Arginase 1: an unexpected mediator of pulmonary capillary barrier dysfunction in models of acute lung injury

The integrity of epithelial and endothelial barriers in the lower airspaces of the lungs has to be tightly regulated, in order to prevent leakage and to assure efficient gas exchange between the alveoli and capillaries. Both G- and G+ bacterial toxins, such as LPS and pneumolysin, respectively, can be released in high concentrations within the pulmonary compartments upon antibiotic treatment of patients suffering from acute respiratory distress syndrome (ARDS) or severe pneumonia. These toxins are able to impair endothelial barrier function, either directly, or indirectly, by induction of pro-inflammatory mediators and neutrophil sequestration. Toxin-induced endothelial hyperpermeability can involve myosin light chain phosphorylation and/or microtubule rearrangement. Endothelial nitric oxide synthase (eNOS) was proposed to be a guardian of basal barrier function, since eNOS knock-out mice display an impaired expression of inter-endothelial junction proteins and as such an increased vascular permeability, as compared to wild type mice. The enzyme arginase, the activity of which can be regulated by the redox status of the cell, exists in two isoforms - arginase 1 (cytosolic) and arginase 2 (mitochondrial) - both of which can be expressed in lung microvascular endothelial cells. Upon activation, arginase competes with eNOS for the substrate L-arginine, as such impairing eNOS-dependent NO generation and promoting ROS generation by the enzyme. This mini-review will discuss recent findings regarding the interaction between bacterial toxins and arginase during acute lung injury and will as such address the role of arginase in bacterial toxin-induced pulmonary endothelial barrier dysfunction

L-citrulline protects from kidney damage in type 1 diabetic mice.

Rationale. Diabetic nephropathy is a major cause of end-stage renal disease, associated with endothelial dysfunction. Chronic supplementation of L-arginine (L-arg), the substrate for endothelial nitric oxide synthase (eNOS), failed to improve vascular function. L-citrulline (L-cit) supplementation not only increases L-arg synthesis, but also inhibits cytosolic arginase I (Arg I), a competitor of eNOS for the use of L-arg, in the vasculature. Aims. To investigate whether L-cit treatment reduces diabetic nephropathy in streptozotocin (STZ)-induced type 1 diabetes in mice and rats and to study its effects on arginase II (ArgII) function, the main renal isoform. Methods. STZ-C57BL6 mice received L-cit or vehicle supplemented in the drinking water. For comparative analysis, diabetic ArgII knock out mice and L-cit-treated STZ-rats were evaluated. Results. L-cit exerted protective effects in kidneys of STZ-rats, and markedly reduced urinary albumin excretion, tubulo-interstitial fibrosis and kidney hypertrophy, observed in untreated diabetic mice. Intriguingly, L-cit treatment was accompanied by a sustained elevation of tubular ArgII at 16 wks and significantly enhanced plasma levels of the anti-inflammatory cytokine IL-10. Diabetic ArgII knock out mice showed greater BUN levels, hypertrophy, and dilated tubules than diabetic wild type mice. Despite a marked reduction in collagen deposition in ArgII knock out mice, their albuminuria was not significantly different from diabetic wild type animals. L-cit also restored NO/ROS balance and barrier function in high glucose-treated monolayers of human glomerular endothelial cells. Moreover, L-cit also has the ability to establish an anti-inflammatory profile, characterized by increased IL-10 and reduced IL-1beta and IL-12(p70) generation in the human proximal tubular cells. Conclusions. L-cit supplementation established an anti-inflammatory profile and significantly preserved the nephron function during type 1 diabetes. <br/