Functional expression of rat renal cortex taurine transporter in Xenopus laevis oocytes: adaptive regulation by dietary manipulation

Glutathione Contributes to Caloric Restriction-Triggered Shift in Taurine Homeostasis

BACKGROUND/OBJECTIVES

Previously, we found that caloric restriction (CR) in mice increases taurine levels by stimulating hepatic synthesis, secretion into the intestine and deconjugation of taurine-conjugated bile acids (BA). Subsequently, in the intestine, taurine conjugates various molecules, including glutathione (GSH). The current study explores the mechanisms behind forming taurine-GSH conjugate and its consequences for taurine, other taurine conjugates, and BA in order to improve understanding of their role in CR.

METHODS

The non-enzymatic conjugation of taurine and GSH was assessed and the uptake of taurine, GSH, and taurine-GSH was verified in five sections of the small intestine. Levels of taurine, gavaged 13C labeled taurine, taurine conjugates, taurine-GSH, and GSH were measured in various tissues of ad libitum and CR mice. Next, the taurine-related CR phenotype was challenged by applying the inhibitors of taurine transporter (SLC6A6) and GSH-S transferases (GST).

RESULTS

The CR-related increase in taurine in intestinal mucosa was accompanied by the uptake and distribution of taurine towards selected organs. A unique composition of taurine conjugates characterized each tissue. Although taurine-GSH conjugate could be formed in non-enzymatic reactions, GST activity contributed to taurine-related CR outcomes. Upon SLC6A6 and GST inhibition, the taurine-related parameters were affected mainly in the ileum rather than the liver. Meanwhile, BA levels were somewhat affected by GST inhibition in the ileum and in the liver by SLC6A6 inhibitor.

CONCLUSIONS

The discovered CR phenotype involves a regulatory network that adjusts taurine and BA homeostasis. GSH supports these processes by conjugating taurine, impacting taurine uptake from the intestine and its availability to form other types of conjugates.

Characterization of Urinary N-Acetyltaurine as a Biomarker of Hyperacetatemia in Mice

Acetate is an important metabolite in metabolic fluxes. Its presence in biological entities originates from both exogenous inputs and endogenous metabolism. Because the change in blood acetate level has been associated with both beneficial and adverse health outcomes, blood acetate analysis has been used to monitor the systemic status of acetate turnover. The present study examined the use of urinary N-acetyltaurine (NAT) as a marker to reflect the hyperacetatemic status of mice from exogenous inputs and endogenous metabolism, including triacetin dosing, ethanol dosing, and streptozotocin-induced diabetes. The results showed that triacetin dosing increased serum acetate and urinary NAT but not other N-acetylated amino acids in urine. The co-occurrences of increased serum acetate and elevated urinary NAT were also observed in both ethanol dosing and streptozotocin-induced diabetes. Furthermore, the renal cortex was determined as an active site for NAT synthesis. Overall, urinary NAT behaved as an effective marker of hyperacetatemia in three experimental mouse models, warranting further investigation into its application in humans.

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.

The blood-brain barrier fatty acid transport protein 1 (FATP1/SLC27A1) supplies docosahexaenoic acid to the brain, and insulin facilitates transport

We purposed to clarify the contribution of fatty acid transport protein 1 (FATP1/SLC 27A1) to the supply of docosahexaenoic acid (DHA) to the brain across the blood-brain barrier in this study. Transport experiments showed that the uptake rate of [¹⁴ C]-DHA in human FATP1-expressing HEK293 cells was significantly greater than that in empty vector-transfected (mock) HEK293 cells. The steady-state intracellular DHA concentration was nearly 2-fold smaller in FATP1-expressing than in mock cells, suggesting that FATP1 works as not only an influx, but also an efflux transporter for DHA. [¹⁴ C]-DHA uptake by a human cerebral microvascular endothelial cell line (hCMEC/D3) increased in a time-dependent manner, and was inhibited by unlabeled DHA and a known FATP1 substrate, oleic acid. Knock-down of FATP1 in hCMEC/D3 cells with specific siRNA showed that FATP1-mediated uptake accounts for 59.2-73.0% of total [¹⁴ C]-DHA uptake by the cells. Insulin treatment for 30 min induced translocation of FATP1 protein to the plasma membrane in hCMEC/D3 cells and enhanced [¹⁴ C]-DHA uptake. Immunohistochemical analysis of mouse brain sections showed that FATP1 protein is preferentially localized at the basal membrane of brain microvessel endothelial cells. We found that two neuroprotective substances, taurine and biotin, in addition to DHA, undergo FATP1-mediated efflux. Overall, our results suggest that FATP1 localized at the basal membrane of brain microvessels contributes to the transport of DHA, taurine and biotin into the brain, and insulin rapidly increases DHA supply to the brain by promoting translocation of FATP1 to the membrane. Read the Editorial Comment for this article on page 324.

Taurine Alters Respiratory Gas Exchange and Nutrient Metabolism in Type 2 Diabetic Rats

OBJECTIVE

To assess the effect of taurine supplementation on respiratory gas exchange, which might reflect the improved metabolism of glucose and/or lipid in the type 2 diabetic Otsuka Long-Evans Tokushima Fatty (OLETF) rats.

RESEARCH METHODS AND PROCEDURES

Male OLETF rats (16 weeks of age) were randomly divided into two groups: unsupplemented group and taurine-supplemented (3% in drinking water) group. After 9 weeks of treatment, indirect calorimetry and insulin tolerance tests were conducted. The amounts of visceral fat pads, tissue glycogen, the blood concentrations of glucose, triacylglycerol, taurine, and electrolytes, and the level of hematocrit were compared between groups. A nondiabetic rat strain (Long-Evans Tokushima Otsuka) was used as the age-matched normal control.

RESULTS

The indirect calorimetry showed that the treatment of OLETF rats with taurine could reduce a part of postprandial glucose oxidation possibly responsible for the increase of triacylglycerol synthesis in the body. Taurine supplementation also improved hyperglycemia and insulin resistance and increased muscle glycogen content in the OLETF rats. Supplementation with taurine increased the blood concentration of taurine and electrolyte and fluid volume, all of which were considered to be related to the improvement of metabolic disturbance in OLETF rats.

DISCUSSION

Taurine supplementation may be an effective treatment for glucose intolerance and fat/lipid accumulation observed in type 2 diabetes associated with obesity. These metabolic changes might be ascribed, in part, to the alteration of circulating blood profiles, where the improved hyperglycemia and/or the blood accumulation of taurine itself would play roles.

Taurine and the renal system

Taurine participates in a number of different physiologic and biologic processes in the kidney, often reflected by urinary excretion patterns. The kidney is key to aspects of taurine body pool size and homeostasis. This review will examine the renal-taurine interactions relative to ion reabsorption; renal blood flow and renal vascular endothelial function; antioxidant properties, especially in the glomerulus; and the role of taurine in ischemia and reperfusion injury. In addition, taurine plays a role in the renal cell cycle and apoptosis, and functions as an osmolyte during the stress response. The role of the kidney in adaptation to variations in dietary taurine intake and the regulation of taurine body pool size are described. Finally, the protective function of taurine against several kidney diseases is reviewed.

Expression and functional analysis of mussel taurine transporter, as a key molecule in cellular osmoconforming

Most aquatic invertebrates adapt to environmental osmotic changes primarily by the cellular osmoconforming process, in which osmolytes accumulated in their cells play an essential role. Taurine is one of the most widely utilized osmolytes and the most abundant in many molluscs. Here, we report the structure, function and expression of the taurine transporter in the Mediterranean blue mussel (muTAUT), as a key molecule in the cellular osmoconforming process. Deduced amino acid sequence identity among muTAUT and vertebrate taurine transporters is lower (47-51%) than that among vertebrate taurine transporters (>78%). muTAUT has a lower affinity and specificity for taurine and a requirement for higher NaCl concentration than vertebrate taurine transporters. This seems to reflect the internal environment of the mussel; higher NaCl and taurine concentrations. In addition to the hyperosmotic induction that has been reported for cloned taurine transporters, the increase in muTAUT mRNA was unexpectedly observed under hypoosmolality, which was depressed by the addition of taurine to ambient seawater. In view of the decrease in taurine content in mussel tissue under conditions of hypoosmolality reported previously, our results lead to the conclusion that muTAUT does not respond directly to hypoosmolality, but to the consequent decrease in taurine content. By immunohistochemistry, intensive expression of muTAUT was observed in the gill and epithelium of the mantle, which were directly exposed to intensive osmotic changes of ambient seawater.

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 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.

Chronic taurine treatment ameliorates reduction in saline-induced diuresis and natriuresis

BACKGROUND

Taurine is an osmolyte found in high concentration in the kidney. Both the modulation of tissue taurine stores and the exogenous administration of taurine are known to affect renal function. Therefore, it is likely that taurine therapy could benefit the dysfunctional kidney.

METHODS

To test this idea, the present study examined the effect of chronic taurine administration on the excretory responses to acute saline volume loading by the unilaterally nephrectomized (UNX) control and the UNX glucose-intolerant rat (ages 3 to 9 months).

RESULTS

Sham-operated animals excreted similar amounts of the administered fluid and sodium loads with age. However, unilateral nephrectomy was associated with a significant reduction in the response to saline volume loading. This defect was prominent at a younger age (that is, 3 months) in the UNX glucose intolerant than the UNX control (6 months old) rat. Chronic taurine treatment ameliorated the reduction in saline-induced diuresis and natriuresis by both the UNX control and the UNX glucose intolerant rat. Both an increase in glomerular filtration and a reduction in tubular reabsorption of fluid and sodium caused this taurine-mediated improvement in renal excretory function.

CONCLUSION

Taurine treatment protects the kidney of the UNX rat against an age-dependent decline in excretory efficiency.

Regulation of high affinity taurine transport in goldfish and rat retinal cells

Adaptive regulation and modulation by phosphorylation are mechanisms by which some cells control taurine transport. Goldfish and rat retinal cells were incubated with the activator of protein kinase C, phorbol 12,13-dibutyrate (PDBu), or the inhibitor of protein phosphatases, okadaic acid (OKA). OKA, 1 nM, inhibited the uptake of taurine at short period of incubation in goldfish retinal cells, and at low concentrations in rat retinal cells incubated with the inhibitor for 1 h. PDBu treatment did not produce significant effects. Isolated Müller cells from the goldfish retina presented a clear adaptive regulation and a decrease of taurine uptake by increasing phosphorylation either by the stimulation of PKC with PDBu or the inhibition of phosphatases with OKA.

Dietary taurine manipulations in aged male Fischer 344 rat tissue: taurine concentration, taurine biosynthesis, and oxidative markers11Abbreviations: CA, cysteic acid; CSA, cysteine sulfinic acid; CSD, cysteine sulfinic acid decarboxylase; CDO, cysteine dioxygenase; DNPH, 2,4-dinitrophenylhydrazin; DPPH, α,α-diphenyl-β-picrylhydrazyl; F344, Fischer 344; HPLC-ECD, high performance liquid chromatography with electrochemical detection; MDCK, Madin Darby canine kidney; PCA, perchloric acid; TAU, taurine; TBARS, thiobarbituric acid; and TCA, trichloroacetic acid

Taurine (TAU) is a ubiquitous sulfur-containing amino acid that has been proposed to be an antioxidant. The concentration of TAU decreases during aging, which may increase susceptibility to oxidative stress. Our study attempted to elucidate the mechanism for the age-dependent decrease in TAU content by examining TAU biosynthesis in aged rats. We also examined the effects of dietary TAU manipulations on TAU content and oxidative markers in aged male Fischer 344 (F344) rats. Adult (9 months) and aged (26 months) rats fed control diets, aged rats fed control diet and TAU-supplemented (1.5%) water, and aged rats fed a TAU-deficient diet were used. We observed a significant age-related decrease in TAU content in liver, kidney, and cerebellum. Dietary TAU supplementation increased tissue TAU content, whereas dietary TAU restriction had no effect. Enzyme-dependent TAU synthesis showed an age-dependent reduction in liver that was decreased further by TAU supplementation. Protein carbonyl content was elevated in the cerebral cortex and kidney of aged rats and was attenuated by TAU supplementation. A trend for a decrease in protein and acid-soluble thiol contents in hepatic tissue of aged rats was observed, and this was attenuated with dietary TAU supplementation. Our data show that a decrease in hepatic TAU biosynthesis may cause, in part, the observed decline in tissue TAU content in aged F344 rats, and TAU supplementation can restore TAU levels. Our study indicates that a decline in TAU content may exacerbate oxidative stress in aged rats, which can be reversed by dietary TAU supplementation.

Taurine modulates arginine vasopressin-mediated regulation of renal function

Taurine has been implicated in the regulation of arginine vasopressin (AVP) secretion, and we have previously shown altered renal excretory function in the taurine-depleted rat. To further elucidate the role of taurine in AVP-mediated renal responses, the effects of an antagonist for renal AVP receptors were examined in four groups of conscious rats: control, taurine-supplemented, taurine-depleted, and taurine-repleted. Control and taurine-supplemented rats displayed similar and significant AVP receptor antagonist-induced elevations in fluid excretion, sodium excretion, and free water clearance but a marked reduction in urine osmolality. These effects are consistent with inhibition of endogenous AVP activity. By contrast, in the taurine-depleted rats, the magnitude and the time course of drug-induced renal excretory responses lagged behind those of the control and taurine-supplemented groups. Further, baseline urine osmolality was significantly higher in the taurine-depleted compared with the control or taurine-supplemented groups. However, after administration of the antagonist, taurine-depleted rats manifested a delayed but more marked reduction in urine osmolality, thereby eliminating the baseline differential that existed between the taurine-depleted rats and control or taurine-supplemented groups. Consistent with these observations, plasma AVP was significantly increased in the taurine-depleted compared with the control rats. Interestingly, taurine repletion shifted all responses closer to the control group. Analysis of the data suggests that the effect of the antagonist on renal excretory function is related primarily to altered tubular reabsorption activity. These observations suggest that taurine modulates renal function, and, thereby, body fluid homeostasis, through an AVP-dependent mechanism.

Kainic acid (KA)-induced seizures in Sprague-Dawley rats and the effect of dietary taurine (TAU) supplementation or deficiency

Male Sprague-Dawley rats received TAU supplementation (1.5% in drinking water) or TAU deficient diets for 4 weeks to test for a possible neuroprotective role of TAU in KA-induced (10 mg/kg s.c.) seizures. TAU supplementation significantly increased serum and hippocampal TAU levels, but not TAU content in temporal cortex or striatum. TAU deficient diets did not attenuate serum or tissue TAU levels. Dietary TAU supplementation failed to decrease the number or latency of partial or clonic-tonic seizures or wet dog shakes, whereas a TAU deficient diet decreased the number of clonictonic and partial seizures. This study does not support previous observations of an anticonvulsant effect of TAU against KA-induced seizures. KA-treatment decreased alpha 2-adrenergic receptor binding sites and TAU content in the temporal cortex across all dietary treatment groups, supporting previous evidence of severe KA-induced damage and neuronal loss in this brain region.

Molecular cloning and functional expression of an LLC-PK1 cell taurine transporter that is adaptively regulated by taurine

Studies have shown that the renal tubular epithelium adapts to alterations in the sulfur amino acid composition of the diet. The renal adaptive response has been described in man, mouse, rat, dog, and pig. The observed phenomenon involves increased or decreased initial rate activity of the NaCl-dependent taurine transporter at the brush border membrane surface of the proximal tubule following dietary manipulation of taurine. A cDNA encoding a taurine transporter has been isolated from LLC-PK1 cells, designated pTAUT, and its functional properties have been examined in Xenopus laevis oocytes. The nucleotide sequence of the clone predicts a 621-amino acid protein with about 90% homology to other cloned taurine transporter cDNAs. When expressed in oocytes the transporter displays a Km of 25 microM and is dependent on the presence of external sodium and chloride, characteristics similar to taurine uptake by LLC-PK1 cells. The abundance of pTAUT mRNA and protein were up-regulated in cells cultured in taurine-free medium as compared with cells cultured in medium containing 500 microM taurine. Activation of PKC by PMA had no effect on adaptive regulation of pTAUT mRNA and protein, indicating that down-regulation of LLC-PK1 cell taurine transport activity by PMA occurs at the post-translational level.

Regulation of the taurine transporter gene in the S3 segment of the proximal tubule

Traditionally, bulk amino acid reabsorption in the kidney has been thought to be localized to the early portions of the proximal nephron. Adult Sprague-Dawley rats were fed diets with low, normal, and high taurine content for two weeks. Kidneys were hybridized with an 35S-radiolabeled complementary RNA probe to the rB16a subclone encoding the extracellular and transmembrane domains of the rat brain taurine transporter. Identical fragments were generated by RT-PCR from rat brain and kidneys as confirmed by DNA sequencing. Hybridization was localized to the outer zone of the medulla of all the kidneys. In the normal diet animals, taurine transporter mRNA was localized to the S3 segment of the proximal tubule, to the loop of Henle in the medulla, and to the glomerular epithelial cell layer. With taurine restriction, taurine transporter mRNA expression was up-regulated predominantly in the S3 segment and was virtually absent in this segment in animals supplemented with taurine. These experiments have precisely localized the rat kidney taurine transporter gene, demonstrating regulation that is limited to the S3 segment of the proximal tubule.