Loss of G1/S Checkpoint in Human Immunodeficiency Virus Type 1-Infected Cells Is Associated with a Lack of Cyclin-Dependent Kinase Inhibitor p21/Waf1

Abstract

Productive high-titer infection by human immunodeficiency virus type 1 (HIV-1) requires the activation of target cells. Infection of quiescent peripheral CD4 lymphocytes by HIV-1 results in incomplete, labile reverse transcripts and lack of viral progeny formation. An interplay between Tat and p53 has previously been reported, where Tat inhibited the transcription of the p53 gene, which may aid in the development of AIDS-related malignancies, and p53 expression inhibited HIV-1 long terminal repeat transcription. Here, by using a well-defined and -characterized stress signal, gamma irradiation, we find that upon gamma irradiation, HIV-1-infected cells lose their G 1 /S checkpoints, enter the S phase inappropriately, and eventually apoptose. The loss of the G 1 /S checkpoint is associated with a loss of p21/Waf1 protein and increased activity of a major G 1 /S kinase, namely, cyclin E/cdk2. The p21/Waf1 protein, a known cyclin-dependent kinase inhibitor, interacts with the cdk2/cyclin E complex and inhibits progression of cells into S phase. We find that loss of the G 1 /S checkpoint in HIV-1-infected cells may in part be due to Tat's ability to bind p53 (a known activator of the p21/Waf1 promoter) and sequester its transactivation activity, as seen in both in vivo and in vitro transcriptionassays. The loss of p21/Waf1 in HIV-1-infected cells was specific to p21/Waf1 and did not occur with other KIP family members, such as p27 (KIP1) and p57 (KIP2). Finally, the advantage of a loss of the G 1 /S checkpoint for HIV-1 per se may be that it pushes the host cell into the S phase, which may then allow subsequent virus-associated processes, such as RNA splicing, transport, translation, and packaging of virion-specific genes, to occur.

HIV-1 Tat targets Tip60 to impair the apoptotic cell response to genotoxic stresses

“…The apoptosis of HIV-infected and bystander cells is thought to be the major cause of T-cell loss (Selliah and Finkel, 2001;Bell and Dockrell, 2003). However, emerging data suggest that HIV also developed mechanisms blocking and delaying the programmed cell death, increasing cell survival as well as activating cell proliferation (Fackler and Baur, 2002;Choi and Smithgall, 2004) with a particularly important role for Tat (Clark et al, 2000;Corallini et al, 2002;Deregibus et al, 2002;Chauhan et al, 2003;Bergonzini et al, 2004;Chipitsyna et al, 2004). In fact, it may be beneficial for HIV to inhibit apoptosis until high levels of progeny viruses are produced (Selliah and Finkel, 2001).…”

“…Accordingly, it has been reported that, while Tat induces apoptosis in uninfected T cells in peripheral blood, it decreases apoptosis in HIV-1-infected T cells (McCloskey et al, 1997;Lum and Badley, 2003). In fact, the loss of the G 1 /S checkpoint may provide a selective advantage for HIV by allowing virus transcription and virion production, which require T-cell cycling (Clark et al, 2000). Tat appears to play an important role in this particular process by inactivating the p53 pathway.…”

“…Tat appears to play an important role in this particular process by inactivating the p53 pathway. HIV-1-infected cells have indeed lost the p21 cip1 gene expression, and this is accompanied by a loss of the G 1 /S checkpoint (Clark et al, 2000). Moreover, p53 was shown to inhibit HIV-1 LTR transcription, and Tat could repress p53 gene expression (Duan et al, 1994;Li et al, 1995;Clark et al, 2000).…”

Using peptides to study the interaction between the p53 tetramerization domain and HIV‐1 tat

“…The interaction sites were found to be AA 341–355 in p53, corresponding to the NES in the tetramerization domain, and AA 49–57 in the ARM domain of Tat 15. The C‐terminal part of p53, which includes the tetramerization domain, was shown to bind Tat specifically 30. Subsequent in vivo studies demonstrated that AA 73–86 of Tat bind p53 indirectly, and that the interaction is mediated by cellular proteins 16.…”

Cell cycle kinetic dysregulation in HIV‐infected normal lymphocytes

“…P53 is upregulated by cellular stress and noxious injury and is an important inhibitor of cell cycle progression at numerous check points throughout the cell cycle (20). Clark et al demonstrated that the HIV-1 Tat interaction with p53 results in the loss of p21/Waf1 inhibition of cyclin E-cdk2 activation (21). A potential consequence of these events is the preferential entry of HIV-1-infected cells into S phase, where viral replication can proceed, as has been suggested by others (7).…”