Fernández-Veledo Sonia, Valdés Raquel, Wallenius Ville, Casado F Javier, Pastor-Anglada Marçal
Department de Bioquímica i Biologia Molecular, Universitat de Barcelona, Diagonal 645, E-08028 Barcelona, Spain.
J Hepatol. 2004 Oct;41(4):538-44. doi: 10.1016/j.jhep.2004.06.008.
BACKGROUND/AIMS: Concentrative nucleoside transporter 1 (CNT1), a high affinity transporter for pyrimidine nucleosides, is responsible for their Na+-dependent concentrative uptake into hepatocytes. Though CNT1 protein amounts increase in rat liver soon after partial hepatectomy, the physiological regulators of CNT1 expression have not yet been identified.
Rat hepatoma cell lines and hepatocytes isolated from fetuses and adult rats were used to identify single agents able to up-regulate CNT1 expression and activity in liver. TNF-alpha receptor-I (TNFRI) and IL-6 knock-out mice were also used to study CNT1 regulation in vivo.
TNF-alpha and IL-6 independently induced CNT1 protein expression in cultured liver parenchymal and FAO hepatoma cells by PI-3 kinase- and ERK-dependent mechanisms, respectively. In vivo data showed that transporter protein levels were low in livers from TNFRI knock-out mice, but not in those from IL-6 deficient animals. However, IL-6 administration only partially restored CNT1 expression in the former model.
This study identifies TNF-alpha as a major in vivo modulator of the nucleoside transporter CNT1 and suggests a secondary role for IL-6 in mediating CNT1 up-regulation by TNF-alpha in vivo. Evidence is provided that two independent pathways are involved in the up-regulation of CNT1 by TNF-alpha and IL-6.
背景/目的：浓缩型核苷转运体1（CNT1）是嘧啶核苷的高亲和力转运体，负责其依赖钠离子的浓缩性摄取进入肝细胞。虽然部分肝切除术后大鼠肝脏中CNT1蛋白量很快增加，但CNT1表达的生理调节因子尚未确定。
使用大鼠肝癌细胞系以及从胎儿和成年大鼠分离的肝细胞，来确定能够上调肝脏中CNT1表达和活性的单一因子。还使用肿瘤坏死因子-α受体-I（TNFRI）和白细胞介素-6基因敲除小鼠来研究体内CNT1的调节。
肿瘤坏死因子-α和白细胞介素-6分别通过依赖磷脂酰肌醇-3激酶和细胞外信号调节激酶的机制，独立诱导培养的肝实质细胞和脂肪酸氧化型肝癌细胞中CNT1蛋白表达。体内数据显示，TNFRI基因敲除小鼠肝脏中转运体蛋白水平较低，但白细胞介素-6缺陷动物的肝脏中则不然。然而，在前者模型中，给予白细胞介素-6仅部分恢复了CNT1表达。
本研究确定肿瘤坏死因子-α为核苷转运体CNT1的主要体内调节因子，并提示白细胞介素-6在介导肿瘤坏死因子-α体内上调CNT1方面起次要作用。有证据表明，肿瘤坏死因子-α和白细胞介素-6上调CNT1涉及两条独立途径。
