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Thyroid hormone transporters: recent advances
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Thyroid hormone transporters: recent advances Takaaki Abe, Takehiro Suzuki, Michiaki Unno, Taro Tokui and Sadayoshi Ito Thyroid hormones, being hydrophobic, were thought to enter target cell membranes by passive diffusion. However, recent studies have documented the existence of numerous organic anion transport systems, about half of which also transport thyroid hormones into (and possibly out of) a variety of target cells. Several of the genes encoding thyroid hormone transporters have been characterized by means of molecular approaches. Here, we discuss the classification of thyroid hormone transporters, with emphasis on how they are influenced by their ionic milieu and what their symported organic anions are. Published online: 10 May 2002
Thyroid hormone (TH) plays an essential role in the mammalian central nervous system and in many peripheral tissues. Hypothyroidism causes serious damage to neural cells and leads to mental retardation [1]. The action of thyroid hormone is mediated mainly through the deiodination of thyroxine (T4) to 3,3′,5-triiodo-L-thyronine (T3), followed by the binding of T3 to a specific nuclear receptor. Before reaching its intracellular targets, TH must cross the plasma membrane. Because of the lipophilic nature of TH, it was thought that it traversed the plasma membrane by simple diffusion. However, in the past decade, a membrane transport system for TH has been postulated to exist in various tissues. Takaaki Abe* Takehiro Suzuki Sadayoshi Ito Division of Nephrology, Endocrinology and Vascular Medicine, Dept Medicine, Tohoku University Graduate School of Medicine and PRESTO, Japan Science and Technology Corporation (JST), Japan. *e-mail: takaabe@ mail.cc.tohoku.ac.jp Michiaki Unno Division of Gastroenterological Surgery, Dept Surgery, Tohoku University Graduate School of Medicine, 1-1 Seriyo-cho, Aoba-ku, Sendai 980-8574, Japan. Taro Tokui Drug Metabolism and Pharmacokinetics Research Laboratories, Sankyo Co. Ltd, Tokyo, Japan.
Membrane binding and transport studies
A series of recent studies identified a membrane TH-binding protein [2]. Kato et al. [3] showed that this protein was identical to the protein disulfide isomerase (PDI). Because most PDI is located in the endoplasmic reticular lumen, and has low affinity for TH, it is not believed to be involved in the plasma membrane transport of TH. Moreover, a membrane transport system for TH has been well characterized in liver, brain cells, leukocytes and other tissues (reviewed in Ref. [4]). According to these data, TH transport in these tissues might be mediated by a saturable, stereospecific, energy-dependent transporter, although little is known about its molecular nature. Brain and retinal TH transport
The brain is separated from the bloodstream by the blood–brain and the blood–cerebrospinal fluid (CSF) barriers, which restrict the entry of molecules into brain tissue. It has been suggested that TH enters the brain via the blood–brain barrier [1] or the choroid plexus [5]. The choroid plexus is a major http://tem.trends.com
component of the blood–CSF barrier, in addition to being the major production site of the TH-binding protein, transthyretin, which transports TH into the brain. Free T4 can also cross the choroid plexus intact from blood to the brain [5] and up to one-fifth of the T4 in the brain has been shown to have passed through the choroid plexus [6]. In the retina, the retinal pigment epithelium is the only source of transthyretin, which transports T4 across the blood–retina barrier [7]. Molecular characterization of TH transport was first carried out in the retina, whose pigment epithelium serves as a convenient structural and functional model of the choroid plexus epithelium. oatp2 and oatp3 as TH transporters
oatp1, a recently identified member of a polypeptide family, is a Na+-independent organic aniontransporting polypeptide, and transports organic ions, bile acids and bromosulfophthalein, in addition to conjugated and unconjugated steroid hormones from the bloodstream into the hepatocytes [8]. Physiological studies have suggested the presence of other members of this family in the liver. Abe et al. [9] isolated oatp2 and oatp3 from rat retina and identified them as TH transporters. Hydrophobicity analyses of oatp2 and oatp3 predict 12 hydrophobic segments, which is characteristic of various transporter families. When the Xenopus oocyte expression system was used, it became clear that both oatp2 and oatp3 transport T4 and T3. The Km values for T4 and T3 in oocyteexpressed oatp2 were 6.53 µM and 5.87 µM, respectively. oatp3 also transports TH with a Km of 4.93 µM for T4 and 7.33 µM for T3. Such oatp2- and oatp3-mediated TH uptake is not dependent on extracellular Na+. In addition, a recent immunohistochemical analysis showed that oatp2 is localized mainly in the retinal pigment epithelium, whereas oatp3 is located in optic nerve fibres, suggesting that they might play successive roles in TH transport in the retina and neural cells [10]. Subsequently, rat oatp1 [11] and oatp4 [12,13] were also reported to transport TH in the liver in a similarly Na+-independent manner. Because oatp1 and oatp2 are localized in the apical and basolateral membranes of the choroid plexus epithelium, respectively [14,15], this spatial localization suggests a serial role for these oatps in the transport of TH from blood to CSF and the brain.
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a
Table 1. Organic anion transporters Name
Gene symbol
b
Expression
Localization
TH transport
Refs
Brain Widely Liver only Liver only Widely Widely Brain Widely
Basolateral Basolateral Basolateral Basolateral
Yes NR Yes Yes NR Yes NR NR
[55] [56] [21,56–58] [22,23] [56] [24,56] [59] [60]
Liver and kidney Retina, liver and brain Retina, liver, brain and kidney Liver only
Basolateral Basolateral Basolateral? Basolateral
Yes Yes Yes Yes ? Yes NR NR NR NR Yes
[8] [9,16] [9] [12,13]
Human organic anion transporter family OATP, OATP-A mOATP, OATP-B LST-1, OATP-C, OATP-2 LST-2, OATP8 OATP-D, PGT-2 OATP-E OATP-F PGT
SLC21A3 SLC21A9 SLC21A6 SLC21A8 SLC21A11 SLC21A12 SLC21A14 SLC21A2
Rat organic anion transporter family oatp1 oatp2 oatp3 rlst-1, oatp4 oatp5 oatp-E moat1 PGT PGT2 OAT-K1/K2 TST-1, TST-2
Slc21a1 Slc21a5 Slc21a7 Slc21a10 Slc21a13 Slc21a13
Slc21a4
Widely Widely Widely Widely Kidney Testis
Basolateral Apical
[24] [17] [20] [18,19] UP
Other kinds of transporter ntcp ASBT LAT1–4F2hc a
Slc10a1 Slc10a12
Liver only Ileum and kidney Liver, brain and kidney
Basolateral Apical Basolateral?
Yes NR Yes
[28] [30] [36]
Abbreviations: ASBT, apical Na -dependent bile acid transporter ; LAT; L-amino acid transporter; LST, liver specific organic anion transporter; moat1, multi-specific organic anion transporter-1; NR, not reported; ntcp, Na /taurocholate cotransporting polypeptide; oatp, organic anion-transporting polypeptide; PGT, prostaglandin transporter; rlst, rat liver-specific organic anion transporter; TH, thyroid b hormone; UP, T. Suzuki, unpublished. Widely includes brain, lung, liver and kidney.
Classification of the organic anion transporter family
The oatps comprise a large family of Na+-independent transporters (Table 1; Fig. 1), the members of which have been identified in rats: oatp1–5 [8,9,13,16], rlst-1 (a splicing variant of oatp4) [12], moat1 [17], OAT-K1 [18] and its splice variant OAT-K2 [19], and rat testis-specific TST-1 and TST-2 (T. Suzuki, unpublished). Because rat prostaglandin transporter (PGT) [20] has homology with the organic anion transporter family at the amino acid level (albeit a moderate one of ~30%), it belongs to the same polypeptide family. Of the known rat organic anion transporters, oatp1 [11], oatp2 [9], oatp3 [9] and oatp4 [13] have been reported to transport TH. The tissue distribution of the rat oatp family is rather broad, except for the testis-specific TST series (Table 1). In humans, the constituents and nomenclature of the oatp family are more complicated. The organic anion and prostaglandin transporters are classified within the family of solute carriers (SLC) (Table 1), designated by the Human Gene Nomenclature Committee Database (http://www.gene.ucl.ac.uk/nomenclature/). The phylogenetic tree for organic anion transporters is shown in Fig. 1. The important point is that human clones do not correspond exactly to rat clones. Although OATP2 exists in humans and oatp2 is also present in rat, human OATP2 is found only in the liver, whereas rat oatp2 is found in the brain, liver http://tem.trends.com
and retina. The pharmacological characteristics of oatps vary considerably among species [21]. There is no human OATP3 corresponding to rat oatp3. In addition, although oatp1, oatp2, oatp3, oatp5, OAT-K1 and OAT-K2 are located close to one another in the tree, only human OATP is located apart from the cluster. By contrast, the structures of some oatps are highly conserved between rats and humans: the overall homology between rat oatp-E and human OATP-E is 72.6%, and the amino acid sequence homology between rat PGT2 and OATP-D is 97.6% (T. Abe, unpublished). Several oatps are involved in TH transport in humans
Compared with rat oatps, the expression patterns of the genes encoding human organic anion transporters are tissue specific. Human liver-specific transporters LST-1 and LST-2, which transport TH [21–23], are found exclusively in the liver, and human OATP, which transports T3 (Km = 6.5 µM) and T4 (Km = 8.0 µM), is found exclusively in the brain [21]. Other molecules are also involved in the transport of TH in other tissues; Fujiwara et al. [24] isolated and characterized a novel human oatp, OATP-E. The isolated cDNA encodes a polypeptide of 722 amino acids with 12 transmembrane domains. By homology and analysis of the phylogenetic tree, OATP-E is shown to be a subfamily of oatps. Human OATP-E transports T3 and T4 in a Na+-independent manner, and the
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rlst-1/oatp4 OATP
(a)
oatp5 oatp2 oatp1
LST-2
NH2
oatp3 OAT-K1 OAT-K2
LST-1/OATP-C/OATP2
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1 2 3 4 5 6 7
rat TST-2
COOH PGT
(b)
rat PGT 1 2 3 4 5 6 7 8 9 101112
rat TST-1
COOH
NH2 MOAT-1/OATP-B oatp-E
0.1
moat1 OATP-E
COOH
(c) C
rat PGT2 PGT2/OATP-D C C TRENDS in Endocrinology & Metabolism
Fig. 1. The phylogenetic relationships between the oatp/LST family and PGT. The phylogenetic tree was constructed with the use of CLUSTAL W [54] and TreeView (http://taxonomy.zoology.gla.ac.uk/ rod/treeview.html) using ungapped regions and distance correction [55–60]. Branch lengths are drawn to scale. Rat clone is depicted in red. Abbreviations: LST, liver-specific transporter; MOAT, multispecific organic anion transporter; oatp, organic anion transporting polypeptide; PGT, prostaglandin transporter; rlst, rat liver-specific transporter; TST, testis-specific transporter.
Km value for T3 is 0.9 µM, which is the lowest value among the TH transporters. Abundant amounts of OATP-E are found in various peripheral tissues, including the small intestine. Bile acids, secreted by the liver, enter the intestine, are absorbed in large part by the ileum and are returned to the liver by way of the portal vein. The enterohepatic circulation of TH has already been well established in the rat [25,26]. Because OATP-E, LST-1 and LST-2 transport both TH and taurocholate, it seems that in humans, OATP-E, LST-1 and LST-2 might be involved in the enterohepatic circulation of both bile acids and TH. Walters et al. [27] reported that an antibody against rat oatp3 located it to the apical brush-border membrane of rat jejunal enterocytes, suggesting a role in the uptake of bile acid, steroid hormones and TH. So far, no human counterpart to rat oatp3 has been found. Na+-dependent TH transport system: NTCP and ASBT
In studies of the oatp family, rat Na+/taurocholate cotransporting polypeptide (ntcp) was reported to transport TH [11]. ntcp consists of ~350 amino acids, with seven putative transmembrane domains [28,29] (Fig. 2). ntcp is found only in the basolateral cell membrane of hepatocytes and is the major mechanism for transporting conjugated bile acids, especially taurocholate, into the liver (other bile acids are mainly transported by oatps). In addition, it was shown that Xenopus oocytes injected with cRNA encoding ntcp transported T4, T3, reverse T3 and T2. http://tem.trends.com
1 2 hc 3 4 5 6 7 8 9 101112
NH2
NH2
COOH
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Fig. 2. Membrane topology models of (a) the ntcp family, (b) the oatp family and (c) the LAT1–4F2hc heteromeric amino acid transporter complex. LAT1 and 4F2hc are bound with a disulphide bond (red) between putative cysteine residues. Abbreviations: hc, single transmembrane domain of heavy chain 4F2; ntcp, Na+-dependent taurocholate carrier protein; oatp, organic anion transporter polypeptide.
These results suggest that the Na+-dependent fraction of the hepatic uptake of TH is mediated, at least in part, by ntcp. The ileal apical Na+-dependent bile acid transporter (ASBT), which has a moderate structural homology with ntcp (an amino acid identity of ~35%), is synthesized in the kidney as well, where it is also localized to the apical cell membrane [30]. In the small intestine, the first step in the enterohepatic bile acid circulation is mediated by a Na+-dependent transport system located in the apical brush-border membrane of the ileum [24,25]. Although the involvement of ASBT in the apical absorption of bile acid has been postulated, the transport of TH by ASBT remains to be documented. Amino acid transporter is a new member of the TH transporter family
Amino acids, especially essential amino acids, are required for protein synthesis and as energy sources in all living cells. Because most amino acids are hydrophilic, they require special membrane transport
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systems to penetrate the cell membrane. Recently, many amino acid transporters, each of which has 12 transmembrane regions, have been characterized [31]. The cell-surface glycoprotein 4F2 heavy chain (4F2hc; CD98 in mouse) has also been identified as part of an amino acid transporter. 4F2hc has a single transmembrane domain and a 120-kDa disulfide-linked protein formed by a heteromeric glycosylated 40-kDa light chain, together with an 80-kDa heavy chain [32]. The synthesis of 4F2hc in Xenopus oocytes induces low levels of cysteine, dibasic and neutral amino acid transport [33]. The complete molecular structure of the 4F2hc-mediated amino acid transport mechanism has recently been clarified (i.e. the heterodimeric complex nature of the amino acid transporter, and the fact that 4F2hc functions as a complete amino acid transporter [34]). Furthermore, Ritchie et al. [35] reported the stimulation of T3 transport in Xenopus oocytes injected with cRNAs for 4F2hc and the IU12 Xenopus amino acid transporter LAT1 homolog. These results demonstrated that TH uptake is, in part, mediated by amino acid transporter(s). Friesema et al. [36] found that co-injection of cRNAs for 4F2hc and human LAT1 light chain not only stimulated the transport of amino acids (Phe, Tyr, Leu and Trp), which is characteristic of a system L amino acid transporter (a Na+-independent exchange of large, neutral amino acids), but also of TH, in a Na+-independent manner. The Km values for T4, T3, reverse T3 and T2 were 7.9 µM, 0.8 µM, 12.5 µM and 7.9 µM, respectively. Blondeau et al. [37] showed that aromatic and neutral amino acids are transported into cultured astrocytes via the Na+-independent system L, which is related to the TH transport system. The contribution of the amino acid transporter LAT1–4F2hc complex to TH transport was confirmed by these data. However, other types of amino acid transporters are also assumed to be involved in the uptake of TH. These have yet to be characterized and isolated [38]. Thyroid hormone efflux system
In addition to the uptake of TH into cells, the export of TH from the cell is an important factor in the regulation of the cellular TH content and binding to its nuclear receptor. Few studies have been published on the transport of TH from the cell. Ribeiro et al. [39] reported the existence of a saturable, temperaturesensitive efflux mechanism of TH, which was inhibited by verapamil. Verapamil is a potent inhibitor of the ATP-binding cassette (ABC) transporter superfamily [multidrug resistance (MDR) and multidrug resistance-related protein (mrp)], suggesting the involvement of the ABC transporter(s) in the efflux of TH. In vitro, the rat thyroid cell line FRTL-5, fibroblast NIH-3T3 and rat hepatocytes show a verapamil-sensitive, saturable and stereospecific T3-efflux mechanism [40]. However, neither FRTL-5 nor NIH-3T3 cells synthesized the multidrug resistance protein mdr1b, and the overexpression of http://tem.trends.com
MDR1 (official symbol ABCB7) in mammalian cells did not enhance the verapamil-sensitive T3 efflux. Thus, further experiments are needed to clarify this verapamil-sensitive TH-efflux mechanism. Differences of Km values and Na+ dependency
Several questions arise from the recent work on the identification of TH transporters, the most important being why there are different Km values between cDNA expression systems and in vitro culture systems. The in vitro studies identified two saturable sites for thyroid hormone binding; one is characterized by high affinity and low capacity, and the other by low affinity and high capacity [4,41]. The high-affinity uptake of T3 is mostly in the nM range and is often energy- and extracellular Na+-dependent. However, in the low-affinity uptake process, the Km for T4 and T3 of the fraction is in the µM range, which is similar to that of oatps and amino acid transporters synthesized in Xenopus oocytes [36], hepatocytes [41] and mammalian cells [42]. Little is known about the molecular entity responsible for the high-affinity system, although the differences in the types of assays or experimental procedures used are possible explanations for the discrepant results. Another controversy arises from the fact that the oatp family- and amino acid transporter-mediated uptake of TH is not dependent on extracellular Na+ in vitro, whereas the transporting mechanisms of TH in tissues are heterogeneous in their Na+ dependency: Na+ dependent [41], Na+ independent [42,43], and mixed [44,45]. Although the Na+-independent fraction of TH uptake into tissues could be partly attributed to the oatp family and 4F2hc–L-type amino acid transporter complex, the molecular entities responsible for the Na+-dependent system in various tissues are still unclear, because the Na+-independent TH transporters, ntcp and ASBT, are not synthesized widely. It was reported [46] that Xenopus oocytes injected with rat liver poly(A)+ RNA showed Na+-dependent uptake of T4 and T3. However, the size of the fractionated poly(A)+ RNA that has Na+-dependent TH transport activity is different from that of the known mRNA size of ntcp. In addition, Wang et al. [47] recently identified a new seven transmembrane membrane type Na+-independent bile acid transporter from the skate, Raja erinacea, and also suggested the existence of a mammalian homolog. Therefore, there are several other candidate molecules that might be responsible for transporting TH. Transport direction
The transport mechanism of the oatp series is still unresolved, although it is known that the oatp and LST family mediates the Na+-independent transport of organic anions, including TH. In early studies [48], oatp-mediated taurocholate transport was reported to use HCO3− as a driving force. Although a role for HCO3− has been proposed, another study reported that glutathione is a driving force for oatp1 [49], and
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that oatp2 mediates the bidirectional transport of organic anions [50]. At the cell–colloid interface in the thyroid gland, colloid is engulfed into colloid vesicles pinocytosed into thyroid cells. After the hydrolysis of thyroglobulin by intracellular lysosomes, the release of T4 and T3 occurs. However, the mechanism of TH release is still unclear. The finding of an outward TH transporter should provide an important piece of the puzzle.
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have also been postulated [52]. One possible mechanism is reduced hormone availability to tissues because of impaired TH entry into target cells. However, no structure–function analyses of the TH transporter have been carried out in such patients. Among members of the oatp family, structure–function analysis of PGT only has been performed [53,54]. Thus, the finding of a TH transporter might help identify the genetic basis of a variant of TH unresponsiveness.
TH transport-related diseases
Acknowledgements We thank Kazuo Nunoki for discussions.
Recently, it was reported that liver-specific organic anion transporter LST-2 is present in abundance in gastrointestinal cancers [22]. Because TH is necessary for the expansion or proliferation of cancer cells, the presence of LST-2 in these cells might imply that it plays a functional role. Certain individuals exhibit a syndrome of resistance to TH in many tissues: these patients have reduced activity of TH relative to the circulating hormone level [51]. One well-known molecular basis is an abnormality of the nuclear TH receptor. However, other functional defects in such patients
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Conclusion and future direction of research
Recent molecular cloning has revealed a greater potential for elucidating TH transporter diversity than was expected from the physiology and pharmacology of TH transport. It seems reasonable to suggest that similar experimental approaches will reveal molecular identities of other TH transporter(s). Much less is known about the contribution of such transporters in vivo; however, important insights have been provided through molecular genetics to allow TH transporter function to be manipulated in vitro and in vivo.
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43 Blondeau, J.P. et al. (1988) Characterization of the thyroid hormone transport system of isolated hepatocytes. J. Biol. Chem. 263, 2685–2692 44 Beslin, A. et al. (1995) Relationship between the thyroid hormone transport system and the Na+–H+ exchanger in cultured rat brain astrocytes. Endocrinology 136, 5385–5390 45 Centanni, M. and Robbins, J. (1987) Role of Na+ in thyroid hormone uptake by rat skeletal muscle. J. Clin. Invest. 80, 1068–1072 46 Docter, R. et al. (1997) Expression of rat liver cell membrane transporters for thyroid hormone in Xenopus laevis oocytes. Endocrinology 138, 1841–1846 47 Wang, W. et al. (2001) Expression cloning of two genes that together mediate organic solute and steroid transport in the liver of a marine vertebrate. Proc. Natl. Acad. Sci. U. S. A. 98, 9431–9436 48 Satlin, L.M. et al. (1997) Organic anion transporting polypeptide mediates organic anion/HCO3− exchange. J. Biol. Chem. 272, 26340–26345 49 Li, L. et al. (1998) Identification of glutathione as a driving force and leukotriene C4 as a substrate for Oatp1, the hepatic sinusoidal organic solute transporter. J. Biol. Chem. 273, 16184–16191 50 Li, L. et al. (2000) Oatp2 mediates bidirectional organic solute transport: a role for intracellular glutathione. Mol. Pharmacol. 58, 335–340 51 Refetoff, S. et al. (1993) The syndromes of resistance to thyroid hormone. Endocr. Rev. 14, 348–399 52 Schuster, V.L. et al. (2000) Synthetic modification of prostaglandin F2a indicates different structural determinants for binding to the prostaglandin F receptor versus the
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prostaglandin transporter. Mol. Pharmacol. 58, 1511–1516 Chan, B.S. et al. (1999) Mapping the substrate binding site of the prostaglandin transporter PGT. J. Biol. Chem. 274, 25564–25570 Thompson, J.D. et al. (1994) CLUSTAL W: improving the sensitivity of progressive multiple sequence alignment through sequence weighting, position-specific gap penalties and weight matrix choice. Nucleic Acids Res. 22, 4673–4680 Kullak-Ublick, G-A. et al. (1995) Molecular and functional characterization of an organic anion transporting polypeptide cloned from human liver. Gastroenterology 109, 1274–1282 Tamai, I. et al. (2000) Molecular identification and characterization of novel members of the human organic anion transporter (OATP) family. Biochem. Biophys. Res. Commun. 273, 251–260 König, J. et al. (2000) A novel human organic anion transporting polypeptide localized to the basolateral hepatocyte membrane. Am. J. Physiol. Gastrointest. Liver Physiol. 278, G156–G164 Hsiang, B. et al. (1999) A novel human hepatic organic anion transporting polypeptide (OATP2). Identification of a liver-specific human organic anion transporting polypeptide and identification of rat and human hydroxymethylglutaryl-CoA reductase inhibitor transporters. J. Biol. Chem. 274, 37161–37168 Pizzagalli, F. et al. (2000) Identification of a new human organic anion transporting polypeptide OATP-F. GenBank Accession #AF260704 Lu, R. et al. (1996) Cloning, in vitro expression, and tissue distribution of a human prostaglandin transporter cDNA (hPGT). J. Clin. Invest. 98, 1142–1149
To ERR in the estrogen pathway Vincent Giguère Estrogens control a variety of physiological and disease-linked processes, most notably reproduction, bone remodeling and breast cancer, and their effects are transduced through classic nuclear receptors referred to as estrogen α (ERα α) and ERβ β. Recent results obtained using the estrogen-related receptor-α α, -β β and -γγ), a subfamily of orphan nuclear receptors closely receptors (ERRα related to the ERs, have shown that the ERRs share target genes, coregulatory proteins, ligands and sites of action with the ERs. In addition, the ERRs can actively influence the estrogenic response, suggesting that pharmacological modulation of ERR activity will be clinically useful to prevent and/or treat a variety of conditions related to women’s health. Published online: 30 April 2002
The steroid hormone estradiol is essential for proper development and maintenance of the female reproductive system, in addition to sperm production in the male. However, the roles of estradiol are not limited to the reproductive system in either sex because the hormone has important physiological http://tem.trends.com
functions in the cardiovascular, immune and central nervous systems and in bone. In addition, estradiol action (or lack thereof) has been implicated in numerous diseases, including breast and uterine cancers, osteoporosis, coronary heart disease and loss of cognitive functions. The pleiotropic effects of estradiol are transduced by two receptors known as estrogen receptor-α (ERα; NR3A1) and ERβ (NR3A2), which are members of the superfamily of nuclear receptors. The proteins in this group generally function as ligand-inducible transcription factors, and include receptors for classic high-affinity ligands, such as steroid hormones, vitamin D, retinoids and thyroid hormones. Also included in this family were putative receptors for which no known ligands were known at the time of their discovery, and these proteins were referred to as orphan nuclear receptors [1]. Study of orphan receptors led to the realization that low-affinity ligands, such as long-chain fatty acids,
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Thyroid hormone transporters: recent advances Takaaki Abe, Takehiro Suzuki, Michiaki Unno, Taro Tokui and Sadayoshi Ito Thyroid hormones, being hydrophobic, were thought to enter target cell membranes by passive diffusion. However, recent studies have documented the existence of numerous organic anion transport systems, about half of which also transport thyroid hormones into (and possibly out of) a variety of target cells. Several of the genes encoding thyroid hormone transporters have been characterized by means of molecular approaches. Here, we discuss the classification of thyroid hormone transporters, with emphasis on how they are influenced by their ionic milieu and what their symported organic anions are. Published online: 10 May 2002
Thyroid hormone (TH) plays an essential role in the mammalian central nervous system and in many peripheral tissues. Hypothyroidism causes serious damage to neural cells and leads to mental retardation [1]. The action of thyroid hormone is mediated mainly through the deiodination of thyroxine (T4) to 3,3′,5-triiodo-L-thyronine (T3), followed by the binding of T3 to a specific nuclear receptor. Before reaching its intracellular targets, TH must cross the plasma membrane. Because of the lipophilic nature of TH, it was thought that it traversed the plasma membrane by simple diffusion. However, in the past decade, a membrane transport system for TH has been postulated to exist in various tissues. Takaaki Abe* Takehiro Suzuki Sadayoshi Ito Division of Nephrology, Endocrinology and Vascular Medicine, Dept Medicine, Tohoku University Graduate School of Medicine and PRESTO, Japan Science and Technology Corporation (JST), Japan. *e-mail: takaabe@ mail.cc.tohoku.ac.jp Michiaki Unno Division of Gastroenterological Surgery, Dept Surgery, Tohoku University Graduate School of Medicine, 1-1 Seriyo-cho, Aoba-ku, Sendai 980-8574, Japan. Taro Tokui Drug Metabolism and Pharmacokinetics Research Laboratories, Sankyo Co. Ltd, Tokyo, Japan.
Membrane binding and transport studies
A series of recent studies identified a membrane TH-binding protein [2]. Kato et al. [3] showed that this protein was identical to the protein disulfide isomerase (PDI). Because most PDI is located in the endoplasmic reticular lumen, and has low affinity for TH, it is not believed to be involved in the plasma membrane transport of TH. Moreover, a membrane transport system for TH has been well characterized in liver, brain cells, leukocytes and other tissues (reviewed in Ref. [4]). According to these data, TH transport in these tissues might be mediated by a saturable, stereospecific, energy-dependent transporter, although little is known about its molecular nature. Brain and retinal TH transport
The brain is separated from the bloodstream by the blood–brain and the blood–cerebrospinal fluid (CSF) barriers, which restrict the entry of molecules into brain tissue. It has been suggested that TH enters the brain via the blood–brain barrier [1] or the choroid plexus [5]. The choroid plexus is a major http://tem.trends.com
component of the blood–CSF barrier, in addition to being the major production site of the TH-binding protein, transthyretin, which transports TH into the brain. Free T4 can also cross the choroid plexus intact from blood to the brain [5] and up to one-fifth of the T4 in the brain has been shown to have passed through the choroid plexus [6]. In the retina, the retinal pigment epithelium is the only source of transthyretin, which transports T4 across the blood–retina barrier [7]. Molecular characterization of TH transport was first carried out in the retina, whose pigment epithelium serves as a convenient structural and functional model of the choroid plexus epithelium. oatp2 and oatp3 as TH transporters
oatp1, a recently identified member of a polypeptide family, is a Na+-independent organic aniontransporting polypeptide, and transports organic ions, bile acids and bromosulfophthalein, in addition to conjugated and unconjugated steroid hormones from the bloodstream into the hepatocytes [8]. Physiological studies have suggested the presence of other members of this family in the liver. Abe et al. [9] isolated oatp2 and oatp3 from rat retina and identified them as TH transporters. Hydrophobicity analyses of oatp2 and oatp3 predict 12 hydrophobic segments, which is characteristic of various transporter families. When the Xenopus oocyte expression system was used, it became clear that both oatp2 and oatp3 transport T4 and T3. The Km values for T4 and T3 in oocyteexpressed oatp2 were 6.53 µM and 5.87 µM, respectively. oatp3 also transports TH with a Km of 4.93 µM for T4 and 7.33 µM for T3. Such oatp2- and oatp3-mediated TH uptake is not dependent on extracellular Na+. In addition, a recent immunohistochemical analysis showed that oatp2 is localized mainly in the retinal pigment epithelium, whereas oatp3 is located in optic nerve fibres, suggesting that they might play successive roles in TH transport in the retina and neural cells [10]. Subsequently, rat oatp1 [11] and oatp4 [12,13] were also reported to transport TH in the liver in a similarly Na+-independent manner. Because oatp1 and oatp2 are localized in the apical and basolateral membranes of the choroid plexus epithelium, respectively [14,15], this spatial localization suggests a serial role for these oatps in the transport of TH from blood to CSF and the brain.
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a
Table 1. Organic anion transporters Name
Gene symbol
b
Expression
Localization
TH transport
Refs
Brain Widely Liver only Liver only Widely Widely Brain Widely
Basolateral Basolateral Basolateral Basolateral
Yes NR Yes Yes NR Yes NR NR
[55] [56] [21,56–58] [22,23] [56] [24,56] [59] [60]
Liver and kidney Retina, liver and brain Retina, liver, brain and kidney Liver only
Basolateral Basolateral Basolateral? Basolateral
Yes Yes Yes Yes ? Yes NR NR NR NR Yes
[8] [9,16] [9] [12,13]
Human organic anion transporter family OATP, OATP-A mOATP, OATP-B LST-1, OATP-C, OATP-2 LST-2, OATP8 OATP-D, PGT-2 OATP-E OATP-F PGT
SLC21A3 SLC21A9 SLC21A6 SLC21A8 SLC21A11 SLC21A12 SLC21A14 SLC21A2
Rat organic anion transporter family oatp1 oatp2 oatp3 rlst-1, oatp4 oatp5 oatp-E moat1 PGT PGT2 OAT-K1/K2 TST-1, TST-2
Slc21a1 Slc21a5 Slc21a7 Slc21a10 Slc21a13 Slc21a13
Slc21a4
Widely Widely Widely Widely Kidney Testis
Basolateral Apical
[24] [17] [20] [18,19] UP
Other kinds of transporter ntcp ASBT LAT1–4F2hc a
Slc10a1 Slc10a12
Liver only Ileum and kidney Liver, brain and kidney
Basolateral Apical Basolateral?
Yes NR Yes
[28] [30] [36]
Abbreviations: ASBT, apical Na -dependent bile acid transporter ; LAT; L-amino acid transporter; LST, liver specific organic anion transporter; moat1, multi-specific organic anion transporter-1; NR, not reported; ntcp, Na /taurocholate cotransporting polypeptide; oatp, organic anion-transporting polypeptide; PGT, prostaglandin transporter; rlst, rat liver-specific organic anion transporter; TH, thyroid b hormone; UP, T. Suzuki, unpublished. Widely includes brain, lung, liver and kidney.
Classification of the organic anion transporter family
The oatps comprise a large family of Na+-independent transporters (Table 1; Fig. 1), the members of which have been identified in rats: oatp1–5 [8,9,13,16], rlst-1 (a splicing variant of oatp4) [12], moat1 [17], OAT-K1 [18] and its splice variant OAT-K2 [19], and rat testis-specific TST-1 and TST-2 (T. Suzuki, unpublished). Because rat prostaglandin transporter (PGT) [20] has homology with the organic anion transporter family at the amino acid level (albeit a moderate one of ~30%), it belongs to the same polypeptide family. Of the known rat organic anion transporters, oatp1 [11], oatp2 [9], oatp3 [9] and oatp4 [13] have been reported to transport TH. The tissue distribution of the rat oatp family is rather broad, except for the testis-specific TST series (Table 1). In humans, the constituents and nomenclature of the oatp family are more complicated. The organic anion and prostaglandin transporters are classified within the family of solute carriers (SLC) (Table 1), designated by the Human Gene Nomenclature Committee Database (http://www.gene.ucl.ac.uk/nomenclature/). The phylogenetic tree for organic anion transporters is shown in Fig. 1. The important point is that human clones do not correspond exactly to rat clones. Although OATP2 exists in humans and oatp2 is also present in rat, human OATP2 is found only in the liver, whereas rat oatp2 is found in the brain, liver http://tem.trends.com
and retina. The pharmacological characteristics of oatps vary considerably among species [21]. There is no human OATP3 corresponding to rat oatp3. In addition, although oatp1, oatp2, oatp3, oatp5, OAT-K1 and OAT-K2 are located close to one another in the tree, only human OATP is located apart from the cluster. By contrast, the structures of some oatps are highly conserved between rats and humans: the overall homology between rat oatp-E and human OATP-E is 72.6%, and the amino acid sequence homology between rat PGT2 and OATP-D is 97.6% (T. Abe, unpublished). Several oatps are involved in TH transport in humans
Compared with rat oatps, the expression patterns of the genes encoding human organic anion transporters are tissue specific. Human liver-specific transporters LST-1 and LST-2, which transport TH [21–23], are found exclusively in the liver, and human OATP, which transports T3 (Km = 6.5 µM) and T4 (Km = 8.0 µM), is found exclusively in the brain [21]. Other molecules are also involved in the transport of TH in other tissues; Fujiwara et al. [24] isolated and characterized a novel human oatp, OATP-E. The isolated cDNA encodes a polypeptide of 722 amino acids with 12 transmembrane domains. By homology and analysis of the phylogenetic tree, OATP-E is shown to be a subfamily of oatps. Human OATP-E transports T3 and T4 in a Na+-independent manner, and the
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rlst-1/oatp4 OATP
(a)
oatp5 oatp2 oatp1
LST-2
NH2
oatp3 OAT-K1 OAT-K2
LST-1/OATP-C/OATP2
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1 2 3 4 5 6 7
rat TST-2
COOH PGT
(b)
rat PGT 1 2 3 4 5 6 7 8 9 101112
rat TST-1
COOH
NH2 MOAT-1/OATP-B oatp-E
0.1
moat1 OATP-E
COOH
(c) C
rat PGT2 PGT2/OATP-D C C TRENDS in Endocrinology & Metabolism
Fig. 1. The phylogenetic relationships between the oatp/LST family and PGT. The phylogenetic tree was constructed with the use of CLUSTAL W [54] and TreeView (http://taxonomy.zoology.gla.ac.uk/ rod/treeview.html) using ungapped regions and distance correction [55–60]. Branch lengths are drawn to scale. Rat clone is depicted in red. Abbreviations: LST, liver-specific transporter; MOAT, multispecific organic anion transporter; oatp, organic anion transporting polypeptide; PGT, prostaglandin transporter; rlst, rat liver-specific transporter; TST, testis-specific transporter.
Km value for T3 is 0.9 µM, which is the lowest value among the TH transporters. Abundant amounts of OATP-E are found in various peripheral tissues, including the small intestine. Bile acids, secreted by the liver, enter the intestine, are absorbed in large part by the ileum and are returned to the liver by way of the portal vein. The enterohepatic circulation of TH has already been well established in the rat [25,26]. Because OATP-E, LST-1 and LST-2 transport both TH and taurocholate, it seems that in humans, OATP-E, LST-1 and LST-2 might be involved in the enterohepatic circulation of both bile acids and TH. Walters et al. [27] reported that an antibody against rat oatp3 located it to the apical brush-border membrane of rat jejunal enterocytes, suggesting a role in the uptake of bile acid, steroid hormones and TH. So far, no human counterpart to rat oatp3 has been found. Na+-dependent TH transport system: NTCP and ASBT
In studies of the oatp family, rat Na+/taurocholate cotransporting polypeptide (ntcp) was reported to transport TH [11]. ntcp consists of ~350 amino acids, with seven putative transmembrane domains [28,29] (Fig. 2). ntcp is found only in the basolateral cell membrane of hepatocytes and is the major mechanism for transporting conjugated bile acids, especially taurocholate, into the liver (other bile acids are mainly transported by oatps). In addition, it was shown that Xenopus oocytes injected with cRNA encoding ntcp transported T4, T3, reverse T3 and T2. http://tem.trends.com
1 2 hc 3 4 5 6 7 8 9 101112
NH2
NH2
COOH
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Fig. 2. Membrane topology models of (a) the ntcp family, (b) the oatp family and (c) the LAT1–4F2hc heteromeric amino acid transporter complex. LAT1 and 4F2hc are bound with a disulphide bond (red) between putative cysteine residues. Abbreviations: hc, single transmembrane domain of heavy chain 4F2; ntcp, Na+-dependent taurocholate carrier protein; oatp, organic anion transporter polypeptide.
These results suggest that the Na+-dependent fraction of the hepatic uptake of TH is mediated, at least in part, by ntcp. The ileal apical Na+-dependent bile acid transporter (ASBT), which has a moderate structural homology with ntcp (an amino acid identity of ~35%), is synthesized in the kidney as well, where it is also localized to the apical cell membrane [30]. In the small intestine, the first step in the enterohepatic bile acid circulation is mediated by a Na+-dependent transport system located in the apical brush-border membrane of the ileum [24,25]. Although the involvement of ASBT in the apical absorption of bile acid has been postulated, the transport of TH by ASBT remains to be documented. Amino acid transporter is a new member of the TH transporter family
Amino acids, especially essential amino acids, are required for protein synthesis and as energy sources in all living cells. Because most amino acids are hydrophilic, they require special membrane transport
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systems to penetrate the cell membrane. Recently, many amino acid transporters, each of which has 12 transmembrane regions, have been characterized [31]. The cell-surface glycoprotein 4F2 heavy chain (4F2hc; CD98 in mouse) has also been identified as part of an amino acid transporter. 4F2hc has a single transmembrane domain and a 120-kDa disulfide-linked protein formed by a heteromeric glycosylated 40-kDa light chain, together with an 80-kDa heavy chain [32]. The synthesis of 4F2hc in Xenopus oocytes induces low levels of cysteine, dibasic and neutral amino acid transport [33]. The complete molecular structure of the 4F2hc-mediated amino acid transport mechanism has recently been clarified (i.e. the heterodimeric complex nature of the amino acid transporter, and the fact that 4F2hc functions as a complete amino acid transporter [34]). Furthermore, Ritchie et al. [35] reported the stimulation of T3 transport in Xenopus oocytes injected with cRNAs for 4F2hc and the IU12 Xenopus amino acid transporter LAT1 homolog. These results demonstrated that TH uptake is, in part, mediated by amino acid transporter(s). Friesema et al. [36] found that co-injection of cRNAs for 4F2hc and human LAT1 light chain not only stimulated the transport of amino acids (Phe, Tyr, Leu and Trp), which is characteristic of a system L amino acid transporter (a Na+-independent exchange of large, neutral amino acids), but also of TH, in a Na+-independent manner. The Km values for T4, T3, reverse T3 and T2 were 7.9 µM, 0.8 µM, 12.5 µM and 7.9 µM, respectively. Blondeau et al. [37] showed that aromatic and neutral amino acids are transported into cultured astrocytes via the Na+-independent system L, which is related to the TH transport system. The contribution of the amino acid transporter LAT1–4F2hc complex to TH transport was confirmed by these data. However, other types of amino acid transporters are also assumed to be involved in the uptake of TH. These have yet to be characterized and isolated [38]. Thyroid hormone efflux system
In addition to the uptake of TH into cells, the export of TH from the cell is an important factor in the regulation of the cellular TH content and binding to its nuclear receptor. Few studies have been published on the transport of TH from the cell. Ribeiro et al. [39] reported the existence of a saturable, temperaturesensitive efflux mechanism of TH, which was inhibited by verapamil. Verapamil is a potent inhibitor of the ATP-binding cassette (ABC) transporter superfamily [multidrug resistance (MDR) and multidrug resistance-related protein (mrp)], suggesting the involvement of the ABC transporter(s) in the efflux of TH. In vitro, the rat thyroid cell line FRTL-5, fibroblast NIH-3T3 and rat hepatocytes show a verapamil-sensitive, saturable and stereospecific T3-efflux mechanism [40]. However, neither FRTL-5 nor NIH-3T3 cells synthesized the multidrug resistance protein mdr1b, and the overexpression of http://tem.trends.com
MDR1 (official symbol ABCB7) in mammalian cells did not enhance the verapamil-sensitive T3 efflux. Thus, further experiments are needed to clarify this verapamil-sensitive TH-efflux mechanism. Differences of Km values and Na+ dependency
Several questions arise from the recent work on the identification of TH transporters, the most important being why there are different Km values between cDNA expression systems and in vitro culture systems. The in vitro studies identified two saturable sites for thyroid hormone binding; one is characterized by high affinity and low capacity, and the other by low affinity and high capacity [4,41]. The high-affinity uptake of T3 is mostly in the nM range and is often energy- and extracellular Na+-dependent. However, in the low-affinity uptake process, the Km for T4 and T3 of the fraction is in the µM range, which is similar to that of oatps and amino acid transporters synthesized in Xenopus oocytes [36], hepatocytes [41] and mammalian cells [42]. Little is known about the molecular entity responsible for the high-affinity system, although the differences in the types of assays or experimental procedures used are possible explanations for the discrepant results. Another controversy arises from the fact that the oatp family- and amino acid transporter-mediated uptake of TH is not dependent on extracellular Na+ in vitro, whereas the transporting mechanisms of TH in tissues are heterogeneous in their Na+ dependency: Na+ dependent [41], Na+ independent [42,43], and mixed [44,45]. Although the Na+-independent fraction of TH uptake into tissues could be partly attributed to the oatp family and 4F2hc–L-type amino acid transporter complex, the molecular entities responsible for the Na+-dependent system in various tissues are still unclear, because the Na+-independent TH transporters, ntcp and ASBT, are not synthesized widely. It was reported [46] that Xenopus oocytes injected with rat liver poly(A)+ RNA showed Na+-dependent uptake of T4 and T3. However, the size of the fractionated poly(A)+ RNA that has Na+-dependent TH transport activity is different from that of the known mRNA size of ntcp. In addition, Wang et al. [47] recently identified a new seven transmembrane membrane type Na+-independent bile acid transporter from the skate, Raja erinacea, and also suggested the existence of a mammalian homolog. Therefore, there are several other candidate molecules that might be responsible for transporting TH. Transport direction
The transport mechanism of the oatp series is still unresolved, although it is known that the oatp and LST family mediates the Na+-independent transport of organic anions, including TH. In early studies [48], oatp-mediated taurocholate transport was reported to use HCO3− as a driving force. Although a role for HCO3− has been proposed, another study reported that glutathione is a driving force for oatp1 [49], and
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that oatp2 mediates the bidirectional transport of organic anions [50]. At the cell–colloid interface in the thyroid gland, colloid is engulfed into colloid vesicles pinocytosed into thyroid cells. After the hydrolysis of thyroglobulin by intracellular lysosomes, the release of T4 and T3 occurs. However, the mechanism of TH release is still unclear. The finding of an outward TH transporter should provide an important piece of the puzzle.
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have also been postulated [52]. One possible mechanism is reduced hormone availability to tissues because of impaired TH entry into target cells. However, no structure–function analyses of the TH transporter have been carried out in such patients. Among members of the oatp family, structure–function analysis of PGT only has been performed [53,54]. Thus, the finding of a TH transporter might help identify the genetic basis of a variant of TH unresponsiveness.
TH transport-related diseases
Acknowledgements We thank Kazuo Nunoki for discussions.
Recently, it was reported that liver-specific organic anion transporter LST-2 is present in abundance in gastrointestinal cancers [22]. Because TH is necessary for the expansion or proliferation of cancer cells, the presence of LST-2 in these cells might imply that it plays a functional role. Certain individuals exhibit a syndrome of resistance to TH in many tissues: these patients have reduced activity of TH relative to the circulating hormone level [51]. One well-known molecular basis is an abnormality of the nuclear TH receptor. However, other functional defects in such patients
References 1 Pardridge, W.M. (1998) Targeted delivery of hormones to tissues by plasma proteins. In Handbook of Physiology Section 7: The Endocrine System Volume I: Cellular Endocrinology (Conn, M., ed.), pp. 335–382, Oxford University Press 2 Horiuchi, R. et al. (1982) affinity labeling of the plasma membrane 3, 3′,5-triiodo-L-thyronine receptor in GH3 cells. Proc. Natl. Acad. Sci. U. S. A. 79, 5527–5531 3 Kato, H. et al. (1989) High level expression of p55, a thyroid hormone binding protein which is homologous to protein disulfide isomerase in retroviral vector. Biochem. Biophys. Res. Commun. 164, 238–244 4 Hennemann, G. et al. (2001) Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability. Endocr. Rev. 22, 451–476 5 Southwell, B.R. et al. (1993) Thyroxine transport to the brain: role of protein synthesis by the choroid plexus. Endocrinology 133, 2116–2126 6 Chanoie, J-P. et al. (1992) Role of transthyretin in the transport of thyroxine from the blood to the choroid plexus, the cerebrospinal fluid, and the brain. Endocrinology 130, 933–938 7 Cavallaro, T. et al. (1990) The retinal pigment epithelium is the unique site of transthyretin synthesis in the rat eye. Invest. Ophthalmol. Vis. Sci. 31, 497–501 8 Jacquemin, E. et al. (1994) Expression cloning of a rat liver Na+-independent organic anion transporter. Proc. Natl. Acad. Sci. U. S. A. 91, 133–137 9 Abe, T. et al. (1998) Molecular characterization and tissue distribution of a new organic anion transporter subtype (oatp3) that transport thyroid hormones and taurocholate and comparison with oatp2. J. Biol. Chem. 273, 22395–22401 10 Ito, A. et al. (2002) Distribution of organic anion transporting polypeptide2 (oatp2) and oatp3 in rat retina. Invest. Ophthalmol. Vis. Sci. 43, 58–63
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Conclusion and future direction of research
Recent molecular cloning has revealed a greater potential for elucidating TH transporter diversity than was expected from the physiology and pharmacology of TH transport. It seems reasonable to suggest that similar experimental approaches will reveal molecular identities of other TH transporter(s). Much less is known about the contribution of such transporters in vivo; however, important insights have been provided through molecular genetics to allow TH transporter function to be manipulated in vitro and in vivo.
11 Friesema, E.C. et al. (1999) Identification of thyroid hormone transporters. Biochem. Biophys. Res. Commun. 254, 497–501 12 Kakyo, M. et al. (1999) Molecular characterization and functional regulation of a novel rat liverspecific organic anion transporter rlst-1. Gastroenterology 117, 770–775 13 Cattori, V. et al. (2000) Identification of organic anion transporting polypeptide 4 (Oatp4) as a major full-length isoform of the liver-specific transporter-1 (rlst-1) in rat liver. FEBS Lett. 474, 242–245 14 Angeletti, R.H. et al. (1997) The choroid plexus epithelium is the site of the organic anion transport protein in the brain. Proc. Natl. Acad. Sci. U. S. A. 94, 283–286 15 Gao, B. et al. (2000) Organic anion-transporting polypeptides mediate transport of opioid peptides across blood–brain barrier. J. Pharmacol. Exp. Ther. 294, 73–79 16 Noé, B. et al. (1997) Isolation of a multispecific organic anion and cardiac glucoside transporter from rat. Proc. Natl. Acad. Sci. U. S. A. 94, 10346–10350 17 Nishio, T. et al. (2000) Molecular identification of a rat novel organic anion transporter moat1, which transports prostaglandin D(2), leukotriene C(4), and taurocholate. Biochem. Biophys. Res. Commun. 275, 831–838 18 Saito, H. et al. (1996) Cloning and functional characterization of a novel rat organic anion transporter mediating basolateral uptake of methotrexate in the kidney. J. Biol. Chem. 271, 20719–20725 19 Masuda, S. et al. (1999) Cloning and functional characterization of a new multispecific organic anion transporter, OAT-K2, in rat kidney. Mol. Pharmacol. 55, 743–752 20 Kanai, N. et al. (1995) Identification and characterization of a prostaglandin transporter. Science 268, 866–869 21 Abe, T. et al. (1999) Identification of a novel gene family encoding human liver-specific organic anion transporter LST-1. J. Biol. Chem. 274, 17159–17163
22 Abe, T. et al. (2001) LST-2, a human liver-specific organic anion transporter, determines methotrexate sensitivity in gastrointestinal cancers. Gastroenterology 120, 1689–1699 23 König, J. et al. (2000) Localization and genomic organization of a new hepatocellular organic anion transporting polypeptide. J. Biol. Chem. 275, 23161–23168 24 Fujiwara, K. et al. (2001) Identification of thyroid hormone transporters in humans: different molecules are involved in a tissue-specific manner. Endocrinology 142, 2005–2012 25 Albert, A. and Keating, F.R. (1952) The role of the gastrointestinal tract, including the liver, in the metabolism of radiothyroxine. Endocrinology 51, 427–443 26 DiStefano, J.J., III et al. (1992) Sites and patterns of absorption of 3,5,3′-triiodothyronine and thyroxine along rat small and large intestines. Endocrinology 131, 275–280 27 Walters, H.C. et al. (2000) Expression, transport properties, and chromosomal location of organic anion transporter subtype 3. Am. J. Physiol. Gastrointest. Liver Physiol. 279, G1188–G1200 28 Hagenbuch, B. et al. (1991) Functional expression cloning and characterization of the hepatocyte Na+/bile acid cotransport system. Proc. Natl. Acad. Sci. U. S. A. 88, 10629–10633 29 Hagenbuch, B. and Meier, P.J. (1994) Molecular cloning, chromosomal localization, and functional characterization of a human liver Na+/bile acid cotransporter. J. Clin. Invest. 93, 1326–1331 30 Wong, M.H. et al. (1994) Expression cloning and characterization of the hamster ileal Na+-dependent bile acid transporter. J. Biol. Chem. 269, 1340–1347 31 Malandro, M.S. and Kilberg, M.S. (1996) Molecular biology of mammalian amino acid transporters. Annu. Rev. Biochem. 65, 305–336 32 Haynes, B.F. et al. (1981) Characterization of a monoclonal antibody (4F2) that binds to human monocytes and to a subset of activated lymphocytes. J. Immunol. 126, 1409–1414
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To ERR in the estrogen pathway Vincent Giguère Estrogens control a variety of physiological and disease-linked processes, most notably reproduction, bone remodeling and breast cancer, and their effects are transduced through classic nuclear receptors referred to as estrogen α (ERα α) and ERβ β. Recent results obtained using the estrogen-related receptor-α α, -β β and -γγ), a subfamily of orphan nuclear receptors closely receptors (ERRα related to the ERs, have shown that the ERRs share target genes, coregulatory proteins, ligands and sites of action with the ERs. In addition, the ERRs can actively influence the estrogenic response, suggesting that pharmacological modulation of ERR activity will be clinically useful to prevent and/or treat a variety of conditions related to women’s health. Published online: 30 April 2002
The steroid hormone estradiol is essential for proper development and maintenance of the female reproductive system, in addition to sperm production in the male. However, the roles of estradiol are not limited to the reproductive system in either sex because the hormone has important physiological http://tem.trends.com
functions in the cardiovascular, immune and central nervous systems and in bone. In addition, estradiol action (or lack thereof) has been implicated in numerous diseases, including breast and uterine cancers, osteoporosis, coronary heart disease and loss of cognitive functions. The pleiotropic effects of estradiol are transduced by two receptors known as estrogen receptor-α (ERα; NR3A1) and ERβ (NR3A2), which are members of the superfamily of nuclear receptors. The proteins in this group generally function as ligand-inducible transcription factors, and include receptors for classic high-affinity ligands, such as steroid hormones, vitamin D, retinoids and thyroid hormones. Also included in this family were putative receptors for which no known ligands were known at the time of their discovery, and these proteins were referred to as orphan nuclear receptors [1]. Study of orphan receptors led to the realization that low-affinity ligands, such as long-chain fatty acids,
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