The Role of Nitric Oxide in Type 2 Diabetes

The prevalence of diabetes is increasing worldwide, and this disease has a tremendous financial burden on most countries. Major types of diabetes are type 1 diabetes and type 2 diabetes (T2D); T2D accounts for 90-95% of all diabetic cases. For better management of diabetes, we need to have a better understanding of its pathophysiology. This chapter provides an overview of glucose homeostasis and the underlying pathophysiology of T2D.

Nitric oxide (NO) plays a critical role in many physiological and pathological functions in the human body. Following the discovery in 1986-1987 that endothelium-derived relaxing factor (EDRF) is NO, the number of NO-based publications within all fields of medicine has increased exponentially. This report provides a brief historical view of NO-based research, emphasizing the events in the last two decades of the 20th century.

Abnormal nitric oxide (NO) metabolism has been associated with the development of insulin resistance and type 2 diabetes (T2D). The concept of NO deficiency is supported by human studies on polymorphisms of endothelial NO synthase (eNOS) gene, animal knockout models for NO synthase isoforms (NOSs), and pharmacological evidence, showing detrimental effects of NOS inhibitors and salutary effects of NO donors on carbohydrate metabolism. On the other hand, T2D and insulin resistance may impair NO homeostasis due to hyperglycemia, oxidative stress, and inflammation. Reduced production of NO [i.e., impaired L-arginine-NOS pathway and function of the nitrate (NO3)-nitrite (NO2)-NO pathway], impaired NO transport within the circulation and delivery to target cells, as well as disrupted NO signaling (e.g., via oxidative-induced NO quenching, and impaired NO-cGMP signaling pathway) can all lead to a reduced NO bioactivity in T2D. This chapter focuses on the role of impaired NO metabolism in T2D.

Asymmetric dimethylarginine (ADMA), an endogenous competitive inhibitor of nitric oxide (NO) synthase (NOS) isoenzymes, can substantially inhibit vascular NO production at concentrations that are observed in pathophysiological conditions. Over-production of ADMA (via overexpression and/or activity of class 1 of the protein arginine methyltransferases, PRMT-1) alongside decreased catabolism (due to decreased expression and/or activity of dimethylarginine dimethyloaminohydrolase, DDAH) in type 2 diabetes (T2D) and insulin resistance results in increased circulatory and intracellular ADMA levels. Such pathological elevated ADMA levels lead to a decreased NO bioavailability and the development of diabetes complications, including cardiovascular diseases, nephropathy, and retinopathy; elevated ADMA levels also increase the mortality risk in these patients. Here, we discuss current documents indicating how disrupted ADMA metabolism contributes to the development of T2D and its complications. The role of other endogenous methylarginines, i.e., NGmonomethyl- L-arginine (L-NMMA) and NG, NG′-dimethyl-L-arginine (SDMA) on NO production and T2D are also discussed.

The nitrate (NO3)-nitrite (NO2)-nitric oxide (NO) pathway, as a storage reservoir for endogenous NO production, is dependent on the oral bacteria with NO3- reducing capacity. Undesirable changes of oral microbiota towards a decreased load of health-related NO3-reducing bacteria and an overgrowth of pathogenic species, leading to subsequent decreased NO2 production in the oral cavity and decreased systemic NO availability, are now considered risk factors for the development of insulin resistance and type 2 diabetes (T2D). This chapter discusses available evidence focusing on oral microbiota dysbiosis in T2D, especially NO3-reducing bacteria and their metabolic activity (including NO3-reductase and NO2-reductase activity), affecting net oral NO2 accumulation and the NO3-NO2-NO pathway.

Nitric oxide (NO), a multifunctional gasotransmitter, is now considered an endocrine hormone that essentially contributes to the regulation of glucose and insulin homeostasis. Here, we discuss current genetic data linking NO metabolism to metabolic disorders, especially insulin resistance and type 2 diabetes (T2D). Although several gene variants of NO synthases [NOSs, i.e., neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS)] isoforms have been identified in humans that affect NO bioactivity and metabolism, only the eNOS polymorphisms are reported to be associated with insulin resistance and T2D. Among the functional eNOS gene polymorphisms, the single nucleotide polymorphisms (SNPs) rs2070744 (T786C), rs1799983 (G894T), and rs869109213 (eNOS 4b/4a) are related to the risk of developing insulin resistance and T2D.

Nitric oxide (NO), a gaseous free radical, is a key signaling molecule in the different phases of the normal wound healing process. The beneficial effects of NO in wound healing are related to its antibacterial properties, regulation of inflammatory response, stimulation of proliferation and differentiation of keratinocytes and fibroblasts, and promotion of angiogenesis and collagen deposition. NO deficiency is an important mechanism responsible for poor healing in diabetic wounds. In this chapter, the function of NO in diabetic wound healing and the possible therapeutic significance of NO in the treatment of diabetic wounds are discussed. Current knowledge supports this notion that NO-based intervention is a promising therapeutic approach for diabetic wound healing.

Osteoporosis affects 200 million people worldwide. Osteoporosis in subjects with diabetes is called diabetoporosis, and type 2 diabetes (T2D) contributes to and aggravates osteoporotic fractures. Hyperglycemia, insulin resistance, bone vasculature impairment, increased inflammation, oxidative stress, and bone marrow adiposity contribute to a higher incidence of osteoporotic fractures in T2D. Decreased nitric oxide (NO) bioavailability due to lower endothelial NO synthase (eNOS)-derived NO and higher inducible NOS (iNOS)-derived NO is one of the main mechanisms of the diabetoporosis. Available data indicates that T2D increases osteoclast-mediated bone resorption and decreases osteoblast-mediated bone formation, mediated in part by reducing eNOS-derived NO and increasing iNOS-derived NO. NO donors delay osteoporosis and decrease osteoporotic fractures in subjects with T2D, suggesting the potential therapeutic implication of NO-based interventions for diabetoporosis.

Uric acid (UA) is the end product of purine catabolism in humans. Hyperuricemia, defined as elevated plasma concentrations of UA above 7 mg/dL, is a risk factor for developing hypertension, cardiovascular diseases, chronic kidney disease, and type 2 diabetes. Hyperuricemia can induce pancreatic β-cell death and impaired insulin secretion. It can also disrupt insulin-induced glucose disposal and insulin signaling in different insulin-sensitive tissues, including cardiomyocytes, skeletal muscle cells, adipocytes, hepatocytes, and endothelial cells. These events lead to the development of systemic insulin resistance and impaired glucose metabolism. Induction of inflammation, oxidative stress, and impairment of nitric oxide (NO) metabolism mediate hyperuricemia-induced insulin resistance and dysglycemia. This chapter is focused on the potential mediatory role of NO metabolism on hyperuricemia-induced dysglycemia and insulin resistance.

According to the World Health Organization (WHO), the prevalence of obesity across the globe has nearly tripled since 1975, with 39 million children under the age of 5 being overweight or obese in 2020. Obesity is the most common risk factor for developing type 2diabetes (T2D), which may lead to elevated serum triglycerides, hypertension, and insulin resistance. In the pathogenesis of T2D, there is a reduction in nitric oxide (NO) bioavailability. Restoration of NO levels has been associated with many favorable metabolic effects in T2D. Drugs that potentiate NO levels may have a role in improving T2D-associated adverse effects. Current medications approved for use in the management of T2D include biguanides, thiazolidinediones, sulfonylureas, meglitinides, dipeptidyl peptidase-4 (DPP-4) inhibitors, glucagon-like peptide-1 (GLP- 1) receptor agonists, alpha-glucosidase inhibitors, and sodium-glucose co-transporter 2 (SGLT2) inhibitors. These drugs mitigate the many adverse effects associated with T2D. This chapter discusses these classes of drugs, examines their mechanism of action, and presents evidence that these drugs directly or indirectly modulate NO levels.

Alzheimer’s disease (AD) is a devastating age-related neurodegenerative disease characterized by progressive pathological changes and functional and cognitive impairments. Brain insulin resistance appears to contribute significantly to the pathology and cognitive deficits among several pathological mechanisms. Brain insulin resistance has been demonstrated in animal models of AD and postmortem human brain tissue from patients with AD dementia. Studies conducted in AD models and humans suggest attenuating brain insulin resistance by agents such as glucagon-like peptide1 (GLP-1) analogs and small molecule drug candidate PTI-125 reduces many AD pathologic features and symptoms. Insulin affects NO levels by activating endothelial and neuronal nitric oxide synthase (eNOS, nNOS), and systemic insulin resistance has been linked to reduced nitric oxide (NO) bioavailability. Increasing NO availability reduces systemic insulin resistance, and the insulin signaling pathway is associated with the activation of eNOS, implying a causal relationship. This chapter explores this relationship and the role of impaired NO availability in brain insulin resistance in AD dementia.

L Arginine (Arg), a semi-essential essential amino acid, has received significant research interest over the last two decades as nitric oxide (NO) precursor. Arg is widely used as a complementary treatment in various NO-disrupted conditions, e.g., hypertension, preeclampsia, and endothelial dysfunction. Here, we provide an overview of the potential efficacy of Arg as a NO precursor and its effects on glucose and insulin homeostasis and diabetes-induced cardiovascular complications.

L-citrulline (Cit), a neutral, non-essential, and non-protein amino acid, is a precursor of L-arginine (Arg) and is involved in nitric oxide (NO) synthesis. Since oral ingestion of Cit can effectively elevate total Arg flux in the entire body and promote NO production, its supplementation has recently received much attention in the realm of cardio-metabolic diseases where NO metabolism is disrupted. Although preliminary data obtained from in vitro and in vivo animal experiments indicates that Cit improves glucose and insulin homeostasis and can effectively prevent hyperglycemia-induced complications such as inflammation, oxidative stress, renal dysfunction, and endothelial dysfunction, these findings are yet to be realized in well-designed longterm clinical studies in patients with type 2 diabetes (T2D). If Cit is shown to be an effective anti-diabetic agent with a good safety profile, its supplementation will be superior to that of Arg because it effectively increases systemic Arg availability more than Arg itself, and hence NO production.

Recent research punctuates that the nitrate (NO3)-nitrite (NO2)-nitric oxide (NO) pathway may be a potential therapeutic target in type 2 diabetes (T2D), a NOdisrupted metabolic disorder. Nutritional aspects of the NO3-NO2-NO pathway has been highlighted by focusing on the protective effects of some traditional high-NO3 diet, such as Mediterranean and DASH (Dietary Approaches to Stop Hypertension) diets and their NO3-rich components, i.e., fruits, vegetables, legumes, and green leafy vegetables, against the development of T2D. Both acute and long-term administration of inorganic NO3 and NO2 in animal experiments display anti-diabetic properties; inorganic NO3 decreases fasting blood glucose, glycosylated hemoglobin, and proinsulin to insulin ratio and improves glucose tolerance. In contrast to animal experiments, NO3/NO2 therapy has failed to show anti-diabetic properties and beneficial effects on glucose and insulin homeostasis in humans. This lost-i- -translation remains an open question, and long-term clinical trials are needed to confirm the salutary effects of inorganic NO3 and NO2 as the natural NO boosters in patients with T2D.

Nitric oxide (NO) donors are chemical agents that produce NO-related activity in biological systems, mimic endogenous NO-related responses, or compensate for NO deficiency. NO donors have been increasingly studied as promising therapeutic agents for insulin resistance and type 2 diabetes (T2D). Here, we provide evidence, which investigated the effects of the most frequently studied and implemented NOreleasing compounds, including sodium nitroprusside (SNP), S-nitrosothiols [RSNOs, i.e., S-nitrosoglutathione (GSNO), S-nitroso-N-acetyl-penicillamine, (SNAP)], and NDiazeniumdiolates (NONOates, i.e., spermine NONOate, diethylamine NONOate) on glucose and insulin homeostasis. Available evidence could not draw a clear conclusion regarding therapeutic applications of NO donors in T2D due to different methodological approaches (i.e., in vitro vs. in vivo) and different doses and formulations used to assess the potential effects of NO donors on carbohydrate metabolism. Considering key properties and different kinetic behaviors between various classes of NO donors, targeted compound selection, defining optimum doses, and appropriate use of NO-releasing platforms (topical vs. systemic delivery mode) seem to be critical issues that can accelerate the bench-to-beside translation of NO donors in T2D.