Strict glucose control and artificial regulation of the NO–ADMA–DDAH system in order to prevent endothelial dysfunction

Asymmetric dimethylarginine (ADMA) is a naturally occurring analogue of the conditionally essential amino acid arginine and a metabolic by‐product of continual protein turnover processes in the cytoplasm of all human cells. Arginine is the precursor for nitric oxide (NO), an important regulator of immune function and organ circulation. Physiologically, NO is known to regulate the transport of insulin and uptake of glucose by several tissues including the endothelium, liver, pancreas, and skeletal muscle. ADMA functions as an endogenous inhibitor of the enzyme nitric oxide synthase (NOS) which forms NO and can therefore impair its bioavailability (see Fig. 1). The elimination of ADMA occurs via both urinary excretion and degradation by the enzyme dimethylarginine dimethylaminohydrolase (DDAH) (Brinkmann et al. 2014). The discovery of ADMA and the observation of its reductive effects on NO synthesis in vitro and in vivo led to a large body of research attempting to discover its role in human metabolism.

Figure 1. Schematic overview of the interactions between arginine, asymmetric dimethylarginine (ADMA), dimethylarginine dimethylaminohydrolase (DDAH), and nitric oxide synthase (NOS) .

PRMTs, protein arginine methyltransferases. Adapted from (Brinkmann et al. 2015).

In insulin resistant (IR) subjects, the body fails to properly respond to endogenous insulin and as a result glucose accumulates extracellularly instead of being transported into the cell. Low levels of intracellular glucose diminish or even eliminate enzyme systems such as the NOS–ADMA–DDAH pathway (Lin et al. 2002; Lai & Ghebremariam, 2016). As consequence, IR results in a disturbed NO metabolism and subsequent endothelial dysfunction in the vascular wall, manifested by impaired endothelium‐dependent vasodilatation. In addition to alteration of the NOS–ADMA–DDAH pathway, some evidence points to a disruption of the insulin‐stimulated PI3‐kinase signalling pathway as a trigger for the impaired endothelial function observed in IR (Kobayashi et al. 2004; Li et al. 2010; Zhang et al. 2012), while other findings involve inflammation and oxidative stress in the defective endothelium‐dependent vasodilatation seen in the presence of IR (El et al. 2013).

In a paper by El Assar et al. (2016) published in this issue of The Journal of Physiology it is indeed shown that morbid obese humans with IR show reduced vasodilatation, which responds to arginine exposure. In addition, ADMA in serum and viscera is increased and DDAH1 and 2 levels are decreased. Arginase is significantly higher, which was reversed by an arginase inhibitor. In IR rats, the endothelial dysfunction was ameliorated when giving arginine. More interestingly, ADMA levels were significantly higher in serum and aorta tissue. Arginase activity was also elevated and endothelium‐dependent vasodilatation improved on giving an arginase inhibitor. Although our understanding of the cause of the occurrence of IR remains unclear, El Assar et al demonstrate clearly that these enzyme systems are disturbed during IR. These results are very promising and may form the basis for future understanding of dysregulation of the NO metabolism in IR.

One can only speculate on the mechanism behind their findings, since the underlying mechanism of endothelial dysfunction associated to IR is not fully understood. Preclinical and clinical studies have demonstrated that hypercholesterolaemia, hypertension, coronary artery disease, renal failure, IR, pre‐eclampsia, critical illness, and diabetes mellitus are associated with increased production of reactive oxygen species, decreased expression and/or activity of DDAH, and accumulation of ADMA in cells and tissues (Siroen et al. 2005; Siroen et al. 2006; Lai & Ghebremariam, 2016). These diseases are often associated with hyperglycaemia as a consequence of IR. High glucose levels have also been demonstrated to impair the activity of DDAH, thereby causing ADMA accumulation. In addition, hyperglycaemia stimulates production of tumour necrosis factor‐α and increases free radical generation, whereas insulin has anti‐inflammatory actions (Siroen et al. 2005). The occurrence of IR in several disease states is, with our current knowledge, still unavoidable, but its consequences can be treated.

Understanding the interaction among NO, glucose, and insulin, as well as defining the role of the NOS–ADMA–DDAH pathway, is essential towards the development of therapeutic agents to prevent cardiovascular complications followed by IR. It could be hypothesized that the first step towards normal NO regulation is tight control of glucose levels by intensive insulin treatment. Insulin regulation may preserve DDAH activity, decrease protein breakdown and thereby reduce the production of ADMA. Recent studies already indicate that tight glucose regulation decreases ADMA levels in IR patients (Siroen et al. 2005). However, it is possible that despite tight glucose regulation, additional approaches are necessary, such as upregulation of DDAH and downregulation of ADMA. For instance, it might also be necessary to modulate DDAH enzymatic activity by chemical entities and transcriptional DDAH regulators that upregulate the gene expression of DDAH and downregulate ADMA, e.g. metformin, thiazolidinediones, sulfonylurea, INT‐747, GW4064 and vitamin A (Lai & Ghebremariam, 2016). Providing the precursors of arginine, such as glutamine, and decreasing arginase production by giving an arginase inhibitor could also be an effective strategy. Future human clinical trials should include tight regulation of glucose and when endothelial dysfunction is still present, additional strategies are warranted to balance the NOS–ADMA–DDAH system to ameliorate endothelial function.