Tryptophan Residue Located at the Middle of Putative Transmembrane Domain 11 Is Critical for the Function of Organic Anion Transporting Polypeptide 2B1

Importance of N-Glycosylation for the Expression and Function of Human Organic Anion Transporting Polypeptide 2B1

Human organic anion transporting polypeptide 2B1 (OATP2B1) is a membrane transporter widely expressed in organs crucial for drug absorption and disposition such as the intestine, liver, and kidney. Evidence indicates that OATP2B1 is a glycoprotein. However, the sites of glycosylation and their contribution to the function and expression of OATP2B1 are largely unknown. In this study, by site-directed mutagenesis, we determined that two of four potential N-glycosylation sites in OATP2B1, N176 and N538, are indeed glycosylated. Functional studies revealed that the transport activities of mutants N176Q and N538Q were greatly reduced as compared to that of wild-type OATP2B1. However, the reduced activity was not due to the impairment of transport function per se but due to the decreased surface expression as the Km and normalized Vmax values of N176Q and N538Q were comparable to those of OATP2B1. Quantitative polymerase chain reaction (PCR) revealed that N176Q and N538Q mutations did not affect the expression of OATP2B1 at a transcriptional level. Immunofluorescence analysis showed that deglycosylated OATP2B1 was largely retained in the endoplasmic reticulum, which may activate the endoplasmic reticulum-associated degradation pathway, and the ubiquitin-proteasome system played a major role in the degradation of OATP2B1. Taken together, OATP2B1 is N-glycosylated, and N-glycosylation is essential for the surface expression of OATP2B1 but not critical for the transport function of OATP2B1 per se.

Investigating the interactions of flavonoids with human OATP2B1: inhibition assay, IC50 determination, and structure-activity relationship analysis

Human organic anion transporting polypeptide 2B1 (OATP2B1) is a crucial transporter for the absorption and disposition of many drugs. Its inhibition by small molecules may alter the pharmacokinetic profile of its substrate drugs. In this study, the interactions of 29 common flavonoids with OATP2B1 were explored using the fluorescent substrate 4',5'-dibromofluorescein and structure-activity relationship analysis. Our results showed that flavonoid aglycones interact with OATP2B1 more strongly than their 3-O- and 7-O-glycoside counterparts, as hydrophilic and bulky groups at these two sites are detrimental to flavonoids' binding with OATP2B1. In contrast, hydrogen-bond forming groups at the C-6 position of ring A and the C-3' and C-4' positions of ring B could strengthen the interaction of flavonoids with OATP2B1. However, a hydroxyl or sugar moiety at the C-8 position of ring A is unfavorable. Our results also indicated that flavones usually interact more strongly with OATP2B1 than their 3-hydroxyflavones (flavonols). The obtained information could be useful for the prediction of additional flavonoids for their interaction with OATP2B1.

Human Organic Anion Transporting Polypeptides 1B1, 1B3, and 2B1 Are Involved in the Hepatic Uptake of Phenolsulfonphthalein

Phenolsulfonphthalein (PSP or phenol red), a sulfonphthalein dye, has been used as a diagnostic agent and a pH indicator in cell culture medium. After administered into the body, PSP is excreted into urine and bile. The urinary excretion of PSP is mediated by organic anion transporter 1/3 (OAT1/3) and multidrug resistance protein 2 (MRP2). In biliary excretion, PSP is effluxed from hepatocytes into the bile via MRP2. However, so far, the molecular mechanism for PSP transport from the blood into hepatocytes is unclear. In the present study, six human major hepatic uptake transporters expressed on the basolateral membrane of hepatocytes, namely, organic anion transporting polypeptide 1B1 (OATP1B1), OATP1B3, OATP2B1, Na+/taurocholate cotransporting polypeptide (NTCP), organic cation transporter 1 (OCT1), and OAT2, have been investigated to see whether they are involved in the hepatic uptake of PSP. An in vitro cell-based study demonstrated that PSP is a substrate for OATP1B1, OATP1B3, and OATP2B1, with OATP1B3 showing the highest transport efficiency. The K m values for OATP1B1-, OATP1B3-, and OATP2B1-mediated PSP uptake were 11.3 ± 1.5, 7.0 ± 1.5, and 5.1 ± 1.0 μM, respectively. PSP interacts with known OATP substrates/inhibitors. However, the presence of PSP in cell culture medium has no significant effect on OATP's function. In vivo pharmacokinetic study in wild-type and Oatp1b2-knockout mice showed that Oatp1b2-knockout led to elevated plasma concentration and decreased liver accumulation of PSP. Taken together, the present study showed that in the liver, OATP1B1, OATP1B3, and OATP2B1 are involved in the uptake of PSP from the blood into hepatocytes, which, along with MRP2-mediated efflux of PSP from hepatocytes into the bile, constitute the vectorial transport of PSP from the blood to the bile and may play a critical role in the biliary excretion of PSP.

Thyroid Hormone Transporters

Thyroid hormone transporters at the plasma membrane govern intracellular bioavailability of thyroid hormone. Monocarboxylate transporter (MCT) 8 and MCT10, organic anion transporting polypeptide (OATP) 1C1, and SLC17A4 are currently known as transporters displaying the highest specificity toward thyroid hormones. Structure-function studies using homology modeling and mutational screens have led to better understanding of the molecular basis of thyroid hormone transport. Mutations in MCT8 and in OATP1C1 have been associated with clinical disorders. Different animal models have provided insight into the functional role of thyroid hormone transporters, in particular MCT8. Different treatment strategies for MCT8 deficiency have been explored, of which thyroid hormone analogue therapy is currently applied in patients. Future studies may reveal the identity of as-yet-undiscovered thyroid hormone transporters. Complementary studies employing animal and human models will provide further insight into the role of transporters in health and disease. (Endocrine Reviews 41: 1 - 55, 2020).

The mechanism of Oatp1a5-mediated microcystin-leucine arginine entering into GnRH neurons

Microcystin-leucine arginine (MC-LR) enters into gonadotropin-releasing hormone (GnRH) neurons and induces decline of serum GnRH levels resulting in male reproductive toxicity via hypothalamic-pituitary-testis axis. The organic anion transporting polypeptide 1a5 (Oatp1a5) is a critical transporter for the uptake of MC-LR by GnRH neurons. However, the underlying molecular mechanisms of the transport process are still elusive. In this study, we found that the transmembrane domains 2, 8, and 9 played important roles in transporting function of Oatp1a5. In addition, our data demonstrated that N-linked glycosylation was involved in the transport of MC-LR by Oatp1a5. Moreover, we showed that N-linked glycosylation sites Asn483 and Asn492 were vital for the transport function of Oatp1a5. In summary, the study furthered our understanding of mechanisms that the uptake of MC-LR by GnRH neurons and laid a theoretical foundation for preventing MC-LR from injuring male reproductive health.

Gly45 and Phe555 in Transmembrane Domains 1 and 10 Are Critical for the Activation of Organic Anion Transporting Polypeptide 1B3 by Epigallocatechin Gallate

Organic anion transporting polypeptides (OATPs) 1B1 and 1B3 are two highly homologous transporters expressed in the human liver. However, epigallocatechin gallate (EGCG), which is the most predominant catechin in green tea, has opposite effects on the function of OATP1B1 and OATP1B3. In the present study, the critical structural domains and amino acid residues for the activation of OATP1B3 by EGCG have been determined by characterizing the function of a series of OATP1B3-derived chimeric transporters, site-directed mutagenesis, and kinetic studies. Our results showed that G45 and F555 in transmembrane domains 1 and 10 are the most important amino acid residues for OATP1B3 activation. Kinetic studies showed that the activation of OATP1B3 by EGCG at a low substrate concentration was due to its increased substrate binding affinity. However, EGCG caused increased K_m and decreased V_max for 1B3-G45A and 1B3-F555H. The flexibility at position 45 and aromaticity at position 555 might be important for OATP1B3 activation. While 1B3-G45A and 1B3-F555H could not be activated by EGCG, their transport activity for EGCG was comparable to that of wild-type OATP1B3. In conclusion, the present study elucidated the molecular mechanism for OATP1B3 activation by EGCG.

Transmembrane Domain 1 of Human Organic Anion Transporting Polypeptide 2B1 Is Essential for Transporter Function and Stability

Organic anion transporting polypeptides (OATPs, gene symbol SLCO) are important membrane transporter proteins that mediate the uptake of wide ranges of endogenous and exogenous compounds. OATP2B1 has been found in multiple organs and tissues, including the liver, small intestine, kidney, brain, placenta, heart, skin, as well as skeletal muscle, and is proposed to be involved in the uptake of orally administered drugs. Quite a few reports have demonstrated that transmembrane domains (TMs) are crucial for proper functions of OATP family members. Comparative modeling proposed that TM1, along with TM2, 4, and 5 of the N-terminal half of OATP2B1, may be localized within the substrate interaction pocket and are important for uptake function of the transporter. Alanine scanning of the putative transmembrane domain 1 of OATP2B1 revealed that substitution of L58 with alanine dramatically altered the K_m value, and mutation of V52, H55, Q59, and L69 resulted in significantly reduced substrate turnover number, whereas A61V, Q62A, and S66A exhibited significant change in both K_m and V_max values. In addition, phenylalanine at position 51 seems to play an important role in maintaining proper folding of OATP2B1 because alanine replacement of F51 caused accelerated degradation of the transporter protein. Although proteasome and lysosome inhibitors could partially recover protein level, the mutant transporter remained nonfunctional. Taken together, the identification of nine essential amino acid residues within TM1 of OATP2B1 suggested that the transmembrane domain is important for maintaining proper function of the transporter.