Binding of the HIV envelope protein Gp120 to the host cell CD4 receptor induces a conformational change in the Gp120, allowing binding to the cellular co-receptor, principally CCR5 or CXCR4. This attachment is followed by fusion of the viral and host membranes mediated by Gp41, and release of viral genetic material and a number of virus-associated proteins into the cytoplasm of the cell. The viral proteins released during uncoating of the virus include reverse transcriptase, structural proteins, and the viral accessory proteins Nef, Vif, and Vpr. Since Gp120 ligation of the CD4 receptor and co-receptors induces pro-apoptotic signals through a number of pathways [2
], and has been implicated in uninfected bystander T cell apoptosis, it is likely that these early viral-associated proteins protect the newly infected cell from premature cell death.
For instance, Vpr is an HIV-accessory protein present in high copy number in mature virions, and is released into the cytoplasm upon uncoating of the infecting virus; it has been variously described to have both pro- and anti-apoptotic properties based on the infection model and protein concentration [2
]. Jurkat T cells stably expressing low levels of Vpr are resistant to both intrinsic and extrinsic apoptotic signals compared to control cells, and this is associated with increased Bcl-2 and decreased Bax expression [15
]. Jurkat cells infected with HIV have a transient resistance to apoptosis induced by tumor necrosis factor (TNF) and cyclohexamide at 48 h after infection, which reverses at 72 h [16
]. This apoptotic-resistant phenotype of HIV infection is dependent on Vpr expression, and occurs predominately in productively infected cells as opposed to bystander cells [16
]. However, in later stages of infection, the opposite effect is seen, in that apoptosis in the absence of additional stimuli is increased in productively infected Jurkat cells compared to uninfected cells. Furthermore, inhibition of Vpr using antisense oligodeoxynucleotides reverses this increased spontaneous apoptosis [16
]. Similar results are demonstrated in HIV-infected primary cells. These experiments show that Vpr expressed in low levels early in infection are anti-apoptotic, but in higher concentrations later in infection the same protein is pro-apoptotic.
Vpr also induces G2 cell cycle arrest in infected cells, one effect that has been implicated in HIV-induced apoptosis. However, early in infection, the HIV-accessory protein Vif induces proteosomal degradation of Vpr, similar to Vif’s function of inducing degradation of the ABOBEC3G host restriction factor. Therefore, expression of Vif early in the infection cycle may delay Vpr-induced G2 cell cycle arrest, allowing for early infected cell survival [17
Nef is a polyfunctional HIV-associated protein that downregulates the expression of a number of cell surface receptors on the infected cell, including MHC Class I molecules and CD28. In addition, Nef expression, and to a lesser extent Vpu and Gp120, in HIV-infected cells decreases CD4 surface expression, decreasing the likelihood of superinfection with a second virus [18
]. Superinfection increases infected cell apoptosis [18
]; therefore, this may represent another antiapoptotic mechanism early in the viral life cycle. Nef also directly binds to p53 through its N-terminal end and decreases the intracellular half life of p53 and p53-dependent transcriptional activation in in vitro HIV infection [19
]. As additional evidence of Nef’s anti-apoptotic effects, endogenous expression of Nef in MOLT-4 cells protects against p53-dependent UV-induced apoptosis. Since p53 signaling has been implicated in apoptosis induced by the HIV Gp120 [20
], it is possible that early expression of Nef inhibits pro-apoptotic signals induced by viral attachment and entry.
A number of cellular proteins, in addition to the viral proteins discussed above, are packaged into and accompany infecting HIV virions [21
], and could have an early anti-apoptotic effect. For instance, the cellular protein kinase mitogen-activated protein kinase 1 (MAPK1), also known as extracellular signal-regulated kinase 2 (ERK2), is incorporated into the virion, and has been shown to be necessary for nuclear translocation of the HIV pre-integration complex [22
]. Phosphorylation by MAPK1/ERK2 regulates the activity of a number of apoptotic regulatory proteins, and inhibition of MAPK1/ERK2 activity inhibits cellular proliferation and decreases survival [23
]. It is possible that MAPK1/ERK2 incorporated into HIV virions could have a similar function in early stages of cellular infection with the virus, although this has not been directly studied. Additional examples of cellular proteins that are incorporated into HIV that have anti-apoptotic properties include heat shock protein 70 (Hsp70) and cyclophilin A [21
A recent study published in Cell
by Doitsh and colleagues has caused a true paradigm shift in the understanding of HIV-induced apoptosis, and so-called “bystander” apoptosis [24
]. Using a model of ex vivo infection of human lymphoid tissue, they demonstrated that incomplete reverse transcription of the viral genetic material results in the accumulation of cytotoxic DNA material, resulting in paraptosis (a form of apoptosis) of cells nonpermissive of viral replication. Therefore, bystander cells, instead of being truly uninfected, may be non-productively infected, and this condition of non-productive infection drives cell death. It is possible, then, that the ultimate fate of the early infected cell is dependent upon mechanisms that promote survival in the face of this cytotoxic insult, and may rely on the above-mentioned anti-apoptotic viral or cellular proteins that accompany the infecting virion.