Taylor, Julie

ABSTRACTHuman monocytes and macrophages are persistent reservoirs of human immunodeficiency virus (HIV) type-1. Persistent HIV infection of these cells results in increased levels of NF-κB in the nucleus secondary to increased IκBα, IκBβ, and IκBɛ degradation, a mechanism postulated to regulate viral persistence. To characterize the molecular mechanisms regulating HIV-mediated degradation of IκB, we have sought to identify the regulatory domains of IκBα targeted by HIV infection. Using monocytic cells stably expressing different transdominant molecules of IκBα, we determined that persistent HIV infection of these cells targets the NH2but not the COOH terminus of IκBα. Further analysis demonstrated that phosphorylation at S32and S36is necessary for HIV-dependent IκBα degradation and NF-κB activation. Of the putative N-terminal IκBα kinases, we demonstrated that the Iκκ complex, but not p90rsk, is activated by HIV infection and mediates HIV-dependent NF-κB activation. Analysis of viral replication in cells that constitutively express IκBα negative transdominant molecules demonstrated a lack of correlation between virus-induced NF-κB (p65/p50) nuclear translocation and degree of viral persistence in human monocytes.

ABSTRACT Casp8p41, a novel protein generated when HIV-1 protease cleaves caspase 8, independently causes NF-κB activation, proinflammatory cytokine production, and cell death. Here we investigate the mechanism by which Casp8p41 induces cell death. Immunogold staining and electron microscopy demonstrate that Casp8p41 localizes to mitochondria of activated primary CD4 T cells, suggesting mitochondrial involvement. Therefore, we assessed the dependency of Casp8p41-induced death on Bax/Bak and caspase 9. In wild-type (WT) mouse embryonic fibroblast (MEF) cells, Casp8p41 causes rapid mitochondrial depolarization ( P < 0.001), yet Casp8p41 expression in Bax/Bak double-knockout (DKO) MEF cells does not. Similarly, caspase 9-deficient T cells (JMR cells), which express Casp8p41, undergo minimal cell death, whereas reconstituting these cells with caspase 9 (F9 cells) restores Casp8p41 cytotoxicity ( P < 0.01). The infection of caspase 9-deficient cells with a green fluorescent protein (GFP) HIV-1 reporter virus results in cell death in 32% of infected GFP-positive cells, while the restoration of caspase 9 expression in these cells restores infected-cell killing to 68% ( P < 0.05), with similar levels of viral replication between infections. Our data demonstrate that Casp8p41 requires Bax/Bak to induce mitochondrial depolarization, which leads to caspase 9 activation following either Casp8p41 expression or HIV-1 infection. This understanding allows the design of strategies to interrupt this form of death of HIV-1-infected cells.