Peroxisome proliferator-activated receptors in the modulation of the immune/inflammatory response in atherosclerosis

PPARs in atherosclerosis: The spatial and temporal features from mechanism to druggable targets

BACKGROUND

Atherosclerosis is a chronic and complex disease caused by lipid disorder, inflammation, and other factors. It is closely related to cardiovascular diseases, the chief cause of death globally. Peroxisome proliferator-activated receptors (PPARs) are valuable anti-atherosclerosis targets that showcase multiple roles at different pathological stages of atherosclerosis and for cell types at different tissue sites.

AIM OF REVIEW

Considering the spatial and temporal characteristics of the pathological evolution of atherosclerosis, the roles and pharmacological and clinical studies of PPARs were summarized systematically and updated under different pathological stages and in different vascular cells of atherosclerosis. Moreover, selective PPAR modulators and PPAR-pan agonists can exert their synergistic effects meanwhile reducing the side effects, thereby providing novel insight into future drug development for precise spatial-temporal therapeutic strategy of anti-atherosclerosis targeting PPARs.

KEY SCIENTIFIC

Concepts of Review: Based on the spatial and temporal characteristics of atherosclerosis, we have proposed the importance of stage- and cell type-dependent precision therapy. Initially, PPARs improve endothelial cells' dysfunction by inhibiting inflammation and oxidative stress and then regulate macrophages' lipid metabolism and polarization to improve fatty streak. Finally, PPARs reduce fibrous cap formation by suppressing the proliferation and migration of vascular smooth muscle cells (VSMCs). Therefore, research on the cell type-specific mechanisms of PPARs can provide the foundation for space-time drug treatment. Moreover, pharmacological studies have demonstrated that several drugs or compounds can exert their effects by the activation of PPARs. Selective PPAR modulators (that specifically activate gene subsets of PPARs) can exert tissue and cell-specific effects. Furthermore, the dual- or pan-PPAR agonist could perform a better role in balancing efficacy and side effects. Therefore, research on cells/tissue-specific activation of PPARs and PPAR-pan agonists can provide the basis for precision therapy and drug development of PPARs.

Two-Photon, Ratiometric, Quantitative Fluorescent Probe Reveals Fluctuation of Peroxynitrite Regulated by Arginase 1

Peroxynitrite, a transient reactive oxygen species (ROS), is believed to play a deleterious role in physiological processes. Herein, we report a two-photon ratiometric fluorescent probe that selectively reacts with peroxynitrite yielding a >200-fold change upon reaction. The probe effectively visualized fluctuations in peroxynitrite generation by arginase 1 in vivo and in vitro. This provides evidence that arginase 1 is a critical regulator of peroxynitrite.

The association between zinc and endothelial adhesion molecules ICAMs and VCAM-1 and nuclear receptors PPAR-ɑ and PPAR-γ: A systematic review on cell culture, animal and human studies

BACKGROUND

Cardiovascular health is strongly influenced by diet. The levels of inflammatory factors like ICAM-1 and VCAM-1 are high in patients with atherosclerosis or predisposing factor for heart disease. Antioxidant and anti-inflammatory functions are attributed to zinc. We systematically reviewed cell culture, human or animal studies for determining the relationship between zinc status and ICAMs or VCAM-1 levels.

METHODS

PubMed, Google Scholar, Scopus, and Cochrane databases from database inception till 30th August 2020 were systematically searched to obtain any possible article for inclusion.

RESULTS

After screening and removing unrelated or duplicate articles by the title and abstract by two independent reviewers, 15 articles were included. Results indicating an inverse relationship between zinc status with ICAM-1 or VCAM-1 levels and the development of endothelial inflammation, plaque formation, or atherosclerosis. A direct relationship between zinc status and PPAR-α or γ levels was also observed. Zinc oxide (ZnO), zinc nanoparticles, or ions can cause endothelial activation and increased levels of ICAM-1 and VCAM-1.

CONCLUSION

Normal function of the endothelium is linked with zinc level. Zinc deficiency causes atherosclerosis, most probably via increased production of ICAM-1 and VCAM-1; and decreased expression of PPAR-ɑ and PPAR-γ receptors. Contrarily, endothelial activation and increased ICAM-1 and VCAM-1 levels can be caused by ZnO, zinc nanoparticles, or zinc ions.

The role of glucose homeostasis on immune function in response to exercise: The impact of low or higher energetic conditions

Immune cells are bioenergetically expensive during activation, which requires tightly regulated control of metabolic pathways. Both low and high glycemic conditions can modulate immune function. States of undernourishment depress the immune system, and in the same way, excessive intake of nutrients, such as an obesity state, compromise its functioning. Multicellular organisms depend on two mechanisms to survive: the regulation and ability to store energy to prevent starvation and the ability to fight against infection. Synergic interactions between metabolism and immunity affect many systems underpinning human health. In a chronic way, the breakdown of glycemic homeostasis in the body can influence cells of the immune system and consequently contribute to the onset of diseases such as type II diabetes, obesity, Alzheimer's, and fat and lean mass loss. On the contrary, exercise, recognized as a primary strategy to control hyperglycemic disorders, also induces a coordinated immune-neuro-endocrine response that acutely modulates cardiovascular, respiratory, and muscle functions and the immune response to exercise is widely dependent on the intensity and volume that may affect an immunodepressive state. These altered immune responses induced by exercise are modulated through the "stress hormones" adrenaline and cortisol, which are a threat to leukocyte metabolism. In this context, carbohydrates appear to have a positive acute response as a strategy to prevent depression of the immune system by maintaining plasma glucose concentrations to meet the energy demand from all systems involved during strenuous exercises. Therefore, herein, we discuss the mechanisms through which exercise may promotes changes on glycemic homeostasis in the metabolism and how it affects immune cell functions under higher or lower glucose conditions.

Interactions Between PPARG and AGTR1 Gene Polymorphisms on the Risk of Hypertension in Chinese Han Population

AIMS

To explore the interactions between PPARG and AGTR1 polymorphisms and their associations with hypertension in the Chinese Han population.

METHODS

Seven single nucleotide polymorphisms (SNPs) of the PPARG gene and five SNPs of the AGTR1 gene were selected and genotyped in 1591 unrelated Chinese Han adults. The SNPAssoc package of R was used to analyze the associations between the selected SNPs and hypertension. The potential gene-gene interactions between PPARG and AGTR1 genes were tested by model-based multifactor dimensionality reduction (MB-MDR).

RESULTS

The frequencies of the C allele of rs3856806 and the G allele of rs13433696 in the PPARG gene were significantly lower in hypertensive subjects, whereas the A allele of rs9817428 in the PPARG gene was much higher in hypertensives. In addition, individuals with T allele of rs2933249 in the AGTR1 gene displayed a significantly decreased risk of hypertension. MB-MDR analyses suggested that the two-locus model (rs9817428 and rs2933249) and the three-locus model (rs9817428, rs3856806, and rs2933249) were significantly associated with a decreased risk of hypertension. Moreover, among the eight SNPs not individually associated with hypertension (rs12631819, rs2920502, rs1175543, and rs2972164 in the PPARG gene, and rs2638360, rs1492100, rs5182, and rs275646 in the AGTR1 gene), the two-locus model involving rs12631819 and rs5182 demonstrated increased susceptibility to hypertension, and the five-locus model involving rs12631819, rs2920502, rs2972164, rs5182, and rs2638360 demonstrated a significantly decreased risk of hypertension.

CONCLUSION

Polymorphisms in both the PPARG and AGTR1 genes were found to be significantly associated with hypertension. Moreover, there were significant gene-gene interactions identified between the PPARG and AGTR1 genes in relation to hypertension susceptibility in the Chinese Han population.

Correlations between peroxisome proliferator activator receptor γ, Cystatin C, or advanced oxidation protein product, and atherosclerosis in diabetes patients

We aimed to explore the relationship between peroxisome proliferator activator receptor γ (PPAR γ), Cystatin C or advanced oxidation protein product (AOPP) and atherosclerosis (AS), and identify their diagnostic values for AS. Eighty AS patients above the age of 75 with type 2 diabetes were screened by brachial-ankle pulse wave velocity (baPWV) and ankle brachial index (ABI). The baseline level of patients was firstly analyzed, and then the expression of PPAR γ was detected by reverse transcriptase-polymerase chain reaction (RT-PCR). Meanwhile, a double-antibody sandwich enzyme-linked immunosorbent assay was performed to analyze the concentration of AOPP, and immunonephelometry was carried out to detect the concentration of Cystatin C. The baseline level of patients was basically consistent. The expression of PPAR γ was significantly higher in severe AS than mild AS patients (P < 0.05), while no differences were found in serum Cystatin C and AOPP between severe AS and mild AS patients (P > 0.05). Thus, PPAR γ exhibited a high diagnostic value for severe AS (AUC = 0.850), but not Cystatin C and AOPP (AUC = 0.553, AUC = 0.4780). Moreover, the combination of PPAR γ, Cystatin C and AOPP exhibited a quite high diagnostic value in AS (AUC = 0.961, Sen = 0.9, Spe = 0.975), which was also higher than PPAR γ alone. In conclusion, the contents of PPAR γ, Cystatin C and AOPP were closely related to AS in diabetes, indicating a potential clinical diagnostic value of PPAR γ, Cystatin C and AOPP in diabetes with AS.

Peroxisome proliferator-activated receptor-γ mutations responsible for lipodystrophy with severe hypertension activate the cellular renin-angiotensin system

OBJECTIVE

Inactivating peroxisome proliferator-activated receptor-γ (PPARγ) mutations lead to a syndrome of familial partial lipodystrophy (FPLD3) associated with early-onset severe hypertension. PPARγ can repress the vascular renin-angiotensin system (RAS) and angiotensin II receptor 1 expression. We evaluated the relationships between PPARγ inactivation and cellular RAS using FPLD3 patients' cells and human vascular smooth muscle cells expressing mutant or wild-type PPARγ. Approach and Results- We identified 2 novel PPARG mutations, R165T and L339X, located in the DNA and ligand-binding domains of PPARγ, respectively in 4 patients from 2 FPLD3 families. In cultured skin fibroblasts and peripheral blood mononuclear cells from the 4 patients and healthy controls, we compared markers of RAS activation, oxidative stress, and inflammation, and tested the effect of modulators of PPARγ and angiotensin II receptor 1. We studied the impact of the 2 mutations on the transcriptional activity of PPARγ and on the vascular RAS in transfected human vascular smooth muscle cells. Systemic RAS was not altered in patients. However, RAS markers were overexpressed in patients' fibroblasts and peripheral blood mononuclear cells, as in vascular cells expressing mutant PPARγ. Angiotensin II-mediated mitogen-activated protein kinase activity increased in patients' fibroblasts, consistent with RAS constitutive activation. Patients' cells also displayed oxidative stress and inflammation. PPARγ activation and angiotensin II receptor 1 mRNA silencing reversed RAS overactivation, oxidative stress, and inflammation, arguing for a role of angiotensin II receptor 1 in these processes.

CONCLUSIONS

Two novel FPLD3-linked PPARG mutations are associated with a defective transrepression of cellular RAS leading to cellular dysfunction, which might contribute to the specific FPLD3-linked severe hypertension.

[Molecular mechanisms regulating the activity of macrophages]

This article reviews modern concepts of the most common types of macrophage activation: classical, alternative, and type II. Molecular mechanisms of induction and regulation of these three types of activation are discussed. Any population of macrophages was shown to change its properties depending on its microenvironment and concrete biological situation (the "functional plasticity of macrophages"). Many intermediate states of macrophages were described along with the most pronounced and well-known activation types (classical activation, alternative activation, and type II activation). These intermediate states are characterized by a variety of combinations of their biological properties, including elements of the three afore mentioned types of activation. Macrophage activity is regulated by a complex network of interrelated cascade mechanisms.

Microparticles: targets and tools in cardiovascular disease

Cells communicate with other cells not only via direct cell-cell contact and the production of signaling molecules but also through release of microparticles (MPs). MPs are small vesicles released from stimulated and/or apoptotic cells. They harbor membrane proteins that are characteristic of the original parent cell and intracellular components involved in cell signaling. MPs are considered to be both biomarkers and effectors of cell signaling that maintain and/or initiate cell dysfunction. Thus, MPs can evoke endothelial dysfunction by decreasing nitric oxide (NO) production and promoting vascular inflammation which favor the prothrombotic state in atherosclerosis. Novel pharmacological approaches targeting MP production or properties could be used to treat cardiovascular pathologies. Paradoxically, another useful approach might be to employ engineered MPs with modified compositions as therapeutic agents to correct cardiovascular pathologies. This review is focused on the mechanisms of MP formation and their effects on target cells under physiological or pathophysiological conditions.

PPARγ in coronary atherosclerosis: in vivo expression pattern and correlations with hyperlipidemic status and statin treatment

OBJECTIVE

Peroxisome proliferator-activated receptor-γ (PPARγ) is involved in regulation of macrophage inflammation and in atherosclerosis. Herein we investigate the influence of statin treatment on PPARγ expression in coronary artery disease.

METHOD

PPARγ expression was investigated in coronary atherosclerotic atherectomies (N=48) and arteries (N=12) from patients with stable or unstable coronary syndromes or undergoing cardiac transplantation for end-stage ischemic cardiomyopathy, respectively, by immunohistochemistry. Plaque components and tissue factor immunoreactivity were also investigated. Atherectomies were obtained from de novo culprit lesions of hypercholesterolemic (16 statin-treated and 16 untreated) and normolipidemic (N=16) patients. Furthermore, PPARγ expression was evaluated in patients peripheral blood monocytes and in monocytic U937 cells after atorvastatin incubation, by Western blot analysis.

RESULT

PPARγ expression was higher in coronary plaques and peripheral blood monocytes of statin-treated patients, and it significantly increased in monocytes after 24h atorvastatin incubation (p<0.05). Intra-plaque macrophage content, atheroma, neoangiogenesis and hemorrhage, and circulating CRP levels were lower in statin-treated than untreated hypercholesterolemic patients and comparable with normolipidemic subjects. PPARγ immunoreactivity was localized to neointima and media, its distribution pattern being different from that of tissue factor.

CONCLUSION

PPARγ expression was enhanced in statin-treated patients with different distribution and behavior as compared to atheroma, macrophage content, tissue factor immunoreactivity and serum CRP. In vitro studies showed increased PPARγ expression in monocytes after atorvastatin incubation. These findings provide further evidence as to the protective role of statins in coronary artery disease and their influence on PPARγ expression in coronary plaques and on the inflammatory status of patients.

MicroRNA-21 targets peroxisome proliferators-activated receptor-alpha in an autoregulatory loop to modulate flow-induced endothelial inflammation

Adhesion of circulating monocytes to vascular endothelial cells (ECs) is a critical event leading to vascular inflammation and, hence, development of atherosclerosis. MicroRNAs (miRs) are a class of endogenous, highly conserved, noncoding small RNAs that play important roles in regulating gene expression and cellular function, as well as pathogenesis of atherosclerosis. Here, we showed that oscillatory shear stress (OSS) induces the expression of miR-21 at the transcriptional level in cultured human umbilical vein ECs via an increased binding of c-Jun, which is a component of transcription factor activator protein-1 (AP-1), to the promoter region of miR-21. OSS induction of miR-21 inhibited the translation, but not transcription, of peroxisome proliferators-activated receptor-α (PPARα) by 3'-UTR targeting. Overexpression of miR-21 up-regulated AP-1 activation, which was attenuated by exogenous expression of PPARα. OSS and overexpression of miR-21 enhanced the expression of adhesion molecules vascular cell adhesion molecule-1 and monocyte chemotactic protein-1 and the consequential adhesion of monocytes to ECs. Overexpression of PPARα significantly attenuated the AP-1-mediated miR-21 expression. These results demonstrate a unique mechanism by which OSS induces AP-1-dependent miR-21 expression, which directly targets PPARα to inhibit its expression, thereby allowing activation of AP-1 and the promotion of monocyte adhesion. Our findings suggest the presence of a positive feedback loop that enables the sustained induction of miR-21, thus contributing to the proinflammatory responses of vascular endothelium under OSS.

Fenofibrate, a PPAR{alpha} agonist, decreases atrogenes and myostatin expression and improves arthritis-induced skeletal muscle atrophy

Arthritis is a chronic inflammatory illness that induces cachexia, which has a direct impact on morbidity and mortality. Fenofibrate, a selective PPARα activator prescribed to treat human dyslipidemia, has been reported to decrease inflammation in rheumatoid arthritis patients. The aim of this study was to elucidate whether fenofibrate is able to ameliorate skeletal muscle wasting in adjuvant-induced arthritis, an experimental model of rheumatoid arthritis. On day 4 after adjuvant injection, control and arthritic rats were treated with 300 mg/kg fenofibrate until day 15, when all rats were euthanized. Fenofibrate decreased external signs of arthritis and liver TNFα and blocked arthritis-induced decreased in PPARα expression in the gastrocnemius muscle. Arthritis decreased gastrocnemius weight, which results from a decrease in cross-section area and myofiber size, whereas fenofibrate administration to arthritic rats attenuated the decrease in both gastrocnemius weight and fast myofiber size. Fenofibrate treatment prevented arthritis-induced increase in atrogin-1 and MuRF1 expression in the gastrocnemius. Neither arthritis nor fenofibrate administration modify Akt-FoxO3 signaling. Myostatin expression was not modified by arthritis, but fenofibrate decreased myostatin expression in the gastrocnemius of arthritic rats. Arthritis increased muscle expression of MyoD, PCNA, and myogenin in the rats treated with vehicle but not in those treated with fenofibrate. The results indicate that, in experimental arthritis, fenofibrate decreases skeletal muscle atrophy through inhibition of the ubiquitin-proteasome system and myostatin.

Innate immunity and monocyte-macrophage activation in atherosclerosis

Innate inflammation is a hallmark of both experimental and human atherosclerosis. The predominant innate immune cell in the atherosclerotic plaque is the monocyte-macrophage. The behaviour of this cell type within the plaque is heterogeneous and depends on the recruitment of diverse monocyte subsets. Furthermore, the plaque microenvironment offers polarisation and activation signals which impact on phenotype. Microenvironmental signals are sensed through pattern recognition receptors, including toll-like and NOD-like receptors - the latter of which are components of the inflammasome - thus dictating macrophage behaviour and outcome in atherosclerosis. Recently cholesterol crystals and modified lipoproteins have been recognised as able to directly engage these pattern recognition receptors. The convergent role of such pathways in terms of macrophage activation is discussed in this review.

Peroxisome proliferator-activated receptors, metabolic syndrome and cardiovascular disease

Metabolic syndrome (MetS) is a constellation of risk factors including insulin resistance, central obesity, dyslipidemia and hypertension that markedly increase the risk of Type 2 diabetes (T2DM) and cardiovascular disease (CVD). The peroxisome proliferators-activated receptor (PPAR) isotypes, PPARα, PPARδ/ß and PPARγ are ligand-activated nuclear transcription factors, which modulate the expression of an array of genes that play a central role in regulating glucose, lipid and cholesterol metabolism, where imbalance can lead to obesity, T2DM and CVD. They are also drug targets, and currently, PPARα (fibrates) and PPARγ (thiazolodinediones) agonists are in clinical use for treating dyslipidemia and T2DM, respectively. These metabolic characteristics of the PPARs, coupled with their involvement in metabolic diseases, mean extensive efforts are underway worldwide to develop new and efficacious PPAR-based therapies for the treatment of additional maladies associated with the MetS. This article presents an overview of the functional characteristics of three PPAR isotypes, discusses recent advances in our understanding of the diverse biological actions of PPARs, particularly in the vascular system, and summarizes the developmental status of new single, dual, pan (multiple) and partial PPAR agonists for the clinical management of key components of MetS, T2DM and CVD. It also summarizes the clinical outcomes from various clinical trials aimed at evaluating the atheroprotective actions of currently used fibrates and thiazolodinediones.

Low-intensity exercise enhances expression of markers of alternative activation in circulating leukocytes: roles of PPARγ and Th2 cytokines

OBJECTIVE

Pharmacological activation of the nuclear receptor PPARγ is linked to numerous beneficial effects in the contexts of inflammation, lipid homeostasis, Type-2 Diabetes (T2D) and atherosclerosis. These beneficial effects include priming of circulating monocytes for differentiation towards an 'alternative' anti-inflammatory M2 macrophage phenotype. As we have recently shown that participation in low-intensity exercise increases PPARγ expression and activity in leukocytes from previously sedentary individuals, we aimed to elucidate whether low-intensity exercise elicited a pattern of gene expression similar to that reported for M2 monocyte-macrophage differentiation.

METHODS

17 sedentary individuals undertook an 8-week low-intensity exercise programme (walking 10,000steps/day, three times/week). Changes in expression of PPARs and the PPARγ co-activators PGC-1α and PGC-1β; Th2 (IL-4; IL-10) and Th1 (IL-6) cytokines; and markers for the M2 (AMAC1, CD14, MR, IL-4) and the 'classical' pro-inflammatory M1 (MCP-1, TNFα, IL-6) phenotypes, were determined using RT-PCR (to assess leukocyte mRNA expression) and ELISA (to assess plasma cytokine levels).

RESULTS

Exercise was associated with upregulation of M2 markers, PGC-1α and PGC-1β, and with downregulation of M1 markers. Moreover, plasma levels of Th2 cytokines increased after exercise, while those of Th1 cytokines decreased. However, other PPARs (PPARα; PPARβ/δ) did not undergo marked exercise-induced activation or upregulation. Thus, participation in low-intensity exercise may prime monocytes for differentiation towards an M2 macrophage phenotype via PPARγ/PGC-1α/β.

CONCLUSION

Given the similarities between these effects and pharmacologically induced M2 polarisation, we propose that exercise-induced PPARγ/PGC-1α/β-mediated M2 polarisation may constitute a novel anti-inflammatory benefit of low-intensity exercise.

Exploring the full spectrum of macrophage activation

Macrophages display remarkable plasticity and can change their physiology in response to environmental cues. These changes can give rise to different populations of cells with distinct functions. In this Review we suggest a new grouping of macrophage populations based on three different homeostatic activities - host defence, wound healing and immune regulation. We propose that similarly to primary colours, these three basic macrophage populations can blend into various other 'shades' of activation. We characterize each population and provide examples of macrophages from specific disease states that have the characteristics of one or more of these populations.

Distinct functions of vascular endothelial and smooth muscle PPARgamma in regulation of blood pressure and vascular tone

Thiazolidinediones (TZDs) are peroxisome proliferators-activated receptor gamma (PPARgamma) activators that exhibit antihypertensive and vasculo-protective effects. Here we describe the use of Tie2Cre/flox and SM22Cre/flox mice, which respectively lacked PPARgamma in the endothelium and the smooth muscle, to study vascular function of PPARgamma. Rosiglitazone (RGZ) induced a similar blood pressure (BP)-lowering effect in deoxycorticosterone acetate (DOCA) salt-treated PPARgamma(f/f) and SM22Cre/flox mice, whereas Tie2Cre/flox mice were completely resistant to this effect. The femoral arteries lacking endothelial PPARgamma exhibited increased reactivity to various vasoconstrictors without a significant alteration in acetylcholine-induced relaxation. In sharp contrast, the vasculature lacking smooth muscle PPARgamma had blunted sensitivity to alpha1-adrenergic agents but enhanced sensitivity to acetylcholine. Our results demonstrated endothelium but not smooth muscle as the site for TZD-induced BP-lowering effect and also uncovered distinct functions of endothelial and smooth muscle PPARgamma in regulation of vascular tone.