High glucose, nitric oxide, and adenosine: a vicious circle in chronic hyperglycaemia?

Diabetes mellitus is characterized by accelerated atherosclerosis and increased risk of cardiovascular disease. Metabolic factors associated with the diabetic state itself, such as hyperglycaemia, are likely to contribute to the increased cardiovascular risk in diabetic patients. Endothelial dysfunction is recognized as a major cardiovascular risk factor, being a key determinant of atherosclerosis. Among the number of factors involved in maintaining proper vascular wall homeostasis, nitric oxide (NO) is of pivotal relevance in guaranteeing physiological endothelial function. In fact, impaired NO synthesis and/or availability results in endothelial dysfunction, thus promoting the development of vascular damage. Although it has been demonstrated that diabetes is associated with endothelial dysfunction, and it is known that endothelium-dependent vasodilation is significantly impaired in diabetic patients, it is still a matter of controversy whether and in which way the imbalance of glucose metabolism might affect NO synthesis and bioavailability. The question of whether the impaired endothelium-mediated vasodilation observed in diabetic patients suggests a deficit in NO synthesis in these subjects arises from several studies demonstrating that in vitro elevated glucose concentrations can up-regulate the endothelial NO synthase (eNOS) gene and protein expression and stimulate NO production in both animal and human cells. Likewise, primary cultures of human umbilical vein endothelial cells (HUVEC) obtained from gestational diabetic pregnancies, as well as HUVEC cultured in high glucose, exhibit increased synthesis of NO and reduced uptake of the endogenous nucleoside vasodilator, adenosine. These findings have been explained by a reduced expression and activity of human equilibrative nucleoside transporter type 1 (hENT1), the main mediator of adenosine uptake, which might be down-regulated by NO-dependent reduced SLC29A1 transcription. The study by Farı́as et al. provides the first evidence of NO-dependent down-regulation of SLC29A1 expression by the hCHOP–C/EBPa complex in HUVEC from gestational diabetic pregnancies. The authors have identified the repression of SLC29A1 promoter activity by the transcription factor hCHOP–C/EBPa complex as a potential mechanism of the reduced hENT1 expression in human foetal endothelial cells from gestational diabetes. The SLC29A1 promoter region contains consensus sequences for several transcription factors, including C/EBP homologous protein 10 (hCHOP), a member of the CCAAT/enhancer-binding protein family that forms heterodimers with C/EBP-activating transcription factors. Notably, hCHOP expression is up-regulated in cultured HUVEC by high glucose as well as in diabetic patients. In this respect, the identification of the molecular mechanism underlying the decreased hENT1 expression in gestational diabetes may open up intriguing scenarios and potentially enlarge our understanding of the ability of high glucose to impair endothelial functions in diabetes through the adenosine/L-arginine/NO (ALANO) pathway, which was recently identified by San Martin and Sobrevia. It may be proposed that in gestational diabetes, increased NO release by HUVEC could occur via activation of the ALANO pathway. NO increases formation of the hCHOP-C/EBPa complex, which, in turn, causes the decrease in hENT1 expression (Figure 1). As a consequence, the extracellular accumulation of adenosine, due to the reduction of hENT1 functions, results in increased L-arginine transport via human cationic amino acid transporters (hCATs) and enhanced NO synthesis by increased eNOS activity. The up-regulation of the endothelial L-arginine/NO pathway by adenosine is associated with increased transcription of eNOS and SLC7A1, leading to increased eNOS and hCAT-1 mRNA levels, respectively. The pathways involved in these transcriptional events are still unknown. On the contrary, the mechanism that, through hCHOP-C/EBPa complex formation, leads to reduced hENT1 expression in HUVEC from gestational diabetes has been elucidated by Farı́as et al. This phenomenon is clearly mediated by increased NO levels, which could be due to the activation of the ‘ALANO pathway’, although it is still unclear whether and in which way this pathway could be part of the increased eNOS expression in HUVEC from gestational diabetic pregnancies. In this regard, although limited information is available on cellular mechanisms associated with increased endothelial eNOS levels in