Bauch Christian, Forster Natasha, Loffing-Cueni Dominique, Summa Vanessa, Verrey François
Institute of Physiology and Institute of Anatomy, University of Zürich, Winterthurerstrasse 190, Switzerland.
J Biol Chem. 2003 Jan 10;278(2):1316-22. doi: 10.1074/jbc.M210449200. Epub 2002 Nov 1.
The heteromeric amino acid transporters b(0,+)AT-rBAT (apical), y(+)LAT1-4F2hc, and possibly LAT2-4F2hc (basolateral) participate to the (re)absorption of cationic and neutral amino acids in the small intestine and kidney proximal tubule. We show now by immunofluorescence that their expression levels follow the same axial gradient along the kidney proximal tubule (S1>S2S3). We reconstituted their co-expression in MDCK cell epithelia and verified their polarized localization by immunofluorescence. Expression of b(0,+)AT-rBAT alone led to a net reabsorption of l-Arg (given together with l-Leu). Coexpression of basolateral y(+)LAT1-4F2hc increased l-Arg reabsorption and reversed l-Leu transport from (re)absorption to secretion. Similarly, l-cystine was (re)absorbed when b(0,+)AT-rBAT was expressed alone. This net transport was further increased by the coexpression of 4F2hc, due to the mobilization of LAT2 (exogenous and/or endogenous) to the basolateral membrane. In summary, apical b(0,+)AT-rBAT cooperates with y(+)LAT1-4F2hc or LAT2-4F2hc for the transepithelial reabsorption of cationic amino acids and cystine, respectively. The fact that the reabsorption of l-Arg led to the secretion of l-Leu demonstrates that the implicated heteromeric amino acid transporters function in epithelia as exchangers coupled in series and supports the notion that the parallel activity of unidirectional neutral amino acid transporters is required to drive net amino acid reabsorption.
异源氨基酸转运体b(0,+)AT-rBAT（位于顶端）、y(+)LAT1-4F2hc以及可能的LAT2-4F2hc（位于基底外侧）参与小肠和近端肾小管中阳离子和中性氨基酸的（再）吸收。我们现在通过免疫荧光显示，它们的表达水平沿近端肾小管呈现相同的轴向梯度（S1>S2>S3）。我们在MDCK细胞上皮中重建了它们的共表达，并通过免疫荧光验证了它们的极化定位。单独表达b(0,+)AT-rBAT导致L-精氨酸的净再吸收（与L-亮氨酸一起给予）。基底外侧y(+)LAT1-4F2hc的共表达增加了L-精氨酸的再吸收，并使L-亮氨酸的转运从（再）吸收转变为分泌。同样，单独表达b(0,+)AT-rBAT时L-胱氨酸被（再）吸收。由于LAT2（外源性和/或内源性）转运至基底外侧膜，4F2hc的共表达进一步增加了这种净转运。总之，顶端的b(0,+)AT-rBAT分别与y(+)LAT1-4F2hc或LAT-4F2hc协同作用，实现阳离子氨基酸和胱氨酸的跨上皮再吸收。L-精氨酸的再吸收导致L-亮氨酸的分泌这一事实表明，所涉及的异源氨基酸转运体在上皮细胞中作为串联偶联的交换体发挥作用，并支持以下观点，即单向中性氨基酸转运体的平行活性是驱动氨基酸净再吸收所必需的。
