Three distinct domains contribute to nuclear transport of murine Foxp3

Stimulation Strength Determined by Superantigen Dose Controls Subcellular Localization of FOXP3 Isoforms and Suppressive Function of CD4+CD25+FOXP3+ T Cells

Staphylococcal superantigens induce massive activation of T cells and inflammation, leading to toxic shock syndrome. Paradoxically, increasing evidence indicates that superantigens can also induce immunosuppression by promoting regulatory T cell (Treg) development. In this study, we demonstrate that stimulation strength plays a critical role in superantigen-mediated induction of immunosuppressive human CD4+CD25+FOXP3+ T cells. Suboptimal stimulation by a low dose (1 ng/ml) of staphylococcal enterotoxin C1 (SEC1) led to de novo generation of Treg-like CD4+CD25+FOXP3+ T cells with strong suppressive activity. In contrast, CD4+CD25+ T cells induced by optimal stimulation with high-dose SEC1 (1 µg/ml) were not immunosuppressive, despite high FOXP3 expression. Signal transduction pathway analysis revealed differential activation of the PI3K signaling pathway and expression of PTEN in optimal and suboptimal stimulation with SEC1. Additionally, we identified that FOXP3 isoforms in Treg-like cells from the suboptimal condition were located in the nucleus, whereas FOXP3 in nonsuppressive cells from the optimal condition localized in cytoplasm. Sequencing analysis of FOXP3 isoform transcripts identified five isoforms, including a FOXP3 isoform lacking partial exon 3. Overexpression of FOXP3 isoforms confirmed that both an exon 2-lacking isoform and a partial exon 3-lacking isoform confer suppressive activity. Furthermore, blockade of PI3K in optimal stimulation conditions led to induction of suppressive Treg-like cells with nuclear translocation of FOXP3, suggesting that PI3K signaling impairs induction of Tregs in a SEC1 dose-dependent manner. Taken together, these data demonstrate that the strength of activation signals determined by superantigen dose regulates subcellular localization of FOXP3 isoforms, which confers suppressive functionality.

MicroRNAs as the critical regulators of Forkhead box protein family during gynecological and breast tumor progression and metastasis

Gynecological and breast tumors are one of the main causes of cancer-related mortalities among women. Despite recent advances in diagnostic and therapeutic methods, tumor relapse is observed in a high percentage of these patients due to the treatment failure. Late diagnosis in advanced tumor stages is one of the main reasons for the treatment failure and recurrence in these tumors. Therefore, it is necessary to assess the molecular mechanisms involved in progression of these tumors to introduce the efficient early diagnostic markers. Fokhead Box (FOX) is a family of transcription factors with a key role in regulation of a wide variety of cellular mechanisms. Deregulation of FOX proteins has been observed in different cancers. MicroRNAs (miRNAs) as a group of non-coding RNAs have important roles in post-transcriptional regulation of the genes involved in cellular mechanisms. They are also the non-invasive diagnostic markers due to their high stability in body fluids. Considering the importance of FOX proteins in the progression of breast and gynecological tumors, we investigated the role of miRNAs in regulation of the FOX proteins in these tumors. MicroRNAs were mainly involved in progression of these tumors through FOXM, FOXP, and FOXO. The present review paves the way to suggest a non-invasive diagnostic panel marker based on the miRNAs/FOX axis in breast and gynecological cancers.

FOXP3 and SQSTM1/P62 correlate with prognosis and immune infiltration in hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is one of the most common highly malignant tumours worldwide. FOXP3 and SQSTM1/P62 have been shown to be abnormally expressed in tumour cells, but their function in different tumours remains controversial. The present study was designed to evaluate the expression of FOXP3 and P62 in HCC and their prognostic value as well as their relationship with immune infiltration in HCC patients.

METHODS

The Gene Expression Omnibus (GEO) database and TNMplot.com platform were used to analyse the expression of FOXP3 and P62. The Cancer Genome Atlas (TCGA) database and Kaplan-Meier plotter were used to assess the impacts of FOXP3 and P62 on clinical prognosis. In addition, TCGA database was also used to examine the correlation between the expression of FOXP3 and P62 and tumour immune infiltration using the CIBERSORT algorithm. Finally, immunohistochemistry (IHC) was used to determine expression levels of FOXP3 and P62 in 89 HCC and adjacent normal liver tissues, and their effects on clinicopathological features and prognosis were verified.

RESULTS

FOXP3 expression was downregulated in HCC tissues, while P62 expression was upregulated. FOXP3 underexpression and P62 overexpression were closely related to decreased overall survival (OS) in HCC patients. Additionally, the abnormal expression of FOXP3 and P62 was closely related to the infiltration levels of 12 types of immune cells, including regulatory T cells (Tregs), M2 macrophages, M0 macrophages, and CD8 T cells. Notably, in the validation model, abnormal FOXP3 and P62 expression was significantly associated with adverse clinicopathological factors in HCC patients, including elevated α-fetoprotein (AFP) levels, poor tumour differentiation, and increased Ki67 levels. Furthermore, low FOXP3 and high P62 expression were independent risk factors for predicting OS prognosis in HCC patients.

CONCLUSION

FOXP3 and P62 have been shown to be important prognostic factors in HCC patients and are associated with immune cell infiltration in HCC. These findings suggest that FOXP3 and P62 may be valuable prognostic biomarkers and potential therapeutic targets for HCC treatment.

Inhibition of FOXP3 by stapled alpha-helical peptides dampens regulatory T cell function

Therapies and preclinical probes designed to drug and better understand the specific functions of intracellular protein–protein interactions (PPIs) remain an area of unmet need. This study describes the development of prototype therapeutics against the FOXP3 homodimer, a PPI essential for regulatory T cell suppressive capacity. We demonstrate that hydrocarbon stapled peptides designed to block this interaction can dampen regulatory T cell (Treg cell) suppressive function and lead to genetic signatures of immune reactivation. This work provides strong scientific justification for continued development of FOXP3-specific peptide-based inhibitors and provides mechanistic insights into the design and delivery of specific inhibitors of the coiled-coil region of FOXP3. These studies ultimately could lead to new immunotherapeutic strategies to amplify immune responsiveness in a number of settings.

Despite continuing advances in the development of novel cellular-, antibody-, and chemotherapeutic-based strategies to enhance immune reactivity, the presence of regulatory T cells (Treg cells) remains a complicating factor for their clinical efficacy. To overcome dosing limitations and off-target effects from antibody-based Treg cell deletional strategies or small molecule drugging, we investigated the ability of hydrocarbon stapled alpha-helical (SAH) peptides to target FOXP3, the master transcription factor regulator of Treg cell development, maintenance, and suppressive function. Using the crystal structure of the FOXP3 homodimer as a guide, we developed SAHs in the likeness of a portion of the native FOXP3 antiparallel coiled-coil homodimerization domain (SAH-FOXP3) to block this key FOXP3 protein-protein interaction (PPI) through molecular mimicry. We describe the design, synthesis, and biochemical evaluation of single- and double-stapled SAHs covering the entire coiled-coil expanse. We show that lead SAH-FOXP3s bind FOXP3, are cell permeable and nontoxic to T cells, induce dose-dependent transcript and protein level alterations of FOXP3 target genes, impede Treg cell function, and lead to Treg cell gene expression changes in vivo consistent with FOXP3 dysfunction. These results demonstrate a proof of concept for rationally designed FOXP3-directed peptide therapeutics that could be used as approaches to amplify endogenous immune responsiveness.

FOXP3+ regulatory T cells and age-related diseases

Regulatory T (Treg) cells are critical for the maintenance of immune homeostasis. Dysregulation of Treg cells has been implicated in the pathogenesis of autoimmunity and chronic inflammation, while aging is characterized by an accumulation of inflammatory markers in the peripheral blood, a phenomenon known as 'inflammaging'. The relationship between Treg cells and age-related diseases remains to be further studied. Increasing evidence revealed that Treg cells' dysfunction occurs in aged patients, suggesting that immune therapies targeting Treg cells may be a promising approach to treat diseases such as cancers and autoimmune diseases. Furthermore, drugs targeting Treg cells show encouraging results and contribute to CD8⁺ T-cell-mediated cytotoxic killing of tumor and infected cells. In general, a better understanding of Treg cell function may help us to develop new immune therapies against aging. In this review, we discuss potential therapeutic strategies to modify immune responses of relevance for aging to prevent and treat age-related diseases, as well as the challenges posed by the translation of novel immune therapies into clinical practice.

Foxp3 Post-translational Modifications and Treg Suppressive Activity

Regulatory T cells (Tregs) are engaged in maintaining immune homeostasis and preventing autoimmunity. Treg cells include thymic Treg cells and peripheral Treg cells, both of which can suppress the immune response via multiple distinct mechanisms. The differentiation, proliferation, suppressive function and survival of Treg cells are affected by distinct energy metabolic programs. Tissue-resident Treg cells hold unique features in comparison with the lymphoid organ Treg cells. Foxp3 transcription factor is a lineage master regulator for Treg cell development and suppressive activity. Accumulating evidence indicates that the activity of Foxp3 protein is modulated by various post-translational modifications (PTMs), including phosphorylation, O-GlcNAcylation, acetylation, ubiquitylation and methylation. These modifications affect multiple aspects of Foxp3 function. In this review, we define features of Treg cells and roles of Foxp3 in Treg biology, and summarize current research in PTMs of Foxp3 protein involved in modulating Treg function. This review also attempts to define Foxp3 dimer modifications relevant to mediating Foxp3 activity and Treg suppression. Understanding Foxp3 protein features and modulation mechanisms may help in the design of rational therapies for immune diseases and cancer.

FoxP3 in Treg cell biology: a molecular and structural perspective

Regulatory T cells (T_regs) are specialized in immune suppression and play a dominant role in peripheral immune tolerance. T_reg cell lineage development and function maintenance is determined by the forkhead box protein 3 (FoxP3) transcriptional factor, whose activity is fine‐tuned by its post‐translational modifications (PTMs) and interaction partners. In this review, we summarize current studies in the crystal structures, the PTMs and interaction partners of FoxP3 protein, and discuss how these insights may provide a roadmap for new approaches to modulate T_reg suppression, and new therapies to enhance immune tolerance in autoimmune diseases.

FoxP3 scanning mutagenesis reveals functional variegation and mild mutations with atypical autoimmune phenotypes

FoxP3⁺ regulatory T cells (Tregs) are a central element of immunological tolerance. FoxP3 is the key determining transcription factor of the Treg lineage, interacting with numerous cofactors and transcriptional targets to determine the many facets of Treg function. Its absence leads to devastating lymphoproliferation and autoimmunity in scurfy mutant mice and immunodysregulation polyendocrinopathy enteropathy X-linked (IPEX) patients. To finely map transcriptionally active regions of the protein, with respect to disease-causing variation, we performed a systematic alanine-scan mutagenesis of FoxP3, assessing mutational impacts on DNA binding and transcriptional activation or repression. The mutations affected transcriptional activation and repression in a variegated manner involving multiple regions of the protein and varying between different transcriptional targets of FoxP3. There appeared to be different modalities for target genes related to classic immunosuppressive function vs. those related to atypical or tissue-Treg functions. Relevance to in vivo Treg biology was established by introducing some of the subtle Foxp3 mutations into the mouse germline by CRISPR-based genome editing. The resulting mice showed Treg populations in normal numbers and exhibited no overt autoimmune manifestations. However, Treg functional defects were revealed upon competition or by system stress, manifest as a strikingly heightened susceptibility to provoked colitis, and conversely by greater resistance to tumors. These observations suggest that some of the missense mutations that segregate in human populations, but do not induce IPEX manifestations, may have unappreciated consequences in other diseases.

The regulation of immune tolerance by FOXP3

The proper restraint of the destructive potential of the immune system is essential for maintaining health. Regulatory T (T_reg) cells ensure immune homeostasis through their defining ability to suppress the activation and function of other leukocytes. The expression of the transcription factor forkhead box protein P3 (FOXP3) is a well-recognized characteristic of T_reg cells, and FOXP3 is centrally involved in the establishment and maintenance of the T_reg cell phenotype. In this Review, we summarize how the expression and activity of FOXP3 are regulated across multiple layers by diverse factors. The therapeutic implications of these topics for cancer and autoimmunity are also discussed.

Foxp1 Regulates the Proliferation of Hair Follicle Stem Cells in Response to Oxidative Stress during Hair Cycling

Hair follicle stem cells (HFSCs) in the bugle circularly generate outer root sheath (ORS) through linear proliferation within limited cycles during anagen phases. However, the mechanisms controlling the pace of HFSC proliferation remain unclear. Here we revealed that Foxp1, a transcriptional factor, was dynamically relocated from the nucleus to the cytoplasm of HFSCs in phase transitions from anagen to catagen, coupled with the rise of oxidative stress. Mass spectrum analyses revealed that the S468 phosphorylation of Foxp1 protein was responsive to oxidative stress and affected its nucleocytoplasmic translocation. Foxp1 deficiency in hair follicles led to compromised ROS accrual and increased HFSC proliferation. And more, NAC treatment profoundly elongated the anagen duration and HFSC proliferation in Foxp1-deficient background. Molecularly, Foxp1 augmented ROS levels through suppression of Trx1-mediated reductive function, thereafter imposing the cell cycle arrest by modulating the activity of p19/p53 pathway. Our findings identify a novel role for Foxp1 in controlling HFSC proliferation with cellular dynamic location in response to oxidative stress during hair cycling.

The Regulatory T Cell Lineage Factor Foxp3 Regulates Gene Expression through Several Distinct Mechanisms Mostly Independent of Direct DNA Binding

The lineage factor Foxp3 is essential for the development and maintenance of regulatory T cells, but little is known about the mechanisms involved. Here, we demonstrate that an N-terminal proline-rich interaction region is crucial for Foxp3’s function. Subdomains within this key region link Foxp3 to several independent mechanisms of transcriptional regulation. Our study suggests that Foxp3, even in the absence of its DNA-binding forkhead domain, acts as a bridge between DNA-binding interaction partners and proteins with effector function permitting it to regulate a large number of genes. We show that, in one such mechanism, Foxp3 recruits class I histone deacetylases to the promoters of target genes, counteracting activation-induced histone acetylation and thereby suppressing their expression.

The suppressive activity of regulatory T cells provides the immune system with a mechanism to prevent detrimental immune responses, such as autoimmunity, attack of the beneficial commensal microbiota and rejection of the fetus. Intriguingly, expression of a single lineage factor Foxp3 is sufficient to completely reprogram T cells from a pro-inflammatory to a suppressive phenotype. Here, we show that Foxp3 alters the expression of thousands of genes through several independent mechanisms. In many cases, its own ability to bind to DNA appears to be dispensable, but rather it binds indirectly to the DNA by interaction with other transcription factors. Foxp3 then in turn recruits other proteins that affect gene expression through chromatin modification. For example, Foxp3 indirectly binds to the IL-2 promoter via interaction with the transcriptional activators c-Rel, AML-1 and NFAT. This leads to the Foxp3 mediated recruitment of class I histone deacetylases HDAC1, 2 and 3, which in turn counteracts the activation-induced hyper-acetylation of the promoter, thereby switching the gene off. In a way, Foxp3 hijacks pre-existing regulatory mechanism to reverse the transcriptional expression status of the target gene. By dissecting Foxp3 on a molecular level, we also show that this is only one of several independent mechanism utilised by Foxp3.

Pim-2 Kinase Influences Regulatory T Cell Function and Stability by Mediating Foxp3 Protein N-terminal Phosphorylation

Regulation of the extent of immune responses is a requirement to maintain self-tolerance and limit inflammatory processes. CD4(+)CD25(+)Foxp3(+) regulatory T (Treg) cells play a role in regulation. The Foxp3 transcription factor is considered a dominant regulator for Treg cell development and function. Foxp3 function itself is directly regulated by multiple posttranslational modifications that occur in response to various external stimuli. The Foxp3 protein is a component of several dynamic macromolecular regulatory complexes. The complexes change constituents over time and through different signals to regulate the development and function of regulatory T cells. Here we identified a mechanism regulating Foxp3 level and activity that operates through discrete phosphorylation. The Pim-2 kinase can phosphorylate Foxp3, leading to decreased suppressive functions of Treg cells. The amino-terminal domain of Foxp3 is modified at several sites by Pim-2 kinase. This modification leads to altered expression of proteins related to Treg cell functions and increased Treg cell lineage stability. Treg cell suppressive function can be up-regulated by either pharmacologically inhibiting Pim-2 kinase activity or by genetically knocking out Pim-2 in rodent Treg cells. Deficiency of Pim-2 activity increases murine host resistance to dextran sodium sulfate-induced colitis in vivo, and a Pim-2 small molecule kinase inhibitor also modified Treg cell functions. Our studies define a pathway for limiting the regulation of Foxp3 function because the Pim-2 kinase represents a potential therapeutic target for modulating the Treg cell suppressive activities in controlling immune responses.

Genome-wide analysis of cancer cell-derived Foxp3 target genes in human tongue squamous cell carcinoma cells

The forkhead transcription factor Foxp3 is essential for differentiation and activation of regulatory T cells (Tregs), and used to be regarded as specific transcription factor of Tregs. In recent years, Foxp3 expression in tumor cells (cancer cell-derived Foxp3) has gained great interest, but its function and molecular mechanisms remain incompletely understood. In the present study, we detected dynamic nuclear translocation of Foxp3 in TSCC cells using immunofluorescent staining. Then we performed a genome-wide analysis of Foxp3 in TSCC cells using a combination of ChIP-on-chip and whole-genome microarray assays. We also compared Foxp3 biding sites in TSCC cells with the known binding sites in human Tregs to show the differences in transcriptional regulation profile. Results indicate that Foxp3 in TSCC cells has distinct biological functions compared with that in Tregs. Cancer cell-derived Foxp3 directly regulates the transcription of genes that affect certain internal biological processes of TSCC cells, and indirectly influences the extracellular microenvironment. This study reveals the relationship between direct and indirect targets genes of Foxp3 in TSCC cells and provide molecular basis of cancer cell-derived Foxp3 function.

Molecular cloning and analysis of gonadal expression of Foxl2 in the rice-field eel Monopterus albus

We isolated the complete Foxl2 (Foxl2a) cDNA from the Monopterus albus ovary. An alignment of known Foxl2 amino-acid sequences confirmed the conservation of the Foxl2 open reading frame, especially the forkhead domain and C-terminal region. The expression of Foxl2 was detected in the brain, eyes, and gonads. A high level of Foxl2 expression in the ovary before sex reversal, but its transcripts decreased sharply when the gonad developed into the ovotestis and testis. The correlation between the Foxl2 expression and the process of sex development revealed the important function of Foxl2 during the sex reversal of M. albus. Immunohistochemical analysis showed that Foxl2 was expressed abundantly in granulosa cells and in the interstitial cells of the ovotestis and testis. These results suggest that Foxl2 plays a pivotal role in the development and maintenance of ovarian function. Foxl2 may be also involved in the early development of testis and the development of ocular structures of M. albus.

Transcriptional control of regulatory T cells

Regulatory T cells (Tregs) constitute unique T cell lineage that plays a key role for immunological tolerance. Tregs are characterized by the expression of the forkhead box transcription factor Foxp3, which acts as a lineage-specifying factor by determining the unique suppression profile of these immune cells. Here, we summarize the recent progress in understanding how Foxp3 expression itself is epigenetically and transcriptionally controlled, how the Treg-specific signature is achieved and how unique properties of Treg subsets are defined by other transcription factors. Finally, we will discuss recent studies focusing on the molecular targeting of Tregs to utilize the specific properties of this unique cell type in therapeutic settings.

Genetic polymorphism in FOXP3 gene: imbalance in regulatory T-cell role and development of human diseases

The FOXP3 gene encodes a transcription factor thought to be important for the development and function of regulatory T cells (Treg cells). These cells are involved in the regulation of T cell activation and therefore are essential for normal immune homeostasis. Signals from microenvironment have a profound influence on the maintenance or progression of diseases. Thus, Tregs have an important marker protein, FOXP3, though it does not necessarily confer a Treg phenotype when expressed. FOXP3 polymorphisms that occur with high frequency in the general populations have been studied in common multifactorial human diseases. Dysfunction of FOXP3 gene product could result in lack of Treg cells and subsequently chronically activated CD4+ T cells which express increased levels of several activation markers and cytokines, resulting in some autoimmune diseases. In contrast, high Treg levels have been reported in peripheral blood, lymph nodes, and tumour specimens from patients with different types of cancer. The present study discusses the polymorphisms located in intron, exon and promoter regions of FOXP3 which have already been investigated by many researchers. FOXP3 has received considerable attention in attempts to understand the molecular aspect of Treg cells. Therefore, in the present study, the relationship between genetic polymorphism of FOXP3 in Treg-cell role and in disease development are reviewed considering the interactive effect of genetic factors.

FOXP3 expression is related to high Ki-67 index and poor prognosis in lymph node-positive breast cancer patients

BACKGROUND

Recent preclinical studies have shown that Forkhead box protein 3 (FOXP3) is an important tumor suppressor gene. The clinical and prognostic implication of FOXP3 expression in breast cancer cells still remains controversial.

METHODS

We evaluated the FOXP3 expression status of 183 patients who underwent curative surgery for breast cancer using the immunohistochemical assay of tissue microarray.

RESULTS

We found FOXP3 expression in 51 out of 183 (27.9%) surgically resected breast cancer tumors, and 33 patients were scored as weak positive and 18 as strong positive. FOXP3-positive tumors were associated with significantly higher nuclear grade, higher histologic grade and a more negative estrogen receptor status. The FOXP3 expression level was independently associated with high Ki-67 index in a logistic regression model. In the node-positive subgroup, strong FOXP3 positivity was related to poor disease-free survival and disease-specific survival compared to FOXP3-negative patients, whereas there was no survival difference between FOXP3-negative and FOXP3-weak-positive patients. Multivariate analysis with adjustment for patient age and human epidermal growth factor receptor 2 status demonstrated significantly poor survival of FOXP3-strong-positive patients in node-positive patients.

CONCLUSION

Our results suggest that strong FOXP3 expression in breast cancer cells is associated with poor prognosis and high Ki-67 index.

Forkhead Box P family members at the crossroad between tolerance and immunity: a balancing act

Maintaining an immune balance between a chronic inflammatory state and autoimmunity is regulated at multiple levels by complex cellular signaling mechanisms. Numerous immune stimulatory and inhibitory signals converge on a large variety of transcriptional regulators. One key transcriptional regulator of immune homeostasis is FOXP3, which is a member of the Forkhead Box P subfamily of transcription factors and was shown to be essential for the development and maintenance of regulatory T cells. However, other FOXP members have received less attention in relation to a role in immune regulation. Still, recent developments point toward a general important regulatory role for FOXP proteins in the development and function of the adaptive immune system and establishment of a balanced immune response. Here, we discuss the current knowledge on the role of FOXP proteins in establishing immune homeostasis with an emphasis on T-cell biology. Furthermore, we review and speculate about different modes of regulating general FOXP activity and the function of this in health and disease.

Subcellular localization and self-interaction of plant-specific Nt-4/1 protein

The Nicotiana tabacum Nt-4/1 protein is a plant-specific protein of unknown function. Analysis of bacterially expressed Nt-4/1 protein in vitro revealed that the protein secondary structure is mostly alpha-helical and suggested that it could consist of three structural domains. Earlier studies of At-4/1, the Arabidopsis thaliana-encoded ortholog of Nt-4/1, demonstrated that GFP-fused At-4/1 was capable of polar localization in plant cells, association with plasmodesmata, and cell-to-cell transport. Together with the At-4/1 ability to interact with a plant virus movement protein, these data supported the hypothesis of the At-4/1 protein involvement in viral transport through plasmodesmata. Studies of the Nt-4/1-GFP fusion protein reported in this paper revealed that the protein was localized to cytoplasmic bodies, which were co-aligned with actin filaments and capable of actin-dependent intracellular movement. The Nt-4/1-GFP bodies, being non-membrane structures, were found in association with the plasma membrane, the tubular endoplasmic reticulum and endosome-like structures. Bimolecular fluorescence complementation experiments and inhibition of nuclear export showed that the Nt-4/1 protein was capable of nuclear-cytoplasmic transport. The nuclear export signal (NES) was identified in the Nt-4/1 protein by site-directed mutagenesis. The Nt-4/1 NES mutant was localized to the nucleoplasm forming spherical bodies. Immunogold labeling and electron microscopy of cytoplasmic Nt-4/1-containing bodies and nuclear structures containing the Nt-4/1 NES mutant revealed differences in their fine structure. In mammalian cells, Nt-4/1-GFP formed cytoplasmic spherical bodies similar to those found for the Nt-4/1 NES mutant in plant cell nuclei. Using dynamic laser light scattering and electron microscopy, the Nt-4/1 protein was found to form multimeric complexes in vitro.

The role of FOXP3 in the development and metastatic spread of breast cancer

The transcription factor FOXP3 is widely known for its role in the development and function of immunoregulatory T cells. However, it has been discovered recently that FOXP3 is also expressed in epithelial cells of the normal human breast, ovary and prostate. Aggressive cancer of these epithelial tissues often correlates with abnormal expression of FOXP3, which can be either absent or underexpressed at transcript or protein levels. It is becoming clear that this failure of normal FOXP3 expression can result in dysregulation of the expression of a range of oncogenes which have been implicated in the development and metastasis of cancer. Recent evidence suggests that FOXP3 might also regulate chemokine receptor expression, providing a possible explanation for the chemokine-driven, tissue-specific spread that is characteristic of many cancers. This review first summarises the general structure, function and properties of FOXP3. This is followed by an analysis of the tumour-suppressive properties of this transcription factor, with particular reference to the development and chemokine-mediated spread of human breast cancer. A final section focuses on potential applications of this new knowledge for therapeutic intervention.

Molecular and biological role of the FOXP3 N-terminal domain in immune regulation by T regulatory/suppressor cells

Regulatory T (Treg) cells are essential in preventing the host from developing certain autoimmune diseases and limiting excessive immune responses against pathogens. The normal function of most Treg cells requires sustained expression of functional FOXP3, a member of the FOXP family transcription factors. FOXP3 is distinct from other subfamily members because of its unique proline rich amino (N)-terminal domain. Mutations in this region are occasionally identified in certain patients with X-linked autoimmunity-allergic dysregulation syndrome (XLAAD) and similar mutations also increase susceptibility of autoimmune diseases in rodent models. Previous analyses of the FOXP3 N-terminal domain revealed a role in nuclear import, interaction with other transcription factors, and as sites of specific post-translational modifications of FOXP3 that contribute to FOXP3 stability.

First insight into the kinome of human regulatory T cells

Regulatory T cells (Tregs) are essential for controlling peripheral tolerance by the active suppression of various immune cells including conventional T effector cells (Teffs). Downstream of the T cell receptor (TCR), more than 500 protein kinases encoded by the human genome have to be considered in signaling cascades regulating the activation of Tregs and Teffs, respectively. Following TCR engagement, Tregs posses a number of unique attributes, such as constitutive expression of Foxp3, hyporesponsiveness and poor cytokine production. Furthermore, recent studies showed that altered regulation of protein kinases is important for Treg function. These data indicate that signaling pathways in Tregs are distinctly organized and alterations at the level of protein kinases contribute to the unique Treg phenotype. However, kinase-based signaling networks in Tregs are poorly understood and necessitate further systematic characterization. In this study, we analyzed the differential expression of kinases in Tregs and Teffs by using a kinase-selective proteome strategy. In total, we revealed quantitative information on 185 kinases expressed in the human CD4(+) T cell subsets. The majority of kinases was equally abundant in both T cell subsets, but 11 kinases were differentially expressed in Tregs. Most strikingly, Tregs showed an altered expression of cell cycle kinases including CDK6. Quantitative proteomics generates first comparative insight into the kinase complements of the CD4(+) Teff and Treg subset. Treg-specific expression pattern of 11 protein kinases substantiate the current opinion that TCR-mediated signaling cascades are altered in Tregs and further suggests that Tregs exhibit significant specificities in cell-cycle control and progression.

Two lysines in the forkhead domain of foxp3 are key to T regulatory cell function

BACKGROUND

The forkhead box transcription factor, Foxp3, is master regulator of the development and function of CD4+CD25+ T regulatory (Treg) cells that limit autoimmunity and maintain immune homeostasis. The carboxyl-terminal forkhead (FKH) domain is required for the nuclear localization and DNA binding of Foxp3. We assessed how individual FKH lysines contribute to the functions of Foxp3 in Treg cells.

METHODOLOGY/PRINCIPAL FINDINGS

We found that mutation of FKH lysines at position 382 (K17) and at position 393 (K18) impaired Foxp3 DNA binding and inhibited Treg suppressive function in vivo and in vitro. These lysine mutations did not affect the level of expression of Foxp3 but inhibited IL-2 promoter remodeling and had important and differing effects on Treg-associated gene expression.

CONCLUSIONS/SIGNIFICANCE

These data point to complex effects of post-translational modifications at individual lysines within the Foxp3 FKH domain that affect Treg function. Modulation of these events using small molecule inhibitors may allow regulation of Foxp3+ Treg function clinically.

Immune regulation by histone deacetylases: a focus on the alteration of FOXP3 activity

Several histone deacetylases (HDACs) are involved in the regulation of forkhead box protein P3 (FOXP3) expression and function by affecting features of FOXP3 protein stability. FOXP3, a forkhead family transcription factor specially expressed in regulatory T (Treg) cells, controls the expression of many key immune-regulatory genes. Treg cells are a population of T lymphocytes that have critical roles in the immune system homeostasis and tolerance to self and foreign antigens, the body's response to cancer and infectious agents. FOXP3 forms oligomeric complexes with other proteins, the components of which are believed to be regulated dynamically. In addition, HDAC activities influence FOXP3 interactions with other partners to form transcriptional regulatory complexes. By understanding the details of the biochemical and structural basis of the regulation of FOXP3 acetylation, therapeutic strategies for diseases related to Treg cells may emerge.

Molecular mechanisms underlying the regulation and functional plasticity of FOXP3(+) regulatory T cells

CD4(+) CD25(+) regulatory T (Treg) cells engage in the maintenance of immunological self-tolerance and homeostasis by limiting aberrant or excessive inflammation. The transcription factor forkhead box P3 (FOXP3) is critical for the development and function of Treg cells. The differentiation of the Treg cell lineage is not terminal, as developmental and functional plasticity occur through the sensing of inflammatory signals in the periphery. Here, we review the recent progress in our understanding of the molecular mechanisms underlying the regulation and functional plasticity of CD4(+) CD25(+) FOXP3(+) Treg cells, through the perturbation of FOXP3 and its complex at a transcriptional, translational and post-translational level.

Histone/protein deacetylases control Foxp3 expression and the heat shock response of T-regulatory cells

Lysine ɛ-acetylation is a post-translational modification that alters the biochemical properties of many proteins. The reaction is catalyzed by histone/protein acetyltransferases (HATs), and is reversed by histone/protein deacetylases (HDACs). As a result, HATs and HDACs constitute an important, though little recognized, set of proteins that control the functions of T-regulatory (Treg) cells. Targeting certain HDACs, especially HDAC6, HDAC9, and Sirtuin-1 (Sirt1), can augment Treg suppressive potency by several distinct and potentially additive mechanisms. These involve promoting Forkhead box p3 (Foxp3) gene expression and preserving Foxp3 lysine ɛ-acetylation, which infers resistance to ubiquitination and proteasomal degradation, and increases DNA binding. Moreover, depleting certain HDAC can enhance the heat shock response, which increases the tenacity of Treg to survive under stress, and helps preserve a suppressive phenotype. As a result, HDAC inhibitor therapy can be used to enhance Treg functions in vivo and have beneficial effects on allograft survival and autoimmune diseases.

Forkhead transcription factors: key players in health and disease

Forkhead box (FOX) proteins constitute an evolutionarily conserved family of transcription factors with a central role not only during development, but also in the adult organism. Thus, the misregulation and/or mutation of FOX genes often induce human genetic diseases, promote cancer or deregulate ageing. Indeed, germinal FOX gene mutations cause diseases ranging from infertility to language and/or speech disorders and immunological defects. Moreover, because of their central role in signalling pathways and in the regulation of homeostasis, somatic misregulation and/or mutation of FOX genes are associated with cancer. FOX proteins have undergone diversification in terms of their sequence, regulation and function. In addition to dedicated roles, evidence suggests that Forkhead factors have retained some functional redundancy. Thus, combinations of slightly defective alleles might induce disease phenotypes in humans, acting as quantitative trait loci. Uncovering such variants would be a big step towards understanding the functional interdependencies of different FOX members and their implications in complex pathologies.

The dual role of the X-linked FoxP3 gene in human cancers

The FoxP3 (forkhead box P3) gene is an X-linked gene that is submitted to inactivation. It is an essential transcription factor in CD4(+)CD25(+)FoxP3 regulatory T cells, which are therapeutic targets in disseminated cutaneous melanoma. Moreover, FoxP3 is an important tumor suppressor gene in carcinomas and has putative cancer suppressor gene function in cutaneous melanoma as well. Therefore understanding the structure and function of the FoxP3 gene is crucial to gaining insight into the biology of melanoma to better develop immunotherapeutics and future therapeutic strategies.

FOXP3 and survival in urinary bladder cancer

OBJECTIVE

To investigate the possible impact of FOXP3 expression in T-cells, as well as in tumour cells, on long-term survival in patients with urinary bladder cancer (UBC) invading muscle.

PATIENTS AND METHODS

In a retrospective study, tumour specimens from 37 patients cystectomized for T1-T4 UBC during 1999-2002 at the Karolinska University Hospital were examined by immunohistochemistry for tumour expression and/or infiltration of immune cells expressing FOXP3 as well as CD3. The results obtained were correlated with clinicopathological parameters, where the primary and secondary outcomes investigated were overall survival and progression-free survival, respectively.

RESULTS

Infiltration of CD3(+) and FOXP3(+) lymphocytes (≥3 cells per high-power field) were both correlated with better survival, and this relationship persisted throughout the whole study period (all P < 0.05). Patients with FOXP3(+) tumour cells had decreased long-term survival compared to those patients with FOXP3(-) tumours (P < 0.05). Despite a limited amount of patient material, the results of the present study indicate that FOXP3 expression, in both lymphocytes and tumour cells, is an important prognostic factor in UBC.

CONCLUSIONS

FOXP3 expression in UBC cells is associated with decreased long-term survival and thus may be a novel negative prognostic factor in UBC invading muscle. By contrast, the presence of FOXP3(+) tumour-infiltrating lymphocytes was correlated with a positive prognosis. Because FOXP3 is up-regulated upon activation in human T-cells, FOXP3 may serve more as an activation marker than as a regulatory T-cell indicator in this case. These results support the need for larger prospective studies aiming to confirm the results obtained and to examine the underlying mechanisms in detail.

Genome-wide identification of human FOXP3 target genes in natural regulatory T cells

The transcription factor FOXP3 is essential for the formation and function of regulatory T cells (Tregs), and Tregs are essential for maintaining immune homeostasis and tolerance. This is demonstrated by a lethal autoimmune defect in mice lacking Foxp3 and in immunodysregulation polyendocrinopathy enteropathy X-linked syndrome patients. However, little is known about the molecular basis of human FOXP3 function or the relationship between direct and indirect targets of FOXP3 in human Tregs. To investigate this, we have performed a comprehensive genome-wide analysis for human FOXP3 target genes from cord blood Tregs using chromatin immunoprecipitation array profiling and expression profiling. We have identified 5579 human FOXP3 target genes and derived a core Treg gene signature conserved across species using mouse chromatin immunoprecipitation data sets. A total of 739 of the 5579 FOXP3 target genes were differentially regulated in Tregs compared with Th cells, thus allowing the identification of a number of pathways and biological functions overrepresented in Tregs. We have identified gene families including cell surface molecules and microRNAs that are differentially expressed in FOXP3(+) Tregs. In particular, we have identified a novel role for peptidase inhibitor 16, which is expressed on the cell surface of >80% of resting human CD25(+)FOXP3(+) Tregs, suggesting that in conjunction with CD25 peptidase inhibitor 16 may be a surrogate surface marker for Tregs with potential clinical application.