Role of Arginase 2 in Systemic Metabolic Activity and Adipose Tissue Fatty Acid Metabolism in Diet-Induced Obese Mice

Decreased Proteasomal Function Exacerbates Endoplasmic Reticulum Stress-Induced Chronic Inflammation in Obese Adipose Tissue

Low-grade chronic inflammation contributes to both aging and the pathogenesis of age-related diseases. White adipose tissue (WAT) in obese individuals exhibits chronic inflammation, which is associated with obesity-related disorders. Aging exacerbates obesity-related inflammation in WAT; however, the molecular mechanisms underlying chronic inflammation and its exacerbation by aging remain unclear. Age-related decline in activity of the proteasome, a multisubunit proteolytic complex, has been implicated in age-related diseases. This study employed a mouse model with decreased proteasomal function that exhibits age-related phenotypes to investigate the impact of adipocyte senescence on WAT inflammation. Transgenic mice expressing proteasomal subunit β5t with weak chymotrypsin-like activity experience reduced lifespan and develop age-related phenotypes. Mice fed with a high-fat diet and experiencing proteasomal dysfunction exhibited increased WAT inflammation, increased infiltration of proinflammatory M1-like macrophages, and increased proinflammatory adipocytokine-like monocyte chemoattractant protein-1, plasminogen activator inhibitor-1, and tumor necrosis factor-α, which are all associated with activation of endoplasmic reticulum (ER) stress-related pathways. Impaired proteasomal activity also activated ER stress-related molecules and induced expression of proinflammatory adipocytokines in adipocyte-like cells differentiated from 3T3-L1 cells. Collective evidence suggests that impaired proteasomal activity increases ER stress and that subsequent inflammatory pathways play pivotal roles in WAT inflammation. Because proteasomal function declines with age, age-related proteasome impairment may be involved in obesity-related inflammation among elderly individuals.

The regulatory role of eosinophils in adipose tissue depends on autophagy

Introduction

Obesity is a metabolic condition that elevates the risk of all-cause mortality. Brown and beige adipose tissues, known for their thermogenic properties, offer potential therapeutic targets for combating obesity. Recent reports highlight the role of immune cells, including eosinophils, in adipose tissue homeostasis, while the underlying mechanisms are poorly understood.

Methods

To study the role of autophagy in eosinophils in this process, we used a genetic mouse model lacking autophagy-associated protein 5 (Atg5), specifically within the eosinophil lineage (Atg5 eoΔ).

Results

The absence of Atg5 in eosinophils led to increased body weight, impaired glucose metabolism, and alterations in the cellular architecture of adipose tissue. Our findings indicate that Atg5 modulates the functional activity of eosinophils within adipose tissue rather than their abundance. Moreover, RNA-seq analysis revealed upregulation of arginase 2 (Arg2) in Atg5-knockout eosinophils. Increased Arg2 activity was shown to suppress adipocyte beiging. Furthermore, we observed enrichment of the purine pathway in the absence of Atg5 in eosinophils, leading to a pro-inflammatory shift in macrophages and a further reduction in beiging.

Discussion

The data shed light on the importance of autophagy in eosinophils and its impact on adipose tissue homeostasis by suppressing Arg2 expression and limiting inflammation in adipose tissue.

The AMPK activator metformin improves recovery from demyelination by shifting oligodendrocyte bioenergetics and accelerating OPC differentiation

Multiple Sclerosis (MS) is a chronic disease characterized by immune-mediated destruction of myelinating oligodendroglia in the central nervous system. Loss of myelin leads to neurological dysfunction and, if myelin repair fails, neurodegeneration of the denuded axons. Virtually all treatments for MS act by suppressing immune function, but do not alter myelin repair outcomes or long-term disability. Excitingly, the diabetes drug metformin, a potent activator of the cellular "energy sensor" AMPK complex, has recently been reported to enhance recovery from demyelination. In aged mice, metformin can restore responsiveness of oligodendrocyte progenitor cells (OPCs) to pro-differentiation cues, enhancing their ability to differentiate and thus repair myelin. However, metformin's influence on young oligodendroglia remains poorly understood. Here we investigated metformin's effect on the temporal dynamics of differentiation and metabolism in young, healthy oligodendroglia and in oligodendroglia following myelin damage in young adult mice. Our findings reveal that metformin accelerates early stages of myelin repair following cuprizone-induced myelin damage. Metformin treatment of both isolated OPCs and oligodendrocytes altered cellular bioenergetics, but in distinct ways, suppressing oxidative phosphorylation and enhancing glycolysis in OPCs, but enhancing oxidative phosphorylation and glycolysis in both immature and mature oligodendrocytes. In addition, metformin accelerated the differentiation of OPCs to oligodendrocytes in an AMPK-dependent manner that was also dependent on metformin's ability to modulate cell metabolism. In summary, metformin dramatically alters metabolism and accelerates oligodendroglial differentiation both in health and following myelin damage. This finding broadens our knowledge of metformin's potential to promote myelin repair in MS and in other diseases with myelin loss or altered myelination dynamics.

Endothelial dysfunction due to eNOS uncoupling: molecular mechanisms as potential therapeutic targets

Nitric oxide (NO) is one of the most important molecules released by endothelial cells, and its antiatherogenic properties support cardiovascular homeostasis. Diminished NO bioavailability is a common hallmark of endothelial dysfunction underlying the pathogenesis of the cardiovascular disease. Vascular NO is synthesized by endothelial nitric oxide synthase (eNOS) from the substrate L-arginine (L-Arg), with tetrahydrobiopterin (BH₄) as an essential cofactor. Cardiovascular risk factors such as diabetes, dyslipidemia, hypertension, aging, or smoking increase vascular oxidative stress that strongly affects eNOS activity and leads to eNOS uncoupling. Uncoupled eNOS produces superoxide anion (O₂^-) instead of NO, thus becoming a source of harmful free radicals exacerbating the oxidative stress further. eNOS uncoupling is thought to be one of the major underlying causes of endothelial dysfunction observed in the pathogenesis of vascular diseases. Here, we discuss the main mechanisms of eNOS uncoupling, including oxidative depletion of the critical eNOS cofactor BH₄, deficiency of eNOS substrate L-Arg, or accumulation of its analog asymmetrical dimethylarginine (ADMA), and eNOS S-glutathionylation. Moreover, potential therapeutic approaches that prevent eNOS uncoupling by improving cofactor availability, restoration of L-Arg/ADMA ratio, or modulation of eNOS S-glutathionylation are briefly outlined.

Arginase: Biological and Therapeutic Implications in Diabetes Mellitus and Its Complications

Arginase is a ubiquitous enzyme in the urea cycle (UC) that hydrolyzes L-arginine to urea and L-ornithine. Two mammalian arginase isoforms, arginase1 (ARG1) and arginase2 (ARG2), play a vital role in the regulation of β-cell functions, insulin resistance (IR), and vascular complications via modulating L-arginine metabolism, nitric oxide (NO) production, and inflammatory responses as well as oxidative stress. Basic and clinical studies reveal that abnormal alterations of arginase expression and activity are strongly associated with the onset and development of diabetes mellitus (DM) and its complications. As a result, targeting arginase may be a novel and promising approach for DM treatment. An increasing number of arginase inhibitors, including chemical and natural inhibitors, have been developed and shown to protect against the development of DM and its complications. In this review, we discuss the fundamental features of arginase. Next, the regulatory roles and underlying mechanisms of arginase in the pathogenesis and progression of DM and its complications are explored. Furthermore, we review the development and discuss the challenges of arginase inhibitors in treating DM and its related pathologies.

Virtual Screening for FDA-Approved Drugs That Selectively Inhibit Arginase Type 1 and 2

Arginases are often overexpressed in human diseases, and they are an important target for developing anti-aging and antineoplastic drugs. Arginase type 1 (ARG1) is a cytosolic enzyme, and arginase type 2 (ARG2) is a mitochondrial one. In this study, a dataset containing 2115-FDA-approved drug molecules is virtually screened for potential arginase binding using molecular docking against several ARG1 and ARG2 structures. The potential arginase ligands are classified into three categories: (1) Non-selective, (2) ARG1 selective, and (3) ARG2 selective. The evaluated potential arginase ligands are then compared with their clinical use. Remarkably, half of the top 30 potential drugs are used clinically to lower blood pressure and treat cancer, infection, kidney disease, and Parkinson’s disease thus partially validating our virtual screen. Most notable are the antihypertensive drugs candesartan, irbesartan, indapamide, and amiloride, the antiemetic rolapitant, the anti-angina ivabradine, and the antidiabetic metformin which have minimal side effects. The partial validation also favors the idea that the other half of the top 30 potential drugs could be used in therapeutic settings. The three categories greatly expand the selectivity of arginase inhibition.

Novel Therapeutics for Diabetic Retinopathy and Diabetic Macular Edema: A Pathophysiologic Perspective

Diabetic retinopathy (DR) and diabetic macular edema (DME) are retinal complications of diabetes that can lead to loss of vision and impaired quality of life. The current gold standard therapies for treatment of DR and DME focus on advanced disease, are invasive, expensive, and can trigger adverse side-effects, necessitating the development of more effective, affordable, and accessible therapies that can target early stage disease. The pathogenesis and pathophysiology of DR is complex and multifactorial, involving the interplay between the effects of hyperglycemia, hyperlipidemia, hypoxia, and production of reactive oxygen species (ROS) in the promotion of neurovascular dysfunction and immune cell polarization to a proinflammatory state. The pathophysiology of DR provides several therapeutic targets that have the potential to attenuate disease progression. Current novel DR and DME therapies under investigation include erythropoietin-derived peptides, inducers of antioxidant gene expression, activators of nitric oxide/cyclic GMP signaling pathways, and manipulation of arginase activity. This review aims to aid understanding of DR and DME pathophysiology and explore novel therapies that capitalize on our knowledge of these diabetic retinal complications.

Arginase II protein regulates Parkin-dependent p32 degradation that contributes to Ca2+-dependent eNOS activation in endothelial cells

Aims

Arginase II (ArgII) plays a key role in the regulation of Ca²⁺ between the cytosol and mitochondria in a p32-dependent manner. p32 contributes to endothelial nitric oxide synthase (eNOS) activation through the Ca²⁺/CaMKII/AMPK/p38MAPK/Akt signalling cascade. Therefore, we investigated a novel function of ArgII in the regulation of p32 stability.

Methods and results

mRNA levels were measured by quantitative reverse transcription-PCR, and protein levels and activation were confirmed by western blot analysis. Ca²⁺ concentrations were measured by FACS analysis and a vascular tension assay was performed. ArgII bound to p32, and ArgII protein knockdown using siArgII facilitated the ubiquitin-dependent proteasomal degradation of p32. β-lactone, a proteasome inhibitor, inhibited the p32 degradation associated with endothelial dysfunction in a Ca²⁺-dependent manner. The amino acids Lys154, Lys 180, and Lys220 of the p32 protein were identified as putative ubiquitination sites. When these sites were mutated, p32 was resistant to degradation in the presence of siArgII, and endothelial function was impaired. Knockdown of Pink/Parkin as an E3-ubiquitin ligase with siRNAs resulted in increased p32, decreased [Ca²⁺]c, and attenuated CaMKII-dependent eNOS activation by siArgII. siArgII-dependent Parkin activation was attenuated by KN93, a CaMKII inhibitor. Knockdown of ArgII mRNA and its gene, but not inhibition of its activity, accelerated the interaction between p32 and Parkin and reduced p32 levels. In aortas of ArgII^−/− mice, p32 levels were reduced by activated Parkin and inhibition of CaMKII attenuated Parkin-dependent p32 lysis. siParkin blunted the phosphorylation of the activated CaMKII/AMPK/p38MAPK/Akt/eNOS signalling cascade. However, ApoE^−/− mice fed a high-cholesterol diet had greater ArgII activity, significantly attenuated phosphorylation of Parkin, and increased p32 levels. Incubation with siArgII augmented p32 ubiquitination through Parkin activation, and induced signalling cascade activation.

Conclusion

The results suggest a novel function for ArgII protein in Parkin-dependent ubiquitination of p32 that is associated with Ca²⁺-mediated eNOS activation in endothelial cells.

Inhibition of p38mapk Reduces Adipose Tissue Inflammation in Aging Mediated by Arginase-II

BACKGROUND

Adipose tissue inflammation occurs not only in obesity but also in aging and is mechanistically linked with age-associated diseases. Studies show that ablation of the l-arginine-metabolizing enzyme arginase-II (Arg-II) reduces adipose tissue inflammation and improves glucose tolerance in obesity. However, the role of Arg-II in aging adipose tissue inflammation is not clear.

OBJECTIVE

This study investigated the role of Arg-II in age-associated adipose tissue inflammation.

METHODS

Visceral adipose tissues of young (3-6 months) and old (20-24 months) wild-type (WT) and Arg-II-/- mice were investigated. Immunofluorescence confocal microscopy was performed for analysis of macrophage accumulation and cellular localization of arginase and cytokines; expression of arginase and cytokines was analyzed by qRT-PCR or immunoblotting or ELISA; activation of mitogen-activated protein kinases in adipose tissues was analyzed by immunoblotting; and arginase activity was measured by colorimetric determination of urea production.

RESULTS

In the old WT mice, there is more macrophage accumulation in the visceral adipose tissues than in Arg-II knockout animals. An age-associated increase in arginase activity and Arg-II expression in adipose tissues of WT mice is observed. Arg-II knockout enhances Arg-I expression and activity, but inhibits interleukin (IL)-6 expression and secretion and reduces active p38mapk in aging adipose tissue macrophages and stromal cells. Treatment of aging adipose tissues of WT mice with a specific p38mapk inhibitor SB203580 reduces IL-6 secretion.

CONCLUSIONS

Arg-II promotes IL-6 production in aging adipose tissues through p38mapk. The results suggest that targeting Arg-II or inhibiting p38mapk could be beneficial in reducing age-associated adipose tissue inflammation.

Is the Arginase Pathway a Novel Therapeutic Avenue for Diabetic Retinopathy?

Diabetic retinopathy (DR) is the leading cause of blindness in working age Americans. Clinicians diagnose DR based on its characteristic vascular pathology, which is evident upon clinical exam. However, extensive research has shown that diabetes causes significant neurovascular dysfunction prior to the development of clinically apparent vascular damage. While laser photocoagulation and/or anti-vascular endothelial growth factor (VEGF) therapies are often effective for limiting the late-stage vascular pathology, we still do not have an effective treatment to limit the neurovascular dysfunction or promote repair during the early stages of DR. This review addresses the role of arginase as a mediator of retinal neurovascular injury and therapeutic target for early stage DR. Arginase is the ureohydrolase enzyme that catalyzes the production of L-ornithine and urea from L-arginine. Arginase upregulation has been associated with inflammation, oxidative stress, and peripheral vascular dysfunction in models of both types of diabetes. The arginase enzyme has been identified as a therapeutic target in cardiovascular disease and central nervous system disease including stroke and ischemic retinopathies. Here, we discuss and review the literature on arginase-induced retinal neurovascular dysfunction in models of DR. We also speculate on the therapeutic potential of arginase in DR and its related underlying mechanisms.

Role of Arginase 2 in Murine Retinopathy Associated with Western Diet-Induced Obesity

Western diet-induced obesity is linked to the development of metabolic dysfunctions, including type 2 diabetes and complications that include retinopathy, a leading cause of blindness. Aberrant activation of the inflammasome cascade leads to the progression of obesity-induced pathologies. Our lab showed the critical role of arginase 2 (A2), the mitochondrial isoform of this ureahydrolase, in obesity-induced metabolic dysfunction and inflammation. A2 deletion also has been shown to be protective against retinal inflammation in models of ischemic retinopathy and multiple sclerosis. We investigated the effect of A2 deletion on western diet-induced retinopathy. Wild-type mice fed a high-fat, high-sucrose western diet for 16 weeks exhibited elevated retinal expression of A2, markers of the inflammasome pathway, oxidative stress, and activation of microglia/macrophages. Western diet feeding induced exaggerated retinal light responses without affecting visual acuity or retinal morphology. These effects were reduced or absent in mice with global A2 deletion. Exposure of retinal endothelial cells to palmitate and high glucose, a mimic of the obese state, increased expression of A2 and inflammatory mediators and induced cell death. These effects, except for A2, were prevented by pretreatment with an arginase inhibitor. Collectively, our study demonstrated a substantial role of A2 in early manifestations of diabetic retinopathy.

Semen Cuscutae Administration Improves Hepatic Lipid Metabolism and Adiposity in High Fat Diet-Induced Obese Mice

Since arginase has been shown to compete with nitric oxide (NO) synthase, emerging evidence has reported that arginase inhibition improves obesity by increasing NO production. Semen cuscutae (SC), which is a well-known Chinese medicine, has multiple biological functions such as anti-oxidant function and immune regulation. In this study, we investigated whether the SC as a natural arginase inhibitor influences hepatic lipid abnormalities and whole-body adiposity in high-fat diet (HFD)-induced obese mice. The lipid accumulation was significantly reduced by SC treatment in oleic acid-induced hepatic steatosis in vitro. Additionally, SC supplementation substantially lowered HFD-induced increases in arginase activity and weights of liver and visceral fat tissue, while increasing hepatic NO. Furthermore, elevated mRNA expressions of sterol regulatory element-binding transcription factor 1 (SREBP-1c), fatty-acid synthase (FAS), peroxisome proliferator-activated receptor-gamma (PPAR-γ)1, and PPAR-γ2 in HFD-fed mice were significantly attenuated by SC supplementation. Taken together, SC, as a novel natural arginase inhibitor, showed anti-obesity properties by modulating hepatic arginase and NO production and metabolic pathways related to hepatic triglyceride (TG) metabolism.

Improvement in endothelial function in cardiovascular disease - Is arginase the target?

Endothelial dysfunction represents an early change in the vascular wall in areas prone to atherosclerotic plaque formation and is present in association with several risk factors for cardiovascular disease. The underlying mechanisms behind endothelial dysfunction are multifactorial and complex. Arginase has emerged as a key player in the regulation of endothelial integrity by the ability of reciprocally inhibits nitric oxide formation and promoting oxidative stress. A chain of evidence suggest that arginase is implicated in the pathogenesis underlying endothelial dysfunction induced by several cardiovascular risk factors and established cardiovascular disease including diabetes, hypercholesteremia, ischemia/reperfusion, atherosclerosis, obesity, ageing and hypertension. Recent data has unveiled a key role of arginase as one of the key mechanisms underlying endothelial dysfunction in diabetes and may serve as a potential therapeutic target in previously overlooked compartments including red blood cells. The current review is devoted to discuss arginase as a key mediator in endothelial dysfunction and the potential for therapeutic possibilities to target this enzyme in various diseases, especially type 2 diabetes, atherosclerosis and ischemia/reperfusion with focus on translational and clinical aspects. Moreover, approaches of how and in which patient group(s) arginase may be targeted in future clinical trials are discussed.