Adenosine deaminase-adenosine pathway in hemolysis-associated pulmonary hypertension

Hemolysis-associated pulmonary hypertension (HA-PH) is a serious clinical complication of various hemolytic disorders, and pulmonary hypertension (PH) is considered the greatest risk factor for death in patients with a hemolytic disorder. It is now well established that hemolysis causes the release of soluble hemoglobin and arginase from injured erythrocytes into plasma. This leads to nitric oxide (NO) deficiency, oxidative stress and a state of endothelial dysfunction that is associated with clinical development of PH. We challenge this concept and propose that in addition to the NO-arginase pathway, the adenosine deaminase-adenosine pathway plays a significant role in HA-PH and that modulation of this pathway may offer protective/therapeutic effects in HA-PH. Our preliminary data suggest that in HA-PH adenosine deaminase (ADA) is released from injured erythrocytes into plasma and that metabolic conversion of adenosine (ADO) to inosine by ADA reduces extracellular ADO levels. Adenosine, mainly via activation of adenosine A_2A receptors, mediates a number of biological responses that may reduce hemolysis-induced vasculopathy and the risk of PH. Hypoxia is the strongest stimulus for ADO synthesis, and this increased ADO production counteracts some of the tissue/vascular injury caused by hypoxia itself. Unfortunately, under hypoxic conditions (anemia, vasoconstriction, and vaso-occlusion) in HA-PH, this "ADO negative-feed back" is abolished and the vascular protective effects of ADO are severely diminished by ADA released from injured erythrocytes. We hypothesize that in hemolytic anemia the repetitive release of ADA increases the risk for vaso-occlusive events and PH. We also propose that increase in extracellular ADO levels or activation of adenosine A_2A receptors attenuates HA-PH, and we suggest further preclinical and clinical investigation of ADA inhibitors and ADO agonists in HA-PH.