Antagonistic interaction of HIV-1 Vpr with Hsf-mediated cellular heat shock response and Hsp16 in fission yeast (Schizosaccharomyces pombe)
- Zsigmond Benko1,
- Dong Liang1, 2,
- Emmanuel Agbottah4,
- Jason Hou1,
- Lorena Taricani†3,
- Paul G Young3,
- Michael Bukrinsky4 and
- Richard Y Zhao1, 2Email author
© Benko et al; licensee BioMed Central Ltd. 2007
Received: 05 January 2007
Accepted: 07 March 2007
Published: 07 March 2007
Expression of the HIV-1 vpr gene in human and fission yeast cells displays multiple highly conserved activities, which include induction of cell cycle G2 arrest and cell death. We have previously characterized a yeast heat shock protein 16 (Hsp16) that suppresses the Vpr activities when it is overproduced in fission yeast. Similar suppressive effects were observed when the fission yeast hsp16 gene was overexpressed in human cells or in the context of viral infection. In this study, we further characterized molecular actions underlying the suppressive effect of Hsp16 on the Vpr activities.
We show that the suppressive effect of Hsp16 on Vpr-dependent viral replication in proliferating T-lymphocytes is mediated through its C-terminal end. In addition, we show that Hsp16 inhibits viral infection in macrophages in a dose-dependent manner. Mechanistically, Hsp16 suppresses Vpr activities in a way that resembles the cellular heat shock response. In particular, Hsp16 activation is mediated by a heat shock factor (Hsf)-dependent mechanism. Interestingly, vpr gene expression elicits a moderate increase of endogenous Hsp16 but prevents its elevation when cells are grown under heat shock conditions that normally stimulate Hsp16 production. Similar responsive to Vpr elevation of Hsp and counteraction of this elevation by Vpr were also observed in our parallel mammalian studies. Since Hsf-mediated elevation of small Hsps occurs in all eukaryotes, this finding suggests that the anti-Vpr activity of Hsps is a conserved feature of these proteins.
These data suggest that fission yeast could be used as a model to further delineate the potential dynamic and antagonistic interactions between HIV-1 Vpr and cellular heat shock responses involving Hsps.
Human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr), a virion-associated protein with a calculated molecular weight of 12.7 kilodalton (kD), is highly conserved among HIV, simian immunodeficiency virus (SIV) and other lentiviruses [1–3]. During the acute phase of the viral infection, Vpr is preferentially targeted by the HIV-specific CD8 T-lymphocytes [4, 5]. Increasing evidence suggests that Vpr plays an important role in the viral life cycle and pathogenesis. For example, Vpr is required both in vitro and in vivo for viral pathogenesis and efficient viral infection of non-dividing host cells such as monocytes and macrophages [6, 7]. Rhesus monkeys, chimpanzees and human subjects infected with Vpr-defective viruses have a slower disease progression often accompanied by reversion of the mutated vpr genes back to the wild type phenotype [8–12].
Vpr displays several distinct activities in host cells. These include induction of cell cycle G2 arrest [13–17] and cell killing . The cell cycle G2 arrest induced by Vpr is thought to suppress human immune functions by preventing T cell clonal expansion  and to provide an optimized cellular environment for maximal levels of viral replication . In addition, Vpr induces cell death, which may contribute to the depletion of CD4+ T-cells in HIV-infected patients [12, 18]. Whether Vpr-induced G2 arrest and cell death are functionally independent of each other is currently of controversial. There are reports suggested that these two activities are separable both in fission yeast and mammalian cells [20–24]; others suggested that Vpr-induced apoptosis is cell cycle dependent [25, 26]. Reasons for these discrepancies are not clear at the moment. In an earlier report, we demonstrated that overexpression of fission yeast (Schizosaccharomyces pombe) Hsp16 specifically suppresses Vpr activities, resembling cellular stress responses to heat shock, . Here, we further show that this suppression is mediated by a heat shock factor (Hsf)-mediated mechanism. Furthermore, we have also tested the suppressive effect of Hsp16 on wild type and a F34I mutant Vpr. The wild type Vpr induces cell cycle G2 arrest and cell death, the F34IVpr mutant is incapable of inducing cell death but retains its ability to induce cell cycle G2 arrest both in fission yeast [21, 27, 28] and mammalian cells (; our unpublished data) Thus, examination of the wild type and the F34I mutant Vpr enable us to investigate these two Vpr activities separately. In addition, the highly conserved Vpr effect on cell cycle G2/M regulation and cell survival makes fission yeast a particularly useful model to study mechanisms of these Vpr activities (For review of this subject, see [30–34]). Interestingly, vpr gene expression appears to trigger a moderate increase in Hsp16 levels but counteracts heat shock-mediated elevation of Hsp16. Together, our findings suggest a highly conserved and dynamic interplay between vpr gene expression and cellular heat shock response involving heat shock proteins.
Endogenous Hsp16 is responsive to vpr gene expression
We next tested whether the expression of endogenous hsp16 is responsive to vpr gene expression. Both the wild type vpr and F34I mutant vpr genes were induced by depleting thiamine from the EMM medium as previously described [36, 37]. As shown in Fig. 1B, expression of wild type vpr or mutant vpr' under normal growth conditions elicited a moderate increase of the Hsp16 protein level (Fig. 1B, lanes 3 and 5). The faint protein band in lane 2 could possibly be due to low level of vpr expression even when the inducible promoter is repressed . Together, these observations suggest that Hsp16 production is responsive to vpr gene expression. These results are consistent with our studies in mammalian cells where vpr gene expression stimulates expression of HSP27, a human paralogue of Hsp16 (Our unpublished data).
Overproduction of Hsp16 suppresses viral infection in CD4-positive T-cells and macrophages
Vpr activities have been implicated as positive factors for HIV-1 replication [6, 8, 38]. Consistent with these activities, Vpr has been shown to increase viral replication 2 to 4 fold in proliferating T lymphocytes [8, 39, 40] but its activities are required for viral infection in non-dividing cells such as macrophages [6, 7]. Responsive expression of human HSP27 and yeast hsp16 to Vpr suggest a possible and highly conserved cellular activity against Vpr. Indeed, we have showed previously that overproduction of Hsp16 reduces viral replication in CD4-positive T-cells in a Vpr-dependent manner .
To examine whether Hsp16 retains its suppressive effect on viral replication when 24 aa of the C-terminal Vpr is removed, we repeated the same infection experiments in the H9 and CEM-SS cells using the C-terminal truncated Vpr-carrying viral strain IIIB. As shown in Fig. 2B-b and Fig. 2B–d, the kinetics of viral replication were essentially indistinguishable between cells with or without Hsp16, suggesting that Hsp16 has lost its inhibitory effect on viral replication in the absence of C-terminal end of Vpr.
The above data suggest the suppressive effect of Hsp16 is specific to Vpr. Since Vpr is required for viral infection in non-dividing cells such as macrophages, we next tested the potential effect of Hsp16 on HIV infection in macrophages. Purified fission yeast Hsp16 protein was added to primary human macrophages infected with HIV-1ADA with increasing concentration from 1, 5 to 10 μg/ml of cells. Viral replication was followed 7 and 10 days after infection by measuring the reverse transcriptase (RT) activities in culture supernatants. To avoid potential interference of endotoxin that often presents in purified recombinant proteins , purified Hsp16 was treated with 10 μg/ml Polymyxin B (PMB)-agarose that was shown to efficiently remove endotoxin . As shown in Fig. 2C-a, infected macrophages without removing endotoxin (-PMB) almost completely eliminated viral replication at day 7 after infection; about 3.5 to 7-fold decrease of viral infection was observed at day 10 with 1 or 5 μg/ml of Hsp16. No viral activity was detectable at 10 μg/ml level. After removing the possible endotoxin from Hsp16, reduced but still significant reduction of viral replication was observed both at day 7 and day 10 after infection. 2.6 to 8.0-fold decrease of viral replication were seen at day 7 with 1.7 to 5.5-fold reduction of viral replication was observed in day 10. These data suggest a dose-dependent suppression of viral replication by Hsp16 in macrophage. To ensure the observed effect was indeed due to Hsp16, as a control, we also tested the potential effect of purified HSP27. 10 μg/ml of HSP27 with the same level of PMB (10 μg/ml) was added to HIV-1ADA-infected macrophages the same way as we did for Hsp16. RT activities were measured over time. As shown in Fig. 2C-b, no significant differences were seen during the entire 24 days after infection. These data suggest a dose-dependent suppression of viral replication by Hsp16 in macrophage. Together, these data show that overproduction of Hsp16 specifically inhibits HIV-1 infection possibly by targeting the Vpr activates.
Heat shock factor is the key regulator for the elevation of Hsp16 and heat shock-mediated suppression of Vpr
These observations suggested that Hsf1 is probably the major cellular factor that contributes to the anti-Vpr activities. To verify this finding, we further examined whether heat shock treatment can induce additional shortening of cells besides the suppressive Hsf1 effect. If additional cellular factors are involved in suppressing Vpr' during the cellular heat shock response, we would expect to see shorter cell length than when Hsf is overexpressed alone. The same experiment as described above was repeated at elevated temperature (36°C). No additional shortening of cells beyond the length observed with Hsf1 overexpression at normal temperature was seen when vpr-expressing cells were grown at 36°C with overproduced Hsf1 (Fig. 3B). Together, results of these experiments suggest that Hsf is the key cellular regulator of heat shock-mediated suppression of the Vpr activities.
Vpr counteracts Hsp16 elevation induced by heat treatment at the transcriptional level
Under the normal growth conditions, Hsp16 protein expression is typically very low or undetectable (FI = 0.1 ± 0.3; Fig. 5A-a; Fig. 5B-a,b, lane 1 ). When these cells were subjected to an acute heat shock (45°C for 15 min), a significant increase (FI = 5.9 ± 0.2) in the Hsp16 protein level was observed 2 hr after heat shock in cells that either had no vpr-containing plasmid (Fig. 5A-d; Fig. 5B-a, b, lane 2) or vpr gene expression was suppressed (Fig. 5B-a, lanes 3,5). In contrast, the level of Hsp16 (FI = 3.1 ± 0.6) was markedly decreased when wild type vpr was expressed under the same heat shock conditions (Fig. 5A-e; Fig. 5B-a, lane 4). Similar Hsp16 elevation (Fig. 5A-g; Fig. 5B-b, lane 2–4) was also observed in cells grown under constant high temperature at 36°C. Consistent with the observation shown in acute heat shock experiment, Hsp16 protein level was diminished in the vpr-expressing cells cultured at 36°C for 48 hrs (Fig. 5B-b, lane 5). Thus, wild type Vpr indeed inhibited heat-mediated activation of Hsp16. Interestingly, no obvious decrease of Hsp16 was observed with the RE076 cells carrying a mutant Vpr' during the early hours (23 hrs) of heat treatments (Fig. 5A-i; Fig. 5B-a, lane 6).
However, after prolonged (48 hrs) incubation of vpr-expressing cells at constant high temperature, both the wild type and mutant Vpr were able to eliminate Hsp16 elevation (Fig. 5B-b, lane 5–6). Taken together, these observations provide an explanation to our finding that heat treatment suppresses the Vpr'-induced cell cycle defect but does not protect against Vpr-induced cell killing because the F34I mutation in Vpr' may have attenuated the ability of Vpr to down-regulate Hsp16 thus allowing elevated Hsp16 to suppress activity of Vpr'. Therefore, wild type Vpr specifically counteracts activation of Hsp16 in response to vpr gene expression or heat treatment.
It is of interest to note that overexpression of hsp16 under the control of an exogenous nmt1 promoter suppressed Vpr-induced cell killing in the wild type cells (Fig. 4A-2 bottom panel; ) suggesting that the counteracting effect of Vpr on Hsp16 is specifically targeted to the hsp16 promoter, i.e., occurs at the transcriptional level. Attempting to confirm this possibility, we further tested whether overexpression of hsp16 under the same nmt1 promoter was also capable of suppressing Vpr-induced cell death at 36°C when Vpr has the strongest counteracting effect on Hsp16. As shown in the bottom panel of Fig. 4A-4, overproduction of Hsp16 was indeed capable of blocking Vpr-induced cell death at both high (36°C) and normal growth temperature (30°C). Therefore, Vpr counteracts Hsp16 elevation induced by heat treatment most likely at the transcriptional level.
In this report, we provide evidence that Hsf1 is the main regulator responsible for Hsp16 elevation and anti-Vpr responses in fission yeast cells. The fact that Hsf1 is responsible for Hsp16-mediated response to Vpr indicates that the effect of Hsp16 on Vpr resembles the cellular heat shock responses. Indeed, overexpression of hsf1 completely reduced Vpr'-induced cell cycle G2 arrest as shown by reversion of the cell elongation (Fig. 3A-d vs. b), shift of the cellular DNA content from G2 to G1 (Fig. 3A-b) and additional heat treatment of hsf-expressing cells did not significantly enhance the suppressive effect of Hsf1 on Vpr (Fig. 3B-h vs. d).
Even though vpr gene expression triggers Hsp16 elevation, Vpr appears to prevent further elevation induced by heat treatment (Fig. 5A-e; Fig. 5B-a, lane 4; Fig. 5B-b, lane 5), suggesting a counteracting effect of Vpr on heat stress-like cellular response possibly through transcriptional regulation of hsp16. This notion is supported by our observations that induction of Hsp16 by heat treatment failed to counteract Vpr-induced cell death (Fig. 4A-3, bottom plate). However, overexpression of hsp16 under the control of an exogenous nmt1 promoter completely suppressed Vpr-induced cell death under the same heat shock conditions (; Fig. 4A-4). Possible transcriptional down-regulation of hsp16 by Vpr is further evidenced by the results shown in Fig. 5A, in which a GFP reporter was fused with the endogenous hsp16 promoter  and expression of vpr eliminated the Hsp16 elevation (Fig. 5A-e, h). Although the molecular mechanism underlying this transcriptional suppression of hsp16 is unclear at the moment, the fact that Hsf activates Hsps through binding of the Hsp promoters  and overexpression of hsf1 or hsp16 through an exogenous adh or nmt1 promoter alleviates the Vpr activity (Fig. 3A; ) support the idea that Vpr may affect expression of hsp16 through competition with Hsf for control of hsp16 expression. One possible scenario is that Vpr may inhibit Hsf1 that results in reduced transcription of hsp16. Alternatively, since Vpr is a weak transcriptional activator through binding to the transcriptional factor Sp1 , it is also possible that Vpr may compete with Hsf1 by binding to the Sp1 region of the hsp16 promoter. Obviously additional tests are needed to elucidate these possibilities. Interestingly, only the wild type Vpr was able to inhibit Hsp16 at early hours (23 hrs) after induction, as a single amino acid substitution from phenylalanine to isoleucine at position 34 of Vpr attenuated its ability to suppress the increase of Hsp16 after acute heat shock (Fig. 5A-i vs. h; Fig. 5B, lane 6 vs. lane 4). In fact, an even higher level of Hsp16 was observed. This is presumably due to the inability of Vpr' to compete with Hsf-mediated Hsp16 elevation. Thus, assuming that expression of hsp16 is responsive to both the presence of Vpr and heat shock treatment, this larger increase of Hsp16 could be an additive effect. Since amino acid substitution at residue 34 of Vpr diminishes the ability of Vpr to induce cell death but retains induction of G2 arrest [20, 28, 29], a plausible possibility is that suppression of Hsp16 and induction of cell death by Vpr share common pathways.
It should be mentioned that whether Vpr-induced G2 arrest and cell death are two functionally independent activities is still of debate. Earlier reports suggested that these two activities are separable both in fission yeast and mammalian cells [20–24]. However, recent reports indicated Vpr-induced apoptosis is cell cycle dependent [25, 26]. Although reasons for these discrepancies are not completely clear at the moment, it is noticed that apoptosis shown in the Andersen's study describes a late event as cells were collected 48–72 hrs after viral infection . Prolonged cell cycle G2 arrest results in apoptosis. Thus, it is not surprising to find that apoptosis described in the Andersen's study is ANT-independent and ANT-dependent apoptosis was documented previously . Additional difference between the apoptosis described by Andersen et al from others is also noticed in the examination of two Vpr mutations. The R77Q and I74A mutants, which separate the apoptosis and G2 arrest induced by Vpr [24, 54], showed no separation between the G2 induction and apoptosis. In our study, the F34IVpr mutant is unable to induce cell death but retains its ability to induce cell cycle G2 arrest both in fission yeast [21, 27, 28] and mammalian cells (; our unpublished data) It thus allowed us to differentiate the effect of a wild type Vpr vs. a mutant Vpr that only confers the inhibitory effect on cell cycle regulation.
Responsive elevation of fission yeast Hsp16 and its human paralogue HSP27 (our unpublished data) suggests that the cellular heat stress-like responses might be antagonistic to Vpr. Indeed, we previously showed that overexpression of hsp16 and human HSP27 suppress the Vpr activities, including cell cycle G2 arrest and cell killing, both in fission yeast and human cells (; our unpublished data). However, the suppressive effect of yeast Hsp16 and human HSP27 on Vpr are not identical. Overproduction of Hsp16 completely eliminated all of the Vpr activities including the positive role of Vpr in supporting viral replication in macrophages (Fig. 2C-a). Under the same condition, however, HSP27 has no clear suppressing effect against Vpr in macrophages (Fig. 2C-b). One possible difference between these two HSPs is that Hsp16 associates directly with Vpr  but no clear HSP27-Vpr interaction was detected both in vitro and in vivo (our unpublished data). Unlike Hsp16, overexpression of HSP27 is unable to block nuclear transport capacity of Vpr (our unpublished data). Since nuclear transport of Vpr is required for HIV-1 infection in non-dividing cells such as macrophages [7, 55, 56], the inability of HSP27 to block nuclear import of Vpr could potentially explain why it has no effect on HIV-1 infection in macrophages.
There appears to be a dynamic interaction between vpr gene expression and activation of Hsp16 in fission yeast. Results of our parallel studies in mammalian cells indicated a similar dynamic and antagonistic interaction between Vpr and HSP27 (our unpublished data). This finding is not surprising because activation of heat shock proteins by Hsf1 is a highly conserved cellular process among all eukaryotic cells . All eukaryotes encode at least one heat shock factor that is believed to regulate transcription of heat shock genes. This protein binds to a regulatory sequence, i.e., the heat shock element, that is absolutely conserved among eukaryotes . Based on the data presented, we hypothesize that expression of vpr or HIV infection elicits a transient activation of the small heat shock proteins (sHsps) of eukaryotes through an Hsf-mediated pathway. Activation of these sHsps is most likely a part of the cellular antiviral reaction to HIV infection and specifically to Vpr. However, these stress responses are normally not sufficient to suppress the Vpr activities because of active counteraction from Vpr. Importantly, however, the Vpr activities could be completely blocked when sHsp's are produced under control of an exogenous promoter thus avoiding transcriptional inhibition by Vpr. Since the Vpr-specific activities have been linked to such clinical manifestation of AIDS as activation of viral replication , suppression of host immune responses  and depletion of CD4+ T-lymphocytes [12, 58], this finding could potentially provide a new approach to reducing Vpr-mediated detrimental effects in HIV-infected patients by stimulating expression of sHsps.
Maintenance and growth of mammalian and yeast cells
Genotypes and sources of S. pombe strains, mammalian cell lines and plasmids used in this study are summarized in Table 1. CD4-positive H9 and CEM-SS cells were grown in RPMI 1640 medium supplemented with 10% heat-inactivated fetal calf serum (FCS) and 100 unit/ml of penicillin/streptomycin. Gene inductions of HIV-1 vpr and other cellular genes under the control of the nmt1 promoter in fission yeast have been described previously [27, 37]. Cells containing the plasmid with the nmt1 promoter were first grown to stationary phase in the presence of 20 μM thiamine. Cells were then washed three times with distilled water, diluted to a final concentration of approximately 2 × 105 cells/ml in 10 ml of the appropriately supplemented EMM medium with or without thiamine. Cells were examined approximately 24 hours after gene induction. Fission yeast cells were normally grown at 30°C with constant shaking at 250 rpm unless otherwise specified.
Induction of cellular heat shock responses were conducted as previously described [27, 35, 59]. Briefly, cultures were first grown as mentioned above to fully express vpr and then exposed to either an acute heat shock at 45°C for 15 min or grown at consistent high temperature at 36°C for an indicated period of time.
Measurement of the Vpr-specific activities in fission yeast
All of the functional assays used to measure Vpr-specific activities, i.e. cell cycle G2 arrest and cell death induced by Vpr, have been described previously [20, 37, 60]. Vpr-induced G2 arrest can be specifically measured in fission yeast based on a number of cellular endpoints [21, 37]. Here we used F34IVpr (Vpr')-induced cell elongation as a marker of G2 arrest [47, 61]. The mutant Vpr', in which phenylalanine was replaced with isoleucine in position 34 (F34IVpr) was used instead of the wild type Vpr because Vpr' has lost its ability to induce cell killing but retains its capacity to induce G2 arrest, as previously shown both in human (our unpublished data) and fission yeast cells [21, 27, 28]. The use of cell elongation, also known as "cdc phenotype", as a marker for cell cycle G2/M delay is a standard approach in fission yeast [47, 61–63]. Cell images were first captured on a Leica microscope and the cell length was determined using OpenLab software. Average and standard deviation of cell length were calculated based on three independent experiments, each counting at least 100 cells. To verify Vpr-induced cell cycle G2 arrest, flow cytometric analysis is carried out as previously described 
Induction of cell death by Vpr was detected by inability of vpr-expressing cells to form colonies on agar plates as previously described . Briefly, S. pombe cells containing the pYZ1N::vpr constructs were first grown on a selective leucine-free minimal EMM plate under vpr-repressing condition. A loopful of viable cells was streaked onto vpr-inducing or vpr-repressing EMM plates and incubated at 30°C for 3–4 days. Inability to form colonies on the vpr-inducing EMM plates but normal growth on the vpr-repressing EMM plates is indicative of Vpr-induced cell killing [20, 64].
To evaluate the suppressive effect of Hsp16 on viral replication in proliferating CD4-positive T-lymphocytes, H9 and CEM-SS cells  that stably express a plasmid control or hsp16 were established . 3 × 106 to 5 × 106 of these H9 or CEM-SS cells were either mock infected or infected with 2.0 × 103 TCID50 of HIV-1LAI or IIIB. The HIV-1 LAI strain carries a wild type vpr gene; the vpr gene in the IIIB strain has a frame shift mutation at codon 73, which results in a truncated Vpr protein missing its C-terminus [8, 11, 41]. Equal infection of the cells was further verified by measuring viral RNA levels 24 hr after viral inoculation using the Roche Monitor assay following the manufacturer's instructions. Viral replication was determined by p24 antigen levels using a commercially available HIVAG-1 polyclonal antigen kit (Abbott Laboratories, Abbott Park, IL).
Monocyte-derived macrophages (MDMs) were prepared from peripheral blood mononuclear cells (PBMCs) by adherence to plastic as described previously . Briefly, human PBMCs were isolated from buffy coats of healthy seronegative donors by Ficoll density gradient centrifugation (Ficoll-Paque PLUS; Pharmacia Biotech, Piscataway, NJ). PBMCs were cultured in Primaria flasks (Becton Dickinson, Franklin Lakes, NJ) in culture medium (Dulbecco modified Eagle medium [DMEM]) supplemented with 10% heat-inactivated normal human serum, 2 mM glutamine, 50 U/mL penicillin, and 50 μg/mL streptomycin (all from Life Technologies, Bethesda, MD) at 37°C in a CO2 incubator at a cell density of 8 × 106 cells/mL. At 2 hours after plating, nonadherent cells were aspirated, and the adherent cells were cultured overnight in DMEM supplemented with macrophage colony-stimulating factor (M-CSF, 2 ng/mL). At 18 hours after initial plating, adherent cells were detached with 10 mM EDTA (ethylenediaminetetraacetic acid)/phosphate-buffered saline (PBS) and plated in 24-well Primaria plates (Becton Dickinson) at a density of 106 cells/mL for 7 days in the presence of recombinant human M-CSF (at 2 ng/mL). After 7 days, final MDM cultures were composed of about 98% macrophages as judged by morphology and nonspecific esterase activity. A macrophage-tropic HIV-1 strain ADA  was used for infection of MDM. Cells were infected with viral inoculae equalized according to the reverse transcriptase (RT) activity (5 × 105 counts per minute/106 cells).
A Leica fluorescence microscope DMR equipped with a high performance CCD camera (Hamamatsu) and OpenLab software (Improvision, Inc., Lesington, MA) was used for all imaging analyses. Fission yeast cells were collected onto a regular glass slide and covered with cover slip. For the observation of green fluorescent protein, we used a Leica L5 filter, which has an excitation of 480/40 (460–500 nm) and emission of 527/30 (512–542 nm). Induction of cellular heat shock responses were conducted as previously described [35, 59]. The level of hsp16 gene expression was quantified by measuring fluorescent signal emitted from the GFP-Hsp16 fusion protein as described previously [35, 48, 49]. For DNA staining, cells were counterstained with 1 μg/ml DAPI, which was observed with a Leica A8 filter with an excitation of 360/40 (340–380 nm) and emission of 470/40 (450–490 nm). Cell length was measured individually on the captured images using the OpenLab software. Statistical significance of differences in cell length was determined using the t-test for paired samples.
Western blot analysis
For Western blot analysis, mammalian cells were harvested and rinsed with ice-cold HEPES-buffered saline (pH 7.0), then lysed in an ice-cold cell lysis buffer [20 mM Tris-HCl, pH7.6, 150 mM NaCl, 1 mM EDTA, 0.5% Nonidet P-40, 1 mM DTT, 5 μM Trichostatin A, 1 mM sodium orthovanadate, 1 mM PMSF, 1 mM NaF and complete protease inhibitors (Roche Applied Science)]. Cellular lysates were prepared and the protein concentration was determined using the Pierce protein assay kit. For immunoblotting, an aliquot of total lysate (50 μg of proteins) in 2× SDS-PAGE sample buffer (1:1 v/v) was electrophoresed and transferred to a nitrocellulose filter. Filters were incubated with appropriate primary antibody in Tris-buffered saline (TBS, pH 7.5) and 5% skim milk or 5% BSA overnight. After washing, the filter was incubated with secondary antibody in TBS-Tween-20 (TBS-T) buffer for 1 h. Protein bands were visualized by an ECL detection system.
For Western blot analysis of fission yeast proteins, cells were washed once with water prior to adding cold stop buffer (150 mM NaCl, 50 mM NaF, 10 mM EDTA, 1 mM NaN3, pH 8.0). Cells were then collected by centrifugation and resuspended in 3 volumes of HB buffer (25 mM MOPS, pH7.2, 60 mM β-glycerophosphate, 15 mM p-nitrophenyl phosphate, 15 mM MgCI2, 15 mM EGTA, 1% Triton X-100, 1 mM DTT). A mixture of protease inhibitors (1 mM PMSF, 20 μg/ml leupeptin, 40 μg/ml aprotinin and 0.1 mM sodium vanadate) and a commercial complete mini protease inhibitor cocktail (Roche, one tablet per 7 ml) was added immediately before lysis by glass-bead agitation. The cells were disrupted for 60 sec using a bead beater (Biospec Products, Bartlesville, OK). Cell breakage was checked under the microscope and disruption was repeated 3–5 times if necessary. Protein concentration was measured using the BCA protein assay kit (Pierce). Equal amounts of protein (30 μg) were resolved on a SDS-polyacrylamide gel and transferred to a nitrocellulose membrane. HIV-1 Vpr or Hsp16 protein levels were assayed by immunoblotting procedures using anti-Vpr serum (generated in our laboratory) and anti-Hsp16 serum (Paul Young's laboratory), respectively. Immunoblots were developed using the enhanced chemiluminescent (ECL) system (Pierce).
List of abbreviations
-human immunodeficiency virus
-viral protein R
-heat shock protein
-heat shock factor
- S. pombe :
-green fluorescent protein
This study was supported in part by grants from National Institute of Health AI40891 and GM63080 (RYZ), and the Natural Sciences and Engineering Research Council of Canada (PGY). The authors would like to thank Dr. R.E. Kingston of Harvard University for the pART1-hsf1 gene construct, Tina Cordero and Hannah Koh for technical assistance and the Zhaolab staff for critical discussion of this manuscript. Content described in this manuscript is covered under a provisional patent application No. U.S. 60/698,624.
- Tristem M, Purvis A, Quicke DL: Complex evolutionary history of primate lentiviral vpr genes. Virol. 1998, 240: 232-237. 10.1006/viro.1997.8929.View Article
- Tristem M, Marshall C, Karpas A, Hill F: Evolution of the primate lentiviruses: evidence from vpx and vpr. EMBO J. 1992, 11: 3405-3412.PubMed CentralPubMed
- Le Rouzic E, Benichou S: The Vpr protein from HIV-1: distinct roles along the viral life cycle. Retrovirology. 2005, 2: 11-10.1186/1742-4690-2-11.PubMed CentralView ArticlePubMed
- Altfeld M, Addo MM, Eldridge RL, Yu XG, Thomas S, Khatri A, Strick D, Phillips MN, Cohen GB, Islam SA, et al: Vpr is preferentially targeted by CTL during HIV-1 infection. J Immunol. 2001, 167: 2743-2752.View ArticlePubMed
- Mothe BR, Horton H, Carter DK, Allen TM, Liebl ME, Skinner P, Vogel TU, Fuenger S, Vielhuber K, Rehrauer W, et al: Dominance of CD8 responses specific for epitopes bound by a single major histocompatibility complex class I molecule during the acute phase of viral infection. J Virol. 2002, 76: 875-884. 10.1128/JVI.76.2.875-884.2002.PubMed CentralView ArticlePubMed
- Di Marzio P, Choe S, Ebright M, Knoblauch R, Landau NR: Mutational analysis of cell cycle arrest, nuclear localization and virion packaging of human immunodeficiency virus type 1 Vpr. J Virol. 1995, 69: 7909-7916.PubMed CentralPubMed
- Heinzinger N, Bukinsky M, Haggerty S, Ragland A, Kewalramani V, Lee M, Gendelman H, Ratner L, Stevenson M, Emerman M: The Vpr protein of human immunodeficiency virus type 1 influences nuclear localization of viral nucleic acids in nondividing host cells. Proc Nat Acad Sci USA. 1994, 91: 7311-7315. 10.1073/pnas.91.15.7311.PubMed CentralView ArticlePubMed
- Goh WC, Rogel ME, Kinsey CM, Michael SF, Fultz PN, Nowak MA, Hahn BH, Emerman M: HIV-1 Vpr increases viral expression by manipulation of the cell cycle: a mechanism for selection of Vpr in vivo. Nat Med. 1998, 4: 65-71. 10.1038/nm0198-065.View ArticlePubMed
- Gibbs JS, Lackner AA, Lang SM, Simon MA, Sehgal PK, Daniel MD, Desrosiers RC: Progression to AIDS in the absence of a gene for vpr or vpx. J Virol. 1995, 69: 2378-2383.PubMed CentralPubMed
- Lang SM, Weeger M, Stahl-Hennig C, Coulibaly C, Hunsmann G, Muller J, Muller-Hermelink H, Fuchs D, Wachter H, Daniel MM, et al: Importance of vpr for infection of rhesus monkeys with simian immunodeficiency virus. J Virol. 1993, 67: 902-912.PubMed CentralPubMed
- Zhao Y, Chen M, Wang B, Yang J, Elder RT, Song X-q, Yu M, Saksena N: Functional conservation of HIV-1 Vpr and variability in a mother-child pair of long-term non-progressors. Virus Res. 2002, 89: 103-121. 10.1016/S0168-1702(02)00127-2.View ArticlePubMed
- Somasundaran M, Sharkey M, Brichacek B, Luzuriaga K, Emerman M, Sullivan JL, Stevenson M: Evidence for a cytopathogenicity determinant in HIV-1 Vpr. Proc Natl Acad Sci USA. 2002, 99: 9503-9508. 10.1073/pnas.142313699.PubMed CentralView ArticlePubMed
- He J, Choe S, Walker R, Di Marzio PD, Morgan DO, Landau NR: Human immunodeficiency virus type 1 viral protein R (Vpr) arrests cells in the G2 phase of the cell cycle by inhibiting p34cdc2 activity. J Virol. 1995, 69: 6705-6711.PubMed CentralPubMed
- Jowett JB, Planelles V, Poon B, Shah NP, Chen M, Chen ISY: The human immunodeficiency virus type 1 vpr gene arrests infected T cells in the G2 + M phase of the cell cycle. J Virol. 1995, 69: 6304-6313.PubMed CentralPubMed
- Li G, Elder RT, Qin K, Park HU, Liang D, Zhao RY: PP2A dependent and independent pathways for ATR phosphorylation of Chk1. J Biol Chem. 2007, 282: 7287-7298. 10.1074/jbc.M607951200.View ArticlePubMed
- Re F, Braaten D, Franke EK, Luban J: Human immunodeficiency virus type 1 Vpr arrests the cell cycle in G2 by inhibiting the activation of p34cdc2-cyclin B. J Virol. 1995, 69: 6859-6864.PubMed CentralPubMed
- Yao XJ, Rougeau N, Duisit G, Lemay J, Cohen EA: Analysis of HIV-1 Vpr determinants responsible for cell growth arrest in Saccharomyces cerevisiae. Retrovirology. 2004, 1: 21-10.1186/1742-4690-1-21.PubMed CentralView ArticlePubMed
- Poon B, Jowett JB, Stewart SA, Armstrong RW, Rishton GM, Chen IS: Human immunodeficiency virus type 1 vpr gene induces phenotypic effects similar to those of the DNA alkylating agent, nitrogen mustard. J Virol. 1997, 71: 3961-3971.PubMed CentralPubMed
- Poon B, Grovit-Ferbas K, Stewart SA, Chen ISY: Cell cycle arrest by Vpr in HIV-1 virions and insensitivity to antiretroviral agents. Science. 1998, 281: 266-269. 10.1126/science.281.5374.266.View ArticlePubMed
- Chen M, Elder RT, Yu M, O'Gorman MG, Selig L, Benarous R, Yamamoto A, Zhao Y: Mutational analysis of Vpr-induced G2 arrest, nuclear localization, and cell death in fission yeast. J Virol. 1999, 73: 3236-3245.PubMed CentralPubMed
- Elder RT, Yu M, Chen M, Edelson S, Zhao Y: Cell cycle G2 arrest induced by HIV-1 Vpr in fission yeast (Schizosaccharomyces pombe) is independent of cell death and early genes in the DNA damage checkpoint. Virus Res. 2000, 68: 161-173. 10.1016/S0168-1702(00)00167-2.View ArticlePubMed
- Nishizawa M, Kamata M, Katsumata R, Aida Y: A carboxy-terminally truncated form of the human immunodeficiency virus type 1 vpr protein induces apoptosis via G(1) cell cycle arrest. J Virol. 2000, 74: 6058-6067. 10.1128/JVI.74.13.6058-6067.2000.PubMed CentralView ArticlePubMed
- Waldhuber MG, Bateson M, Tan J, Greenway AL, McPhee DA: Studies with GFP-Vpr fusion proteins: induction of apoptosis but ablation of cell-cycle arrest despite nuclear membrane or nuclear localization. Virology. 2003, 313: 91-104. 10.1016/S0042-6822(03)00258-7.View ArticlePubMed
- Lum JJ, Cohen OJ, Nie Z, Weaver JG, Gomez TS, Yao XJ, Lynch D, Pilon AA, Hawley N, Kim JE, et al: Vpr R77Q is associated with long-term nonprogressive HIV infection and impaired induction of apoptosis. J Clin Invest. 2003, 111: 1547-1554. 10.1172/JCI200316233.PubMed CentralView ArticlePubMed
- Andersen JL, Zimmerman ES, DeHart JL, Murala S, Ardon O, Blackett J, Chen J, Planelles V: ATR and GADD45alpha mediate HIV-1 Vpr-induced apoptosis. Cell Death Differ. 2005, 12: 326-334. 10.1038/sj.cdd.4401565.View ArticlePubMed
- Andersen JL, Dehart JL, Zimmerman ES, Ardon O, Kim B, Jacquot G, Benichou S, Planelles V: HIV-1 Vpr-Induced Apoptosis Is Cell Cycle Dependent and Requires Bax but Not ANT. PLoS Pathog. 2006, 2: e127-10.1371/journal.ppat.0020127.PubMed CentralView ArticlePubMed
- Benko Z, Liang D, Agbottah E, Hou J, Chiu K, Yu M, Innis S, Reed P, Kabat W, Elder RT, et al: Anti-Vpr activity of a yeast chaperone protein. J Virol. 2004, 78: 11016-11029. 10.1128/JVI.78.20.11016-11029.2004.PubMed CentralView ArticlePubMed
- Gu J, Emerman M, Sandmeyer S: Small heat shock protein suppression of Vpr-induced cytoskeletal defects in budding yeast. Mol Cell Biol. 1997, 17: 4033-4042.PubMed CentralView ArticlePubMed
- Vodicka MA, Koepp DM, Silver PA, Emerman M: HIV-1 Vpr interacts with the nuclear transport pathway to promote macrophage infection. Genes Dev. 1998, 12: 175-185.PubMed CentralView ArticlePubMed
- Zhao Y, Lieberman HB: Schizosaccharomyces pombe: a model for molecular studies of eukaryotic genes. DNA Cell Biol. 1995, 14: 359-371.View ArticlePubMed
- Zhao Y, Elder RT: Yeast perspectives on HIV-1 Vpr. Front Biosci. 2000, 5: D905-916.View ArticlePubMed
- Elder RT, Benko Z, Zhao Y: HIV-1 Vpr modulates cell cycle G2/M transition through an alternative cellular mechanism other than the classic mitotic checkpoints. Front Biosci. 2002, 7: D349-357.View ArticlePubMed
- Zhao RY, Bukrinsky M, Elder RT: HIV-1 viral protein R (Vpr) & host cellular responses. Indian J Med Res. 2005, 121: 270-286.PubMed
- Bukrinsky M, Zhao Y: Heat-shock proteins reverse the G2 arrest caused by HIV-1 viral protein R. DNA Cell Biol. 2004, 23: 223-225. 10.1089/104454904773819806.View ArticlePubMed
- Taricani L, Feilotter HE, Weaver C, Young PG: Expression of hsp16 in response to nucleotide depletion is regulated via the spc1 MAPK pathway in Schizosaccharomyces pombe. Nucleic Acids Res. 2001, 29: 3030-3040. 10.1093/nar/29.14.3030.PubMed CentralView ArticlePubMed
- Maundrell K: Thiamine-repressible expression vectors pREP and pRIP for fission yeast. Gene. 1993, 123: 127-130. 10.1016/0378-1119(93)90551-D.View ArticlePubMed
- Zhao Y, Cao J, O'Gorman MRG, Yu M, Yogev R: Effect of human immunodeficiency virus Type 1 protein R (vpr) gene expression on basic cellular functions of fission yeast Schizosaccharomyces pombe. J Virol. 1996, 70: 5821-5826.PubMed CentralPubMed
- Gummuluru S, Emerman M: Cell cycle- and Vpr-mediated regulation of human immunodeficiency virus type 1 expression in primary and transformed T-cell lines. J Virol. 1999, 73: 5422-5430.PubMed CentralPubMed
- Yao XJ, Mouland AJ, Subbramanian RA, Forget J, Rougeau N, Bergeron D, Cohen EA: Vpr stimulates viral expression and induces cell killing in human immunodeficiency virus type 1-infected dividing Jurkat T cells. J Virol. 1998, 72: 4686-4693.PubMed CentralPubMed
- Vanitharani R, Mahalingam S, Rafaeli Y, Singh SP, Srinivasan A, Weiner DB, Ayyavoo V: HIV-1 Vpr transactivates LTR-directed expression through sequences present within -278 to -176 and increases virus replication in vitro. Virology. 2001, 289: 334-342. 10.1006/viro.2001.1153.View ArticlePubMed
- Popovic M, Sarngadharan MG, Read E, Gallo RC: Detection, isolation, and continuous production of cytopathic retroviruses (HTLV-III) from patients with AIDS and pre-AIDS. Science. 1984, 224: 497-500. 10.1126/science.6200935.View ArticlePubMed
- Bourbigot S, Beltz H, Denis J, Morellet N, Roques BP, Mely Y, Bouaziz S: The C-terminal domain of the HIV-1 regulatory protein Vpr adopts an antiparallel dimeric structure in solution via its leucine-zipper-like domain. Biochem J. 2005, 387: 333-341. 10.1042/BJ20041759.PubMed CentralView ArticlePubMed
- Roumier T, Vieira HL, Castedo M, Ferri KF, Boya P, Andreau K, Druillennec S, Joza N, Penninger JM, Roques B, Kroemer G: The C-terminal moiety of HIV-1 Vpr induces cell death via a caspase-independent mitochondrial pathway. Cell Death Differ. 2002, 9: 1212-1219. 10.1038/sj.cdd.4401089.View ArticlePubMed
- Nara PL, Fischinger PJ: Quantitative infectivity assay for HIV-1 and -2. Nature. 1988, 332: 469-470. 10.1038/332469a0.View ArticlePubMed
- Gao B, Tsan MF: Endotoxin contamination in recombinant human heat shock protein 70 (Hsp70) preparation is responsible for the induction of tumor necrosis factor alpha release by murine macrophages. J Biol Chem. 2003, 278: 174-179. 10.1074/jbc.M208742200.View ArticlePubMed
- Gallo GJ, Prentice H, Kingston RE: Heat shock factor is required for growth at normal temperatures in the fission yeast Schizosaccharomyces pombe. Mol Cell Biol. 1993, 13: 749-761.PubMed CentralView ArticlePubMed
- Elder RT, Yu M, Chen M, Zhu X, Yanagida M, Zhao Y: HIV-1 Vpr induces cell cycle G2 arrest in fission yeast (Schizosaccharomyces pombe) through a pathway involving regulatory and catalytic subunits of PP2A and acting on both Wee1 and Cdc25. Virology. 2001, 287: 359-370. 10.1006/viro.2001.1007.View ArticlePubMed
- Albano CR, Randers-Eichhorn L, Bentley WE, Rao G: Green fluorescent protein as a real time quantitative reporter of heterologous protein production. Biotechnol Prog. 1998, 14: 351-354. 10.1021/bp970121b.View ArticlePubMed
- Li X, Zhao X, Fang Y, Jiang X, Duong T, Fan C, Huang CC, Kain SR: Generation of destabilized green fluorescent protein as a transcription reporter. J Biol Chem. 1998, 273: 34970-34975. 10.1074/jbc.273.52.34970.View ArticlePubMed
- Gallo GJ, Schuetz TJ, Kingston RE: Regulation of heat shock factor in Schizosaccharomyces pombe more closely resembles regulation in mammals than in Saccharomyces cerevisiae. Mol Cell Biol. 1991, 11: 281-288.PubMed CentralView ArticlePubMed
- Wang L, Mukherjee S, Jia F, Narayan O, Zhao LJ: Interaction of virion protein Vpr of human immunodeficiency virus type 1 with cellular transcription factor Sp1 and trans-activation of viral long terminal repeat. J Biol Chem. 1995, 270: 25564-25569. 10.1074/jbc.270.43.25564.View ArticlePubMed
- Zhao Y, Yu M, Chen M, Elder RT: Cell cycle G2 arrest and cell death are independent functions of HIV-1 Vpr. Proceedings of 12th World AIDS Conference, Geneva, Switzerland . 1998, Monduzzi Editore, Vol. 1:31-35
- Jacotot E, Ravagnan L, Loeffler M, Ferri KF, Vieira HL, Zamzami N, Costantini P, Druillennec S, Hoebeke J, Briand JP, et al: The HIV-1 viral protein R induces apoptosis via a direct effect on the mitochondrial permeability transition pore. J Exp Med. 2000, 191: 33-46. 10.1084/jem.191.1.33.PubMed CentralView ArticlePubMed
- Rajan D, Wildum S, Rucker E, Schindler M, Kirchhoff F: Effect of R77Q, R77A and R80A changes in Vpr on HIV-1 replication and CD4 T cell depletion in human lymphoid tissue ex vivo. Aids. 2006, 20: 831-836. 10.1097/01.aids.0000218546.31716.7f.View ArticlePubMed
- Connor RI, Chen BK, Choe S, Landau NR: Vpr is required for efficient replication of human immunodeficiency virus type-1 in mononuclear phagocytes. Virol. 1995, 206: 935-944. 10.1006/viro.1995.1016.View Article
- Sherman MP, de Noronha CM, Eckstein LA, Hataye J, Mundt P, Williams SA, Neidleman JA, Goldsmith MA, Greene WC: Nuclear export of Vpr is required for efficient replication of human immunodeficiency virus type 1 in tissue macrophages. J Virol. 2003, 77: 7582-7589. 10.1128/JVI.77.13.7582-7589.2003.PubMed CentralView ArticlePubMed
- Levy DN, Refaeli Y, MacGregor RR, Weiner DB: Serum Vpr regulates productive infection and latency of human immunodeficiency virus type 1. Proc Nat Acad Sci USA. 1994, 91: 10873-10877. 10.1073/pnas.91.23.10873.PubMed CentralView ArticlePubMed
- Stewart SA, Poon B, Jowett JB, Chen IS: Human immunodeficiency virus type 1 Vpr induces apoptosis following cell cycle arrest. J Virol. 1997, 71: 5579-5592.PubMed CentralPubMed
- Humphrey T, Enoch T: Sum1, a highly conserved WD-repeat protein, suppresses S-M checkpoint mutants and inhibits the osmotic stress cell cycle response in fission yeast. Genetics. 1998, 148: 1731-1742.PubMed CentralPubMed
- Zhao Y, Yu M, Chen M, Elder RT, Yamamoto A, Cao J: Pleiotropic effects of HIV-1 protein R (Vpr) on morphogenesis and cell survival in fission yeast and antagonism by pentoxifylline. Virology. 1998, 246: 266-276. 10.1006/viro.1998.9208.View ArticlePubMed
- Masuda M, Nagai Y, Oshima N, Tanaka K, Murakami H, Igarashi H, Okayama H: Genetic studies with the fission yeast Schizosaccharomyces pombe suggest involvement of wee1, ppa2, and rad24 in induction of cell cycle arrest by human immunodeficiency virus type 1 Vpr. J Virol. 2000, 74: 2636-2646. 10.1128/JVI.74.6.2636-2646.2000.PubMed CentralView ArticlePubMed
- Lee M, Nurse P: Cell cycle control genes in fission yeast and mammalian cells. Trends Genet. 1988, 4: 287-290. 10.1016/0168-9525(88)90171-0.View ArticlePubMed
- Nurse P, Thuriaux P, Nasmyth K: Genetic control of the cell division cycle in the fission yeast Schizosaccharomyces pombe. Mol Gen Genet. 1976, 146: 167-178. 10.1007/BF00268085.View ArticlePubMed
- Zhao Y, Elder RT, Chen M, Cao J: Fission yeast expression vectors adapted for large scale cloning and GFP fusion with positive screening. BioTechniques. 1998, 25: 438-444.PubMed
- Schmidtmayerova H, Nuovo GJ, Bukrinsky M: Cell proliferation is not required for productive HIV-1 infection of macrophages. Virology. 1997, 232: 379-384. 10.1006/viro.1997.8584.View ArticlePubMed
- Gendelman HE, Orenstein JM, Martin MA, Ferrua C, Mitra R, Phipps T, Wahl LA, Lane HC, Fauci AS, Burke DS, et al: Efficient isolation and propagation of human immunodeficiency virus on recombinant colony-stimulating factor 1-treated monocytes. J Exp Med. 1988, 167: 1428-1441. 10.1084/jem.167.4.1428.View ArticlePubMed
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.