Induction of cystine-glutamate transporter X_c^- by human immunodeficiency virus type 1 transactivator protein Tat in retinal pigment epithelium

PURPOSE. The transactivator protein Tat encoded by the human immunodeficiency virus-1 (HIV-1) genome reduces glutathione levels in mammalian cells. Because the retina contains large amounts of glutathione, a study was undertaken to determine the influence of Tat on glutathione levels, γ-glutamyl transpeptidase activity, and the expression and activity of the cystine-glutamate transporter x_c^- in the human retinal pigment epithelial cell line ARPE-19 and in retina from Tat-transgenic mice. METHODS. The transport function of x_c^- was measured as glutamate uptake in the absence of Na⁺. mRNA levels for xCT and 4F2hc, the two subunits of system x_c^-, were monitored by RT-PCR and Northern blot and protein levels by Western blot. The expression pattern of xCT and 4F2hc in the mouse retina was analyzed by immunofluorescence. RESULTS. Expression of Tat in ARPE-19 cells led to a decrease in glutathione levels and an increase in γ-glutamyl transpeptidase activity. The transport function of x_c^- was upregulated, and this increase was accompanied by increases in the levels of mRNAs for xCT and 4F2hc and in corresponding protein levels. The influence of Tat on the expression of x_c^- was independent of the cellular status of glutathione. Most of these findings were confirmed in the retina of Tat-transgenic mice. CONCLUSIONS. Expression of HIV-1 Tat in the retina decreases glutathione levels and increases γ-glutamyl transpeptidase activity. Tat also upregulates the expression of system x_c^-. Glutathione levels may be decreased and the expression of x_c^- enhanced in the retina of patients with HIV-1 infection, leading to oxidative stress and excitotoxicity.