Role of calmodulin in HIV-potentiated Fas-mediated apoptosis

The recently demonstrated extraordinary rate of turnover of T cells in human immunodeficiency virus (HIV)-1-infected patients and the apparently concomitant high rate of viral production and death are consistent with a large amount of cell death directly due to infection. Apoptosis may be one of the major forms of T cell death in HIV-1 infection. Many apoptotic pathways depend on calcium and therefore would be expected to involve calmodulin. As the HIV-1 envelope glycoprotein, gp160, contains two known calmodulin-binding domains, we investigated the possibility that the cytoplasmic domain of the HIV-1 envelope protein gp160 could enhance Fas-mediated apoptosis, the major form of apoptosis in lymphocytes. Our studies have shown that 1) transfection of H9 and MOLT-4 cells with a non-infectious HIV proviral clone, pFN, which expresses wild-type gp160, leads to enhanced Fas-mediated apoptosis, 2) transfection of MOLT-4 cells with a pFN construct pFN delta 147, which expresses a carboxyl-terminally truncated gp160 lacking the calmodulin-binding domains, produces less Fas-mediated apoptosis than transfection with pFN, and 3) the calmodulin antagonists trifluoperazine and tamoxifen completely inhibit the pFN enhancement of Fas-mediated apoptosis in MOLT-4 cells. We have replicated all of these results using the vectors pSRHS and pSRHS delta 147, which express wild-type gp160 and truncated gp160, respectively, in the absence of other viral proteins. These investigations provide a mechanism by which HIV-1 may induce apoptosis and a possible intracellular target for future therapeutics.