The red blood cell as a mediator of vascular dysfunction in type 2 diabetes and COVID-19

Abstract

Background: The number of patients diagnosed with Type 2 Diabetes (T2D) is steadily increasing and accounts for a large proportion of cardiovascular complications. Endothelial dysfunction represents an early disturbance in the development of atherosclerosis and precedes clinical signs of vascular disease. The mechanisms underlying endothelial dysfunction in T2D are complex but are characterized by decreased bioavailability of the vasodilator nitric oxide (NO) and an imbalance between reactive oxygen species (ROS) and anti-oxidants causing a state of oxidative stress. Arginase has emerged as an important regulator of NO and ROS by consuming the NO substrate L-arginine. Cardiovascular complications arising from coronavirus disease 2019 (COVID-19) have also created a clinical challenge.

Red blood cells (RBCs) have long been considered as innocent nuclei free bystanders, mainly serving the purpose of nutrient transport and gas exchange. Recent evidence has unveiled an important role of these cells in modulating vascular tone through their fascinating ability to release NO bioactivity and their highly developed anti-oxidant defence. Studies have claimed that the structure and function of the RBCs are altered, not only in chronic inflammation as in T2D, but also in acute inflammation in COVID-19. However, the functional implications of these disturbances have not been explored.

Objectives: To explore the importance of dysfunctional RBCs as important mediators of vascular dysfunction in T2D and COVID-19.

Methods and results: In Studies I and II, ex vivo incubations of internal mammary arteries or rat aortic rings with RBCs from patients with T2D resulted in attenuated endothelium-dependent relaxation (EDR) but not endothelium-independent relaxation (EIDR). Mechanistic experiments including expression and enzymatic assays revealed that this impairment was mediated by increased arginase activity and ROS including hydrogen peroxide and peroxynitrite. In Study III, the effect of arginase inhibition in vivo in patients with T2D before and after improvement in glycaemic control was performed using venous occlusion plethysmography. Forearm endothelial dysfunction persisted despite marked improvement in glycaemic control. Two hours of intra-arterial infusion of the arginase inhibitor Nω-hydroxy-nor-L-arginine markedly improved forearm endothelial function both at poor glycaemic control and following four months of improvement in glycaemic control. In Study IV, RBCs were collected from patients with T2D before and after improvement in glycaemic control to assess the impact of dysglycaemia on RBC-mediated endothelial function. The degree of endothelial dysfunction induced by RBCs remained despite clear-cut improvement in glycaemic control. In Study V, in vivo microvascular endothelial function was markedly impaired in patients with COVID-19, both in the acute phase of the infection and at four months follow-up. RBCs from patients with COVID-19 only in the acute phase markedly attenuated both EDR and EIDR through an increase in arginase and ROS and decreased NO bioavailability.

Conclusions: The RBC represents a novel mediator of endothelial dysfunction in patients with T2D (independent of glucose levels) and COVID-19 through alterations in arginase activity and ROS production. Targeting dysfunctional RBCs and the disturbed pathways might represent a previously overlooked therapeutic strategy for improving vascular function and preventing vascular complications in acute and chronic inflammation.