Received for publication March 9, 2006.
Revised June 1, 2006.
Accepted for publication June 5, 2006.

ORGANIC CATION/CARNITINE TRANSPORTER OCTN2 (Slc22a5) IS RESPONSIBLE FOR CARNITINE TRANSPORT ACROSS APICAL MEMBRANES OF SMALL INTESTINAL EPITHELIAL CELLS IN MOUSE

Abstract

The organic cation/carnitine transporter OCTN2 is responsible for renal tubular reabsorption of its endogenous substrate, carnitine, although its physiological role in small intestine remains controversial. Here we present direct evidence for a predominant role of OCTN2 in small intestinal absorption of carnitine, based on experiments with juvenile visceral steatosis (jvs) mice, which have a hereditary deficiency of the octn2 gene. Uptake of carnitine, assessed with an Ussing-type chamber system, from the apical surface of the small intestine was saturable and higher than that from the basal surface in wild-type mice, whereas carnitine uptake having these characteristics was almost absent in jvs mice. Saturable uptake of carnitine was also confirmed in isolated enterocytes obtained from wild-type mice, the Km value obtained (~20 µM) being close to that reported for carnitine uptake by HEK293 cells stably expressing mouse OCTN2 (Slc22a5). The carnitine uptake by enterocytes was decreased in the presence of various types of organic cations, this inhibition profile being similar to that of mouse OCTN2, whereas uptake of carnitine was quite small and unsaturable in enterocytes obtained from jvs mice. Immunohistochemical and immunoprecipitation analyses suggested colocalization of OCTN2 with PDZK1, an adaptor protein that functionally regulates OCTN2. Immunoelectron microscopy visualized both OCTN2 and PDZK1 in microvilli of absorptive epithelial cells. These findings indicate that OCTN2 is predominantly responsible for the uptake of carnitine from the apical surface of mouse small intestinal epithelial cells, and it may therefore be a promising target for oral delivery of therapeutic agents that are OCTN2 substrates.

Key words: Amino Acid, MDR/p-Glycoprotein, Organic anion

This article has been cited by other articles:

				A. Koch, B. Konig, G. I. Stangl, and K. Eder PPAR{alpha} Mediates Transcriptional Upregulation of Novel Organic Cation Transporters-2 and -3 and Enzymes Involved in Hepatic Carnitine Synthesis Experimental Biology and Medicine, March 1, 2008; 233(3): 356 - 365. [Abstract] [Full Text] [PDF]

				S. R. Srinivas, P. D. Prasad, N. S. Umapathy, V. Ganapathy, and P. S. Shekhawat Transport of butyryl-L-carnitine, a potential prodrug, via the carnitine transporter OCTN2 and the amino acid transporter ATB0,+ Am J Physiol Gastrointest Liver Physiol, November 1, 2007; 293(5): G1046 - G1053. [Abstract] [Full Text] [PDF]