Detrimental Role of Excess Adenosine-Mediated 2,3-Diphosphoglycerate Induction in Erythrocyte Sickling and Novel Mechanism-Based Therapies.

Sickle cell disease (SCD) is a devastating inherited hemolytic disorder due to a single missense mutation in the β-globin gene resulting in the production of hemoglobin S (HbS). Despite our knowledge of the molecular defect in HbS, we remain unable to control HbS polymerization and erythrocyte sickling under hypoxic conditions, which are central to the pathophysiology of the disease.

Here we report that adenosine, a cellular metabolite that is well known to be induced under hypoxic/ischemic conditions, is increased in the blood circulation of patients with SCD and in SCD transgenic (Tg) mice. To our surprise, we found that adenosine contributes to hypoxia-induced sickling of cultured erythrocytes from humans with SCD. Next, to evaluate the pathogenic role of increased adenosine in erythrocyte signaling in vivo, we took advantage of SCD transgenic (Tg) mice, a well accepted animal model of SCD. Intriguingly, treatment of SCD Tg mice with polyethylene glycol-modified adenosine deaminase (PEG-ADA) enzyme therapy to lower circulating adenosine levels reduced sickling and attenuated multiple tissue damage (including lung, kidney, liver and spleen) seen in SCD Tg mice. These findings revealed a previously unrecognized role for excess adenosine in sickling and the progression to multiple life-threatening complications. Next, by screening erythrocytes for small metabolites that are induced by adenosine and may contribute to sickling, we determined that 2,3-diphosphoglycerate (2,3-DPG), an erythroid specific metabolite known to promote O₂ release from Hb, is elevated in erythrocytes of both SCD Tg mice and humans. Using both pharmacological and genetic approaches, we demonstrated that adenosine-mediated 2,3-DPG induction is through A_2B receptor signaling and that this signaling pathway is a major contributor to hypoxia-induced erythrocyte sickling.

Overall, we have identified for the first time that elevated adenosine induces 2,3-DPG via A_2BR signaling and contributes to sickling in both human and mouse with SCD. Thus, our studies provide strong support for the new concept that adenosine-mediated 2,3-DPG induction is beneficial for normal erythrocytes by promoting O₂ release from Hb to hypoxic tissues. However, for SCD patients, the beneficial effect of adenosine-mediated 2,3-DPG induction becomes detrimental by inducing erythrocyte sickling due to 2,3-DPG mediated increased O₂ release and increased deoxyHbS polymerization. Our findings reveal a novel therapeutic possibility to treat and prevent sickling and progression to multiple life-threatening complications by targeting on the adenosine signaling pathway.

No relevant conflicts of interest to declare.