Are organic cation transporters capable of transporting prostaglandins?

The non-neuronal monoamine transporters OCT1, OCT2 and EMT (human gene symbols SLC22A1-A3) efficiently transport a number of positively-charged monoamines and some small organic cations across the plasma membrane, and thus are implicated in the inactivation of released monoamine transmitters (e.g. noradrenaline, histamine, agmatine) in vivo. Although prostaglandins are full anions at physiological pH, data from a recent publication suggest efficient transport of the prostaglandins PGE2 and PGF2alpha by OCT1 and OCT2. In the present study we have reexamined transport of PGE2 by OCT2 from human (OCT2h). Uptake of substrate into monolayers of 293 cells, stably transfected to express OCT2h, was compared to uptake into non-transfected control cells. Efficiency of transport of the established substrate 3H-1-methyl-4-phenylpyridinium (MPP+), expressed as clearance, was high at 81 microl min(-1) mg protein(-1) on average. By contrast, uptake of 3H-PGE2 was virtually identical for control cells and OCT2h cells. The efficiency of transport was 0.1+/-0.6, 1.0+/-0.3, and 0.7+/-0.4 microl min(-1) mg protein(-1) for cell lysis with methanol, HClO4, and Triton X-100 respectively. Similar results were obtained with unlabeled MPP+ (192+/-12 microl min(-1) mg protein(-1)) and PGE2 (0.3+/-0.1 microl min(-1) mg protein(-1)) in LC-MS/MS analysis. We conclude that OCT2h is not capable of transporting prostaglandins. The data from the previous report may represent binding rather than transport. Our comparison of transport efficiencies confirms the notion that relevant substrates of OCT1, OCT2, and EMT must carry a positive charge.