Cloning and characterization of the promoter region of the rat taurine transporter (TauT) gene

Targeting Taurine Transporter (TauT) for Cancer Immunotherapy of p53 Mutation Mediated Cancers - Molecular Basis and Preclinical Implication

Taurine transporter (TauT) has been identified as a target gene of p53 tumor suppressor. TauT is also found to be overexpressed in variety type of human cancers, such as leukemia. This study showed that expression of TauT was upregulated by c-Myc and c-Jun oncogenes. To explore whether blocking of TauT inhibits tumor development, the RNA interference (RNAi) and immune targeting approaches were tested in tumor cells in vitro and in p53 mutant mice in vivo. Knockdown of TauT expression by RNAi resulted in cell cycle G2 arrest and suppressed human breast cancer MCF-7 cells proliferation determined by colonies production and cell migration assays. Knockdown of TauT also rendered MCF-7 cells more susceptible to chemotherapeutic drug-induced apoptosis. An antibody specifically against TauT blocked taurine uptake and induced cell cycle G2 arrest leading to cell death of variety type of tumor cells without affecting the viability of normal mammalian cells. TauT peptide vaccination significantly increased median lifespan (1.5-fold) of the p53 null mice and rescued p53+/- mice by extending the median lifespan from 315 days to 621 days. Furthermore, single dose treatment of tumor-bearing (thymic lymphoma) p53 null mice with TauT peptide reduced tumor size by about 50% and significantly prolonged survival of these mice from average 7 days (after observing the thymic lymphoma) to 21 days. This finding demonstrates that a novel TauT peptide vaccine can delay, inhibit, and/or treat p53 mutation related spontaneous tumorigenesis in vivo. Therefore, TauT peptide may be used as a universal cancer vaccine to prevent and/or treat patients with p53 mutation-mediated cancers.

Chondrocytic cells express the taurine transporter on their plasma membrane and regulate its expression under anisotonic conditions

Taurine is a small organic osmolyte which participates in cell volume regulation. Chondrocytes have been shown to accumulate and release taurine; in bone, taurine participates in bone metabolism. However, its role in skeletal cells is poorly understood, especially in chondrocytes. This study investigated the regulation of taurine transporter in chondrocytic cells. We examined the transcriptional regulation of the taurine transporter under anisotonia by reporter gene and real-time RT-PCR assays. The effect of providing supplementary taurine on cell viability was evaluated with the lactate dehydrogenase release assay. The localization of the taurine transporter in human chondrosarcoma cells was studied by overexpressing a taurine transporter-enhanced green fluorescent protein. We observed that the transcription of the taurine transporter gene was up-regulated in hypertonic conditions. Hyperosmolarity-related cell death could be partly abolished by taurine supplementation in the medium. As expected, the fluorescently labeled taurine transporter localized at the plasma membrane. In polarized epithelial MDCK cells, the strongest fluorescence signal was located in the lateral cell membrane area. We also observed that the taurine transporter gene was expressed in several human tissues and malignant cell lines. This is the first study to present information on the transcriptional regulation of taurine transporter gene and the localization of the taurine transporter protein in chondrocytic cells.

Regulation of taurine transport at the blood-placental barrier by calcium ion, PKC activator and oxidative stress conditions

BACKGROUND

In the present study, we investigated the changes of uptake and efflux transport of taurine under various stress conditions using rat conditionally immortalized syncytiotrophoblast cell line (TR-TBT cells), as in vitro blood-placental barrier (BPB) model.

METHODS

The transport of taurine in TR-TBT cells were characterized by cellular uptake study using radiolabeled taurine. The efflux of taurine was measured from the amount of radiolabeled taurine remaining in the cells after the uptake of radiolabeled taurine for 60 min.

RESULTS

Taurine uptake was significantly decreased by phosphorylation of protein kinase C (PKC) activator in TR-TBT cells. Also, calcium ion (Ca2+) was involved in taurine transport in TR-TBT cells. Taurine uptake was inhibited and efflux was enhanced under calcium free conditions in the cells. In addition, oxidative stress induced the change of taurine transport in TR-TBT cells, but the changes were different depending on the types of oxidative stress inducing agents. Tumor necrosis factor-alpha (TNF-alpha), lipopolysaccharide (LPS) and diethyl maleate (DEM) significantly increased taurine uptake, but H2O2 and nitric oxide (NO) donor decreased taurine uptake in the cells. Taurine efflux was down-regulated by TNF-alpha in TR-TBT cells.

CONCLUSION

Taurine transport in TR-TBT cells were regulated diversely at extracellular Ca2+ level, PKC activator and oxidative stress conditions. It suggested that variable stresses affected the taurine supplies from maternal blood to fetus and taurine level of fetus.

Functional TauT protects against acute kidney injury

Nephrotoxicity is common with the use of the chemotherapeutic agent cisplatin, but the cellular mechanisms that modulate the extent of injury are unknown. Cisplatin downregulates expression of the taurine transporter gene (TauT) in LLC-PK1 proximal tubular renal cells, and forced overexpression of TauT protects against cisplatin-induced apoptosis in vitro. Because the S3 segments of proximal tubules are the sites of both cisplatin-induced injury and adaptive regulation of the taurine transporter, we hypothesized that TauT functions as an anti-apoptotic gene and protects renal cells from cisplatin-induced nephrotoxicity in vivo. Here, we studied the regulation of TauT in cisplatin nephrotoxicity in a human embryonic kidney cell line and in LLC-PK1 cells, as well as in TauT transgenic mice. Cisplatin-induced activation of p53 repressed TauT and overexpression of TauT prevented the progression of cisplatin-induced apoptosis and renal dysfunction in TauT transgenic mice. Although cisplatin activated p53 and PUMA (a p53-responsive proapoptotic Bcl-2 family protein) in the kidneys of both wildtype and TauT transgenic mice, only wildtype animals demonstrated acute kidney injury. These data suggest that functional TauT plays a critical role in protecting against cisplatin-induced nephrotoxicity, possibly by attenuating a p53-dependent pathway.

TauT protects against cisplatin-induced acute kidney injury (AKI) established in a TauT transgenic mice model

Cisplatin is a commonly used chemotherapeutic agent that has a major limitation because of its nephrotoxicity. Taurine is an important osmolyte that has been found to protect against cisplatin-induced apoptosis in renal cells in vitro. To determine whether over-expression of hTauT protects against cisplatin-induced acute kidney injury (AKI) in vivo, animals (wt and transgenic) were injected with cisplatin and the levels of BUN and serum creatinine were measured. Saline-injected mice were used as a control. The results showed that the levels of BUN and creatinine were significantly increased in the cisplatin-injected wild-type mice (110 +/- 12 mg/dl and 0.98 +/- 0.05 mg/dl) as compared to the saline-injected wild type mice (20 +/- 2.5 mg/dl and 0.52 +/- 0.06 mg/dl). However, over-expression of hTauT effectively prevented the progression of cisplatin-induced AKI in hTauT transgenic mice, as measured by the levels of BUN and serum creatininelevels (23 +/- 3.5 mg/dl and 0.6 +/- 0.05 mg/dl).

Myogenic differentiation induces taurine transporter in association with taurine-mediated cytoprotection in skeletal muscles

Skeletal muscle homoeostasis is maintained by a variety of cytoprotective mechanisms. Since ablation of the TauT (taurine transporter) gene results in susceptibility to exercise-induced muscle weakness in vivo, it has been suggested that TauT is essential for skeletal muscle function. However, the regulatory mechanisms of TauT expression remain to be elucidated. In the present study, we demonstrated that TauT was up-regulated during myogenesis in C2C12 cells. Treatment with bFGF (basic fibroblast growth factor), which inhibited muscle differentiation, abrogated myogenic induction of TauT. The promoter activities of TauT were up-regulated during muscle differentiation in C2C12 cells. Database analyses identified an MEF2 (myocyte enhancer binding factor 2) consensus sequence at -844 in the rat TauT gene. Truncation of the promoter region containing the MEF2 site significantly reduced the promoter activity, demonstrating the functional importance of the MEF2 site. Electrophoretic mobility-shift assays confirmed that MEF2 bound to the MEF2 consensus sequence and that DNA-protein complex levels were increased during differentiation. Promoter analyses using mutated promoter-reporter plasmids demonstrated that this site was functional. Importantly, transfection with a MyoD expression vector markedly enhanced TauT promoter activity in the (non-myogenic) 10T1/2 cells. Moreover, co-transfection with an MEF2 expression vector augmented MyoD-induced TauT promoter activity, suggesting that MEF2 is required for full activation of TauT expression. Finally, we examined the effects of taurine on myotube atrophy to clarify the biological significance of the up-regulation of TauT, and demonstrated that taurine attenuated muscle atrophy induced by dexamethasone. TauT expression is regulated under the control of the myogenic programme, and we propose that this is the mechanism for taurine-mediated resistance to muscle atrophy.

The taurine transporter: mechanisms of regulation

Taurine transport undergoes an adaptive response to changes in taurine availability. Unlike most amino acids, taurine is not metabolized or incorporated into protein but remains free in the intracellular water. Most amino acids are reabsorbed at rates of 98-99%, but reabsorption of taurine may range from 40% to 99.5%. Factors that influence taurine accumulation include ionic environment, electrochemical charge, and post-translational and transcriptional factors. Among these are protein kinase C (PKC) activation and transactivation or repression by proto-oncogenes such as WT1, c-Jun, c-Myb and p53. Renal adaptive regulation of the taurine transporter (TauT) was studied in vivo and in vitro. Site-directed mutagenesis and the oocyte expression system were used to study post-translational regulation of the TauT by PKC. Reporter genes and Northern and Western blots were used to study transcriptional regulation of the taurine transporter gene (TauT). We demonstrated that (i) the body pool of taurine is controlled through renal adaptive regulation of TauT in response to taurine availability; (ii) ionic environment, electrochemical charge, pH, and developmental ontogeny influence renal taurine accumulation; (iii) the fourth segment of TauT is involved in the gating of taurine across the cell membrane, which is controlled by PKC phosphorylation of serine 322 at the post-translational level; (iv) expression of TauT is repressed by the p53 tumour suppressor gene and is transactivated by proto-oncogenes such as WT1, c-Jun, and c-Myb; and (v) over-expression of TauT protects renal cells from cisplatin-induced nephrotoxicity.

Regulation of the expression and subcellular localization of the taurine transporter TauT in mouse NIH3T3 fibroblasts

The cellular level of the organic osmolyte taurine is a balance between active uptake and passive leak via a volume sensitive pathway. Here, we demonstrate that NIH3T3 mouse fibroblasts express a saturable, high affinity taurine transporter (TauT, Km = 18 microm), and that taurine uptake via TauT is a Na+- and Cl(-)-dependent process with an apparent 2.5 : 1 : 1 Na+/Cl-/taurine stoichiometry. Transport activity is reduced following acute administration of H2O2 or activators of protein kinases A or C. TauT transport activity, expression and nuclear localization are significantly increased upon serum starvation (24 h), exposure to tumour necrosis factor alpha (TNFalpha; 16 h), or hyperosmotic medium (24 h); conditions that are also associated with increased localization of TauT to the cytosolic network of microtubules. Conversely, transport activity, expression and nuclear localization of TauT are reduced in a reversible manner following long-term exposure (24 h) to high extracellular taurine concentration. In contrast to active taurine uptake, swelling-induced taurine release is significantly reduced following preincubation with TNFalpha (16 h) but unaffected by high extracellular taurine concentration (24 h). Thus, in NIH3T3 cells, (a) active taurine uptake reflects TauT expression; (b) TauT activity is modulated by multiple stimuli, both acutely, and at the level of TauT expression; (c) the subcellular localization of TauT is regulated; and (d) volume-sensitive taurine release is not mediated by TauT.

Expression of taurine transporter is regulated through the TonE (tonicity-responsive element)/TonEBP (TonE-binding protein) pathway and contributes to cytoprotection in HepG2 cells

In hypertonic environment, taurine accumulates in cells via activation of TauT (taurine transporter) as an adaptive regulation. Recent studies revealed that TonE (tonicity-responsive element)/TonEBP (TonE-binding protein) pathway regulated the expression of various molecules which protect cells against hypertonic stress. In the present study, we investigated the osmoregulatory mechanisms of TauT expression. TauT was up-regulated at both functional and transcriptional levels in HepG2 under hypertonic condition. The TonE site was identified in the promoter region of TauT gene. Reporter gene assay revealed that promoter activity was increased under hypertonic conditions, whereas deletion or mutation of TonE sequence abolished the induction of the promoter activity in response to hypertonicity. By using the reporter gene plasmids containing a TonE site of TauT promoter (p2xTonE-Luc), it was demonstrated that a TonE site was sufficient for the hypertonicity-mediated activation of TauT promoter. Importantly, co-transfection of TauT promoter gene plasmid with wild-type TonEBP expression vector enhanced promoter activity under isotonic conditions, whereas dominant-negative TonEBP abrogated the TauT promoter activity induced by hypertonicity. Finally, treatment with taurine prevented HepG2 cells from cell death induced by hypertonic medium. These findings suggested that induction of TauT by hypertonicity is mediated by the activation of the TonE/TonEBP pathway and confers resistance to hypertonic stress.

Taurine biosynthetic enzymes and taurine transporter: molecular identification and regulations

Many biological effects of taurine rely upon its cellular concentration, which is primarily controlled by taurine biosynthetic enzymes cysteine dioxygenase (CDO) and cysteine sulfinate decarboxylase (CSD) and taurine transporter (TauT). The cloning of CDO, CSD and TauT in various species provided first-hand information on these proteins, as well as molecular tools to investigate their regulations. CDO upregulation in hepatocytes in response to high sulfur amino acids appears clearly as the most spectacular among the regulations of the biosynthetic enzymes. Downregulation of TauT activity by activation of PKC appears particularly well documented. A unique serine residue could be identified as a phosphorylation site that leads to an inactive form of TauT. The previously revealed downregulation of TauT expression by taurine and hypertonicity-induced upregulation of TauT expression were shown to result from a modified transcription rate of TauT gene, but the precise molecular mechanisms are not yet formally established. Other regulations of taurine transporter expression were more recently reported, which involve glucose, tumor suppressor protein p53, tumor necrosis factor-alpha, and nitric oxide. This review reports the experimental models and data that support these various regulations but also points out the aspects that remain poorly understood or unknown concerning their molecular basis and physiological significance.

Regulation of the cellular content of the organic osmolyte taurine in mammalian cells

Change in the intracellular concentration of osmolytes or the extracellular tonicity results in a rapid transmembrane water flow in mammalian cells until intracellular and extracellular tonicities are equilibrated. Most cells respond to the osmotic cell swelling by activation of volume-sensitive flux pathways for ions and organic osmolytes to restore their original cell volume. Taurine is an important organic osmolyte in mammalian cells, and taurine release via a volume-sensitive taurine efflux pathway is increased and the active taurine uptake via the taurine specific taurine transporter TauT decreased following osmotic cell swelling. The cellular signaling cascades, the second messengers profile, the activation of specific transporters, and the subsequent time course for the readjustment of the cellular content of osmolytes and volume vary from cell type to cell type. Using Ehrlich ascites tumor cells, NIH3T3 mouse fibroblasts and HeLa cells as biological systems, it is revealed that phospholipase A2-mediated mobilization of arachidonic acid from phospholipids and subsequent oxidation of the fatty acid via lipoxygenase systems to potent eicosanoids are essential elements in the signaling cascade that is activated by cell swelling and leads to release of osmolytes. The cellular signaling cascade and the activity of the volume-sensitive taurine efflux pathway are modulated by elements of the cytoskeleton, protein tyrosine kinases/phosphatases, GTP-binding proteins, Ca2+/calmodulin, and reactive oxygen species and nucleotides. Serine/threonine phosphorylation of the active taurine uptake system TauT or a putative regulator, as well as change in the membrane potential, are important elements in the regulation of TauT activity. A model describing the cellular sequence, which is activated by cell swelling and leads to activation of the volume-sensitive efflux pathway, is presented at the end of the review.

Finding of TRE (TPA responsive element) in the sequence of human taurine transporter promoter

Activity of the taurine transporter (TAUT) is regulated by signal transduction in response to diverse stimuli including tumor promoters such as phobol ester. Regulation of the transcription rate of TAUT appears to play an important role in exerting biological roles of taurine in mammalian tissues in adverse environments. Although cDNA of human TAUT has been cloned and sequenced in placenta, thyroid cells, and retinal pigment epithelial cells, the promoter region of TAUT has never been reported. In order to clone the upstream region of the human TAUT promoter, we have compared TAUT cDNA sequences with the entire human genome sequence. Polymerase chain reaction (PCR) was performed from genomic DNA prepared from a SK-Hep-1 cell line for the amplification of the TAUT promoter region including the partial exon (150 bp) and the 5' untranslated region (UTR, 380 bp). The PCR product of the promoter region, which was 1800 bp long, was ligated into the pGEM-T vector, and sequenced. The 5' flanking region of the TAUT promoter was analysed for the identification of enhancer and regulation motifs. Surprisingly we found the consensus TPA responsive element (TGAGTCAG) which is responsible for gene regulation by the protein kinase C (PKC)-mediated signal transduction pathway. The well known fact that proto-oncogene AP1 (cFos/cJun heterodimer or cJun/cJun homodimer) binds to TRE implies that TAUT expression might be closely linked to tumor promotion. Since AP1 activity is also tightly regulated in nerve cells, AP1-regulated TAUT transcription might be an important step in nerve cell function. Furthermore, the TFIID binding site, cap signal for transcription initiation, PEA3 motif, heat shock factor binding motif, and many other motifs were found in the TAUT promoter region, and require characterization.

Taurine: new implications for an old amino acid

Taurine is a semi-essential amino acid and is not incorporated into proteins. In mammalian tissues, taurine is ubiquitous and is the most abundant free amino acid in the heart, retina, skeletal muscle, brain, and leukocytes. In fact, taurine reaches up to 50 mM concentration in leukocytes. Taurine has been shown to be tissue-protective in many models of oxidant-induced injury. One possibility is that taurine reacts with hypochlorous acid, produced by the myeloperoxidase pathway, to produce the more stable but less toxic taurine chloramine (Tau-Cl). However, data from several laboratories demonstrate that Tau-Cl is a powerful regulator of inflammation. Specifically, Tau-Cl has been shown to down-regulate the production of pro-inflammatory mediators in both rodent and human leukocytes. Taurolidine, a derivative of taurine, is commonly used in Europe as an adjunctive therapy for various infections as well as for tumor therapy. Recent molecular studies on the function of taurine provide evidence that taurine is a constituent of biologic macromolecules. Specifically, two novel taurine-containing modified uridines have been found in both human and bovine mitochondria. Studies investigating the mechanism of action of Tau-Cl have shown that it inhibits the activation of NF-kappaB, a potent signal transducer for inflammatory cytokines, by oxidation of IkappaB-alpha at Met45. Key enzymes for taurine biosynthesis have recently been cloned. Cysteine sulfinic acid decarboxylase, a rate-limiting enzyme for taurine biosynthesis, has been cloned and sequenced in the mouse, rat and human. Another key enzyme for cysteine metabolism, cysteine dioxygenase (CDO), has also been cloned from rat liver. CDO has a critical role in determining the flux of cysteine between cysteine catabolism/taurine synthesis and glutathione synthesis. Taurine transporter knockout mice show reduced taurine, reduced fertility, and loss of vision due to severe apoptotic retinal degeneration. Apoptosis induced by amino chloramines is a current and important finding since oxidants derived from leukocytes play a key role in killing pathogens. The fundamental importance of taurine in adaptive and acquired immunity will be unveiled using genetic manipulation.

Regulation of taurine transporter gene (TauT) by WT1

In the present study we have demonstrated that WT1 (Wilms tumor suppressor gene) enhances the expression of TauT (taurine transporter gene) in human embryonic kidney 293 cells in a dose-dependent manner. TauT promoter activity was increased five-fold by cotransfection of a full-length TauT promoter-reporter construct with WT1. Electrophoretic mobility shift assays (EMSAs) using nuclear extracts from WT1-overexpressing 293 cells showed a putative WT1-binding site in the basal promoter region of TauT, which bound to WT1 in EMSAs. Mutation of this WT1 consensus sequence abolished binding of WT1. These results demonstrate that TauT may represent a downstream target gene of WT1 during renal development.

Regulation of taurine transport at the blood-brain barrier by tumor necrosis factor-alpha, taurine and hypertonicity

Taurine is the abundant sulfur-containing beta-amino acid in brain where it exerts a neuroprotective effect. Although it is known that the blood-brain barrier (BBB) mediates taurine transport, the regulation of taurine transport have not been clarified yet. A conditionally immortalized rat brain capillary endothelial cells (TR-BBB13), an in vitro model of the BBB, exhibited [3H]taurine uptake, which was dependent on both Na+ and Cl-, and inhibited by beta-alanine. Taurine transporter (TAUT) mRNA was detected in TR-BBB13 cells, and TAUT protein was also expressed at 70 kDa. TR-BBB13 cells exposed to 20 ng/mL TNF-alpha and under hypertonic conditions showed a 1.7-fold and 3.2-fold increase in [3H]taurine uptake, respectively. In contrast, lipopolysaccharide and diethyl maleate did not significantly affect taurine uptake. The taurine uptake was reduced by pre-treatment with excess taurine (50 mm). The mRNA level of the TAUT in TNF-alpha and following hypertonic treatment was greater than that in control cells, whereas that under excess taurine conditions was lower than in controls. Therefore, taurine transport activity at the BBB appears to be regulated at the transcriptional level by cell damage, osmolality and taurine in the brain.

Transcriptional repression of taurine transporter gene (TauT) by p53 in renal cells

Taurine, an intracellular osmolyte whose body pool size is adaptively regulated by the kidney, is required for normal renal development. Overexpression of the p53 tumor suppressor gene in p53 transgenic mice results in renal malformation, suggesting that altered expression of certain p53 target gene(s) involved in renal development may be responsible. This study shows that the taurine transporter gene (TauT) is a transcriptional target of p53. Expression of TauT was decreased after activation of p53 by doxorubicin, a DNA-damaging drug, in 293 and NRK-52E renal cells. TauT promoter activity was decreased 5-10-fold by cotransfection of a full-length TauT promoter-reporter construct with p53, which was reversed by cotransfection with a mutant p53 (p53-281). Electrophoretic mobility shift assays using nuclear extracts from p53-expressing (10)1val cells showed a putative p53-binding site in the TauT promoter region, which bound to the p53 in electrophoretic mobility shift assays. Mutation of this p53 consensus sequence abolished binding of p53. These results demonstrate that TauT may represent a downstream target gene of p53 that could link the roles of p53 in renal development and apoptosis.