Human Equilibrative Nucleoside Transporters 1 and 2 may be Differentially Modulated by A_2B Adenosine Receptors in Placenta Microvascular Endothelial Cells from Pre-eclampsia

Abstract 

Pre-eclampsia is associated with elevated maternal blood pressure and proteinuria, altered fetal growth, and increased plasma adenosine concentration in the mother and the fetus. Human equilibrative nucleoside transporters 1 (hENT1) and hENT2 are crucial to maintain physiological plasma levels of adenosine, thus modulating its several biological effects through adenosine receptor activation. However, it is unknown whether hENTs and adenosine receptors are expressed in human placental microvascular endothelium (hPMEC). To assay whether the increased fetal plasma adenosine concentration in pre-eclampsia results from altered hENT-mediated transport, and the potential involvement of adenosine receptors in this phenomenon, we investigated hENTs and A_2A and A_2B adenosine receptors expression and function in hPMEC. Cells were isolated and cultured from normal pregnancies (n=17) or pre-eclampsia with adequate-for-gestational age fetuses (n=7). hENT1, hENT2, A_2A and A_2B adenosine receptors were expressed and functional in hPMEC. Extracellular adenosine concentration was higher (4-fold) in pre-eclampsia versus normal pregnancies. hPMEC from pre-eclampsia exhibit increased total transport (hENT1+hENT2), and maximal velocity (V_max) for hENT2- (2-fold), but reduced V_max for hENT1-mediated adenosine transport (75%), with no changes in apparent K_m. hENT2 expression was increased (4.5-fold), but hENT1 protein abundance was reduced (80%) in pre-eclampsia. Equally, A_2A expression was reduced (50–80%) in pre-eclampsia. CGS-21680 (A_2A agonist) did not alter hENTs expression or activity, but ZM-241385 (A_2A antagonist) blocked pre-eclampsia effects and increased hENT1-mediated transport in normal pregnancies. Thus, A_2B adenosine receptors may differentially modulate hENTs in hPMEC, which could be a mechanism attempting to re-establish physiological extracellular adenosine levels in pre-eclampsia.

Keywords: Human, Placenta, Adenosine, Endothelium, Pre-eclampsia