- Open Access
HTLV-1 Tax mutants that do not induce G1arrest are disabled in activating the anaphase promoting complex
© Merling et al; licensee BioMed Central Ltd. 2007
- Received: 31 January 2007
- Accepted: 29 May 2007
- Published: 29 May 2007
HTLV-1 Tax is a potent activator of viral transcription and NF-κB. Recent data indicate that Tax activates the anaphase promoting complex/cyclosome (APC/C) ahead of schedule, causing premature degradation of cyclin A, cyclin B1, securin, and Skp2. Premature loss of these mitotic regulators is accompanied by mitotic aberrations and leads to rapid senescence and cell cycle arrest in HeLa and S. cerevisiae cells. Tax-induced rapid senescence (tax-IRS) of HeLa cells is mediated primarily by a dramatic stabilization of p27 KIP and is also accompanied by a great surge in the level of p21 CIP1 mRNA and protein. Deficiencies in p27 KIP prevent Tax-IRS. A collection of tax point mutants that permit normal growth of S. cerevisiae have been isolated. Like wild-type tax, many of them (C23W, A108T, L159F, and L235F) transactivate both the HTLV-LTR and the NF-κB reporters. One of them, V19M, preferentially activates NF-κB, but is attenuated for LTR activation. None of the mutants significantly elevated the levels of p21 CIP1 and p27 KIP1 , indicating that the dramatic surge in p21CIP1/WAF1and p27 KIP 1 induced by Tax is brought about by a mechanism distinct from NF-κB or LTR activation. Importantly, the ability of these mutants to activate APC/C is attenuated or abrogated. These data indicate that Tax-induced rapid senescence is causally associated with APC/C activation.
- HeLa Cell
- A108T Cell
- DsRed Expression
- MluI Site
- Mitotic Aberration
Human T-lymphotropic virus type I (HTLV-I) is the etiologic agent of adult T-cell leukemia and lymphoma, which occurs in approximately 5% of infected individuals after a long latency period lasting up to 20–40 years. The HTLV-1 viral transactivator/onco-protein Tax is thought to play an important role in T-cell malignancy and HAM/TSP. Tax transactivates the HTLV-1 LTR promoter through its interaction with CREB/ATF-1 [1–6], CBP/p300 [7–11], and the Tax-responsive 21-bp repeat element, and activates the NF-κB pathway [12–17] through the interaction with PP2A/IKKγ . In addition to its transactivation functions, Tax also impacts on many aspects of the cell cycle: activating G1/S transition [19–21], inactivating p53 functions , inducing p21CIP1/WAF1mRNA transcription [23–26], and inhibiting apoptosis and DNA repair [27, 28]. Recent data have indicated that Tax can dramatically perturb mitotic regulation, causing micronuclei formation, cytokinesis failure, and chromosome instability [29, 30]. ATL cells are often aneuploid with complex chromosomal abnormalities including trisomy 3, trisomy 7, a partial deletion of 6q, and abnormalities of 14q11 . Large lymphocytes with cleaved/cerebriform nuclei are also frequently seen in HTLV-I-positive individuals [32–35]. These pathological findings are likely to be associated with Tax-induced mitotic aberrations.
Indeed, in tax-expressing HeLa, MT4, and S. cerevisiae cells, the levels of cyclin A, cyclin B and the anaphase inhibitor: securin/Pds1p (precocious dissociation of sister chromatids) were found to be significantly reduced . We have found that the loss of cell cycle regulators and the mitotic defects induced by Tax may be causally linked and are associated with premature activation of the anaphase promoting complex/cyclosome (referred to as APC/C henceforth), a multiprotein E3-ubiquitin ligase that controls the onset of anaphase and mitotic exit by targeting mitotic cyclins and other cell cycle regulators for degradation . More recently, we have shown that the cell cycle dysregulation induced by tax does not end with mitotic abnormalities. Tax-transduced HeLa cells, after passage through a faulty cell division cycle, immediately entered into a senescence-like G1 arrest termed tax-induced rapid senescence, tax-IRS . These cells expressed high levels of Cdk2 inhibitors: p21CIP1/WAF1and p27 KIP 1 as a consequence of Tax-mediated activation of p21CIP1/WAF1mRNA transcription, and increased stabilization of p21CIP1/WAF1and p27 KIP 1 proteins. Consistent with these findings, Tripp et al have also reported that expression of tax can cause CD34+ hematopoietic cells to cease proliferation .
During normal cell cycle progression, p21CIP1/WAF1and p27 KIP 1 transiently accumulate during G1, but become degraded in S. The destruction of p21CIP1/WAF1and p27 KIP 1 during S phase is regulated by the multisubunit E3 ubiquitin ligase, SCF (Skp-Cullin-F box), together with its substrate-targeting subunit, Skp2 [39–44] and the cell cycle regulatory protein, Cks1 [39, 44, 45]. Recent evidence indicates that Skp2 and Cks1 are both substrates of the Cdh1-associated APC/C (APCCdh1). They become polyubiquitinated and degraded in late M and early G1 when APCCdh1 is highly active. This renders SCFSKP2 inactive and allows p21CIP1/WAF1and p27 KIP 1 levels to build up in G1. When tax is expressed, APC/C becomes prematurely activated. This causes Skp2 to be polyubiquitinated and degraded starting in S, through G2/M and in subsequent G1. The drastic reduction in Skp2 and possibly Cks1, apparently inactivated SCFSKP2, profoundly stabilized p21CIP1/WAF1and p27 KIP1 , thereby committing cells to senescence. The stabilization and surge of p21CIP1/WAF1and p27 KIP 1 in tax-expressing cells, therefore, is temporally and causally linked to premature APC/C activation. In essence, Tax activates the cellular program for mitotic exit far ahead of schedule, thereby driving cells into a state of permanent arrest. Interestingly and as might be predicted, we have found that HTLV-1 transformed T-cells invariably express lower levels of p27 KIP1 . Indeed, a loss of p27 KIP 1 function allows cells to evade tax-IRS .
Our earlier results have indicated that expression of Tax in S. cerevisiae also leads to unscheduled, APC-mediated degradation of Clb2p and Pds1p, G2/M delay, chromosome aneuploidy, growth arrest, and loss of cell viability . Considering the highly conserved nature of the cellular machineries that control mitosis in eukaryotes, this is probably not surprising. The powerful genetics available for S. cerevisiae provides an opportunity to dissect the mechanism by which Tax dysregulates APC/C and mitosis, which is otherwise difficult to perform in human cells. Here we report the isolation of a collection of 26 tax point mutants whose expression in S. cerevisiae did not lead to growth arrest. Five mutants (V19M, C23W, A108T, L159F, and L235F) – with amino acid substitutions that span the majority of Tax protein sequence – were chosen for in-depth analyses. C23W, A108T, L159F, and L235F transactivated both the HTLV-LTR and the NF-κB reporters. One mutant, V19M, preferentially activated NF-κB, but was attenuated in LTR activation. All became impaired or abrogated in their ability (i) to activate APC, (ii) to increase the levels of p21CIP1/WAF1and p27 KIP1 , and (iii) to cause tax-IRS. These data strongly suggest that tax-IRS, with the associated mitotic aberrations and the accompanying rise in p21 CIP1 / WAF1 and p27 KIP 1 levels, is coupled to APC/C activation, and is mechanistically unrelated to the CREB/ATF-CBP/p300 or IKK-NF-κB pathway.
Isolation of tax mutants that do not cause growth arrest in S. cerevisiae
DNA sequence analyses of the tax coding sequence revealed that each of the 26 tax mutants contained a single amino acid substitution that resulted from a G to A or C to T transition, as might be expected for hydroxylamine mutagenesis. The altered amino acid residues in the Tax protein sequence are listed in Fig. 1A. Many of the amino acid substitutions are clustered in the NH2-terminal half of Tax (20/26). Consistent with the notion that the amino acid substitutions had occurred in important regions of Tax, we noticed that the T130I substitution overlap with the dual amino acid substitutions – T130A L131S – in a well characterized tax mutant known as M22, which is partially defective in dimerization and is severely impaired in IKKγ/NEMO-binding and NF-κB activation. Two distinct mutations (G61E and G61R) and (A108T and A108V) were isolated for each of the amino acid residues 61 and 108, suggesting the importance of these residues in protein-protein interactions that mediate Tax functions. Finally, the expression levels of all mutants in S. cerevisiae were comparable as judged by immunoblotting (data not shown).
Tax mutants selected in W303a are functional in HTLV-1 LTR and NF-κB trans-activation
Expression of taxmutants in HeLa cells
We next selected 5 mutants (V19M, C23W, A108T, L159F, and L235F) that retained the ability to transactivate LTR and/or NF-κB for further analysis. Lentivirus vectors (LV) capable of transducing the mutant tax alleles were generated by co-transfection of the respective HR'-CMV-tax-SV40-puro vectors together with packaging plasmids that encode HIV structural proteins and VSV G protein as previously reported [37, 47]. A stable HeLa cell line, HeLa-18 × 21-DsRed, which expresses DsRed under the control of a Tax-inducible enhancer/promoter cassette containing 18 copies of the 21-bp repeat upstream of a minimal HTLV-1 promoter , was used as the cellular background for introducing the tax alleles. As the expression of DsRed in HeLa-18 × 21-DsRed is strictly Tax-dependent, cells that express Tax after gene transduction can be readily detected by fluorescence microscopy (Fig. 3). HeLa-18 × 21-DsRed cells were infected with LV carrying the wild-type, V19M, C23W, A108T, L159F, L235F tax alleles, or the EGFP gene. The LV-transduced cells were then selected in media containing 1 μg/ml puromycin for 2–3 days. Drug-resistant colonies were then grown in puromycin-free medium for 1 day and observed under a fluorescence microscope for DsRed expression. In agreement with the LTR-Luc reporter activities described above (Fig. 2), C23W, A108T, L159F, and L235F, but not V19M activated DsRed expression (Fig. 3B). As expected, HeLa-18 × 21-DsRed transduced with the LV-EGFP control did not express DsRed. Previously, we have demonstrated that Tax expression in HeLa cells greatly elevated the levels of p21CIP1/WAF1and p27 KIP 1 cyclin-dependent kinase inhibitors, thereby causing HeLa cells to enter into a senescence-like G1 arrest termed Tax-induced rapid senescence (Tax-IRS) . The HeLa cells in Tax-IRS are flat, enlarged, vacuolated, often binucleated, and stained positive for the senescence associated β-galactosidase. Indeed, in agreement with previous results, microscopic examination of the HeLa-18 × 21-DsRed cell line transduced with LV-Tax (wild-type) revealed a prevalence of enlarged and binucleated cells, consistent with the notion that they were in the state of Tax-IRS (Fig. 3A). By contrast, the morphology of cells transduced with mutant tax alleles, with the exception of A108T, resembled those of control cells transduced with the EGFP gene. Finally, we noted that despite some similarity of A108T cells to Tax (wild-type) cells, the extent of arrest and morphological changes in A108T cells appeared to be attenuated (Fig. 3A).
Tax mutants whose expression is permissible in S. cerevisiaedo not cause, or are attenuated in inducing cell cycle arrest in HeLa cells, but remain functional in activating I-κB degradation and p100 processing
S. cerevisiae-viable taxmutants are attenuated in APC/C activation
In this study, we have described a collection of 26 tax single-point mutants that are disabled in causing cell cycle arrest in S. cerevisiae. A total of 21 tax alleles were analyzed further by luciferase reporter assays for LTR and NF-κB transactivation. Approximately half of the amino acid substitutions likely have impacted on critical regions of Tax so as to render it severely defective. Special attention was directed to five mutants (V19M, C23W, A108T, L159F, and L235F) that remained strong LTR and NF-κB transactivators. Their phenotypes in HeLa cells were largely consistent with those seen in S. cerevisiae – all were attenuated or significantly impaired in causing p21CIP1/WAF1and p27 KIP 1 accumulation, but remained able to induce I-κB degradation and p100 NF-κB2 processing. Whereas the majority of HeLa cells transduced with wild-type tax entered into tax-IRS, cells transduced with each of the 5 mutant tax alleles continued to proliferate, albeit at rates that varied dependent on the levels of p21CIP1/WAF1and p27 KIP 1 expressed. These results validated the utility of S. cerevisiae as a model for dissecting the mitotic abnormalities and rapid senescence/cell cycle arrest induced by Tax.
The levels of p21CIP1/WAF1and p27 KIP 1 are regulated through transcription, phosphorylation (by cyclinE/Cdk2), subcellular localization, ubiquitination, and proteasome-mediated degradation [40, 49, 50]. The E3 ubiquitin ligase, SCF, together with its substrate-recognition subunit, Skp2, mediates the ubiquitination and degradation of p21CIP1/WAF1and p27 KIP 1 [39, 44, 45]. The level of Skp2 oscillates in a cell cycle-dependent manner. Recent data have indicated that Skp2 and another SCF subunit, Cks1, are substrates of the Cdh1-associated APC/C (APCCdh1) [39, 44]. Both become ubiquitinated and degraded in late M and early G1 when APCCdh1 is highly active. This inactivates SCF and allows p21CIP1/WAF1and p27 KIP 1 to accumulate transiently in G1. We have shown recently that in HeLa cells transduced with tax, early APC/C activation sets in motion premature loss of cyclin A, cyclin B, securin, and Skp2, and causes a dramatic build-up of p21CIP1/WAF1and p27 KIP 1 during S phase. After an aberrant mitotic division cycle, the great surge in p21CIP1/WAF1and p27 KIP 1 in tax-expressing cells then commits cells into a state of irreversible cell cycle arrest. Results from the present analysis, i.e. mutations in tax that abrogated induction/stabilization of p21CIP1/WAF1and p27 KIP 1 also disabled APC/C activation, are in agreement with that conclusion.
Activation of p21CIP1/WAF1mRNA transcription by Tax has been reported previously [23, 51–53]. Because a Tax mutant, M47, which is deficient in LTR activation, became disabled in activating p21CIP1/WAF1promoter, some of these earlier studies have suggested that Tax-induced increase in p21CIP1/WAF1resulted from transactivation via the CREB/ATF-CBP/p300 pathway [23, 53]. Since four of the five mutants analyzed here activated LTR-luciferase reporter at levels (70–80%) comparable to that of the wild-type tax, yet were substantially impaired in causing p21CIP1/WAF1and p27 KIP 1 increase, we conclude that the CREB/ATF-CBP/p300 pathway is most likely not the principal determinant in the accumulation of p21CIP1/WAF1and p27 KIP1 . Likewise, several of the mutants – A108T, V19M and C23W, in particular – are potent activators of IKK-NF-κB as indicated by luciferase reporter assays and the extent of I-κB degradation and p100 processing. These mutants are nevertheless impaired in elevating p21CIP1/WAF1and p27 KIP 1 levels. These results support the notion that the NF-κB pathway is not directly responsible for the Tax-induced increase in p21CIP1/WAF1and p27 KIP1 . Our earlier data suggest that a major factor for Tax-induced p21CIP1/WAF1increase is correlated with premature APC/C activation . Therefore, promoter transactivation by Tax may only contribute moderately to the overall build-up of p21CIP1/WAF1protein. This would explain the discrepancy between the data reported here and the earlier studies which relied heavily on p21CIP1/WAF1promoter-luciferase reporter assays. Finally, we have shown that via a tripartite interaction, Tax, PP2A and IKKγ form a stable ternary complex wherein PP2A activity is inhibited or diminished by Tax. In essence, PP2A inhibition by IKKγ-bound Tax maintains IKK in an active, phosphorylated state, causing constitutive phosphorylation and degradation of I-κB, which, in turn, leads to potent activation of genes under NF-κB/Rel control. Since PP2A regulates many critical cellular processes, it is conceivable that APC/C activation by Tax is also mediated through an inhibition of PP2A. In this sense, a tax mutant deficient in PP2A interaction will be disabled for both NF-κB and APC/C activation, but may continue to transactivate LTR. Several tax mutants, H43Q, K85N, and M22 (T130A, L131S) have been shown previously to be disabled for PP2A binding . Both H43Q and M22 are deficient in NF-κB but not in LTR transactivation, while K85N is defective for both. Preliminary analyses suggest that these three mutants are also impaired in inducing cell cycle arrest. Mutants like H43Q and M22, with the possible exception of T130I, however, are not highly represented in the current collection. This may be due, in part, to the importance of the PP2A-binding domain of Tax in mediating other critical protein-protein interaction.
The biological/virological relevance for the profound cell cycle arrest induced by Tax remains unclear. It is possible that HTLV-1-infected T-cells that are arrested in a senescence-like state may persist longer in vivo and may, in this condition, be co-opted to devote significant cellular resources to virus replication. Alternatively, the dramatic morphological changes associated with the senescence-like arrest may facilitate virus assembly and/or transmission. Many of the tax mutants characterized here that remain functional in transactivating viral LTR and NF-κB can be incorporated into an infectious molecular clone of HTLV-1 to address this question.
Tax mutagenesis and selection for taxmutants
The CEN plasmid, pRS315 Gal10-Tax, that contains tax under the control of a galactose-inducible promoter derived from Gal10 gene has been previously described . To introduce mutations into tax sequence, pRS315 Gal10-Tax plasmid DNA was exposed to hydroxylamine (1 mg/ml) overnight at 37°C . The plasmid was then purified using a PCR purification kit (Qiagen), and used (1 μg) to transform S. cerevisiae. The transformants were plated on leucine-dropout plates that contained galactose as the sole carbon source. Colonies that appeared on galactose plates were then picked and seeded in grids on galactose plates, transferred to nitrocellulose filters, lysed, and screened for Tax expression using a monoclonal antibody (4C5) against Tax. Only positive clones were chosen for plasmid extraction/isolation. The plasmids extracted from W303-1a were than used to transform competent E. coli for DNA preparation and sequence analysis.
Cloning mutant taxalleles into a lentiviral vector
The lentivirus vector, LV-Tax-SV-Puro, which contains the wild-type tax gene under the control of the CMV enhancer promoter and the puromycin-resistance gene expressed from the SV40 enhancer/promoter (SV-Puro), has been previously reported . A mutant tax allele, M47, which carries a diagnostic BglII restriction site in the tax coding sequence, was cloned into LV-Tax-SV-Puro via the BamHI (located immediately upstream of the translational initiation codon) and SmaI (downstream of the M47 mutations) restriction sites to generate LV-M47-SV-Puro. Most mutant tax alleles were cloned into LV-M47-SV-puro similarly except that an internal MluI site and an XmaI site, located at the aforementioned SmaI site, were used. The recombinants were identified by a loss of the diagnostic BglII site from the recombinant, and confirmed by DNA sequence analysis. For NH2-terminal mutations that lie upstream of the MluI site, DNA fragments harboring the mutations were generated by PCR and cloned into LV-Tax-SV-puro via the BamHI and MluI sites. Primers used to amplify the NH2 terminal coding region of Tax are 5'TaxBamHI 5'-CGCGGATCCGCCACCATGGCCCACTTCCCAGGGTT-3' (with the translational start site underlined) and 3'TaxXmaI 5'-GCTCTAAGCCCCCGGGGGATA-3'.
Construction of the Tax-inducible reporter, 18 × 21-DsRed and derivation of the 18 × 21-DsRed indicator cell line
A highly Tax-inducible enhancer/promoter cassette that contains 18 copies of the Tax responsive 21 bp repeat element upstream of a minimal HTLV-1 promoter (18 × 21) has been reported previously . A blunt-ended BamHI fragment containing the 18 × 21 cassette was inserted upstream of the DsRed reporter gene in the pDsRed2-C1 (Stratagene) plasmid (blunt-ended at AseI and AgeI sites) to make p18 × 21DsRed-Neo. A HeLa reporter cell line for Tax was derived by transfecting cells with the p18 × 21-DsRed-Neo plasmid, followed by G418 (1 μg/ml, Invitrogen) selection. G418-resistant clones were expanded and transduced with the HR'CMV-Tax-puro lentiviral vector and observed for DsRed expression. One clone that had low basal expression but high DsRed expression in the presence of Tax was chosen for this study.
DNA transfection and luciferase reporter assay
Approximately 105 293T cells/well in a 12-well plate were transfected with the expression construct for each of the tax alleles (0.5 μg) together with either HTLV- LTR-Luc (0.1 μg) or E-selectin-Luc reporter plasmid (0.5 μg) using a calcium phosphate transfection kit (Invitrogen). Forty eight hours after transfection, cells were lysed using 250 μl of reporter lysis butter and 20 μl lysate from each transfection was used for the luciferase assay. After injection of 100 ul luciferase substrate (Promega), the luciferase activity was measured by a MLX microtiter plate luminometer. Transactivation functions of V19M, C23W, A108T, L159F, and L235F mutants were further confirmed by including in the transfection mixture 0.5 μg of a control plasmid, pRL-TK, that contains the renilla luciferase reporter gene driven by the herpesvirus thymidine kinase promoter.
Lentiviral vector production and gene transduction
Lentiviral vectors (LV) were produced as previously described after transfection of 293T cells . Culture supernatants were harvested at 24, 48 and 72 h after transfection, pooled, filtered, aliquoted, and stored at -80 °C. Viral titers were measured by adding serially diluted LV stocks to 2 × 105 (HeLa 18 × 21-DsRed-Neo) cells were seeded in a 24-well plate. Polybrene (8 μg/ml, Sigma) was added to the medium together with the vector stocks to facilitate infection. Three days post-transduction, the number of RFP-positive cells in each well was counted as a measure of viral titer. To transduce tax alleles, 2 × 105 HeLa cells were first plated in a 6-well plate similar as in . They were then transduced with LV (m.o.i. = 2) the next day. After 24 h, the medium was removed and fresh DMEM containing puromycin (1 μg/ml, Sigma) was added. The selection medium and cell debris were removed 48 h after selection by 1× PBS washes, and replaced with fresh puromycin-free DMEM.
Immunoblot analyses of cells transduced with different taxalleles
HeLa cells transduced with LV-Tax or LV-mutant Tax were grown to approximately 70% confluency 4–5 days after initial seeding. SDS sample buffer (2×, 60 μl) was added to each well to lyse the cells. Cell lysates were scraped and transferred to an Eppendorf tubes and heated at 100°C for 5 minutes. Total cell proteins were then resolved by SDS/12% PAGE, transferred to nitrocellulose membrane and probed with antibodies (Santa Cruz Biotechnology) against cyclin B1 (sc-752), actin (sc-1616), I-κBα (sc-1643), p52-NFκB2 (sc-7386), Skp2 (sc-7164), p21CIP1/WAF1(sc-397), and p27 KIP 1 (sc-1641). For detection of Tax, a mouse hybridoma antibody, 4C5, which reacts with the COOH terminal region of Tax, was used.
Cell cycle analysis of HeLa cells transduced with taxalleles
HeLa cells transduced with LV containing either the wild-type or each of the mutants tax alleles were selected with puromycin and maintained as described above. Cells were harvested 4–5 days post-transduction for flow cytometry as previously described .
Detection of Clb2p in S. cerevisia
Detection of Clb2p in S. cerevisia was previously described  except that yeast cracking buffer (8M urea, 5% SDS, Tris-HCl (pH6.8), EDTA 0.1 mM, Bromophenol blue 0.4 mg/ml, β-mercaptoethanol 10 μl/ml) was used as the lysis buffer. Immunoblots were carried out with anti-HA (Santa Cruz Biotechnology), anti-PP2A-C (Upstate) and 4C5 (Tax) monoclonal antibodies.
Detection of polyubiquitinated cyclin B
HEK 293T cells were co-transfected with HA-tagged ubiquitin and the various tax alleles as above. Cells are washed the next day and harvested 48 h later for immunoprecipitation using the cyclin B1 antibody (Santa Cruz Biotechnology) as previously described  except that RIPA buffer (Upstate protocol Tris-HCl, NP40 1%, Na-deoxycholate 0.25%, NaCl 150 mM, EDTA 1 mM, PMSF 1 mM, protease inhibitor cocktail (1 μg/ml), Na3VO4 1 mM, and NaF 1 mM) was used instead of the lysis buffer previously described. Immunoblots were carried out with anti-HA (Santa Cruz), anti-cyclin B1 (Santa Cruz Biotechnology) and 4C5 (Tax) monoclonal antibodies.
We thank Austin Lin and Alan Giam for their technical assistance during their summer internship. We also thank all the members of the lab for their contributions. This work was supported by grants from the National Institutes of Health to C.-Z.G.
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