the intracellular levels of cyclooxygenase. Under basal conditions mTOR inhibition significantly reduced microglial viability. Interestingly, mTOR inhibitors did not display any relevant effect on astrocyte NOS2 activity or cell viability. In conclusion, mTOR selectively controls microglial activation in response to pro-inflammatory cytokines and appears to play a crucial role in microglial viability; thus these drugs may be a useful pharmacological tool to reduce neuroinflammation.

Effect of depletion of monocytes/macrophages on early aortic valve lesion in experimental hyperlipidemia

Monocytes/macrophages are key players throughout atheroma development. The aim of this study was to determine the role of macrophages in lesion formation in heart valves in hyperlipidemia. We examined whether systemic depletion of monocytes/macrophages had a beneficial or adverse effect on the development of lesions in hyperlipemic hamsters injected twice weekly (for 2 months) with clodronate-encapsulated liposomes (H+Lclod), a treatment that selectively induces significant monocyte apoptosis. Hyperlipemic hamsters were employed as controls, as were hyperlipemic hamsters treated with plain liposomes. We assayed serum cholesterol (CH) and triglycerides (TG), the lipid and collagen contents and the size of the valve lesions, the matrix metalloproteinases (MMPs) in the serum and vessel wall, apolipoprotein E (ApoE), interleukin-1beta (IL-1beta), and superoxide anion production. In comparison with controls, H+Lclod hamsters exhibited: (1) increased lipid and collagen accumulation within the lesions, (2) decreased activity of MMP-9 and MMP-2 in sera and aortic homogenates, (3) decreased serum CH and TG and decreased expression of ApoE in sera and liver, (4) reduced expression of IL-1beta in aorta and liver homogenates, and (5) no change in the level of superoxide anion in the aorta. Thus, initially, the presence of the macrophages is beneficial in valvular lesion formation. Depletion of monocytes/macrophages is a two-edged sword having a beneficial effect by decreasing the expression of IL-1beta and MMP activities but an adverse effect by inducing a significant increase in the lipid and collagen content and expansion of valvular lesions.

The protein synthesis inhibitor anisomycin induces macrophage apoptosis in rabbit atherosclerotic plaques through p38 mitogen-activated protein kinase

Because macrophages play a major role in atherosclerotic plaque destabilization, selective removal of macrophages represents a promising approach to stabilize plaques. We showed recently that the protein synthesis inhibitor cycloheximide, in contrast to puromycin, selectively depleted macrophages in rabbit atherosclerotic plaques without affecting smooth muscle cells (SMCs). The mechanism of action of these two translation inhibitors is dissimilar and could account for the differential effects on SMC viability. It is not known whether selective depletion of macrophages is confined to cycloheximide or whether it can also be achieved with translation inhibitors that have a similar mechanism of action. Therefore, in the present study, we investigated the effect of anisomycin, a translation inhibitor with a mechanism of action similar to cycloheximide, on macrophage and SMC viability. In vitro, anisomycin induced apoptosis of macrophages in a concentration-dependent manner, whereas SMCs were only affected at higher concentrations. In vivo, anisomycin selectively decreased the macrophage content of rabbit atherosclerotic plaques through apoptosis. The p38 mitogen-activated protein kinase (MAPK) inhibitor SB202190 [4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)-1H-imidazole] prevented anisomycin-induced macrophage death, without affecting SMC viability. SB202190 decreased anisomycin-induced p38 MAPK phosphorylation, did not alter c-Jun NH(2)-terminal kinase (JNK) phosphorylation, and increased extracellular signal-regulated kinase (ERK) 1/2 phosphorylation. The latter effect was abolished by the mitogen-activated protein kinase kinase 1/2 inhibitor U0126 [1,4-diamino-2,3-dicyano-1,4-bis(2-aminophynyltio)butadiene ethanolate], although the prevention of anisomycin-induced macrophage death by SB202190 remained unchanged. The JNK phosphorylation inhibitor SP600125 did not affect anisomycin-induced macrophage or SMC death. In conclusion, anisomycin selectively decreased the macrophage content in rabbit atherosclerotic plaques, indicating that this effect is not confined to cycloheximide. p38 MAPK, but not ERK1/2 or JNK, plays a major role in anisomycin-induced macrophage death.

Autophagy in atherosclerosis: a cell survival and death phenomenon with therapeutic potential

Autophagy is a reparative, life-sustaining process by which cytoplasmic components are sequestered in double-membrane vesicles and degraded on fusion with lysosomal compartments. A growing body of evidence suggests that autophagy is stimulated in advanced atherosclerotic plaques by oxidized lipids, inflammation, and metabolic stress conditions. However, despite the increasing interest in autophagy in various pathophysiological situations such as neurodegeneration, cancer, and cardiac myopathies, the process remains an underestimated and overlooked phenomenon in atherosclerosis. As a consequence, its role in plaque formation and stability is poorly understood. Most likely, autophagy safeguards plaque cells against cellular distress, in particular oxidative injury, by degrading damaged intracellular material. In this way, autophagy is antiapoptotic and contributes to cellular recovery in an adverse environment. An interesting observation is that basal autophagy can be intensified by specific drugs. Excessively stimulated autophagic activity is capable of destroying major proportions of the cytosol, leading finally to type II programmed cell death that lacks several hallmarks of apoptosis or necrosis. Because atherosclerosis is an inflammatory disorder of the arterial intima, pharmacological approaches could be developed to stabilize vulnerable, rupture-prone lesions through selective induction of macrophage autophagic death.

Differential effect of the protein synthesis inhibitors puromycin and cycloheximide on vascular smooth muscle cell viability

Recent evidence indicates that the protein synthesis inhibitor cycloheximide triggers selective macrophage death in rabbit atheroma-like lesions without affecting smooth muscle cells (SMCs) or the endothelium, thereby favoring a stable plaque phenotype. In this study, we report that puromycin, a protein synthesis inhibitor with a different mode of action but with similar ability to inhibit de novo protein synthesis, did not reveal plaque-stabilizing effects. The macrophage and the SMC content readily decreased in puromycin-treated atheroma-like lesions in rabbit carotid arteries. Moreover, puromycin induced apoptosis in macrophages and SMCs in vitro. Puromycin-treated SMCs showed signs of endoplasmic reticulum (ER) stress, as demonstrated by CCAAT/enhancer-binding protein homologous protein (CHOP) protein expression, splicing of X-box-binding protein 1 mRNA, and phosphorylation of eukaryotic translation initiation factor 2alpha. The ER stress inducer thapsigargin up-regulated CHOP protein expression in SMCs without affecting their viability, indicating that ER stress not necessarily results in cell death. Puromycin, but not thapsigargin, activated the ER stress-related caspase-12. Treatment of SMCs with a combination of cycloheximide and puromycin inhibited ER stress and partially improved SMC viability. In addition, puromycin, but not cycloheximide or thapsigargin, induced intracellular accumulation of polyubiquitinated proteins in SMCs, whereas the proteasome function was not affected. Taken together, puromycin, in contrast to cycloheximide, induces SMC apoptosis, thereby favoring an unstable plaque phenotype. SMC death upon puromycin treatment could only be partially prevented by cycloheximide, which completely blocked ER stress. However, other or additional mechanisms, such as increased polyubiquitination of proteins, might be involved in puromycin-induced SMC death.

Selective depletion of macrophages in atherosclerotic plaques via macrophage-specific initiation of cell death

Macrophages play a central role in atherosclerotic plaque destabilization, leading to acute coronary syndromes and sudden death. Removal of macrophages from plaques via pharmacological therapy may therefore represent a promising approach to stabilize vulnerable, rupture-prone lesions. In this review, we summarize the current therapeutic means to induce macrophage cell death in atherosclerotic plaques without affecting smooth muscle cell viability, and their potential pitfalls.

Autophagy in cardiovascular disease

Autophagy is a major cytoprotective pathway that eukaryotic cells use to degrade and recycle cytoplasmic contents. Recent evidence indicates that autophagy under baseline conditions represents an important homeostatic mechanism for the maintenance of normal cardiovascular function and morphology. By contrast, excessive induction of the autophagic process by environmental or intracellular stress has an important role in several types of cardiomyopathy by functioning as a death pathway. As a consequence, enhanced autophagy represents one of the mechanisms underlying the cardiomyocyte dropout responsible for the worsening of heart failure. Successful therapeutic approaches that regulate autophagy have been reported recently, suggesting that the autophagic machinery can be manipulated to treat heart failure or to prevent rupture of atherosclerotic plaques and sudden death.

Nitric oxide selectively depletes macrophages in atherosclerotic plaques via induction of endoplasmic reticulum stress

BACKGROUND AND PURPOSE

Macrophages in atherosclerotic plaques have a tremendous impact on atherogenesis and plaque destabilization. We previously demonstrated that treatment of plaques in cholesterol-fed rabbits with the nitric oxide (NO) donor molsidomine preferentially eliminates macrophages, thereby favouring features of plaque stability. In this study, we investigated the underlying mechanism.

EXPERIMENTAL APPROACH

Macrophages and smooth muscle cells (SMCs) were treated in vitro with the NO donors, spermine NONOate or S-nitroso-N-acetylpenicillamine (SNAP) as well as with the well-known endoplasmic reticulum (ER) stress inducers thapsigargin, tunicamycin, dithiothreitol or brefeldin A. Cell viability was analysed by Neutral Red viability assays. Cleavage of caspase-3, DNA fragmentation and ultrastructural changes were examined to characterize the type of macrophage death. Induction of ER stress was evaluated by measuring C/EBP homologous protein (CHOP) expression, phosphorylation of eukaryotic initiation factor 2 alpha (eIF2a), splicing of X-box binding protein 1 (XBP1) and inhibition of protein synthesis.

KEY RESULTS

Macrophages and SMCs treated with spermine NONOate or SNAP showed several signs of ER stress, including upregulation of CHOP expression, hyperphosphorylation of eIF2 alpha, inhibition of de novo protein synthesis and splicing of XBP1 mRNA. These effects were similar in macrophages and SMCs, yet only macrophages underwent apoptosis. Plaques from molsidomine-treated atherosclerotic rabbits showed a 2.7-fold increase in CHOP expression as compared to placebo. Beside NO, selective induction of macrophage death could be initiated with thapsigargin and tunicamycin.

CONCLUSIONS AND IMPLICATIONS

Induction of ER stress explains selective depletion of macrophages in atherosclerotic plaques by a NO donor, probably via inhibition of protein synthesis.

P2X7 receptor-mediated phosphatidic acid production delays ATP-induced pore opening and cytolysis of RAW 264.7 macrophages

In macrophages, extracellular ATP (ATPe) stimulation of P2X7 receptors (P2X7R) results in cation channel opening, non-specific pore formation, secretion of cytokines, killing of intracellular bacteria and cytolysis. Signaling pathways controlling these diverse responses are currently under investigation. Among these pathways, phospholipase D (PLD) has been implicated in P2X7R-activated macrophages killing of intracellular pathogenic bacteria. Here we present evidence that early P2X7R-mediated PLD activation reduces pore opening and delays cytolysis of RAW 267.4 macrophages induced by ATPe. Use of inhibitors of PA metabolic enzymes suggests that PA, and not one of its metabolites, is the bioactive lipid. This is strengthened by the observation that addition of exogenous PA also reduces pore formation and cytolysis of RAW 264.7 macrophages. However, the beneficial effects of PA are only transient, due to its conversion into diacylglycerol through PA phosphatase-1 activity during prolonged P2X7R stimulation. Revealing that the PLD/PA pathway mediates survival of macrophages provides a potent strategy to inhibit P2X7R-mediated cytolysis by controlling PA metabolism. This will be important in the case of P2X7R-induced killing of intracellular bacteria which is lately associated with macrophage death, limiting the potency of ATPe to eliminate pathogenic bacteria.