Expression and Functional Features of NaCT, a Sodium-coupled Citrate Transporter, in Human and Rat Livers and Cell Lines

In this article, we report on the expression and function of a Na(+)-coupled transporter for citrate, NaCT, in human and rat liver cell lines and in primary hepatocytes from the rat liver. We also describe the polarized expression of this transporter in human and rat livers. Citrate uptake in human liver cell lines HepG2 and Huh-7 was obligatorily dependent on Na+. The uptake system showed a preference for citrate over other intermediates of the citric acid cycle and exhibited a Michaelis constant of approximately 6 mM for citrate. The transport activity was stimulated by Li+, and the activation was associated with a marked increase in substrate affinity. Citrate uptake in rat liver cell line MH1C1 was also Na+ dependent and showed a preference for citrate. The Michaelis constant for citrate was approximately 10 microM. The transport activity was inhibited by Li+. Primary hepatocytes from the rat liver also showed robust activity for Na+-coupled citrate uptake, with functional features similar to those described in the rat liver cell line. Immunolabeling with a specific anti-NaCT antibody showed exclusive expression of the transporter in the sinusoidal membrane of hepatocytes in human and rat livers. This constitutes the first report on the expression and function of NaCT in liver cells.