Takahisa Matsuda

Taurine depletion caused by knocking out the taurine transporter gene leads to cardiomyopathy with cardiac atrophy

The sulfur-containing beta-amino acid, taurine, is the most abundant free amino acid in cardiac and skeletal muscle. Although its physiological function has not been established, it is thought to play an important role in ion movement, calcium handling, osmoregulation and cytoprotection. To begin examining the physiological function of taurine, we generated taurine transporter- (TauT-) knockout mice (TauTKO), which exhibited a deficiency in myocardial and skeletal muscle taurine content compared with their wild-type littermates. The TauTKO heart underwent ventricular remodeling, characterized by reductions in ventricular wall thickness and cardiac atrophy accompanied with the smaller cardiomyocytes. Associated with the structural changes in the heart was a reduction in cardiac output and increased expression of heart cardiac failure (fetal) marker genes, such as ANP, BNP and beta-MHC. Moreover, ultrastructural damage to the myofilaments and mitochondria was observed. Further, the skeletal muscle of the TauTKO mice also exhibited decreased cell volume, structural defects and a reduction of exercise endurance capacity. Importantly, the expression of Hsp70, ATA2 and S100A4, which are upregulated by osmotic stress, was elevated in both heart and skeletal muscle of the TauTKO mice. Taurine depletion causes cardiomyocyte atrophy, mitochondrial and myofiber damage and cardiac dysfunction, effects likely related to the actions of taurine. Our data suggest that multiple actions of taurine, including osmoregulation, regulation of mitochondrial protein expression and inhibition of apoptosis, collectively ensure proper maintenance of cardiac and skeletal muscular structure and function.

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.

Signals through gp130 upregulate Wnt5a and contribute to cell adhesion in cardiac myocytes

Glycoprotein 130 (gp130), a common receptor of IL‐6 family cytokines, plays critical roles in cardiac functions. Here, we demonstrate that the stimulation of gp130 with leukemia inhibitory factor (LIF) promoted cell adhesion in a cadherin‐dependent manner in cultured cardiomyocytes. Wnt5a was upregulated by the stimulation of gp130 with IL‐6 family cytokines, accompanied by N‐cadherin protein upregulation. Wnt5a was not induced by LIF in cardiomyocytes expressing dominant‐negative STAT3. Ablation of Wnt5a by antisense cDNA inhibited LIF‐induced cell adhesion. Collectively, signals through gp130 upregulate Wnt5a through STAT3, promoting the N‐cadherin‐mediated cell adhesion.

Platelet activating factor induces cytoskeletal reorganization through Rho family pathway in THP‐1 macrophages

In the process of atherosclerosis, platelet activating factor (PAF) promotes the infiltration of inflammatory cells into atherosclerotic plaque by modulating their cytoskeleton. Here, we examined whether Rho family proteins are involved in PAF‐induced cytoskeletal reorganization in THP‐1 macrophages. PAF stimulation rapidly induced cell elongation, accompanied by filopodia formation. The inhibition of Rho family proteins by the overexpression of Rho‐GDI attenuated the PAF‐mediated morphological changes. Both RhoA and Cdc42 were activated in response to PAF. Inhibition of RhoA or Cdc42 by dominant negative mutants abrogated morphological changes induced by PAF. Collectively, PAF regulates cytoarchitecture through Rho family proteins in macrophages.

Myogenic Induction of Taurine Transporter Prevents Dexamethasone-Induced Muscle Atrophy

We demonstrate for the first time that TauT expression is upregulated during differentiation in C2C12 cells. In addition, taurine have a protective effect on muscle atrophy. Thus, taurine may be a therapeutic agent for cachexic state of diverse pathology.

Molecular mechanisms of cardioprotection by taurine on ischemia-induced apoptosis in cultured cardiomyocytes.

An integral part of the pathogenesis of heart failure is myocyte loss. The traditional explanation for myocyte loss was cell necrosis but there has been a surge of evidence affirming the role of apoptosis in the genesis of heart failure (Garg et al., 2005). Evidence for apoptotic cell death was shown in clinical cases of myocardial infarction, as well as in rabbit, rat, and mouse models of continuous ischemia or ischemia/reperfusion (Garg et al., 2005). It has been shown that the mitochondrial pathways participate in apoptosis induced by ischemia (Garg et al., 2005). Taurine (2-aminoethanesulfonic acid), the -amino acid, is one of the factors that regulates the degree of apoptosis during ischemia (Roysommuti et al., 2003; Schaffer et al., 2003). However, little is known about the cytoprotective signalling pathways mediate this response. We have previously reported that isolated neonatal cardiomyocytes become resistant to ischemia-induced apoptosis when exposed to medium containing 20 mM taurine (Takahashi et al., 2003). In this study, the interaction between taurine and mitochondria-mediated apoptosis is investigated in a newly developed simulated ischemia model utilizing isolated cardiomyocytes that are incubated with medium containing and lacking taurine and then sealed within cultured flasks (Takahashi et al., 2003, Takatani et al., 2004a,b).

Minoxidil attenuates ischemia-induced apoptosis in cultured neonatal rat cardiomyocytes

The effects of minoxidil (a mitochondrial K+ATP channel opener) on ischemia-induced necrosis and apoptosis were examined using a cardiomyocyte model of simulated ischemia, since mito-chondrial K+ATP channel openers have been suggested to be involved in the mechanisms of cardioprotective action against ischemia/reperfusion injury. In the absence of minoxidil, simulated ischemia led to cellular release of creatine phosphokinase (CPK), morphologic degeneration, and beating cessation within 24 to 72 hours. Based on the Hoechst 33258 staining pattern, a significant number of cells placed in sealed flasks underwent apoptosis. Myocytes treated with 5 μM of minoxidil failed to alter the degree of ischemia-induced CPK loss for 48 to 72 hours. However, minoxidil treatment prevented the loss of beating function in many of the ischemic cells, and attenuated the decline in intracellular ATP content after a 48-hour ischemic incubation. The number of nuclear fragmentation was significantly reduced in minoxidil-treated cells after a 72-hour ischemic insult compared with untreated ischemic cells. This effect was blocked by the mitochondrial K+ATP channel antagonist 5-HD. The data suggest that minoxidil renders the cell resistant to ischemia-induced necrosis and apoptosis. The beneficial effects of minoxidil appear to be related to the opening of mitochondrial K+ATP channels.

TauT gene expression is regulated by TonEBP and plays a role in cell survival.

In summary, this study shows that TonEBP regulates TauT expression in response to hypertonicity, and that the TonEBP/TauT pathway plays an important role in cytoprotection from hypertonic stress. The activation of TonEBP/TauT pathway could contribute to the protective signal pathway against hypertonicity-related tissue damage.

Protective effect of taurine on the detachment of cultured cardiac fibroblasts from the substratum induced by calcium depletion

Summary.Removal of Ca2+ from the incubation medium of cultured rat cardiac fibroblasts causes cellular morphological changes, such as the formation of blebs, the ballooning of the cell membrane and the detachment from the culture dish. A 24 hr preincubation with 20 mM taurine blocked the Ca2+ depletion-induced detachment of the cardiac fibroblasts. However, taurine treatment did not prevent other morphological changes induced by Ca2+ depletion. The data suggest that taurine plays an important role in cell adhesion in the heart.

Modulation of taurine on CYP3A4 induction by rifampicin in a HepG2 cell line.

There has recently been an increased demand for nutritional supplements and health drinks containing taurine. The Japanese diet also provides high levels of taurine. In addition, 60% of the over-the-counter (OTC) drugs classified as vitamin-mixture medication contain taurine (Figure 1). Because taurine intake is so high, there has been concern that it might interact with other drugs. A common form of drug-drug interaction involves the induction or inhibition of specific drug-metabolizing enzymes such as cytochrome P450 (CYP)1. However, little is known about the influence of taurine on the metabolism of clinically important drugs. The antibiotics rifampicin (RFP) is a strong CYP3A4 inducer in humans2. In the present study we examined whether taurine could affect the levels of CYP3A4 mRNA in the presence and absence of RFP.