Identification of a Novel System L Amino Acid Transporter Structurally Distinct from Heterodimeric Amino Acid Transporters

Abstract

A cDNA that encodes a novel Na ؉ -independent neutral amino acid transporter was isolated from FLC4 human hepatocarcinoma cells by expression cloning. When expressed in Xenopus oocytes, the encoded protein designated LAT3 (L-type amino acid transporter 3) transported neutral amino acids such as L-leucine, L-isoleucine, L-valine, and L-phenylalanine. The LAT3-mediated transport was Na ؉ -independent and inhibited by 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid, consistent with the properties of system L. Distinct from already known system L transporters LAT1 and LAT2, which form heterodimeric complex with 4F2 heavy chain, LAT3 was functional by itself in Xenopus oocytes. The deduced amino acid sequence of LAT3 was identical to the gene product of POV1 reported as a prostate cancer-upregulated gene whose function was not determined, whereas it did not exhibit significant similarity to already identified transporters. The Eadie-Hofstee plots of LAT3-mediated transport were curvilinear, whereas the low affinity component is predominant at physiological plasma amino acid concentration. In addition to amino acid substrates, LAT3 recognized amino acid alcohols. The transport of L-leucine was electroneutral and mediated by a facilitated diffusion. In contrast, L-leucinol, Lvalinol, and L-phenylalaninol, which have a net positive charge induced inward currents under voltage clamp, suggesting these compounds are transported by LAT3. LAT3-mediated transport was inhibited by the pretreatment with N-ethylmaleimide, consistent with the property of system L2 originally characterized in hepatocyte primary culture. Based on the substrate selectivity, affinity, and N-ethylmaleimide sensitivity, LAT3 is proposed to be a transporter subserving system L2. LAT3 should denote a new family of organic solute transporters.

The mechanism of transmembraneS-nitrosothiol transport

“…It is ubiquitous and is essential in cellular nutrition and transcellular transport of neutral amino acids. Various subtypes with different substrate specificities and transport properties have been described for system L (30)(31)(32)(33)(34). For example, LAT1 prefers neutral amino acid with branched or aromatic side chains (31), whereas LAT2 has a broad substrate specificity toward all the L-isomers of neutral amino acids, including L-cysteine (32,33).…”

Relationship between [ 14 C]MeAIB uptake and amino acid transporter family gene expression levels or proliferative activity in a pilot study in human carcinoma cells: Comparison with [ 3 H]methionine uptake

“…Moreover, amino acids and amino acid transporters play important roles other than in energy metabolism, such as in macromolecular synthesis, mTOR activation, and ROS homeostasis beyond energy metabolism [49]. There are approximately fifty different types of amino acid transporters, but only LAT1 [50], LAT3 [51], ASCT2 [52], ATB 0, + [53] and xCT [54] have been reported to be expressed at high levels on the surface of cancer cells. Recently, there have been numerous reports in cancer cells related to glutamine transport via ASCT2 and LAT1 [55][56][57].…”

The ErbB3-binding protein Ebp1 suppresses androgen receptor-mediated gene transcription and tumorigenesis of prostate cancer cells

“…The kallikrein 2 gene is regulated by AR and being evaluated as a marker for prostate cancer progression (25). The POV-1 gene encodes a transcript for an L amino acid transporter (26) that is up-regulated in aggressive prostate carcinoma (27). The TMPRSS2 gene is androgen-regulated and also highly expressed in the prostate and prostate cancer (28).…”