Loss of G ₁ /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 transcription assays. 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 Interaction with RNA Polymerase II C-terminal Domain (CTD) and a Dynamic Association with CDK2 Induce CTD Phosphorylation and Transcription from HIV-1 Promoter

“…These data indicate that Tat-dependent transcription may in part be regulated by CDK2, which activity is highest at the G 1 phase. Finally, we have observed that the activity of CDK2/cyclin E is increased in HIV-1-infected quiescent peripheral CD4 lymphocytes (27). Collectively, our published data (9 -11, 26, 27) points to a possibility that Tat-dependent transcription may be regulated by CDK2.…”

Biochemical mechanisms of HIV induced T cell apoptosis

“…Recently, Clark et al reported that HIV infected T cells bypass the G1/S checkpoint by inhibiting p21 Waf1 , a known cyclin dependent kinase inhibitor. 89 This inhibition is mediated by the binding of p53 by Tat, and sequestration of its transactivation activity. The authors postulate that this loss of the G1/S checkpoint provides a selective advantage for HIV by allowing virus associated transcription and production of virions, processes which require T cell cycling.…”

HIV-1 Tat increases cell survival in response to cisplatin by stimulating Rad51 gene expression

“…The observed enhancement in the level of Rad51 in PC12-Tat cells is not restricted (Duan et al, 1994;Li et al, 1995;Clark et al, 2000;Magnusson et al, 2000), an event that can lead to enhancement of homologous recombination (Sengupta et al, 2003). Elevation of the level of Rad51 by Tat can have biological relevance and can be beneficial for HIV-1 and its lytic infection in many ways.…”