Hypoxia can act as an initial trigger to induce erythrocyte sickling and eventual end organ damage in sickle cell disease (SCD). Many factors and metabolites are altered in response to hypoxia and may contribute to the pathogenesis of the disease. Using metabolomic profiling, we found that the steady-state concentration of adenosine in the blood was elevated in a transgenic mouse model of SCD. Adenosine concentrations were similarly elevated in the blood of humans with SCD. Increased adenosine levels promoted sickling, hemolysis and damage to multiple tissues in SCD transgenic mice and promoted sickling of human erythrocytes. Using biochemical, genetic and pharmacological approaches, we showed that adenosine A2B receptor (A2BR)-mediated induction of 2,3-diphosphoglycerate, an erythrocyte-specific metabolite that decreases the oxygen binding affinity of hemoglobin, underlies the induction of erythrocyte sickling by excess adenosine both in cultured human red blood cells and in SCD transgenic mice. Thus, excessive adenosine signaling through the A2BR has a pathological role in SCD. These findings may provide new therapeutic possibilities for this disease.

Chronic kidney disease (CKD) is a prevalent life-threatening disease frequently associated with hypertension, progression to renal fibrosis and eventual renal failure. While the pathogenesis of CKD remains largely unknown, an increased inflammatory response is known to be associated with the disease and has long been speculated to contribute to disease development. However, the causative factors, the exact role of the increased inflammatory cascade in CKD and the underlying mechanisms for its progression remain unidentified. Here we report that interleukin-6 (IL-6) expression levels were significantly increased in the kidneys collected from CKD patients and further elevated in CKD patients characterized with hypertension. Functionally, we determined that angiotensin II (Ang II) is a causative factor responsible for IL-6 induction in the mouse kidney and that genetic deletion of IL-6 significantly reduced hypertension and key features of CKD including renal injury and progression to renal fibrosis in Ang II-infused mice. Mechanistically, we provide both human and mouse evidence that IL-6 is a key cytokine functioning downstream of Ang II signaling to directly induce fibrotic gene expression and preproendothelin-1 (prepro-ET-1) mRNA expression in the kidney. Overall, both the mouse and human studies reported here provide evidence that Ang II induces IL-6 production in the kidney and that, in addition to its role in hypertension, increased IL-6 may play an important pathogenic role in CKD by inducing fibrotic gene expression and ET-1 gene expression. These findings immediately suggest the IL-6 signaling is a novel therapeutic target to manage this devastating disorder affecting millions worldwide.

Sphingosine-1-phosphate (S1P) is a bioactive lipid that regulates multicellular functions through interactions with its receptors on cell surfaces. S1P is enriched and stored in erythrocytes; however, it is not clear whether alterations in S1P are involved in the prevalent and debilitating hemolytic disorder sickle cell disease (SCD). Here, using metabolomic screening, we found that S1P is highly elevated in the blood of mice and humans with SCD. In murine models of SCD, we demonstrated that elevated erythrocyte sphingosine kinase 1 (SPHK1) underlies sickling and disease progression by increasing S1P levels in the blood. Additionally, we observed elevated SPHK1 activity in erythrocytes and increased S1P in blood collected from patients with SCD and demonstrated a direct impact of elevated SPHK1-mediated production of S1P on sickling that was independent of S1P receptor activation in isolated erythrocytes. Together, our findings provide insights into erythrocyte pathophysiology, revealing that a SPHK1-mediated elevation of S1P contributes to sickling and promotes disease progression, and highlight potential therapeutic opportunities for SCD.