The Essential Role Of Adenosine Signaling In Priapism And Penile Fibrosis

Priapism is a condition of persistent penile erection lasting at least 4 hours in the absence of sexual excitation. Although uncommon in the general population, it was recognized as a serious complication of sickle cell disease (SCD) as early as 1934. In general, about 40% of men with SCD display priapism.Priapism is a urological emergency requiring a prompt and an accurate diagnosis and treatment because it is associated with erectile tissue damage and penile fibrosis, finally leads to erectile dysfunction (ED). However, the preventive approaches or effective treatment options for the disorder are limited because of poor understanding of its pathogenesis. Many patients have to get surgical treatment which rarely restores normal erectile function.Recently we unexpectedly found that adenosine deaminase (ADA) deficient mice display features of priapism seen in humans. ADA is a purine metabolic enzyme that catalyzes the conversion of adenosine to inosine. As a result of ADA deficiency, mice exhibit a marked increase in adenosine concentrations, particularly in the penis, which has the highest level of adenosine among all the tissues examined. This finding revealed adenosine as a causative factor contributing to priapism, and indicated that ADA-deficient mice represented a novel and important animal model to study the role of adenosine signaling in priapism. Subsequently, the significance of excess adenosine in priapism was extended and confirmed by a well-accepted animal model of priapism, SCD transgenic (Tg) mice. These findings raise the intriguing possibility of the contributory role of elevated adenosine signaling in priapism in general and that this signaling pathway represents a potentially novel therapeutic target for the prevention and treatment of priapism.The pathogenesis of penile fibrosis associated with priapism probably results from a combination of prolonged penile erection, ischemia-mediated inflammatory response, vascular damage, and attempted tissue repair. Multiple factors are released from locally insulted penile tissue and responding cells. However, the causative factors and underlying mechanism of penile fibrosis associated with priapism remain unknown.Because priapism is commonly associated with hypoxic conditions, one of the best-known signaling molecules to be induced under hypoxic conditions is adenosine. Previous studies indicated that adenosine functions as a profibrotic molecule playing a critical role in the pathogenesis of pulmonary fibrosis as well as hepatic fibrosis via adenosine receptors. Thus, we speculate that increased adenosine probably contributes to penile fibrosis.ADA deficient mice and SCD Tg mice, two well-accepted priapism animal models, were involved in this study to identify the critical role of adenosine signaling in priapism and penile fibrosis.ADA deficient mice display all of the features seen in ADA deficient human including severe T and B cell lymphopeniasevere, combined immunodeficiency disease (SCID), bone abnormalities, severe pulmonary insufficiency and so on. Without ADA replacement treatment, the mice will die in several weeks after birth due to immunodeficiency. Because ADA deficient human could survive with Polyethylene Glycol-modified Adenosine Deaminase (PEG-ADA) treatment, we routinely treat the ADA deficient mice with PEG-ADA to make them survive. In this study, zymogram analysis was employed to detect the ADA activity to determine the newborn mice's genotyping.SCD Tg mice express exclusively human sickle hemoglobin (HbS,α2βS2) and have the major features (irreversibly sickled red cells, anemia, multiorgan pathology, priapism) found in humans with sickle cell disease. For genotyping of these mice, two methods including blood smear test and PCR are used.Previous studies have demonstrated that elevated adenosine level in penile tissues leads to priapism in both ADA deficient mice and SCD Tg mice. In this study, different dosages of PEG-ADA were used to lower down penile adenosine levels in ADA deficient mice and SCD Tg mice, and then we use electrical field stimulation (EFS)-induced corporal cavernosal strip (CCS) relaxation experiments to evaluate the penile erectile function. We found that a high dosage regimen of PEG-ADA enzyme therapy from birth in ADA-deficient mice prevented the increased EFS-induced CCS relaxation associated with priapism by lowering penile adenosine levels to normal. Intriguingly, in both ADA-deficient mice and SCD Tg mice with established priapism, we found that normalization of adenosine levels in penile tissues by PEG-ADA treatment relieved the heightened EFS-induced cavernosal relaxation in priapism.Our studies firstly identify that PEG-ADA, a FDA approved drug for ADA deficient human, is a novel, safe, and mechanism-based drug to prevent and correct excess adenosine-mediated increased cavernosal relaxation seen in two independent priapic animal models, and suggested its therapeutic possibility in men suffering from priapism.Penile fibrosis is one of the serious complications of priapism, which could lead to erectile dysfunction. In this study, we demonstrate that increased adenosine is a novel causative factor contributing to penile fibrosis in two independent animal models of priapism, ADA deficient mice and SCD transgenic mice. An important finding is that chronic reduction of adenosine by PEG-ADA treatment successfully prevented and attenuated penile fibrosis in both mouse models, indicating an essential role of increased adenosine in penile fibrosis and a novel therapeutic possibility for this serious complication. In addition, we also identified that both mice models share a similar fibrotic gene expression profile in penile tissue (including procollagen I, TGF-β1, and plasminogen activator inhibitor-1 mRNA), suggesting that they share similar signaling pathways for progression to penile fibrosis.Our studies firstly demonstrated that increased adenosine in penile tissues contributes to penile fibrosis in ADA deficient mice and SCD Tg mice, two priapism animal models and PEG-ADA is novel, safe and effective drug to prevent and treat penile fibrosis.In an effort to decipher specific cell types and underlying mechanism responsible for adenosine-mediated penile fibrosis, we purified corpus cavernosal fibroblast cells (CCFCs), the major cell type involved in this process, from wild-type mice. Quantitative RT-PCR showed that the major receptor expressed in these cells is the adenosine receptor A2BR-Based on this fact, we further purified CCFCs from A2BR-deficient mice and demonstrated that A2BR is essential for excess adenosine-mediated penile fibrosis. Finally, we revealed that TGF-P functions downstream of the A2BR to increase CCFC collagen secretion and proliferation.Our studies firstly identify an essential role of increased adenosine in the pathogenesis of penile fibrosis via A2BR signaling and offer a potential target for prevention and treatment of penile fibrosis by PEG-ADA treatment.