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{"project":"bionlp-st-ge-2016-coref","denotations":[{"id":"T11285","span":{"begin":23493,"end":23513},"obj":"Anaphor"},{"id":"T11286","span":{"begin":23514,"end":23534},"obj":"Antecedent"},{"id":"T11287","span":{"begin":23541,"end":23545},"obj":"Antecedent"},{"id":"T5118","span":{"begin":3956,"end":3961},"obj":"Anaphor"},{"id":"T5119","span":{"begin":3949,"end":3954},"obj":"Antecedent"},{"id":"T13274","span":{"begin":27596,"end":27609},"obj":"Anaphor"},{"id":"T13275","span":{"begin":27561,"end":27565},"obj":"Antecedent"},{"id":"T13276","span":{"begin":27585,"end":27590},"obj":"Antecedent"},{"id":"T13277","span":{"begin":27817,"end":27820},"obj":"Anaphor"},{"id":"T13278","span":{"begin":27791,"end":27795},"obj":"Antecedent"},{"id":"T8225","span":{"begin":14637,"end":14655},"obj":"Anaphor"},{"id":"T8226","span":{"begin":14604,"end":14607},"obj":"Antecedent"},{"id":"T8227","span":{"begin":15683,"end":15696},"obj":"Anaphor"},{"id":"T8228","span":{"begin":15722,"end":15725},"obj":"Antecedent"}],"relations":[{"id":"R4097","pred":"boundBy","subj":"T5118","obj":"T5119"},{"id":"R8839","pred":"boundBy","subj":"T11285","obj":"T11286"},{"id":"R8840","pred":"boundBy","subj":"T11285","obj":"T11287"},{"id":"R10140","pred":"boundBy","subj":"T13274","obj":"T13275"},{"id":"R10141","pred":"boundBy","subj":"T13274","obj":"T13276"},{"id":"R10142","pred":"boundBy","subj":"T13277","obj":"T13278"},{"id":"R6551","pred":"boundBy","subj":"T8225","obj":"T8226"},{"id":"R6553","pred":"boundBy","subj":"T8227","obj":"T8228"}],"namespaces":[{"prefix":"_base","uri":"http://bionlp.dbcls.jp/ontology/ge.owl#"}],"text":"Results\n\nFoxp3 Suppresses NF-κB Dependent Transcriptional Activation\nTo ascertain the molecular mechanisms by which Foxp3 functions to promote the regulatory function of CD4+CD25hi T cells, we first confirmed the function of Foxp3 as a repressor of activation of NF-κB, previously implicated as a target of other forkhead/winged-helix family transcription factors (e.g., Foxj1 and Foxo3a) [19,20]. We analyzed the effect of Foxp3 overexpression on NF-κB activation in HEK 293T cells in dose-response and time course analyses. Transfection of HEK 293T cells with an NF-κB luciferase reporter vector in the presence or absence of increasing concentrations of a Foxp3 expression vector or a control vector (enhanced green fluorescent protein [EGFP]) was performed, and cells were harvested after 24 h to assay for luciferase activity and Foxp3 mRNA expression. Results indicated that as the concentration of Foxp3 transfected into cells increases (from 50 to 2,400 ng), the level of NF-κB activation decreases proportionally (Figure 1A). Foxp3 mRNA was also assayed to monitor activity of the Foxp3 expression vector (Figure 1B). Since NF-κB activation was partially affected by transfection of high concentrations of the control vector, we determined the fold inhibition of NF-κB activation by Foxp3 compared to the control vector at each concentration (Figure 1A). Fold inhibition of NF-κB activation was directly proportional to the level of Foxp3 mRNA expression detected by real-time RT-PCR. To determine the level of Foxp3-mediated suppression of NF-κB activation over time, HEK 293T cells were transfected with an NF-κB luciferase reporter vector and an expression vector encoding Foxp3 or EGFP (control vector) and harvested over 4 d. As shown in Figure 1C, NF-κB activation was suppressed by overexpression of Foxp3 at all time points. Extending these results from established, in vitro HEK cell lines to primary human lymphocytes, overexpression of Foxp3 in purified CD4+ T cells from three healthy donors also down-regulated the steady-state level of NF-κB activation (Figure 1D). These results recapitulate those from Bettelli and colleagues [15] demonstrating that Foxp3 functions, in part, to block NF-κB-dependent transcription in human cell lines as well as in primary human CD4+ T cells.\nFigure 1 Foxp3 Suppresses NF-κB-Dependent Transcription\n(A and B) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with an NF-κB luciferase reporter vector (200 ng) in the presence or absence of indicated concentrations of a Foxp3 expression vector or a control vector and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested 24 h posttransfection and analyzed for luciferase activity (A) and Foxp3 mRNA expression (B).\n(C) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with an NF-κB luciferase reporter vector (200 ng) in the presence or absence of a Foxp3 expression vector or a control vector (1,500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested at 24, 48, 72, and 96 h posttransfection and analyzed for luciferase activity.\n(D) CD4+ T cells (2 × 106) from three healthy donors were nucleofected with an NF-κB luciferase reporter vector (1,000 ng) and Foxp3 or control expression vectors (2,000 ng) and internal reference plasmid (1,000 ng). Cells were harvested 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity shown in A, C, and D was normalized to the internal reference control.\n\nThe Carboxyl-Terminal FKH Domain Is Not Required for Suppression of NF-κB Activation in T Cells\nTo define the requirements of Foxp3 with respect to inhibition of NF-κB-dependent transcription, we utilized a mutant of Foxp3 lacking the FKH domain (Figure 2A) [16], similar to the scurfy mutant Foxp3 of mice, and a mutant Foxp3 protein from a patient with IPEX [4,11,14,17]. Unlike full-length Foxp3, which localizes almost exclusively to the nucleus and can bind in a sequence-specific manner to forkhead binding sites, the ΔFKH mutant fails to localize to the nucleus and thus cannot interact with promoter elements or nuclear proteins [16]. Therefore, we utilized the ΔFKH mutant to determine whether nuclear localization (or other function associated with the FKH domain) of Foxp3 was a prerequisite for inhibition of NF-κB activation. Although Foxp3 interaction with NF-κB presumably takes place in the nucleus, it may also be possible for a cytoplasmic Foxp3 protein to bind to NF-κB in the cytoplasm and prevent localization to the nucleus following an activation stimulus. Overexpression of full-length Foxp3, but not of ΔFKH, was able to suppress activation of a cotransfected NF-κB reporter vector in HEK 293T cells (Figure 2B). Both Foxp3 and ΔFKH were expressed at very high levels following transfection as detected by real-time RT-PCR (unpublished data). These data appear to suggest that the carboxyl-terminal FKH domain is critically important for Foxp3 to down-regulate NF-κB-dependent transcription. However, NF-κB activation was blocked to a similar extent by both full-length Foxp3 and ΔFKH in Jurkat T cells (Figure 2C) and primary human CD4+ T cells (Figure 2D). Western blot analysis of NF-κB p65 expression demonstrated that Foxp3 and ΔFKH does not block NF-κB activation at the level of p65 protein expression (Figure 2E). These results are very interesting with respect to Foxp3 function, because they suggest that the carboxyl-terminal FKH domain, and possibly nuclear localization, are dispensable for Foxp3 function in T cell populations. Alternative interpretations may include the possibility that the localization of ΔFKH differ between epithelial cells and T cells. In either case, these results suggest a cell type-specific mechanism of action for this Foxp3 mutant.\nFigure 2 FKH Domain of Foxp3 Is Required to Inhibit NF-κB Activation in HEK 293T Cells but Not CD4+ T Cells\n(A) Schematic representation of human Foxp3, including proline-rich (P-P-P), zinc finger (ZNF), leucine zipper (ZIP), and forkhead (FKH) domains. Mutations within the Foxp3 gene associated with IPEX have been previously described [4,14,43]. The FKH domain contains a nuclear localization signal (NLS), which is required for Foxp3 expression in the nucleus.\n(B and C) HEK 293T cells (5 × 105) (B) were seeded into six-well plates 1 d prior to transfection or Jurkat T cells (1 × 106) (C) were seeded into six-well plates on the day of transfection with an NF-κB luciferase reporter vector (200 ng) in the presence or absence of a Foxp3, ΔFKH, or control expression vector (1,000 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested at 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity was normalized to the internal reference control.\n(D) CD4+ T cells (2 × 106) from healthy donors were nucleofected with an NF-κB reporter plasmid (2,000 ng), together with either a Foxp3, ΔFKH, or control expression vector (2,000 ng), and an internal reference plasmid (1,000 ng). Cells were harvested 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity was normalized to the internal reference control. Results from one healthy donor are shown.\n(E) Foxp3, ΔFKH, and NF-κB p65 expression in whole-cell extracts (20 μg) derived from HEK 293T cells transfected with a control expression vector (EGFP), Foxp3, or ΔFKH expression vectors were analyzed by Western blot analysis. β-actin expression was analyzed as a loading control.\n\nFoxp3 Suppresses HIV-1 Gene Expression in Part through Blocking Activation of NF-κB\nIf Foxp3 functions as a repressor of NF-κB-dependent gene expression, then we hypothesized that Foxp3 overexpression could selectively down-regulate transcription from promoters previously shown to be responsive to NF-κB. To address this question, we examined the transcriptional activation of the HIV-1 LTR, which contains two tandem cis-acting NF-κB binding sites located between positions −102 and −81 with respect to the transcription initiation site [21]. NF-κB plays a crucial role in regulating gene expression directed from the HIV-1 LTR in CD4+ T cells [21]. Overexpression of full-length Foxp3, but not ΔFKH, in HEK 293T cells was able to inhibit basal activation of the HIV-1 LTR (Figure 3A), similar to what was previously demonstrated with the synthetic NF-κB reporter vector (Figure 2B). Furthermore, HIV-1 LTR activation was suppressed by full-length Foxp3 and ΔFKH in Jurkat T cells (Figure 3B). To demonstrate that Foxp3-mediated HIV-1 LTR repression was associated with interactions with NF-κB bound to the HIV-1 LTR, we compared basal activation of the HIV-1 LTR or an identical HIV-1 LTR lacking the NF-κB sites located between −102 and −81 (HIV-1 Δ-κB LTR) (Figure 3C). This mutant HIV-1 LTR construct exhibited reduced levels of transcription compared to the parental HIV-1 LTR in purified healthy donor CD4+ T cells (unpublished data). However, directly comparing the effect of Foxp3 overexpression on the activation of these two viral promoters demonstrated that Foxp3 was more capable of suppressing transcriptional activation of the HIV-1 LTR (Figure 3D) compared to the mutated HIV-1 LTR (Figure 3E). These results suggest that Foxp3 down-regulation of HIV-1 LTR activation was mediated at least in part by cis-acting NF-κB binding sites. Residual levels of inhibition of the HIV-1 Δ-κB LTR by Foxp3 may be due to NF-AT binding sites located upstream of the NF-κB sites within the HIV-1 LTR [22,23].\nFigure 3 Foxp3 Inhibits Basal Activation of the HIV-1 LTR\n(A and B) HEK 293T cells (5 × 105) (A) were seeded into six-well plates 1 d prior to transfection or Jurkat T cells (1 × 106) (B) were seeded into six-well plates on the day of transfection with an HIV-1 LTR luciferase reporter vector (200 ng) in the presence or absence of a Foxp3, ΔFKH, or control expression vector (1,000 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested at 24 h posttransfection and analyzed for luciferase activity.\n(C) Schematic representation of HIV-1 LTR luciferase reporter constructs pHIV-1 LTR-luc and pHIV-1 Δ-κB LTR-luc.\n(D and E) CD4+ T cells (2 × 106) from two healthy donors were nucleofected with either parental HIV-1 LTR (D) or HIV-1 LTR lacking NF-κB binding sites (at −102 to −81) (E) (1,000 ng), Foxp3 or control expression vectors (2,000 ng), and an internal reference plasmid (1,000 ng). Cells were harvested 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity shown in A, B, D, and E was normalized to the internal reference control.\n\nThe Transactivation Functions of HTLV-I Tax Are Suppressed by Foxp3\nPrevious studies by Bettelli and colleagues have demonstrated that Foxp3 can repress both the basal levels of NF-κB activation as well as tumor necrosis factor-α-stimulated NF-κB activation [15]. Our next step was to determine whether Foxp3 could also suppress the activation of NF-κB caused by a strong viral transactivator protein. HTLV-I encodes a multifunctional transactivator protein, Tax, capable of activating both the NF-κB and CREB pathways [24–29]. Since the HTLV-I Tax protein can function at multiple levels in both the cytoplasm and the nucleus to stimulate activation of NF-κB [28,29], we hypothesized that overexpression of Foxp3 may interfere with this process. However, since Tax-dependent HTLV-I gene expression is independent of NF-κB [18], we also hypothesized that Foxp3 would not affect Tax-dependent activation of the HTLV-I LTR. To test these hypotheses, we overexpressed HTLV-I Tax, full-length Foxp3, and/or ΔFKH in HEK 293T cells cotransfected with an HTLV-I LTR or NF-κB reporter vector. As shown in Figure 4A, HTLV-I Tax strongly up-regulated NF-κB-dependent transcriptional activation (~60-fold). Interestingly, overexpression of Foxp3, but not ΔFKH, suppressed Tax-mediated activation of NF-κB-dependent transcription. These observations further suggest that the carboxyl-terminal FKH domain is required for inhibiting activation of NF-κB in the presence of Tax in HEK 293T cells, strikingly similar to the requirements of Foxp3 inhibition of basal NF-κB activation shown in Figure 2B. Transactivation of the HTLV-I LTR was stimulated about 55-fold by overexpression of Tax (Figure 4B), while transfection of Foxp3 suppressed Tax-dependent HTLV-I LTR activation, although HTLV-I LTR activation in the presence or absence of Tax is independent of NF-κB or NF-AT (another transcriptional activator known to interact with Foxp3). Furthermore, overexpression of ΔFKH also led to suppression of HTLV-I transactivation by Tax to a similar extent as full-length Foxp3 (Figure 4B). The suppressive effects shown in Figure 4A and 4B were not the result of Foxp3 down-regulating the expression of the transfected Tax plasmid as determined by real-time RT-PCR (Figure 4C). These results strongly suggest that Foxp3 interacts with transcriptional regulators in addition to NF-κB and NF-AT, and that the carboxyl-terminal FKH domain, and therefore localization to the nucleus, are not required for inhibition of Tax-mediated HTLV-I LTR activation (even in HEK 293T cells).\nFigure 4 Transactivation Functions of HTLV-I Tax Are Blocked by Overexpression of Foxp3\n(A and B) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with NF-κB (A) or HTLV-I LTR (B) luciferase reporter vectors (200 ng) in the presence or absence of an HTLV-I Tax, Foxp3, ΔFKH, or control expression vector (500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested at 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity was normalized to the internal reference control.\n(C) Expression of HTLV-I Tax in (A) and (B) was measured by quantitative RT-PCR (TaqMan) to demonstrate that Foxp3 did not suppress Tax transactivation of NF-κB and the HTLV-I LTR by down-regulating the expression of Tax.\n(D) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with a Gal4 luciferase reporter vector (200 ng) in the presence or absence of a Gal4-BD or Gal4-BD-Tax expression vector (500 ng), Foxp3, ΔFKH, or control expression vector (500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested at 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity was normalized to the internal reference control. To determine whether Foxp3 inhibited the transactivation functions of Tax by directly associating with this viral protein, we generated an expression vector in which HTLV-I Tax was fused in-frame to the carboxyl terminus of the Gal4 DNA-binding domain (Gal4-BD). This Gal4-BD-Tax fusion protein activated transcription of a synthetic promoter containing five Gal4 binding sites, while Gal4-BD was insufficient to stimulate transcription by itself (Figure 4D). Transactivation of the Gal4-resposive promoter by Gal4-BD-Tax remained relatively unaffected by overexpression of either EGFP (control), Foxp3, or ΔFKH, suggesting that Foxp3 does not repress Tax transactivation by directly interfacing with the HTLV-I Tax protein. To confirm that Foxp3 had a direct effect on HTLV-I replication, we transfected HEK 293T cells with a well-characterized HTLV-I infectious molecular clone (termed ACH) [30] in the presence of full-length Foxp3, ΔFKH, or control vector. ACH has been previously shown to direct the expression of viral antigens, produce infectious virus, and transform CD4+ T cells both in vitro and in vivo [31,32]. After 24 h, the amount of viral antigen expression, in this case Tax mRNA, was detected by a sensitive real-time RT-PCR assay. As illustrated in Figure 5, the level of Tax mRNA synthesized from ACH was down-regulated in the presence of Foxp3 compared to the level produced in the presence of the control vector. ΔFKH did not have a discernable affect on Tax expression. These data indicate that Foxp3 is capable of repressing the expression of Tax from an infectious HTLV-I molecular clone.\nFigure 5 Foxp3 Inhibits the Expression of HTLV-I Tax Directed from an Infectious Molecular Clone\nHEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection in the absence or presence of an HTLV-I infectious molecular clone (pACH; 800 ng) and either a control (EGFP), Foxp3, or ΔFKH expression vector (800 ng). Expression of HTLV-I Tax from the molecular clone was measured by quantitative RT-PCR (TaqMan).\n\nIncreased Foxp3 Protein Expression Is Associated with Low HTLV-I Proviral Load\nSince Foxp3 is expressed almost exclusively within CD4+CD25+ T cells, a major viral reservoir for HTLV-I [33], it was important to determine whether there was an association between Foxp3 and HTLV-I replication in infected patients. We therefore quantitated the Foxp3 protein expression in CD4+CD25+ T cells by flow cytometry and the HTLV-I proviral load (a surrogate marker of viral replication) by real-time PCR from eight patients with HAM/TSP and eight asymptomatic carriers (ACs). The data from this analysis is summarized in Table 1. As expected, patients with HAM/TSP exhibited significantly higher proviral loads (indicated as HTLV-I proviral DNA copies/100 cells) (34.68 ± 23.19) compared to ACs (4.75 ± 5.47) (p = 0.0008). The percentage of Foxp3+ cells within the CD4+CD25+ T cell population was significantly greater in ACs (43.23 ± 12.95) than in HAM/TSP patients (18.59 ± 5.77) (p = 0.0033). These data suggest that high levels of Foxp3 protein expression are associated with reduced HTLV-I replication in vivo. They also support our recent reports that high proviral loads, which have been shown to correlate with high Tax mRNA in HTLV-I-infected patients, are associated with reduced Foxp3 expression [8,34].\nTable 1 Expression of Foxp3 in CD4+CD25+ T Cells in HTLV-I-Infected Patients\n\nCREB Is a Target for Transcriptional Repression by Foxp3\nAlthough Foxp3 could down-regulate Tax-dependent transactivation of the HTLV-I LTR (Figure 4B) and inhibit Tax expression from an infectious molecular clone (Figure 5), Foxp3 failed to modulate Tax function in the absence of the viral promoter (Figure 4D). These results led us to hypothesize that Foxp3 acts on HTLV-I gene expression by interacting with proteins important for driving HTLV-I LTR activity in vivo. Previous studies have demonstrated that the Tax-responsive elements within the HTLV-I LTR play a crucial role in driving Tax-mediated transactivation of the HTLV-I LTR [18]. The Tax-responsive elements have been shown to resemble CREB binding sites, bind CREB in vitro and in vivo, and facilitate HTLV-I LTR activation both in the presence and in the absence of Tax [35,36]. Ching and colleagues [18] demonstrated that addition of a dominant-negative CREB expression vector resulted in nearly complete inhibition of Tax-mediated activation of the HTLV-I LTR, while blocking NF-κB activation by addition of a dominant-negative IKKβ expression vector had no effect on Tax transactivation of the HTLV-I LTR. Therefore, we hypothesized that Foxp3 may inhibit Tax transactivation of the HTLV-I LTR via disruption of the CREB signaling pathway. To test this possibility, HEK 293T cells were transfected with an HTLV-I LTR or synthetic CREB reporter vector along with a control expression vector (EGFP) or expression vectors encoding Foxp3 or ΔFKH. As shown in Figure 6A, Foxp3 down-regulated basal activation of the HTLV-I LTR and transcription of a synthetic CREB reporter vector, suggesting that Foxp3 down-regulates HTLV-I LTR activation by targeting the CREB pathway. Deletion of the FKH domain of Foxp3 dampened the suppressive effect of Foxp3, but did not completely abrogate suppression, as is seen with NF-κB-responsive promoters in HEK 293T cells. Like NF-κB activation, CREB transcriptional activation was also suppressed by expression of Foxp3, and to a similar extent ΔFKH, in healthy donor CD4+ T cells (Figure 6B). Similarly, Foxp3 and ΔFKH also repressed basal HTLV-I LTR activation in primary human CD4+ T cells (Figure 6C). To our knowledge, this is the first evidence implicating CREB as a molecular target of Foxp3. As observed with NF-κB activation, ΔFKH was a more potent inhibitor of CREB activation in CD4+ T cells than in HEK 293T cells, further indicating that a cell type-specific mechanism of action may govern the function of this Foxp3 mutant.\nFigure 6 Foxp3 Inhibits CREB-Dependent Transcription\n(A) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with a CREB or an HTLV-I LTR luciferase reporter vector (200 ng) in the presence or absence of a Foxp3, ΔFKH, or control expression vector (500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng).\n(B) Purified CD4+ T cells (2 × 106) from two healthy donors were transfected with a CREB luciferase reporter vector (2,000 ng) in the presence or absence of a Foxp3, ΔFKH, or control expression vector (2,000 ng) and internal reference plasmid (pGL4-TKhRluc2; 1,000 ng).\n(C) Purified CD4+ T cells (2 × 106) from healthy donors were transfected with an HTLV-I LTR luciferase reporter vector (2,000 ng) in the presence or absence of a Foxp3, ΔFKH, or control expression vector (2,000 ng) and internal reference plasmid (pGL4-TKhRluc2; 1,000 ng).\n(D) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with a Gal4 luciferase reporter vector (200 ng) in the presence or absence of Gal4-BD, Gal4-BD-CREB-1, or Gal4-BD-c-Jun (500 ng) along with Foxp3 or control expression vector (500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells from A, B, C, and D were harvested at 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity was normalized to the internal reference control.\n(E) Foxp3, ΔFKH, and CREB-1 expression in whole-cell extracts (20 μg) derived from HEK 293T cells transfected with a control expression vector (EGFP), Foxp3, or ΔFKH expression vectors were analyzed by Western blot analysis. β-actin expression was analyzed as a loading control. To determine whether Foxp3 functioned by directly signaling through CREB, we utilized expression vectors encoding CREB-1 or c-Jun (a member of the activator protein 1 family of transcription factors) fused in-frame to the Gal4-BD (Gal4-BD-CREB-1 and Gal4-BD-c-Jun). As shown in Figure 6D, activation of a Gal4-responsive reporter vector by Gal4-BD-CREB-1 was down-regulated by Foxp3 compared to control vector (EGFP), indicating that Foxp3 functions by directly or indirectly interacting with CREB-1. However, Foxp3 failed to markedly affect transcriptional activation of Gal4-BD-c-Jun (c-Jun has also been demonstrated to bind to the HTLV-I LTR) and Gal4-BD-Tax (see Figure 5). Importantly, the mechanism of Foxp3-mediated inhibition of CREB-dependent transcription was not due to a block in CREB-1 protein expression, as determined by Western blot analysis (Figure 6E). Although these results demonstrate that Foxp3 functions as a co-repressor of CREB activation (in addition to NF-κB and NF-AT), we were unable to detect a direct physical interaction between CREB-1 and Foxp3 by coimmunoprecipitation or mammalian two-hybrid analysis (unpublished data). Therefore, our data suggest that Foxp3 may interfere with CREB signaling at an upstream event, such as phosphorylation of CREB or recruitment/function of coactivator proteins CREB-binding protein (CBP)/p300.\n\nFoxp3 Antagonizes CREB Transcriptional Activation by Disrupting Coactivator Recruitment\nStimulation of CREB-dependent transcription by reagents that activate adenylate cyclase and increase cAMP levels (e.g., forskolin) increase the transactivation potential of CREB through phosphorylation of serine 133 by protein kinase A, which permits binding and recruitment of coactivators CBP/p300 to the promoter [37,38]. Phosphorylation of serine 133 does not, however, affect the DNA-binding activity of CREB in most cases [39–41]. Addition of forskolin to HEK 293T cells stimulated activation of a CREB reporter vector about 65 fold (Figure 7A). Overexpression of Foxp3 was capable of down-regulating forskolin-induced CREB transcriptional activation. The functional interaction between Foxp3 and CREB did not affect the DNA-binding activity of CREB-1, but did show a modest decrease in activating transcription factor 2 (ATF-2) DNA-binding activity in the presence of forskolin as determined by transcription factor ELISA (Figure 7B).\nFigure 7 Foxp3 Antagonizes CREB Activation by Blocking Recruitment of Coactivator Protein p300\n(A and B) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with a CREB or an HTLV-I LTR luciferase reporter vector (200 ng) in the presence or absence of a Foxp3, DFKH, or control expression vector (500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Forskolin (10 μM) was added to the appropriate reactions 20 h posttransfection. Reactions were assayed 24 h posttransfection for luciferase activity (A) and CREB-1 and ATF-2 DNA-binding activity (B).\n(C) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with a Gal4 luciferase reporter vector (200 ng) in the presence or absence of Gal4-BD-CREB-1 (500 ng) along with a control, Foxp3, or p300 expression vector (500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested at 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity in (A and C) was normalized to the internal reference control.\n(D) Whole-cell extracts from HEK 293T cells transfected with a control (EGFP) or Foxp3 expression vector together with a p300-HA expression vector were immunoprecipitated with anti-HA monoclonal antibody. Proteins were then separated by SDS-PAGE and immunoblotted with anti-Foxp3 (ab10563) to detect immunoprecipitates and with anti-HA for the lysates. While Foxp3 has been shown to bind to and repress activation of both NF-κB and NF-AT, exactly how Foxp3 functions to bring about this affect has not been elucidated. To determine how Foxp3 blocks CREB-dependent transcription, we examined whether Foxp3 was capable of (1) disrupting the recruitment of coactivator proteins and/or (2) preventing phosphorylation of CREB at serine 133 (which is a prerequisite for coactivator recruitment). Since both of these events are required for CREB-dependent gene expression, we hypothesized that Foxp3 may affect CREB activation at both steps. To determine whether Foxp3 can disrupt the function/recruitment of the coactivator protein p300, we introduced a Gal4 reporter vector and a Gal4-BD-CREB-1 expression vector into HEK 293T cells in the absence or presence of Foxp3, p300, and/or control expression vectors (Figure 7C). As expected, p300 overexpression stimulated transcription of the Gal4-BD-CREB-1 fusion protein. Foxp3, again, repressed basal levels of Gal4-BD-CREB-1 activation by more than 2-fold, while effectively neutralizing Gal4-BD-CREB-1 activation in the presence of p300. We next analyzed the effect of Foxp3 on phosphorylation of CREB at serine 133 in forskolin-treated HEK 293T cells by Western blot analysis. Overexpression of Foxp3 failed to reduce the detectable levels of CREB phosphorylation using a phosphospecific antibody for CREB-1 (unpublished data). However, when we attempted to determine whether Foxp3 could physically interact with the coactivator protein p300, we found that p300 immunoprecipitated Foxp3 when both proteins were overexpressed in HEK 293T cells (Figure 7D). Collectively, these results suggest that Foxp3 antagonizes CREB-dependent gene expression by directly interacting with coactivator p300 and interfering with its function and/or recruitment to CREB-responsive promoter sequences.\n"}

bionlp-st-ge-2016-spacy-parsed

UBERON-AE

{"project":"UBERON-AE","denotations":[{"id":"T2971","span":{"begin":322,"end":328},"obj":"http://purl.obolibrary.org/obo/UBERON_0000023"},{"id":"T2972","span":{"begin":329,"end":334},"obj":"http://purl.obolibrary.org/obo/UBERON_0002488"}],"text":"Results\n\nFoxp3 Suppresses NF-κB Dependent Transcriptional Activation\nTo ascertain the molecular mechanisms by which Foxp3 functions to promote the regulatory function of CD4+CD25hi T cells, we first confirmed the function of Foxp3 as a repressor of activation of NF-κB, previously implicated as a target of other forkhead/winged-helix family transcription factors (e.g., Foxj1 and Foxo3a) [19,20]. We analyzed the effect of Foxp3 overexpression on NF-κB activation in HEK 293T cells in dose-response and time course analyses. Transfection of HEK 293T cells with an NF-κB luciferase reporter vector in the presence or absence of increasing concentrations of a Foxp3 expression vector or a control vector (enhanced green fluorescent protein [EGFP]) was performed, and cells were harvested after 24 h to assay for luciferase activity and Foxp3 mRNA expression. Results indicated that as the concentration of Foxp3 transfected into cells increases (from 50 to 2,400 ng), the level of NF-κB activation decreases proportionally (Figure 1A). Foxp3 mRNA was also assayed to monitor activity of the Foxp3 expression vector (Figure 1B). Since NF-κB activation was partially affected by transfection of high concentrations of the control vector, we determined the fold inhibition of NF-κB activation by Foxp3 compared to the control vector at each concentration (Figure 1A). Fold inhibition of NF-κB activation was directly proportional to the level of Foxp3 mRNA expression detected by real-time RT-PCR. To determine the level of Foxp3-mediated suppression of NF-κB activation over time, HEK 293T cells were transfected with an NF-κB luciferase reporter vector and an expression vector encoding Foxp3 or EGFP (control vector) and harvested over 4 d. As shown in Figure 1C, NF-κB activation was suppressed by overexpression of Foxp3 at all time points. Extending these results from established, in vitro HEK cell lines to primary human lymphocytes, overexpression of Foxp3 in purified CD4+ T cells from three healthy donors also down-regulated the steady-state level of NF-κB activation (Figure 1D). These results recapitulate those from Bettelli and colleagues [15] demonstrating that Foxp3 functions, in part, to block NF-κB-dependent transcription in human cell lines as well as in primary human CD4+ T cells.\nFigure 1 Foxp3 Suppresses NF-κB-Dependent Transcription\n(A and B) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with an NF-κB luciferase reporter vector (200 ng) in the presence or absence of indicated concentrations of a Foxp3 expression vector or a control vector and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested 24 h posttransfection and analyzed for luciferase activity (A) and Foxp3 mRNA expression (B).\n(C) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with an NF-κB luciferase reporter vector (200 ng) in the presence or absence of a Foxp3 expression vector or a control vector (1,500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested at 24, 48, 72, and 96 h posttransfection and analyzed for luciferase activity.\n(D) CD4+ T cells (2 × 106) from three healthy donors were nucleofected with an NF-κB luciferase reporter vector (1,000 ng) and Foxp3 or control expression vectors (2,000 ng) and internal reference plasmid (1,000 ng). Cells were harvested 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity shown in A, C, and D was normalized to the internal reference control.\n\nThe Carboxyl-Terminal FKH Domain Is Not Required for Suppression of NF-κB Activation in T Cells\nTo define the requirements of Foxp3 with respect to inhibition of NF-κB-dependent transcription, we utilized a mutant of Foxp3 lacking the FKH domain (Figure 2A) [16], similar to the scurfy mutant Foxp3 of mice, and a mutant Foxp3 protein from a patient with IPEX [4,11,14,17]. Unlike full-length Foxp3, which localizes almost exclusively to the nucleus and can bind in a sequence-specific manner to forkhead binding sites, the ΔFKH mutant fails to localize to the nucleus and thus cannot interact with promoter elements or nuclear proteins [16]. Therefore, we utilized the ΔFKH mutant to determine whether nuclear localization (or other function associated with the FKH domain) of Foxp3 was a prerequisite for inhibition of NF-κB activation. Although Foxp3 interaction with NF-κB presumably takes place in the nucleus, it may also be possible for a cytoplasmic Foxp3 protein to bind to NF-κB in the cytoplasm and prevent localization to the nucleus following an activation stimulus. Overexpression of full-length Foxp3, but not of ΔFKH, was able to suppress activation of a cotransfected NF-κB reporter vector in HEK 293T cells (Figure 2B). Both Foxp3 and ΔFKH were expressed at very high levels following transfection as detected by real-time RT-PCR (unpublished data). These data appear to suggest that the carboxyl-terminal FKH domain is critically important for Foxp3 to down-regulate NF-κB-dependent transcription. However, NF-κB activation was blocked to a similar extent by both full-length Foxp3 and ΔFKH in Jurkat T cells (Figure 2C) and primary human CD4+ T cells (Figure 2D). Western blot analysis of NF-κB p65 expression demonstrated that Foxp3 and ΔFKH does not block NF-κB activation at the level of p65 protein expression (Figure 2E). These results are very interesting with respect to Foxp3 function, because they suggest that the carboxyl-terminal FKH domain, and possibly nuclear localization, are dispensable for Foxp3 function in T cell populations. Alternative interpretations may include the possibility that the localization of ΔFKH differ between epithelial cells and T cells. In either case, these results suggest a cell type-specific mechanism of action for this Foxp3 mutant.\nFigure 2 FKH Domain of Foxp3 Is Required to Inhibit NF-κB Activation in HEK 293T Cells but Not CD4+ T Cells\n(A) Schematic representation of human Foxp3, including proline-rich (P-P-P), zinc finger (ZNF), leucine zipper (ZIP), and forkhead (FKH) domains. Mutations within the Foxp3 gene associated with IPEX have been previously described [4,14,43]. The FKH domain contains a nuclear localization signal (NLS), which is required for Foxp3 expression in the nucleus.\n(B and C) HEK 293T cells (5 × 105) (B) were seeded into six-well plates 1 d prior to transfection or Jurkat T cells (1 × 106) (C) were seeded into six-well plates on the day of transfection with an NF-κB luciferase reporter vector (200 ng) in the presence or absence of a Foxp3, ΔFKH, or control expression vector (1,000 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested at 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity was normalized to the internal reference control.\n(D) CD4+ T cells (2 × 106) from healthy donors were nucleofected with an NF-κB reporter plasmid (2,000 ng), together with either a Foxp3, ΔFKH, or control expression vector (2,000 ng), and an internal reference plasmid (1,000 ng). Cells were harvested 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity was normalized to the internal reference control. Results from one healthy donor are shown.\n(E) Foxp3, ΔFKH, and NF-κB p65 expression in whole-cell extracts (20 μg) derived from HEK 293T cells transfected with a control expression vector (EGFP), Foxp3, or ΔFKH expression vectors were analyzed by Western blot analysis. β-actin expression was analyzed as a loading control.\n\nFoxp3 Suppresses HIV-1 Gene Expression in Part through Blocking Activation of NF-κB\nIf Foxp3 functions as a repressor of NF-κB-dependent gene expression, then we hypothesized that Foxp3 overexpression could selectively down-regulate transcription from promoters previously shown to be responsive to NF-κB. To address this question, we examined the transcriptional activation of the HIV-1 LTR, which contains two tandem cis-acting NF-κB binding sites located between positions −102 and −81 with respect to the transcription initiation site [21]. NF-κB plays a crucial role in regulating gene expression directed from the HIV-1 LTR in CD4+ T cells [21]. Overexpression of full-length Foxp3, but not ΔFKH, in HEK 293T cells was able to inhibit basal activation of the HIV-1 LTR (Figure 3A), similar to what was previously demonstrated with the synthetic NF-κB reporter vector (Figure 2B). Furthermore, HIV-1 LTR activation was suppressed by full-length Foxp3 and ΔFKH in Jurkat T cells (Figure 3B). To demonstrate that Foxp3-mediated HIV-1 LTR repression was associated with interactions with NF-κB bound to the HIV-1 LTR, we compared basal activation of the HIV-1 LTR or an identical HIV-1 LTR lacking the NF-κB sites located between −102 and −81 (HIV-1 Δ-κB LTR) (Figure 3C). This mutant HIV-1 LTR construct exhibited reduced levels of transcription compared to the parental HIV-1 LTR in purified healthy donor CD4+ T cells (unpublished data). However, directly comparing the effect of Foxp3 overexpression on the activation of these two viral promoters demonstrated that Foxp3 was more capable of suppressing transcriptional activation of the HIV-1 LTR (Figure 3D) compared to the mutated HIV-1 LTR (Figure 3E). These results suggest that Foxp3 down-regulation of HIV-1 LTR activation was mediated at least in part by cis-acting NF-κB binding sites. Residual levels of inhibition of the HIV-1 Δ-κB LTR by Foxp3 may be due to NF-AT binding sites located upstream of the NF-κB sites within the HIV-1 LTR [22,23].\nFigure 3 Foxp3 Inhibits Basal Activation of the HIV-1 LTR\n(A and B) HEK 293T cells (5 × 105) (A) were seeded into six-well plates 1 d prior to transfection or Jurkat T cells (1 × 106) (B) were seeded into six-well plates on the day of transfection with an HIV-1 LTR luciferase reporter vector (200 ng) in the presence or absence of a Foxp3, ΔFKH, or control expression vector (1,000 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested at 24 h posttransfection and analyzed for luciferase activity.\n(C) Schematic representation of HIV-1 LTR luciferase reporter constructs pHIV-1 LTR-luc and pHIV-1 Δ-κB LTR-luc.\n(D and E) CD4+ T cells (2 × 106) from two healthy donors were nucleofected with either parental HIV-1 LTR (D) or HIV-1 LTR lacking NF-κB binding sites (at −102 to −81) (E) (1,000 ng), Foxp3 or control expression vectors (2,000 ng), and an internal reference plasmid (1,000 ng). Cells were harvested 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity shown in A, B, D, and E was normalized to the internal reference control.\n\nThe Transactivation Functions of HTLV-I Tax Are Suppressed by Foxp3\nPrevious studies by Bettelli and colleagues have demonstrated that Foxp3 can repress both the basal levels of NF-κB activation as well as tumor necrosis factor-α-stimulated NF-κB activation [15]. Our next step was to determine whether Foxp3 could also suppress the activation of NF-κB caused by a strong viral transactivator protein. HTLV-I encodes a multifunctional transactivator protein, Tax, capable of activating both the NF-κB and CREB pathways [24–29]. Since the HTLV-I Tax protein can function at multiple levels in both the cytoplasm and the nucleus to stimulate activation of NF-κB [28,29], we hypothesized that overexpression of Foxp3 may interfere with this process. However, since Tax-dependent HTLV-I gene expression is independent of NF-κB [18], we also hypothesized that Foxp3 would not affect Tax-dependent activation of the HTLV-I LTR. To test these hypotheses, we overexpressed HTLV-I Tax, full-length Foxp3, and/or ΔFKH in HEK 293T cells cotransfected with an HTLV-I LTR or NF-κB reporter vector. As shown in Figure 4A, HTLV-I Tax strongly up-regulated NF-κB-dependent transcriptional activation (~60-fold). Interestingly, overexpression of Foxp3, but not ΔFKH, suppressed Tax-mediated activation of NF-κB-dependent transcription. These observations further suggest that the carboxyl-terminal FKH domain is required for inhibiting activation of NF-κB in the presence of Tax in HEK 293T cells, strikingly similar to the requirements of Foxp3 inhibition of basal NF-κB activation shown in Figure 2B. Transactivation of the HTLV-I LTR was stimulated about 55-fold by overexpression of Tax (Figure 4B), while transfection of Foxp3 suppressed Tax-dependent HTLV-I LTR activation, although HTLV-I LTR activation in the presence or absence of Tax is independent of NF-κB or NF-AT (another transcriptional activator known to interact with Foxp3). Furthermore, overexpression of ΔFKH also led to suppression of HTLV-I transactivation by Tax to a similar extent as full-length Foxp3 (Figure 4B). The suppressive effects shown in Figure 4A and 4B were not the result of Foxp3 down-regulating the expression of the transfected Tax plasmid as determined by real-time RT-PCR (Figure 4C). These results strongly suggest that Foxp3 interacts with transcriptional regulators in addition to NF-κB and NF-AT, and that the carboxyl-terminal FKH domain, and therefore localization to the nucleus, are not required for inhibition of Tax-mediated HTLV-I LTR activation (even in HEK 293T cells).\nFigure 4 Transactivation Functions of HTLV-I Tax Are Blocked by Overexpression of Foxp3\n(A and B) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with NF-κB (A) or HTLV-I LTR (B) luciferase reporter vectors (200 ng) in the presence or absence of an HTLV-I Tax, Foxp3, ΔFKH, or control expression vector (500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested at 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity was normalized to the internal reference control.\n(C) Expression of HTLV-I Tax in (A) and (B) was measured by quantitative RT-PCR (TaqMan) to demonstrate that Foxp3 did not suppress Tax transactivation of NF-κB and the HTLV-I LTR by down-regulating the expression of Tax.\n(D) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with a Gal4 luciferase reporter vector (200 ng) in the presence or absence of a Gal4-BD or Gal4-BD-Tax expression vector (500 ng), Foxp3, ΔFKH, or control expression vector (500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested at 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity was normalized to the internal reference control. To determine whether Foxp3 inhibited the transactivation functions of Tax by directly associating with this viral protein, we generated an expression vector in which HTLV-I Tax was fused in-frame to the carboxyl terminus of the Gal4 DNA-binding domain (Gal4-BD). This Gal4-BD-Tax fusion protein activated transcription of a synthetic promoter containing five Gal4 binding sites, while Gal4-BD was insufficient to stimulate transcription by itself (Figure 4D). Transactivation of the Gal4-resposive promoter by Gal4-BD-Tax remained relatively unaffected by overexpression of either EGFP (control), Foxp3, or ΔFKH, suggesting that Foxp3 does not repress Tax transactivation by directly interfacing with the HTLV-I Tax protein. To confirm that Foxp3 had a direct effect on HTLV-I replication, we transfected HEK 293T cells with a well-characterized HTLV-I infectious molecular clone (termed ACH) [30] in the presence of full-length Foxp3, ΔFKH, or control vector. ACH has been previously shown to direct the expression of viral antigens, produce infectious virus, and transform CD4+ T cells both in vitro and in vivo [31,32]. After 24 h, the amount of viral antigen expression, in this case Tax mRNA, was detected by a sensitive real-time RT-PCR assay. As illustrated in Figure 5, the level of Tax mRNA synthesized from ACH was down-regulated in the presence of Foxp3 compared to the level produced in the presence of the control vector. ΔFKH did not have a discernable affect on Tax expression. These data indicate that Foxp3 is capable of repressing the expression of Tax from an infectious HTLV-I molecular clone.\nFigure 5 Foxp3 Inhibits the Expression of HTLV-I Tax Directed from an Infectious Molecular Clone\nHEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection in the absence or presence of an HTLV-I infectious molecular clone (pACH; 800 ng) and either a control (EGFP), Foxp3, or ΔFKH expression vector (800 ng). Expression of HTLV-I Tax from the molecular clone was measured by quantitative RT-PCR (TaqMan).\n\nIncreased Foxp3 Protein Expression Is Associated with Low HTLV-I Proviral Load\nSince Foxp3 is expressed almost exclusively within CD4+CD25+ T cells, a major viral reservoir for HTLV-I [33], it was important to determine whether there was an association between Foxp3 and HTLV-I replication in infected patients. We therefore quantitated the Foxp3 protein expression in CD4+CD25+ T cells by flow cytometry and the HTLV-I proviral load (a surrogate marker of viral replication) by real-time PCR from eight patients with HAM/TSP and eight asymptomatic carriers (ACs). The data from this analysis is summarized in Table 1. As expected, patients with HAM/TSP exhibited significantly higher proviral loads (indicated as HTLV-I proviral DNA copies/100 cells) (34.68 ± 23.19) compared to ACs (4.75 ± 5.47) (p = 0.0008). The percentage of Foxp3+ cells within the CD4+CD25+ T cell population was significantly greater in ACs (43.23 ± 12.95) than in HAM/TSP patients (18.59 ± 5.77) (p = 0.0033). These data suggest that high levels of Foxp3 protein expression are associated with reduced HTLV-I replication in vivo. They also support our recent reports that high proviral loads, which have been shown to correlate with high Tax mRNA in HTLV-I-infected patients, are associated with reduced Foxp3 expression [8,34].\nTable 1 Expression of Foxp3 in CD4+CD25+ T Cells in HTLV-I-Infected Patients\n\nCREB Is a Target for Transcriptional Repression by Foxp3\nAlthough Foxp3 could down-regulate Tax-dependent transactivation of the HTLV-I LTR (Figure 4B) and inhibit Tax expression from an infectious molecular clone (Figure 5), Foxp3 failed to modulate Tax function in the absence of the viral promoter (Figure 4D). These results led us to hypothesize that Foxp3 acts on HTLV-I gene expression by interacting with proteins important for driving HTLV-I LTR activity in vivo. Previous studies have demonstrated that the Tax-responsive elements within the HTLV-I LTR play a crucial role in driving Tax-mediated transactivation of the HTLV-I LTR [18]. The Tax-responsive elements have been shown to resemble CREB binding sites, bind CREB in vitro and in vivo, and facilitate HTLV-I LTR activation both in the presence and in the absence of Tax [35,36]. Ching and colleagues [18] demonstrated that addition of a dominant-negative CREB expression vector resulted in nearly complete inhibition of Tax-mediated activation of the HTLV-I LTR, while blocking NF-κB activation by addition of a dominant-negative IKKβ expression vector had no effect on Tax transactivation of the HTLV-I LTR. Therefore, we hypothesized that Foxp3 may inhibit Tax transactivation of the HTLV-I LTR via disruption of the CREB signaling pathway. To test this possibility, HEK 293T cells were transfected with an HTLV-I LTR or synthetic CREB reporter vector along with a control expression vector (EGFP) or expression vectors encoding Foxp3 or ΔFKH. As shown in Figure 6A, Foxp3 down-regulated basal activation of the HTLV-I LTR and transcription of a synthetic CREB reporter vector, suggesting that Foxp3 down-regulates HTLV-I LTR activation by targeting the CREB pathway. Deletion of the FKH domain of Foxp3 dampened the suppressive effect of Foxp3, but did not completely abrogate suppression, as is seen with NF-κB-responsive promoters in HEK 293T cells. Like NF-κB activation, CREB transcriptional activation was also suppressed by expression of Foxp3, and to a similar extent ΔFKH, in healthy donor CD4+ T cells (Figure 6B). Similarly, Foxp3 and ΔFKH also repressed basal HTLV-I LTR activation in primary human CD4+ T cells (Figure 6C). To our knowledge, this is the first evidence implicating CREB as a molecular target of Foxp3. As observed with NF-κB activation, ΔFKH was a more potent inhibitor of CREB activation in CD4+ T cells than in HEK 293T cells, further indicating that a cell type-specific mechanism of action may govern the function of this Foxp3 mutant.\nFigure 6 Foxp3 Inhibits CREB-Dependent Transcription\n(A) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with a CREB or an HTLV-I LTR luciferase reporter vector (200 ng) in the presence or absence of a Foxp3, ΔFKH, or control expression vector (500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng).\n(B) Purified CD4+ T cells (2 × 106) from two healthy donors were transfected with a CREB luciferase reporter vector (2,000 ng) in the presence or absence of a Foxp3, ΔFKH, or control expression vector (2,000 ng) and internal reference plasmid (pGL4-TKhRluc2; 1,000 ng).\n(C) Purified CD4+ T cells (2 × 106) from healthy donors were transfected with an HTLV-I LTR luciferase reporter vector (2,000 ng) in the presence or absence of a Foxp3, ΔFKH, or control expression vector (2,000 ng) and internal reference plasmid (pGL4-TKhRluc2; 1,000 ng).\n(D) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with a Gal4 luciferase reporter vector (200 ng) in the presence or absence of Gal4-BD, Gal4-BD-CREB-1, or Gal4-BD-c-Jun (500 ng) along with Foxp3 or control expression vector (500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells from A, B, C, and D were harvested at 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity was normalized to the internal reference control.\n(E) Foxp3, ΔFKH, and CREB-1 expression in whole-cell extracts (20 μg) derived from HEK 293T cells transfected with a control expression vector (EGFP), Foxp3, or ΔFKH expression vectors were analyzed by Western blot analysis. β-actin expression was analyzed as a loading control. To determine whether Foxp3 functioned by directly signaling through CREB, we utilized expression vectors encoding CREB-1 or c-Jun (a member of the activator protein 1 family of transcription factors) fused in-frame to the Gal4-BD (Gal4-BD-CREB-1 and Gal4-BD-c-Jun). As shown in Figure 6D, activation of a Gal4-responsive reporter vector by Gal4-BD-CREB-1 was down-regulated by Foxp3 compared to control vector (EGFP), indicating that Foxp3 functions by directly or indirectly interacting with CREB-1. However, Foxp3 failed to markedly affect transcriptional activation of Gal4-BD-c-Jun (c-Jun has also been demonstrated to bind to the HTLV-I LTR) and Gal4-BD-Tax (see Figure 5). Importantly, the mechanism of Foxp3-mediated inhibition of CREB-dependent transcription was not due to a block in CREB-1 protein expression, as determined by Western blot analysis (Figure 6E). Although these results demonstrate that Foxp3 functions as a co-repressor of CREB activation (in addition to NF-κB and NF-AT), we were unable to detect a direct physical interaction between CREB-1 and Foxp3 by coimmunoprecipitation or mammalian two-hybrid analysis (unpublished data). Therefore, our data suggest that Foxp3 may interfere with CREB signaling at an upstream event, such as phosphorylation of CREB or recruitment/function of coactivator proteins CREB-binding protein (CBP)/p300.\n\nFoxp3 Antagonizes CREB Transcriptional Activation by Disrupting Coactivator Recruitment\nStimulation of CREB-dependent transcription by reagents that activate adenylate cyclase and increase cAMP levels (e.g., forskolin) increase the transactivation potential of CREB through phosphorylation of serine 133 by protein kinase A, which permits binding and recruitment of coactivators CBP/p300 to the promoter [37,38]. Phosphorylation of serine 133 does not, however, affect the DNA-binding activity of CREB in most cases [39–41]. Addition of forskolin to HEK 293T cells stimulated activation of a CREB reporter vector about 65 fold (Figure 7A). Overexpression of Foxp3 was capable of down-regulating forskolin-induced CREB transcriptional activation. The functional interaction between Foxp3 and CREB did not affect the DNA-binding activity of CREB-1, but did show a modest decrease in activating transcription factor 2 (ATF-2) DNA-binding activity in the presence of forskolin as determined by transcription factor ELISA (Figure 7B).\nFigure 7 Foxp3 Antagonizes CREB Activation by Blocking Recruitment of Coactivator Protein p300\n(A and B) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with a CREB or an HTLV-I LTR luciferase reporter vector (200 ng) in the presence or absence of a Foxp3, DFKH, or control expression vector (500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Forskolin (10 μM) was added to the appropriate reactions 20 h posttransfection. Reactions were assayed 24 h posttransfection for luciferase activity (A) and CREB-1 and ATF-2 DNA-binding activity (B).\n(C) HEK 293T cells (5 × 105) were seeded into six-well plates 1 d prior to transfection with a Gal4 luciferase reporter vector (200 ng) in the presence or absence of Gal4-BD-CREB-1 (500 ng) along with a control, Foxp3, or p300 expression vector (500 ng) and internal reference plasmid (pGL4-TKhRluc2; 50 ng). Cells were harvested at 24 h posttransfection and analyzed for luciferase activity. Relative luciferase activity in (A and C) was normalized to the internal reference control.\n(D) Whole-cell extracts from HEK 293T cells transfected with a control (EGFP) or Foxp3 expression vector together with a p300-HA expression vector were immunoprecipitated with anti-HA monoclonal antibody. Proteins were then separated by SDS-PAGE and immunoblotted with anti-Foxp3 (ab10563) to detect immunoprecipitates and with anti-HA for the lysates. While Foxp3 has been shown to bind to and repress activation of both NF-κB and NF-AT, exactly how Foxp3 functions to bring about this affect has not been elucidated. To determine how Foxp3 blocks CREB-dependent transcription, we examined whether Foxp3 was capable of (1) disrupting the recruitment of coactivator proteins and/or (2) preventing phosphorylation of CREB at serine 133 (which is a prerequisite for coactivator recruitment). Since both of these events are required for CREB-dependent gene expression, we hypothesized that Foxp3 may affect CREB activation at both steps. To determine whether Foxp3 can disrupt the function/recruitment of the coactivator protein p300, we introduced a Gal4 reporter vector and a Gal4-BD-CREB-1 expression vector into HEK 293T cells in the absence or presence of Foxp3, p300, and/or control expression vectors (Figure 7C). As expected, p300 overexpression stimulated transcription of the Gal4-BD-CREB-1 fusion protein. Foxp3, again, repressed basal levels of Gal4-BD-CREB-1 activation by more than 2-fold, while effectively neutralizing Gal4-BD-CREB-1 activation in the presence of p300. We next analyzed the effect of Foxp3 on phosphorylation of CREB at serine 133 in forskolin-treated HEK 293T cells by Western blot analysis. Overexpression of Foxp3 failed to reduce the detectable levels of CREB phosphorylation using a phosphospecific antibody for CREB-1 (unpublished data). However, when we attempted to determine whether Foxp3 could physically interact with the coactivator protein p300, we found that p300 immunoprecipitated Foxp3 when both proteins were overexpressed in HEK 293T cells (Figure 7D). Collectively, these results suggest that Foxp3 antagonizes CREB-dependent gene expression by directly interacting with coactivator p300 and interfering with its function and/or recruitment to CREB-responsive promoter sequences.\n"}

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