Background: Critical mediators controlling vascular tone, such as endothelial nitric oxide synthase (eNOS), are expressed not only by endothelial cells but also erythrocytes (red blood cells; RBCs). The bioavailability of nitric oxide (NO) in the vascular system is counterregulated by arginase 1 (ARG1) which also is expressed in both endothelial cells and erythrocytes.

Methods: Two conditional mouse lines were generated, lacking ARG1 in either endothelial cells (END.ARG1-KO) or erythrocytes (RBC.ARG1-KO). To examine the importance of erythrocyte ARG1 in the context of dyslipidemia, RBC.ARG1-KO mice were backcrossed into the apolipoprotein E deficient (apoE^-/-) background.

Results: Confocal microscopy confirmed reduced ARG1 expression in erythrocytes from RBC.ARG1-KO mice compared to RBC.ARG1-WT controls, while ARG1 levels in END.ARG1-WT and -KO mouse erythrocytes were unaffected. Conversely, ARG1 immunofluorescence signals were lower in primary endothelial cells of END.ARG1-KO mice compared to END.ARG1-WT littermates or RBC.ARG1-WT and -KO animals. Vascular relaxation studies in isolated thoracic aorta segments of mice lacking ARG1 in endothelial cells showed only minimal alterations compared to controls. In contrast, endothelium-dependent vasodilation was significantly impaired in RBC.ARG1-KO mice and to an even greater extent under dyslipidemic conditions compared to their wildtype littermate controls. Systemic blood pressure levels (determined using the tail cuff noninvasive blood pressure system in conscious mice), did not differ in END.ARG1-KO mice compared to END.ARG1-WT controls, whereas significantly increased blood pressure was observed in apoE-/- mice lacking ARG1 in erythrocytes. RT² Profiler PCR array analysis using primary endothelial cells isolated from apoE-/- RBC.ARG1-KO and -WT controls revealed significant differences in genes involved in vasoconstriction and vasodilation. Messenger RNA levels of brain natriuretic peptide (Nppb) or E-selectin (Sele) were significantly reduced, suggesting that a general state of endothelial cell activation downstream of the eNOS-ARG1 dysequilibrium contributes, at least in part, to the impaired vascular function and elevated blood pressure observed in those mice. In addition, expression levels of genes not primarily associated with vascular function control, such as Bcl2, Cx3cl1 and Cxcr5, or Tgfb1 and the endothelial acessory TGFβ receptor endoglin, or Vwf as well as many others were significantly upregulated in endothelial cells of apoE^-/- mice lacking ARG1 in erythrocytes.

Conclusions: Our findings highlight the importance of erythrocyte arginase and its crosstalk with endothelial cells to blood pressure and vascular function control, which may extend to a multitude of biological pathways beyond a role in the regulation of NO availability.