Serum fetuin-A and arginase-1 in human obesity model: Is there any interaction between inflammatory status and arginine metabolism?

Alcohol Drinks Induce Acute Lowering in Circulating l-Arginine in Obese and Type 2 Diabetic Subjects

Since low serum l-arginine (Arg) and high asymmetric dimethylarginine (ADMA) can predict microvascular complications in type 2 diabetes mellitus (T2DM), we tested whether Arg and ADMA are affected by diet and physical activity in overweight/obese and T2DM subjects. We tested the effects on serum Arg and ADMA of single loads of dextrose, protein, fat, or alcohol (∼300 calories each); one episode of physical exercise; and 12 weeks of standard lifestyle modification (dietary and physical activity counseling). Alcohol drink was followed by ∼30% lowering in Arg. Arg and ADMA increased after a protein load but remained stable after glucose or fat load or 30 min of treadmill walk. Following 12 weeks of lifestyle modification, ADMA declined only in subjects achieving weight loss >5%. In conclusion, alcohol is a previously unrecognized acute suppressor of serum Arg. Lifestyle modification lowers ADMA in subjects who achieve weight loss >5%. Clinical Trial Registration Number: NCT04406402.

Regulation of carbohydrate metabolism by nitric oxide and hydrogen sulfide: Implications in diabetes

Nitric oxide (NO) and hydrogen sulfide (H₂S) are two gasotransmitters that are produced in the human body and have a key role in many of the physiological activities of the various organ systems. Decreased NO bioavailability and deficiency of H₂S are involved in the pathophysiology of type 2 diabetes and its complications. Restoration of NO levels have favorable metabolic effects in diabetes. The role of H₂S in pathophysiology of diabetes is however controversial; H₂S production is decreased during development of obesity, diabetes, and its complications, suggesting the potential therapeutic effects of H₂S. On the other hand, increased H₂S levels disturb the pancreatic β-cell function and decrease insulin secretion. In addition, there appear to be important interactions between NO and H₂S at the levels of both biosynthesis and signaling pathways, yet clear an insight into this relationship is lacking. H₂S potentiates the effects of NO in the cardiovascular system as well as NO release from its storage pools. Likewise, NO increases the activity and the expression of H₂S-generating enzymes. Inhibition of NO production leads to elimination/attenuation of the cardioprotective effects of H₂S. Regarding the increasing interest in the therapeutic applications of NO or H₂S-releasing molecules in a variety of diseases, particularly in the cardiovascular disorders, much is to be learned about their function in glucose/insulin metabolism, especially in diabetes. The aim of this review is to provide a better understanding of the individual and the interactive roles of NO and H₂S in carbohydrate metabolism.

Mechanisms of obesity-induced metabolic and vascular dysfunctions

Obesity has reached epidemic proportions and its prevalence is climbing. Obesity is characterized by hypertrophied adipocytes with a dysregulated adipokine secretion profile, increased recruitment of inflammatory cells, and impaired metabolic homeostasis that eventually results in the development of systemic insulin resistance, a phenotype of type 2 diabetes. Nitric oxide synthase (NOS) is an enzyme that converts L-arginine to nitric oxide (NO), which functions to maintain vascular and adipocyte homeostasis. Arginase is a ureohydrolase enzyme that competes with NOS for L-arginine. Arginase activity/expression is upregulated in obesity, which results in diminished bioavailability of NO, impairing both adipocyte and vascular endothelial cell function. Given the emerging role of NO in the regulation of adipocyte physiology and metabolic capacity, this review explores the interplay between arginase and NO, and their effect on the development of metabolic disorders, cardiovascular diseases, and mitochondrial dysfunction in obesity. A comprehensive understanding of the mechanisms involved in the development of obesity-induced metabolic and vascular dysfunction is necessary for the identification of more effective and tailored therapeutic avenues for their prevention and treatment.

Anti-obesity and anti-diabetic effects of nitrate and nitrite

Prevalence of obesity is increasing worldwide and type 2 diabetes to date is the most devastating complication of obesity. Decreased nitric oxide bioavailability is a feature of obesity and diabetes that links these two pathologies. Nitric oxide is synthesized both by nitric oxide synthase enzymes from l-arginine and nitric oxide synthase-independent from nitrate/nitrite. Nitric oxide production from nitrate/nitrite could potentially be used for nutrition-based therapy in obesity and diabetes. Nitric oxide deficiency also contributes to pathogeneses of cardiovascular disease and hypertension, which are associated with obesity and diabetes. This review summarizes pathways for nitric oxide production and focuses on the anti-diabetic and anti-obesity effects of the nitrate-nitrite-nitric oxide pathway. In addition to increasing nitric oxide production, nitrate and nitrite reduce oxidative stress, increase adipose tissue browning, have favorable effects on nitric oxide synthase expression, and increase insulin secretion, all effects that are potentially promising for management of obesity and diabetes. Based on current data, it could be suggested that amplifying the nitrate-nitrite-nitric oxide pathway is a diet-based strategy for increasing nitric oxide bioavailability and the management of these two interlinked conditions. Adding nitrate/nitrite to drugs that are currently used for managing diabetes (e.g. metformin) and possibly anti-obesity drugs may also enhance their efficacy.

Asymmetric dimethylarginine (ADMA) elevation and arginase up-regulation contribute to endothelial dysfunction related to insulin resistance in rats and morbidly obese humans

KEY POINTS

The presence of insulin resistance (IR) is determinant for endothelial dysfunction associated with obesity. Although recent studies have implicated the involvement of mitochondrial superoxide and inflammation in the defective nitric oxide (NO)-mediated responses and subsequent endothelial dysfunction in IR, other mechanisms could compromise this pathway. In the present study, we assessed the role of asymmetric dimethylarginine (ADMA) and arginase with respect to IR-induced impairment of endothelium-dependent vasodilatation in human morbid obesity and in a non-obese rat model of IR. We show that both increased ADMA and up-regulated arginase are determinant factors in the alteration of the l-arginine/NO pathway associated with IR in both models and also that acute treatment of arteries with arginase inhibitor or with l-arginine significantly alleviate endothelial dysfunction. These results help to expand our knowledge regarding the mechanisms of endothelial dysfunction that are related to obesity and IR and establish potential therapeutic targets for intervention.

ABSTRACT

Insulin resistance (IR) is determinant for endothelial dysfunction in human obesity. Although we have previously reported the involvement of mitochondrial superoxide and inflammation, other mechanisms could compromise NO-mediated responses in IR. We evaluated the role of the endogenous NOS inhibitor asymmetric dimethylarginine (ADMA) and arginase with respect to IR-induced impairment of l-arginine/NO-mediated vasodilatation in human morbid obesity and in a non-obese rat model of IR. Bradykinin-induced vasodilatation was evaluated in microarteries derived from insulin-resistant morbidly obese (IR-MO) and non-insulin-resistant MO (NIR-MO) subjects. Defective endothelial vasodilatation in IR-MO was improved by l-arginine supplementation. Increased levels of ADMA were detected in serum and adipose tissue from IR-MO. Serum ADMA positively correlated with IR score and negatively with pD2 for bradykinin. Gene expression determination by RT-PCR revealed not only the decreased expression of ADMA degrading enzyme dimethylarginine dimethylaminohydrolase (DDAH)1/2 in IR-MO microarteries, but also increased expression of arginase-2. Arginase inhibition improved endothelial vasodilatation in IR-MO. Analysis of endothelial vasodilatation in a non-obese IR model (fructose-fed rat) confirmed an elevation of circulating and aortic ADMA concentrations, as well as reduced DDAH aortic content and increased aortic arginase activity in IR. Improvement of endothelial vasodilatation in IR rats by l-arginine supplementation and arginase inhibition provided functional corroboration. These results demonstrate that increased ADMA and up-regulated arginase contribute to endothelial dysfunction as determined by the presence of IR in human obesity, most probably by compromising arginine availability. The results provide novel insights regarding the mechanisms of endothelial dysfunction related to obesity and IR and establish potential therapeutic targets for intervention.