Functional Characterization of Rat Organic Anion Transporter 2 in LLC-PK1 Cells

Abstract

Rat organic anion transporter 2 (rOat2) is abundantly expressed in the liver and localized to the basolateral membrane. A previous study using the Xenopus laevis oocyte expression system has shown that rOat2 transports organic anions such as salicylate (Sekine et al., 1998) and, in the present study, rOat2 was characterized using a mammalian expression system. In addition to the substrates previously shown to be transported by rOat2, three substrates, indomethacin [IDM, Michaelis-Menten constant (K_m) of 0.37 μM] and nucleoside derivatives such as 3′-azido-3′-deoxythymidine (AZT,K_m of 26 μM) and 2′,3′-dideoxycytidine (ddC, K_m of 3.08 mM), were also identified for the first time The rank order of rOat2-mediated transport of these substrates was IDM > salicylate > prostaglandin E₂ > AZT > ddC >p-aminohippurate (PAH). Ketoprofen, indocyanine green and glibenclamide are potent inhibitors of the uptake of [¹⁴C]salicylate via rOat2 (K_iof ∼12 μM), while diclofenac, benzoate, verapamil, ibuprofen, and tolbutamide are moderate inhibitors (K_i of ∼150 μM). The affinity of PAH, a common substrate for the OAT family, for rOat2 is low (K_i > 1 mM) compared with the other members of the OAT family (rOat1 and rOat3). Salicylate and IDM are also substrates for rOat1, but their affinity for rOat2 was higher than that for rOat1. The present study shows that rOat2 is a multispecific transporter and suggests that it may be involved at least partly, in the hepatic uptake of IDM, salicylate and nucleoside derivatives.