Adenoviral modulation of the tumor‐associated system L amino acid transporter, LAT1, alters amino acid transport, cell growth and 4F2/CD98 expressionwith cell‐type specific effects in cultured hepatic cells

Abstract

Altered expression of metabolite transporters is observed frequently in tumor cell lines and primary neoplasms. The extent to which these may to contribute to the growth autonomy associated with cancer is not clear. LAT1 is a major L-type amino acid transporter over-expressed in a variety of cancer types and a light chain component of the CD98 heterodimer. We utilized an adenoviral expression system to modulate the level of LAT1 in a hepatic in vitro model to examine phenotypic changes associated with short-term exogenous and blocked expression. LAT1 levels were increased three fold and resulted in increased L-type amino acid transport as a result of adenoviral expression in murine hepatocytes. The protein was expressed on the cell surface and complexed with the CD98 heavy chain known as 4F2. Surprisingly, levels of the total CD98 protein complex were increased 2.4-fold as a result of adenoviral expression of light chain only, suggesting coordinate regulation. Exogenous overexpression was less effective in normal rat liver cells relative to mouse. LAT1 antisense expression in hepatic tumor cells resulted in a modest though statistically significant decrease in cell number, viability and S-phase cells over a 5-day period relative to controls despite the absence of a significant decrease in L-type transport over this period. These studies are preparatory to in vivo efforts focusing on LAT1/CD98 as a potential therapeutic target. ' 2005 Wiley-Liss, Inc.Key words: LAT1; 4F2; CD98; leucine; adenovirus Recent studies have demonstrated that LAT1 is a major L-type amino acid transporter in a variety of cancer cells including hepatic, 1-3 oral, 4 breast, 5 bladder 6 and colon. 7 This predicted 12 transmembrane spanning protein mediates sodium independent transport of large neutral amino acids including several essential amino acids and is a major route for providing tumor cells with branched and aromatic amino acids. Because these amino acids are indispensable for growth and proliferation-dependent protein synthesis, LAT1 represents a target of high potential therapeutic interest. LAT1 exhibits broad substrate selectivity and is responsible for transport not only of naturally occurring amino acids, but also amino acid-related drugs such as l-dopa, melphalan, (an anti-cancer phenylalanine mustard), triiodothyronine and thyroxine and gabapentin, an anti-convulsant. Functional expression of LAT1 in the plasma membrane requires an additional single transmembrane glycoprotein known as 4F2 antigen or heavy chain and together these form a heterodimeric complex known as CD98. [8][9][10][11][12][13] The glycoprotein-associated transporters are a novel class of amino acid transporters recently the subject of much interest not only in relation to transport but also to CD98-linked functions in cell activation, integrin signaling, cell fusion and malignant transformation. [14][15][16][17] Our research has utilized a tetracycline-inducible adenoviral expression system (TET-Off) system to alter levels of LAT1 and 4F2 in vitro. We have...

L‐type amino acid transporter 1 as a potential molecular target in human astrocytic tumors

“…20,22 Thompson and coworkers recently reported the role of LAT1 as a potential therapeutic target in hepatic tumor cells in vitro. 23 The present study suggests that LAT1 may play an important role in human high-grade gliomas. In addition, inhibitors to LAT1 may be an effective therapeutic option for high-grade gliomas.…”

Enteral β-hydroxy-β-methylbutyrate supplementation increases protein synthesis in skeletal muscle of neonatal pigs

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Divergent expression of L-type amino acid transporter 1 during uterine cervical carcinogenesis

“…Functional expression of LAT1 in the plasma membrane requires an additional single transmembrane glycoprotein known as the heavy chain of the 4F2 antigen, and together, these form a heterodimeric complex known as CD98 [2,3]. LAT1 preferentially transports large neutral amino acids including the essential ones such as leucine, isoleucine, valine, phenylalanine, tyrosine, tryptophan, methionine, and histidine [1]; thus, LAT1 plays a critical role in cell growth and proliferation [1,4].…”