The implication of the chemokine receptor CXCR4 in HIV-1 envelope protein-induced apoptosis is independent of the G protein-mediated signalling

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Abstract
The envelope glycoprotein complex (gp120/gp41)n of HIV-1 is one of the viral products responsible for increased apoptosis in HIV infection. Here the role of the chemokine receptor CXCR4 in HIV-1 envelope protein-induced apoptosis was investigated. Apoptosis occurring in cocultures of chronically HIV-1 IIIB-infected cells with CD4 target cells expressing the CXCR4 receptor was quantified by terminal deoxinucleotidyl transferase dUTP nick end labeling (TUNEL) or propidium iodide staining followed by fluorescent antibody cell sorting, which allows the evaluation of single-cell killing. Moreover global (single cell- and syncytium-associated) apoptosis was quantified by a new radioactive TUNEL-derived assay. By using these different techniques it was shown that single and syncytium-forming CD4 T cells die by apoptosis upon contact with envelope protein expressing cells independently of viral replication. Moreover, both the CXCR4 agonist SDF-1a, and the antagonist AMD3100, showed inhibitory effects on HIV-1 envelope protein-induced apoptosis in the CD4 T-cell subset of peripheral blood mononuclear cells and CD4 cell lines. CXCR4 signalling-induced by HIV-1 envelope proteins in CD4 T cells was not detected. Furthermore, it was shown that envelope protein-induced apoptosis can occur after treating target cells with the Gi-protein inhibitor pertussis toxin. Evidence is provided for a role of CXCR4 in the mechanisms of HIV envelope protein-induced pathogenesis, contributing to selective CD4 cell killing. The results suggest that CXCR4 is involved in HIV-1-induced apoptosis; however, this role does not appear to involve G-protein-mediated CXCR4 signalling.