Digoxin, which is one of the most commonly prescribed drugs for the treatment of heart failure, is mainly eliminated from the circulation by the kidney. P-glycoprotein is well characterized as a digoxin pump at the apical membrane of the nephron. However, little is known about the transport mechanism at the basolateral membrane. We have isolated an organic anion transporter (OATP4C1) from human kidney. Human OATP4C1 is the first member of the organic anion transporting polypeptide (OATP) family expressed in human kidney. The isolated cDNA encodes a polypeptide of 724 aa with 12 transmembrane domains. The genomic organization consists of 13 exons located on chromosome 5q21. Its rat counterpart, Oatp4c1, is also isolated from rat kidney. Human OATP4C1 transports cardiac glycosides (digoxin, K m ‫؍‬ 7.8 M and ouabain, K m ‫؍‬ 0.38 M), thyroid hormone (triiodothyronine, Km ‫؍‬ 5.9 M and thyroxine), cAMP, and methotrexate in a sodiumindependent manner. Rat Oatp4c1 also transports digoxin (K m ‫؍‬ 8.0 M) and triiodothyronine (Km ‫؍‬ 1.9 M). Immunohistochemical analysis reveals that rat Oatp4c1 protein is localized at the basolateral membrane of the proximal tubule cell in the kidney. These data suggest that human OATP4C1͞rat Oatp4c1 might be a first step of the transport pathway of digoxin and various compounds into urine in the kidney.

We have isolated and characterized a novel human and rat organic anion transporter subtype, OATP-D. The isolated cDNA from human brain encodes a polypeptide of 710 amino acids (Mr 76,534) with 12 predicted transmembrane domains. The rat clone encodes 710 amino acids (Mr 76,821) with 97.6% amino acid sequence homology with human OATP-D. Human and rat OATP-D have moderate amino acid sequence homology with LST-l/rlst-1, the rat oatp family, the prostaglandin transporter, and moatl/MOAT1/KIAA0880/OATP-B. Phylogenetic tree analysis revealed that OATP-D is branched in a different position from all known organic anion transporters. OATP-D transports prostaglandin E1 (Km 48.5 nM), prostaglandin E2 (Km 55.5 nM), and prostaglandin F2,, suggesting that, functionally, OATP-D encodes a protein that has similar characteristics to those of the prostaglandin transporter. Rat OATP-D also transports prostaglandins. The expression pattern of OATP-D mRNA was abundant mainly in the heart, testis, brain, and some cancer cells. Immunohistochemical analysis further revealed that rat OATP-D is widely expressed in the vascular, renal, and reproductive system at the protein level. These results suggest that OATP-D plays an important role in translocating prostaglandins in specialized tissues and cells.

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We have isolated three novel organic anion transporter cDNAs designated rat GST-1 (gonad-specific transporter), rat GST-2, and human GST, expressed at high levels in the testis. Rat GST-1, GST-2, and human GST consist of 748, 702, and 719 amino acids, respectively, and all molecules possess the 12 predicted transmembrane domains, which is a common structure of organic anion transporters. Northern blot analyses and in situ hybridization revealed that both of the rat molecules are highly expressed in the testis, especially in Sertoli cells, spermatogonia, and Leydig cells. Weak signals are also detected in the epididymis and ovary in adult rat. The exclusive expression of human GST mRNA in the testis was confirmed by RT-PCR. The pharmacological experiments of Xenopus laevis oocytes injected with the respective rat GST-1- and GST-2-cRNAs revealed that both rat GST-1 and GST-2 transport taurocholic acid, dehydroepiandrosterone sulfate, and T4 with Michaelis-Menten kinetics (taurocholic acid, Km = 8.9 and 2.5 microm, dehydroepiandrosterone sulfate, Km = 25.5 and 21.microm, and T4, Km = 6.4 and 5.8 for rat GST-1 and GST-2, respectively). T3 was also transported by rat GST-1 and GST-2. These data suggest that rat GST-1 and GST-2 might be one of the molecular entities responsible for transporting dehydroepiandrosterone sulfate and thyroid hormones involved in the regulation of sex steroid transportation and spermatogenesis in the gonad.