Human organic anion transporter 1 mediates cellular uptake of cysteine-S conjugates of inorganic mercury

Background. The epithelial cells lining the renal proximal tubule have been shown to be the primary cellular targets where mercuric ions gain entry, accumulate, and induce pathologic effects in vivo. Recent data have implicated at least one of the organic anion transport systems in the basolateral uptake of inorganic mercury (Hg2+). Methods. Using a line of Madin-Darby canine kidney (MDCK) II cells transfected stably with the human organic anion transporter 1 (hOAT1), and oocytes from Xenopus laevis microinjected with cRNA for hOAT1, we tested the hypothesis that hOAT1 can transport biologically relevant mercuric conjugates of cysteine (Cys). Results. Indeed, MDCK II cells expressing a functional form of hOAT1 gained the ability to transport the mercuric conjugate 2-Amino-3-(2-amino-2-carboxy- ethylsulfanyl-mercuricsulfanyl)-propionic acid (Cys-S-Hg-S-Cys), but not the corresponding di-glutathione S-conjugate of Hg2+ (G-S-Hg-SG). Moreover, p-aminohippurate (PAH), adipate, and glutarate (but not succinate or malonate) inhibited individually the uptake of Cys-S-Hg-S-Cys in a dose-dependent manner. Uptake of Cys-S-Hg-S-Cys, but not G-S-Hg-S-G, was also documented in Xenopus oocytes expressing hOAT1. Conclusion. These data represent ostensibly the most direct line of evidence implicating a specific membrane protein (i.e., hOAT1) in the transport of a biologically relevant molecular species of Hg2+ in a mammalian cell. Moreover, these data indicate that the organic anion transporter(s) likely play a prominent role in the basolateral transport of mercuric ions by proximal tubular cells and in the nephropathy induced by Hg2+.