Adenosine signaling in normal and sickle erythrocytes and beyond

Abstract

Sickle cell disease (SCD) is a debilitating hemolytic genetic disorder with high morbidity and mortality affecting millions of individuals worldwide. Although SCD was discovered more than a century ago, no effective mechanism-based prevention and treatment are available due to poorly understood molecular basis of sickling, the fundamental pathogenic process of the disease. SCD patients constantly face hypoxia. One of the best-known signaling molecules to be induced under hypoxic conditions is adenosine. Recent studies demonstrate that hypoxia-mediated elevated adenosine signaling plays an important role in normal erythrocyte physiology. In contrast, elevated adenosine signaling contributes to sickling and multiple life threatening complications including tissue damage, pulmonary dysfunction and priapism. Here, we summarize recent research on the role of adenosine signaling in normal and sickle erythrocytes, progression of the disease and therapeutic implications. In normal erythrocytes, both genetic and pharmacological studies demonstrate that adenosine can enhance 2,3-bisphosphoglycerate (2,3-BPG) production via A(2B) receptor (ADORA2B) activation, suggesting that elevated adenosine has an unrecognized role in normal erythrocytes to promote O(2) release and prevent acute ischemic tissue injury. However, in sickle erythrocytes, the beneficial role of excessive adenosine-mediated 2,3-BPG induction becomes detrimental by promoting deoxygenation, polymerization of sickle hemoglobin and subsequent sickling. Additionally, adenosine signaling via the A(2A) receptor (ADORA2A) on invariant natural killer T (iNKT) cells inhibits iNKT cell activation and attenuates pulmonary dysfunction in SCD mice. Finally, elevated adenosine coupled with ADORA2BR activation is responsible for priapism, a dangerous complication seen in SCD. Overall, the research reviewed here reveals a differential role of elevated adenosine in normal erythrocytes, sickle erythrocytes, iNK cells and progression of disease. Thus, adenosine signaling represents a potentially important therapeutic target for the treatment and prevention of disease.

Figure 1. Adenosine metabolism and signaling

Cells release ATP in response to hypoxia and cell damage. The extracellular ATP is converted to adenosine (A) by the consecutive action of the ecto-nucleotidases, CD39 and CD73. The resulting adenosine can activate adenosine receptors (AR), be a substrate for extracellular adenosine deaminase (ADA), or reenter cells via equilibrative nucleoside transporters (ENTs). Within cells adenosine has multiple fates: 1) conversion to inosine via deamination, 2) conversion to adenosylhomocysteine (AdoHcy) via S-adenosylhomocytesine hydrolase (SAHH), or conversion to AMP by adenosine kinase (ADK). Adenosine can also be derived from AMP by the action of cytosolic 5’-nucleotidase (Cyto 5’NT).

Figure 2. Novel role of adenosine signaling in normal and sickle erythrocytes

Extracellular levels of adenosine increase in response to hypoxia (see Figure 1). Elevated adenosine activates ADORA2B adenosine receptors on erythrocytes, thereby activating downstream signaling pathways resulting in increased intracellular 2,3-bisphosphoglycerate (2,3-BPG), an allosteric regulator of hemoglobin (Hb) that reduces oxygen-binding affinity. This signaling pathway is beneficial to for normal individuals leading to increased oxygen release to hypoxic tissues. However, this process is detrimental for individuals with sickle cell disease by promoting the release of oxygen from sickle hemoglobin (HbS), resulting in increased concentrations of deoxy-HbS, increased deoxy-HbS polymerization and sickling.

Figure 3. Beneficial and detrimental effects of adenosine signaling in sickle cell disease

Elevated adenosine associated with SCD has beneficial effects by activating ADORA2A receptors on invariant natural killer T (iNKT) cells, a process that prevents iNKT cells activation and pulmonary inflammation. However, elevated adenosine activates ADORA2B receptors on erythrocytes thereby activating a signaling pathway leading to deoxy-HbS polymerization and erythrocyte sickling (see Fig. 2). The activation of ADORA2B receptors on penile endothelial cells and cavernosal smooth muscle cells results in priapism, a serious complication of SCD, leading to penile fibrosis and erectile dysfunction.

Figure 4. Sickle cell trait (SCT), purinergic signaling and malaria

SCT is associated with increased concentrations of free heme in the plasma, possibly due to accelerated autooxidation and heme loss due to instability of β^S. This feature may contribute to a mild hemolysis and hypoxia that activates purinergic remodeling in nucleated cells, a transcriptional process that promotes changes in gene expression that favor the breakdown of ATP and the accumulation of adenosine. The breakdown of extracellular ATP may result in reduced parasite P2Y signaling on and impaired parasitic invasion of the RBCs.