Induction of ATB0, an Amino Acid Transporter/Retroviral Receptor by HIV-1 TAT in RPE

Abstract

Abstract: : Purpose: There is an increased incidence of virus-induced neoplasms in humans infected with the AIDS virus HIV-1. ATB⁰ is an amino acid transporter that also functions as a receptor for certain retroviruses. The purpose of this study was to determine if the expression of this amino acid transporter/retroviral receptor is increased in the human retinal pigment epithelial cell line ARPE-19 that has been made to express the HIV-1 TAT protein. Methods: ARPE-19 cells expressing the HIV-1 TAT were made by stable transfection. Control cells and TAT-expressing cells were then used for the measurement of amino acid transport activity that is specific for ATB⁰. This was done by monitoring the Na⁺-dependent uptake of glutamine. Results: The uptake of glutamine increased 2.2-fold in TAT-expressing cells compared to control cells. The TAT-induced transport activity was inhibitable by several neutral amino acids but not by basic and acidic amino acids. The increase in transport activity was detectable only in the presence of Na⁺. The activation of TAT-induced transport activity by Na⁺ showed hyperbolic relationship. The induced transport activity was saturable with increasing concentrations of glutamine with a K_m value of 127 ± 25 µM. The substrate kinetics, substrate specificity and Na⁺-activation kinetics were comparable between control cells and TAT-expressing cells. But, the maximal velocity of the transport process increased 2-fold in TAT-expressing cells compared to control cells. Interestingly, the transport activity in control cells was not influenced by Cl^– whereas the TAT-induced transport activity was abolished in the absence of Cl^–. ATB^0,+ is another transporter that can transport glutamine and this transporter is Na⁺- and Cl^–-dependent. Since the TAT-induced glutamine transport activity is not inhibited by basic amino acids, the observed increase in glutamine transport in TAT-expressing cells is not mediated by ATB^0,+. Conclusions: The expression of HIV-1 TAT in ARPE-19 cells induces the expression of the amino acid transporter/retroviral receptor ATB⁰. The Cl^–-dependency of the transporter appears to be altered by TAT, suggesting some hitherto unidentified post-translational modifications in the transport protein in addition to the induction of the transporter gene. The increase in the cell surface expression of this protein by TAT may provide the molecular basis for the increased incidence of virus-induced neoplasms associated with AIDS.

Keywords: AIDS/HIV • gene/expression • retinal pigment epithelium