Erythrocyte Angiotensin Type 1 Receptor Enhances Oxygen Delivery to Combat Tissue Hypoxia, Damage and Fibrosis

Introduction: The renin-angiotensin system (RAS) is an important regulator of blood pressure and fluid balance through the activation of AT receptors (ATRs) by angiotensin II (Ang II). Dysregulation of RAS leads to hypertension, inflammation, organ damage, fibrosis and progression of multiple chronic diseases. It is known that Ang II induces erythropoiesis. However, as the most abundant and only cells responsible for oxygen delivery within our body, the function of RAS and its underlying mechanism in mature erythrocytes remains unknown.

Aim: We sought to define the specific expression of ATRs in mature erythrocytes, its function of and underlying mechanisms in multiple chronic disease conditions.

Methods: Using gene expression profiling and Western blot analyses, we probed the expression of ATRs in both human and mouse erythroblasts and mature erythrocytes. Using genetic approaches, we generated the specific ablation of AT1aR in erythrocytes (eAT1aR^-/-) by crossbreeding AT1aR^loxP/loxP mice with EpoR-Cre⁺ mice. Using two well accepted chronic hypertension and chronic kidney disease experimental models including Ang II infusion and unilateral ureteral obstruction (UUO), we compared the erythrocyte function and its life span as well as peripheral tissue hypoxia, function and fibrosis among eAT1aR^-/- mice and controls with or without Ang II infusion and UUO challenge. Using comprehensive metabolomics and U-¹³C₆ isotopically labeled glucose flux analysis coupled with multidisciplinary cellular and molecular approaches, we determined its molecular and metabolic bases.

Results: We precisely defined that only AT1aRs but not AT1bRs or AT2Rs were expressed in both human and murine erythroblasts and mature erythrocytes. Genetically, eAT1aR^-/- mice were fertile and morphologically indistinguishable from littermates with the similar systolic blood pressure and erythrocyte lifespan as control mice. Ang II infusion induced hypertension, severe cardiorenal hypoxia, myocardial hypertrophy, cardiorenal damage and fibrosis but less oxygen delivery from erythrocytes in eAT1aR^-/- mice comparing to the Ang II-infused controls. Metabolically, we revealed that Ang II failed to induce glucose metabolic reprogramming channeled toward glycolysis over pentose phosphate pathway, resulting in less erythroid-specific Rapoport-Luebering Shunt and reduced 2,3-bisphosphoglycerate (2,3-BPG) production and O₂ delivery from erythrocytes of Ang II infused eAT1aR^-/- mice comparing to the controls. Similar as Ang II challenge, UUO murine models exhibited severe renal hypoxia, inflammation and fibrotic changes without effects on blood pressure in eAT1aR^-/- mice comparing to the controls, indicating the common beneficial role of eAT1-mediated oxygen delivery in two independent animal models of hypertension and obstructive induced cardiorenal damages and progression. Molecularly, we defined that Ang II directly induced oxygen offload by activating AT1R-mediated SphK1-S1P-dependent BPGM activation in both primary cultured human and murine erythrocytes.

Conclusion: We revealed a previously unrecognized protective role of eAT1 to combat tissue hypoxia, damage and fibrosis in two distinct experimental models via SphK1-S1P signaling cascade-mediated BPGM activation and enhanced O₂ delivery from erythrocytes. These findings add a significant new chapter to the importance of erythrocyte RAS system in counteracting tissue hypoxia, cardiorenal damage and disease progression and suggest new therapeutic avenues.