Nef protein of HIV‐1 induces apoptotic cytolysis of murine lymphoid cells independently of CD95 (Fas) and its suppression by serine/threonine protein kinase inhibitors

113

The effects of soluble Nef protein on CD4؉ T cells were examined. CD4 ؉ -T-cell cultures exposed to soluble Nef were analyzed for apoptosis by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling and hallmarks of apoptosis including cytoplasmic shrinkage, nuclear fragmentation, DNA laddering, and caspase activation. We observed dose-and time-dependent inductions of apoptosis. DNA laddering and activated caspase 3 were also evident. Cells treated with Nef/protein kinase inhibitor complexes were protected from Nef-induced apoptosis, suggesting possible roles for protein kinases in the apoptosis pathway. Similarly, cells treated with Nef/anti-Nef antibody complexes were protected from Nef-induced apoptosis. The cellular receptor responsible for Nef-induced apoptosis was identified through antibody-and ligand-blocking experiments as a receptor commonly involved in viral entry. CXCR4 antibodies, as well as the endogenous ligand SDF-1␣, were effective in blocking Nef-induced apoptosis, while CCR5 and CD4 antibodies were ineffective. Moreover, a CXCR4-deficient cell line, MDA-MB-468, which was resistant to Nef-induced apoptosis, became sensitive upon transfection with a CXCR4-expressing vector. This study suggests that extracellular Nef protein could contribute to the decline of CD4 counts prior to and during the onset of AIDS in patients with human immunodeficiency virus type 1 infections.

Section: Discussionsupporting

confidence: 92%

Section: Resultsmentioning

confidence: 99%

Section: Finkel Et Al Reported That Apoptosis Was Observed In Cd4mentioning

confidence: 99%

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Extracellular Nef Protein Targets CD4⁺T Cells for Apoptosis by Interacting with CXCR4 Surface Receptors

James¹,

Huang²,

Khan³

et al. 2004

113

“…Vpr arrests the cell cycle of infected cells at the G 2 /M phase, which can lead to caspase activation and apoptosis (49). The regulatory protein Nef has been suggested to induce apoptosis through a serine/threonine kinase-dependent signaling pathway (42). Studies of HIV-1 deletion mutants, however, have demonstrated that the expression of these regulatory proteins is either not necessary or insufficient for the major cytopathic effects of virus infection (14,45,47).…”

Section: Human Immunodeficiency Virus Type 1 (Hiv-1) Infection Of Hummentioning

confidence: 99%

Importance of Membrane Fusion Mediated by Human Immunodeficiency Virus Envelope Glycoproteins for Lysis of Primary CD4-Positive T Cells

LaBonte

Patel

Hofmann

et al. 2000

“…These observations appear to be inconsistent with a model in which antiviral immune responses represent the primary mechanism underlying the loss of CD4 ϩ T cells. Many studies have suggested cytotoxic effects of HIV-1 proteins, including Tat, Vpr, Nef, and protease, in tissue culture systems (5,12,36,44,56,69,70,73,74,80,85,86,89,93). However, vpr-or nef-deleted HIV-1 still exhibits significant cytotoxicity in tissue culture, and SIV mutants lacking these genes are still capable of causing AIDS in some monkeys (2,35,42,49,50,63).…”

mentioning

confidence: 99%

Cytolysis by CCR5-Using Human Immunodeficiency Virus Type 1 Envelope Glycoproteins Is Dependent on Membrane Fusion and Can Be Inhibited by High Levels of CD4 Expression

LaBonte

Madani

Sodroski

2003

T-tropic (X4) and dualtropic (R5X4) human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins kill primary and immortalized CD4؉ CXCR4 ؉ T cells by mechanisms involving membrane fusion. However, because much of HIV-1 infection in vivo is mediated by M-tropic (R5) viruses whose envelope glycoproteins use CCR5 as a coreceptor, we tested a panel of R5 and R5X4 envelope glycoproteins for their ability to lyse CCR5 ؉ target cells. As is the case for CXCR4؉ target cells, HIV-1 envelope glycoproteins expressed by single-round HIV-1 vectors killed transduced CD4؉ CCR5 ؉ cells in a membrane fusion-dependent manner. Furthermore, a CD4-independent R5 HIV-1 envelope glycoprotein was able to kill CD4-negative target cells expressing CCR5, demonstrating that CD4 is not intrinsically required for the induction of death. Interestingly, high levels of CD4 expression protected cells from lysis and syncytium formation mediated by the HIV-1 envelope glycoproteins. Immunoprecipitation experiments showed that high levels of CD4 coexpression inhibited proteolytic processing of the HIV-1 envelope glycoprotein precursor gp160. This inhibition could be overcome by decreasing the CD4 binding ability of gp120. Studies were also undertaken to investigate the ability of virion-bound HIV-1 envelope glycoproteins to kill primary CD4 ؉ T cells. However, neither X4 nor R5X4 envelope glycoproteins on noninfectious virions caused death in primary CD4 ؉ T cells. These results demonstrate that the interaction of CCR5 with R5 HIV-1 envelope glycoproteins capable of inducing membrane fusion leads to cell lysis; overexpression of CD4 can inhibit cell killing by limiting envelope glycoprotein processing.