Background
Persistent human immunodeficiency virus type 1 (HIV-1) infection causes a progressive loss of CD4+ T cells, followed by acquired immunodeficiency syndrome (AIDS) [
1,
2]. Although highly active antiretroviral therapy (HAART) can improve the life-span of HIV-1 infected patients by successful suppression of the HIV-1 replication, HAART is not able to eliminate the latent reservoir established early during infection [
3]. Accordingly, latent reservoirs of HIV-1 infection have importantly been recognized as a major barrier to HIV-1 eradication [
4,
5].
Numerous strategies for the elimination of latent HIV-1 infection have been investigated. These strategies include intensive ART regimens, transplantation of CCR5-deleted bone marrow, and reactivation of HIV-1 expression. One of the most innovative strategies aims to awaken the latent virus and thereby increase the killing of the latent infection reservoir by the immune system, known as “shock and kill”, and uses T-cell activating cytokines (IL-7), T-cell receptor signaling pathway activators (bryostatin and prostratin as a protein kinase C activator), histone deacetylase inhibitors (valproic acid, vorinostat (suberoylanilide hydroxamic acid)), DNA methylase inhibitors, and disulfiram [
6]. Although these reactivating strategies have recently been shown to increase viral RNA in patients, reduction of reservoir size has not been consistently successful.
Tumor suppressor p53 plays a key role in the stimulation of cell apoptosis and growth arrest through a cooperative signaling network of genotoxic stress caused by treatment with anticancer drugs, irradiation, ultraviolet light, and glucose starvation [
7]. HIV-1 infection also induces genotoxic stresses linked to p53 activation in CD4+ T cells by integration mediated- dsDNA strands break, secretion of type I interferons and expression of HIV-1 accessory proteins Vpr/Vpu, which may be considered as intracellular markers of HIV-1 infection [
8‐
11]. The p53 activation by HIV-1 infection also plays an important role in host-restriction mechanisms against HIV-1 replication, which suppress long terminal repeat (LTR)-mediated viral transcription through Tat modulation [
12].
With regard to apoptosis of HIV-l infected cells, an earlier report showed that latently HIV-l infected H9, J1.1 and U1 cells that are p53 null were more resistant to apoptosis than their uninfected parent cells under treatment with H
2O
2 and staurosporine (STS) which leads to an increase in oxidative stress rather than genotoxic stress [
13]. In a recent study, ACH2 and U1 cells showed induction of HIV-1 reactivation by an alternative replication program (ARP) during cell apoptosis after cytotoxic drug treatment [
14]. Although these studies suggested that apoptosis might be an important characteristic of latent HIV-1 infection, these data might be missed by an examination of p53 status and comparative analysis of the apoptotic ratio between the latently infected and uninfected cells. Currently, the important roles of p53 have been discussed in HIV-1 infection. However, the p53-mediated apoptosis of latently infected cells by treatment of anticancer drugs has not been considered in developing strategies for eliminating cells latently infected with HIV-1. Herein, we hypothesize that apoptosis may be more sensitively induced in latently infected cells encoding competent p53 than uninfected cells upon treatment with anticancer drugs. We found that anticancer drugs greatly enhanced apoptosis of latently infected cells compared with uninfected cells, and knockdown of p53 significantly diminished the apoptosis. Our data suggest that anticancer drugs may be useful agents for selectively purging HIV-1 reservoirs.
Discussion
During the clinical course of HIV-1 infection, latently HIV-1 infected provirus is present in the long-lived CD4+ T memory cells that play a role as a steady source of infectious viral particles after interruption of treatment and as a major obstacle to HIV-1 eradication [
4,
5]. Eliminating such HIV-1 reservoirs is a great challenge, because a selective marker by which to target the reservoir latently infected with HIV-1 has not yet been discovered.
HIV-1 infection can facilitate p53 expression and activation in T cells by integration-mediated host chromosomal abnormality and/or Vpr-induced DNA damaged-stress and Vpu-mediated stabilization of p53 [
8,
10,
11,
23]. The induced p53 may play roles in the host restriction of HIV-1 replication and in the apoptosis of cells acutely infected with HIV-1. Anticancer drugs such as 5-FU, doxorubicin, and etoposide are well known to induce genotoxic stress linked to p53 activation followed by cell apoptosis. Although important roles for p53 in HIV-1 infection have been considered, utilization of anticancer drugs to activate p53 and selectively eliminate cells latently infected with HIV-1 has not been addressed.
In the present study, 5-FU induced the early and late apoptosis of latently infected ACH2 cells compared with latently infected J1.1 and uninfected A3.01 cells. Moreover, the levels of intracellular apoptotic markers such as cleaved caspase-3 and PARP were markedly increased in ACH2 cells compared with those in other cells (Fig.
1). The uninfected A3.01 cells that is p53 competent also showed significantly increased apoptosis upon 5-FU treatment compared with that of p53 defective J1.1 cells, while the levels were lower than those of the latently infected ACH2 cells derived from A3.01 cells. These data reveal that the differences in status of p53 among uninfected and latently infected cells (Fig.
2a) may lead to p53 dependent tendencies toward apoptosis upon genotoxic stress (Figs.
1,
2 and
3). Larrosa et al. showed that H9, J1.1, and U1 cells latently infected with HIV-1 were resistant to cytotoxic substances such as H
2O
2 and STP when compared with their uninfected parent cells [
13]. However, their findings may do not support the resistance to apoptosis of latently infected cells in general, because the cells they used were all p53 null cell lines, which showed resistance to apoptosis upon substance-induced stress. Consistent with a previous report, our data also showed that p53 null J1.1 cells were more resistant to apoptosis upon genotoxic stress than uninfected A3.01 (Figs.
1 and
3) and Jurkat cells (data not shown). Together, these data indicate that latent HIV-1 infection may prevent apoptosis of the host cell to promote longer survival of the cells by an as yet unknown mechanism in the absence of p53, but HIV-1 infection may facilitate the apoptosis of cells susceptible to HIV infection that have competent p53. Although p53 is known to be a key factor in inducing apoptosis in anticancer drug treated cells, the p53 null J1.1 cells still showed small amounts of cell death. These data indicated that the cells were undergoing apoptosis by the p53-independent pathway such as p73 [
24], Bid [
25], MCL2 [
26] and ATM/E2F1 [
27] that were showed in its parent Jurkat cells upon the genotoxic stresses.
Western blot analysis of p53 target gene proteins showed that the levels of Bax, puma, and p21, which are the best-known p53 targets, were greatly increased in latently infected cells expressing p53 upon 5-FU treatment. As such, these gene products may contribute to the sensitivity of cells to apoptosis. Although the 5-FU treated A3.01 cells and the J1.1 cells expressed with ectopic p53 also showed increased levels of apoptosis compared with the untreated cells, the levels were much weaker than those of the p53 competent cells infected with HIV-1. The data indicated that the insufficient level of p53 activation and the alternative pathway of p53 could not greatly increase apoptosis, as shown in the ACH2 and NCHA2 cells. CD4+ T lymphocyte depletion linked to HIV-1 infection has been explained by numerous mechanisms including abortive infection-induced pyroptosis [
28], DNA-dependent protein kinase [
29], Fas-ligand [
30], p53 activation [
8] and viral proteins Nef and Vpr [
31,
32]. These observations sufficiently explained the apoptosis as a result of acute HIV-1 infection. Genini et al. suggested that activation of p53 by acute HIV-1 infection may contribute to the depletion of HIV-1 infected T cells [
8]. The present data also showed that latently infected cells, which are p53 competent, showed an increased level of phosphorylated p53 and its target genes in the absence of drugs, similarly to acute infection. However, the slight enhancement of p53 activation in the latently infected cells might not be sufficient to trigger apoptosis. Accordingly, genotoxic stress induced by anticancer drugs promoted the p53 activation synergistically leading to apoptosis of latently infected cells.
Numerous anticancer drugs have been developed and used for the cure of cancerous disease. Among them, 5-FU, an inhibitor of thymine synthetase, doxorubicin, an inhibitor of topoisomerase II, and etoposide, an inhibitor of topoisomerase I, are widely used in the treatment of cancers [
33]. Although the drugs act on different enzymes linked to chromosomal DNA, these drugs commonly induce DNA stress followed by p53 activation-induced apoptosis. Based on the mechanisms of action of these drugs, 5-FU and doxorubicin/etoposide efficiently induced p53 activation in latently infected cells, followed by apoptosis, but not in p53 null J1.1 cells (Figs.
1 and
3). A new generation of anticancer drug, such as taxane, which inhibit the microtubule synthesis during mitosis unlinked to DNA-damaging stress, are used widely [
34]. Therefore, p53-mediated apoptosis by the drug should determine whether the drug selectively induces the apoptosis of latently infected cells.
To determine the exact role of p53 in apoptosis of latently infected cells upon treatment with an anticancer drug, p53 siRNAs and a p53 inhibitor were employed. Both could reduce the apoptosis of the cells upon 5-FU treatment. The diminished apoptosis in the ACH2 and NCHA2 cells by p53 siRNA and the inhibitor indicated that p53 played an important role in the anticancer drug-induced apoptosis (Fig.
4). However, the siRNA and the inhibitor could not completely reduce the apoptosis. The results might have been caused by the p53-independent pathways and the incomplete efficacy of the siRNA and the inhibitor. A mechanism underlying viral reactivation in response to anticancer drugs has been proposed [
14]. However, the data presented did not show distinctive apoptotic fates between cells because they focused on viral reactivation in the reservoir cells during anticancer drug-induced apoptosis without comparison to uninfected parent cells or assessing p53 status.
Although several methodologies for isolation of latently infected cells ex vivo have been developed, studying the functional role of those cells is so far difficult. There are lot of hurdles, such as the rarity of reservoirs (<1 per 10
6 CD4+ T cells), the absence of suitable markers by which to isolate cells and T-cell activator treatments for surmounting the rarity, which may activate a variety of cellular signals [
35]. Therefore, evaluation of p53-mediated apoptosis of latently infected cells in vivo and ex vivo remains for further study.
Acknowledgments
We are grateful to the AIDS Research and Reference Reagent Program (ARRRP, NIH, USA) for providing cell lines (A3.01, ACH2, Jurkat and J1.1 cells).