Submitted on August 9, 2006
Accepted on September 6, 2006
Expression and functional features of NaCT, a sodium-coupled citrate transporter, in human and rat liver and cell lines
Elangovan Gopal1, Seiji Miyauchi1, Pamela M Martin1, Sudha Ananth1, Sonne R Srinivas1, Sylvia B. Smith2, Puttur Prasad3, and Vadivel Ganapathy1*
1 Biochemistry and Molecular Biology, Medical College of Georgia, Augusta, Georgia, United States
2 Cell Biology and Anatomy, Medical College of Georgia, Augusta, Georgia, United States
3 Biochemistry and Molecular Biology, Med Coll Georgia, Augusta, Georgia, United States; Med Coll Georgia
* To whom correspondence should be addressed. E-mail: vganapat{at}mail.mcg.edu.
Here we report on the expression and function of NaCT, a Na+-coupled transporter for citrate in human and rat liver cell lines and in primary hepatocytes from rat liver. We also describe the polarized expression of this transporter in human and rat liver. Citrate uptake in the human liver cell lines HepG2 and Huh-7 was obligatorily dependent on Na+. The uptake system showed preference for citrate over other intermediates of the citric acid cycle and exhibited a Michaelis constant of ~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 the rat liver cell line MH1C1 was also Na+-dependent and showed preference for citrate. The Michaelis constant for citrate was ~10 µM. The transport activity was inhibited by Li+. Primary hepatocytes from 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 the hepatocytes in human liver and rat liver. This constitutes the first report on the expression and function of NaCT in liver cells.
