Differential responses to prostacyclin analogs in platelets and erythrocytes from humans with type 2 diabetes.

摘要

Erythrocytes release ATP in response to low oxygen tension as well as activation of the prostacyclin receptor (IPR). Both stimuli activate distinct signaling pathways for ATP release that require an increase in cAMP. ATP release in response to low oxygen tension is defective in erythrocytes from humans with type 2 diabetes (DM2) while IPR-mediated ATP release is enhanced. The goal of this project was to determine the mechanism(s) responsible for augmented IPR-mediated ATP release from erythrocytes of humans with DM2. Incubation of healthy human (HH) and DM2 erythrocytes with prostacyclin analogs stimulated greater increases in cAMP in DM2 erythrocytes. Levels of cAMP in this signaling pathway are regulated by phosphodiesterase 3 (PDE3). Studies using a selective PDE3 inhibitor suggested that PDE3 activity is reduced in DM2 erythrocytes compared to HH erythrocytes. The resultant increases in cAMP levels could mediate the enhanced IPR-induced ATP release from DM2 erythrocytes. In addition, we identified that, in HH erythrocytes, PDE3 is endogenously regulated by PDE5. Since both PDE3A and PDE5 are cytosolic PDEs, this suggests that PDE3A is the PDE3 isoform involved in the IPR-mediated pathway for ATP release from the erythrocyte. In addition to alterations in PDE3 activity, increased IPR expression and/or receptor binding could result in enhanced ATP release in DM2 erythrocytes. Western blot analysis of IPR protein expression showed no differences between DM2 (n=4) and HH (n=3) erythrocytes and attempts to measure erythrocyte IPR receptor binding were unsuccessful. In subsequent studies using platelets, it was determined that both expression of the IPR protein and receptor binding were decreased in DM2 versus HH platelets as was IPR activation-induced increases in cAMP. Although these results suggested that platelets were not a useful surrogate for erythrocytes, they did provide important insights into the excessive platelet aggregation identified in humans with DM2. Since in platelets, aggregation is prevented by increases in cAMP, the findings that platelets from humans with DM2 have significantly smaller increases in cAMP associated with IPR activation as well as decreased IPR binding sites and expression suggest a previously unrecognized mechanism for the excessive platelet aggregation observed in these individuals.