LAT2, a New Basolateral 4F2hc/CD98-associated Amino Acid Transporter of Kidney and Intestine

Abstract

Glycoprotein-associated amino acid transporters (gpaAT) are permease-related proteins that require heterodimerization to express their function. So far, four vertebrate gpaATs have been shown to associate with 4F2hc/CD98 for functional expression, whereas one gpaAT specifically associates with rBAT. In this study, we characterized a novel gpaAT, LAT2, for which mouse and human cDNAs were identified by expressed sequence tag data base searches. The encoded ortholog proteins are 531 and 535 amino acids long and 92% identical. They share 52 and 48% residues with the gpaATs LAT1 and y ؉ LAT1, respectively. When mouse LAT2 and human 4F2hc cRNAs were co-injected into Xenopus oocytes, disulfide-linked heterodimers were formed, and an L-type amino acid uptake was induced, which differed slightly from that produced by LAT1-4F2hc: the apparent affinity for L-phenylalanine was higher, and L-alanine was transported at physiological concentrations. In the presence of an external amino acid substrate, LAT2-4F2hc also mediated amino acid efflux. LAT2 mRNA is expressed mainly in kidney and intestine, whereas LAT1 mRNA is expressed widely. Immunofluorescence experiments showed colocalization of 4F2hc and LAT2 at the basolateral membrane of kidney proximal tubules and small intestine epithelia. In conclusion, LAT2 forms with LAT1 a subfamily of L-type gpaATs. We propose that LAT1 is involved in cellular amino acid uptake, whereas LAT2 plays a role in epithelial amino acid (re)absorption.

Basolateral LAT-2 Has a Major Role in the Transepithelial Flux of L-Cystine in the Renal Proximal Tubule Cell Line OK

“…Moreover, among the heteromeric amino acid transporters that mediate exchange of neutral amino acids, only LAT-2 transport activity, but not LAT-1, asc-1 and asc-2, is expressed in the basolateral domain of OK cells. This is in full agreement with the expression of these transporters in the epithelial cells of the renal proximal tubule; LAT-1 is barely expressed in human and mouse kidney (38,39), asc-1 is not expressed in kidney (28), asc-2 is localized in collecting ducts in mouse kidney (29), and only LAT-2 is conspicuously expressed in the epithelial cells of the renal proximal tubule and the small intestine (21)(22)(23)40).…”

“…The following criteria were used to detect LAT-2 activity: (1) L-alanine is transported via LAT-2, asc-1, and asc-2 (22,23,28,29), but it is not transported via LAT-1 (30,31); (2) transport of L-alanine via LAT-2, but not via asc-1 and asc-2, is inhibited by BCH and L-tyrosine (23) To check the amino acid exchanger activity of the LAT-2-related L-system (21,22,24), polarized cells were loaded on the basolateral and apical membranes with the radioactive substrate (L-[ 3 H] leucine or L-[ 3 H] alanine), and the efflux across the basolateral membrane was measured. The efflux of Lleucine or L-alanine trans-stimulated by LAT-2 substrates was mostly sodium independent.…”

“…In these cells, like it is believed to occur during renal reabsorption (1), taken-up cystine is reduced to cysteine for efflux (19,20). LAT-2 is expressed in the basolateral plasma membrane of the renal epithelial cells of the upper part of the proximal tubule (S1 and S2 segments) (21), where most of amino acid reabsorption occurs (1). LAT-2/4F2hc co-expressed in Xenopus oocytes show exchange of neutral amino acids of any size (21)(22)(23)(24).…”

“…LAT-2 is expressed in the basolateral plasma membrane of the renal epithelial cells of the upper part of the proximal tubule (S1 and S2 segments) (21), where most of amino acid reabsorption occurs (1). LAT-2/4F2hc co-expressed in Xenopus oocytes show exchange of neutral amino acids of any size (21)(22)(23)(24). These characteristics suggest that LAT-2 may play a role in renal reabsorption, but which amino acids use this transporter for net efflux or influx in the context of a renal epithelial cell is unknown.…”

CD98hc (SLC3A2) regulation of skin homeostasis wanes with age

“…CD98hc is highly expressed on the surface of epithelial cells of most embryonic and adult tissues, including epidermis, the choroid plexus in the brain and retina, and in intestinal, renal, and thymic epithelium (Nakamura et al, 1999;Rossier et al, 1999;Dave et al, 2004). CD98hc overexpression in intestinal epithelial cells induces gut homeostatic defects linked to changes in cell proliferation and survival consequent to integrin signaling alterations (Nguyen et al, 2011).…”