Satsu Hideo, Terasawa Eriko, Hosokawa Yu, Shimizu Makoto
Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan.
Biochem J. 2003 Oct 15;375(Pt 2):441-7. doi: 10.1042/BJ20030535.
We investigated the characterization and the regulation of TAUT (taurine transporter) and CDO (cysteine dioxygenase), one of the key enzymes of taurine biosynthesis, in human hepatoblastoma HepG2 cells. The activity of TAUT in the HepG2 cells was evaluated by means of a sodium- and chloride-dependent high-affinity transport system, the characteristics of which were similar to those of the beta amino-acid-specific taurine transport system described previously for various tissues [Uchida, Kwon, Yamauchi, Preston, Marumo and Handler (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 8230-8234; Ramamoorthy, Leibach, Mahesh, Han, Yang-Feng, Blakely and Ganapathy (1994) Biochem. J. 300, 893-900; and Satsu, Watanabe, Arai and Shimizu (1997) J. Biochem. (Tokyo) 121, 1082-1087]. By culturing in a hypertonic medium, the intracellular taurine content of HepG2 cells was markedly increased. Under hypertonic conditions, the activity of TAUT was up-regulated, and the results of the kinetic analysis suggested that this up-regulation was associated with an increase in the amount of TAUT. The expression level of TAUT mRNA was markedly higher than that of the control cells. The expression level of CDO mRNA was also up-regulated under the hypertonic conditions. Culturing the cells in a taurine-rich medium resulted in both the activity of TAUT and the expression level of TAUT mRNA being down-regulated in HepG2 cells. On the other hand, the expression level of CDO mRNA was not affected under a taurine-rich condition. The present results show that both TAUT and CDO were co-operatively regulated in response to hypertonicity, but did not co-operatively respond to the change in extracellular taurine concentration. Generally, the TAUT and taurine biosynthetic enzymes have independent regulatory systems, but under certain conditions, they could be regulated in harmony with each other.
我们研究了人肝癌HepG2细胞中TAUT（牛磺酸转运体）和CDO（半胱氨酸双加氧酶，牛磺酸生物合成的关键酶之一）的特性及调控。通过钠和氯依赖性高亲和力转运系统评估HepG2细胞中TAUT的活性，其特性与先前描述的各种组织中的β氨基酸特异性牛磺酸转运系统相似[内田、权、山内、普雷斯顿、丸茂和汉德勒（1992年）《美国国家科学院院刊》89卷，8230 - 8234页；拉马穆尔蒂、莱巴赫、马赫什、韩、杨 - 冯、布莱克利和加纳帕蒂（1994年）《生物化学杂志》300卷，893 - 900页；以及佐藤、渡边、新井和清水（1997年）《生物化学杂志（东京）》121卷，1082 - 1087页]。通过在高渗培养基中培养，HepG2细胞内的牛磺酸含量显著增加。在高渗条件下，TAUT的活性上调，动力学分析结果表明这种上调与TAUT量的增加有关。TAUT mRNA的表达水平明显高于对照细胞。在高渗条件下，CDO mRNA的表达水平也上调。在富含牛磺酸的培养基中培养细胞导致HepG2细胞中TAUT的活性和TAUT mRNA的表达水平均下调。另一方面，在富含牛磺酸的条件下，CDO mRNA的表达水平不受影响。目前的结果表明，TAUT和CDO在对高渗性的反应中协同调节，但对细胞外牛磺酸浓度的变化没有协同反应。一般来说，TAUT和牛磺酸生物合成酶有独立的调节系统，但在某些条件下，它们可以相互协调调节。
