Molecular Mechanisms Controlling Foxp3 Expression in Health and Autoimmunity: From Epigenetic to Post-translational Regulation

Abstract

The discovery of the transcription factor Forkhead box-p3 (Foxp3) has shed fundamental insights into the understanding of the molecular determinants leading to generation and maintenance of T regulatory (Treg) cells, a cell population with a key immunoregulatory role. Work over the past few years has shown that fine-tuned transcriptional and epigenetic events are required to ensure stable expression of Foxp3 in Treg cells. The equilibrium between phenotypic plasticity and stability of Treg cells is controlled at the molecular level by networks of transcription factors that bind regulatory sequences, such as enhancers and promoters, to regulate Foxp3 expression. Recent reports have suggested that specific modifications of DNA and histones are required for the establishment of the chromatin structure in conventional CD4⁺ T (Tconv) cells for their future differentiation into the Treg cell lineage. In this review, we discuss the molecular events that control Foxp3 gene expression and address the associated alterations observed in human diseases. Also, we explore how Foxp3 influences the gene expression programs in Treg cells and how unique properties of Treg cell subsets are defined by other transcription factors.

Keywords: Foxp3; Foxp3 stability; Treg cells; autoimmunity; epigenetic regulation.

Figure 1

Histone modifications, transcription factors, and miRNAs regulating Foxp3 expression. (A) Enzymes catalyzing histone modifications: histone acetyltransferase (HAT), histone deacetylases (HDAC), histone methyltransferase (HMT), and histone demethylase (HDM). Transcription factors binding promoter and conserved non-coding sequences (CNS0, CNS1, CNS2, and CNS3) at Foxp3 locus. Promoter: nuclear factor of activated T-cells (NFAT), p65, cAMP response element binding protein (CREB), mothers against decapentaplegic (Smad)3, c-Rel, mixed lineage leukemia (MLL)4, Enolase (Eno)-1, Forkhead box-p3-Exon2 (Foxp3-E2), miR-15a/16, and miR-4281. CNS0: special AT-rich sequence binding protein (Satb)1. CNS1: Smad2 and Smad3. CNS2: Foxp3, signal transducer and activator of transcription (STAT)5, runt-related transcription factor (Runx)1-core binding factor (CBF)-β, CREB, the methyl-CpG-binding domain (Mbd)2, the chromatin-remodeling complex tet methylcytosine dioxygenase (Tet)2, Eno-1, and Foxp3-E2. CNS3: Forkhead transcription factor of the O class (Foxo)1, Foxo3, and c-Rel. (B) Schematic representation of miRNAs modulating Foxp3 expression: miR-17, miR15a/16, miR-210, miR-24, and miR-31. Interferon regulatory factor (Irf)4.

Figure 2

Post-translational modifications regulating Foxp3 expression. Covalent post-translational modifications in Foxp3 amino acids: ubiquitination (Ub) mediated by deubiquitinase (DUB) ubiquitin-specific-processing protease (USP)7, STIP1 homology and U-Box containing protein (Stub)1, TNF receptor associated factor (TRAF)6, chemokine (C-C motif) ligand (CCL)3, and hypoxia-inducible factor (HIF)-1α; acetylation (Ac)/deacetylation mediated by Histone deacetylase (HDAC)7, HDAC9, tat-interactive protein 60 kDa (TIP60), sirtuin (Sirt)1, and short-chain fatty acids (SCFAs); phosphorylation (p) mediated by cyclin-dependent kinase (CDK)2, Nemo-like kinase (NLK), proto-oncogene serine/threonine-protein kinase (PIM)1, and PIM2.

Figure 3

Foxp3 alterations in inflammation, autoimmunity, and IPEX. Inflammation, autoimmune diseases and IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked) syndrome are associated with alteration of Foxp3 gene expression, stability, and function. Lipopolysaccharides (LPS), tumor necrosis factor (TNF)-α, interleukin (IL)-18, STIP1 homology and U-Box containing protein (Stub)1, Forkhead box-p3 (Foxp3), signal transducer activator of transcription (STAT)3, Id2, and interferon (IFN)-γ, Deleted in breast cancer 1 (DBC1) factor, enhancer of zeste (Ezh)2, DNA-methyltransferase 3a (Dnmt3a), Foxp3-Exon2 (Foxp3-E2), interleukin-2 receptor alpha chain (IL-2RA), tat-interactive protein 60 kDa (TIP60), and RAR-related orphan receptor (ROR)γt.