Epigenetic regulation and T-cell responses in endometriosis - something other than autoimmunity

Abstract

Endometriosis is defined as the presence of endometrial-like glands and stroma located outside the uterine cavity. This common, estrogen dependent, inflammatory condition affects up to 15% of reproductive-aged women and is a well-recognized cause of chronic pelvic pain and infertility. Despite the still unknown etiology of endometriosis, much evidence suggests the participation of epigenetic mechanisms in the disease etiopathogenesis. The main rationale is based on the fact that heritable phenotype changes that do not involve alterations in the DNA sequence are common triggers for hormonal, immunological, and inflammatory disorders, which play a key role in the formation of endometriotic foci. Epigenetic mechanisms regulating T-cell responses, including DNA methylation and posttranslational histone modifications, deserve attention because tissue-resident T lymphocytes work in concert with organ structural cells to generate appropriate immune responses and are functionally shaped by organ-specific environmental conditions. Thus, a failure to precisely regulate immune cell transcription may result in compromised immunological integrity of the organ with an increased risk of inflammatory disorders. The coexistence of endometriosis and autoimmunity is a well-known occurrence. Recent research results indicate regulatory T-cell (Treg) alterations in endometriosis, and an increased number of highly active Tregs and macrophages have been found in peritoneal fluid from women with endometriosis. Elimination of the regulatory function of T cells and an imbalance between T helper cells of the Th1 and Th2 types have been reported in the endometria of women with endometriosis-associated infertility. This review aims to present the state of the art in recognition epigenetic reprogramming of T cells as the key factor in the pathophysiology of endometriosis in the context of T-cell-related autoimmunity. The new potential therapeutic approaches based on epigenetic modulation and/or adoptive transfer of T cells will also be outlined.

Keywords: T cells; T-cell reprogramming; autoimmunity; endometriosis; epigenetic mechanisms.

Figure 1

Overview of the main epigenetic mechanisms that regulate gene expression and may establish potentially heritable changes in gene expression without altering the underlying DNA nucleotide sequence. (A) Histone (chromatin) modifications. On the left. Chromatin remodeling is the dynamic modification of chromatin architecture to allow access of condensed genomic DNA to the regulatory transcription machinery proteins, and thereby control gene expression. For example, histone acetylation by HAT (histone acetyl transferase) increases DNA (chromatin) accessibility because acetylated histones cannot pack as well together as deacetylated histones. HDAC – histone deacetylase; On the right. Each nucleosome consists of two subunits, both made of histones H2A, H2B, H3 and H4, also known as core histones, with the linker histone H1 acting as a stabilizer. Histone post-translational modifications are covalent modifications of histones by phosphorylation on serine or threonine residues, methylation on lysine or arginine, acetylation and deacetylation of lysines, ubiquitylation of lysines and sumoylation of lysines. Histone modifications affect chromosome structure and function, especially during transcription and chromatin remodelling processes. (B) DNA methylation and hydroxymethylation. DNA can be modified at cytosine and adenine residues by the addition of chemical groups. Cytosines can be modified by methylation (5mc) or hydroxymethylation (5hmC), while adenines are modified by methylation. CpG islands (regions of the genome that contain a large number of CpG dunucleotide repeats) are DNA methylations regions in promoters known to regulate gene expression through transcriptional silencing of the corresponding gene. DNA methylation at CpG islands is crucial for gene expression and tissue-specific processes. DNMT – DNA methyltransferase; SAM – S-adenosylmethionine; SAH – S-adenosylhomocysteine; TET – ten-eleven-translocation (methylcytosine dioxygenase). (C) Non-coding RNAs (ncRNAs) and RNA modifications. - ncRNAs play an important role in transcription regulation by epigenetic machinery. Within RNA-induced silencing complexes (RISCs), miRNAs mediate the recognition and binding of RNAs that become targeted for degradation. lncRNAs are associated with other complexes and can activate or repress transcription. - RNA methylation is a post-transcriptional level of regulation. At present, more than 150 kinds of RNA modifications have been identified. They are widely distributed in messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA), noncoding small RNA (sncRNA) and long-chain non-coding RNA (lncRNA). - Alternative splicing (AS) of pre-mRNAs serves as an additional regulatory process for gene expression after transcription, and it generates distinct mRNA species, and even noncoding RNAs (ncRNAs), from one primary transcript. AS contributes to the diversity of proteins in eukaryotes as cells respond to signals from the environment. AS may lead to generation of ncRNAs, especially long noncoding RNAs (lncRNAs). RNA modifications, such as the RNA N6-methyladenosine (m6A) modification, have been found to regulate AS. - RNA editing is an important mechanism of genetic regulation that amplifies genetic plasticity by allowing the production of alternative protein products from a single gene. RNA editing involves the post-transcriptional insertion and deletion of nucleotides (e.g., uridylate – UMP) within nascent transcripts. RNA editing has been observed in mRNAs, tRNAs, and rRNAs, in mitochondrial and chloroplast encoded RNAs, as well as in nuclear encoded RNAs.

Figure 2

Structure of the αβ T cell receptor (TCR) and the TCR-CD3 complex (the area within the dashed line) including main signaling pathways. - TCR structure: ❶ - variable region; ❷ - constant region; ❸ - hinge region; ❹ - transmembrane region; ❺ - cytoplasmatic tail. - The core TCR signaling complex consists of two TCR chains (αβ heterodimer) that are noncovalently coupled to three dimeric signaling molecules named the cluster of differentiation 3 (CD3): CD3ϵδ, CD3ϵγ, and CD3ζζ. - Signaling via the TCR/CD3 antigen receptor complex is activated after interaction of the TCR with cognate peptide antigen bound to a major histocompatibility complex (MHC) molecule on the surface of antigen-presenting cells (APC), and co-stimulation by co-receptor molecules such as CD28. An early event in the proximal signaling of TCR is the involvement and activation of a set of protein tyrosine kinases (PTKs), such as LCK, FYN, and ZAP-70, that are important components required for activation of TCR signaling through tyrosine phosphorylation on CD3. The proximal TCR signaling is followed by the activation of multiple distal signaling cascades, such as: • Ca²⁺–calmodulin (CaM) – calcineurin (CaN) – nuclear factor of activated T-cells (NFAT); • diacylglycerol (DAG) – Ras guanyl nucleotide releasing protein (RasGRP) – Ras – proto-oncogene serine/threonine-protein kinase (Raf) – dual-specificity tyrosine/threonine protein kinases (MEK1/2) – extracellular signal-regulated kinases 1/2 (ERK1/2); • protein kinase C-theta (PKCθ) – I kappa B kinases (IKKα, IKKβ, IKKγ) – nuclear factor kappa B (NF-κB). These signaling cascades, regulated largely by epigenetic mechanisms, finally bring out the diverse phenotypic effects, as they control many aspects of T cell biology. For the sake of clarity of the diagram, the presentation of the negative regulation (downregulation) of TCR-mediated signaling has been abandoned. See the main text (2.2. TCR signaling) for details. α1, α2, β1, β2 – domains α1 and α2 and β1 and β2 of the chains (α and β, respectively) that form heterodimeric MHC-II complex; Ag – antigen; AP-1 – activator protein 1; Bcl10 – B cell lymphoma 10; Ca²⁺ – calcium; CaM – calmodulin; CaN – calcineurin; Carma1 – caspase recruitment domain membrane-associated guanylate kinase protein 1; c-Fos/c-Jun –AP-1-forming dimer of proto-oncogenes; CRAC – calcium release-activated Ca²⁺; DAG – diacylglycerol; IκB – kinase (IKK) complex containing IKKα, IKKβ, and IKKγ; IKKα – I kappa B kinase α; IKKβ – I kappa B kinase β; IKKγ – I kappa B kinase γ; IP₃ – inositol trisphosphate; ITAM –immunoreceptor tyrosine-based activation motif; ITK – interleukin-2 inducible tyrosine kinase; LAT – linker activation of T cells; Lck – leukocyte-specific tyrosine kinase; Malt 1 – mucosa-associated lymphoid tissue protein 1; Nck – adaptor protein non-catalytic region of tyrosine kinase (Nck); NFAT – nuclear factors of activated T cells; NF-κB – nuclear factor kappa B; P –phosphorylated proteins; p50 – regulatory subunit of the NF-kB complex; p65 – subunit of NF-κB; PI3K – phosphatidylinositol-3 kinase; PIP2 – phosphatidylinositol bisphosphate; PKC-theta – protein kinase C-theta; PLCγ1 – phospholipase C gamma 1; RasGRP – Ras guanyl nucleotide releasing protein; Slp76 – SH2-domain containing leukocyte protein of 76 kDa; ZAP70 – zeta-activated protein 70 kDa.

Figure 3

The essential role of epigenetic changes in regulatory T cell (Treg) development and function on the example of forkhead box P3 (Foxp3) - the master regulatory protein involved in Treg-mediated immune system responses. Noticeable that IL-2 action does not involve TCR signaling pathway. See the main text (Chapter 2.2.1. Epigenetic mechanisms influencing TCR signaling and autoimmunity) for details. AML1/Runx1 – Acute myeloid leukemia 1 protein or Runt-related transcription factor 1; AP-1 – activator protein 1; CD – cluster of differentiation (cell surface marker); CNS1-3 – conserved non-coding sequences; APC – antigen-presenting cells; DNMT – DNA methyl-transferase; Eos – transcription factor, member of the Ikaros Zinc Finger (IkZF) family of transcription factors; EZH2 – Enhancer of Zeste Homolog 2; Foxp3 – forkhead box P3 protein; GSK3β – glycogen synthase kinase-3 beta; H3K27Ac – acetylation of the lysine residue at N-terminal position 27 of the histone H3; H3K27Me3 – tri-methylation of lysine residue at N-terminal position 27 on the histone H3; IL-2 – interleukin 2; IL-2R – interleukin 2 receptor; Irf4 – Interferon regulatory factor 4; miRNA/miRNAs –microRNA/microRNAs; mTOR – mammalian target of rapamycin kinase; mTORC2 – mTOR Complex 2; NF-κB – nuclear factor kappa-light-chain-enhancer of activated B cells; NFAT – nuclear factor of activated T cells; p38MAP – p38 mitogen-activated protein kinase; PRC2 – polycomb repressive complex 2; Rictor – Rapamycin-insensitive companion of mammalian target of rapamycin; RelB – NF-κB subunit proto-oncogene RelB; RORγt – Retinoic acid-related orphan receptor gamma t; Satb1 – Special AT-rich sequence-binding protein 1; STAT – signal transducer and activator of transcription; TCR – T-cell receptor; TGFβR1 – transforming growth factor beta-receptor 1; Th17 – T helper 17 cells.

Figure 4

Regulatory T cells and altered immune response: autoimmune disease vs. endometriosis. Maintenance of Foxp3 protein expression in regulatory Tregs is crucial for a balanced immune response. The rationales for pharmacological modulation of SIRT1 activity in Tregs as an example of novel therapeutic approach for controlling immune responses in endometriosis and autoimmunity are also showed. ^# Restored to normal Foxp3 expression after the treatment; * It can also be opposite: increased number and activity of Tregs may be considered as an insufficient compensatory mechanism to overcome inflammation (194, 195). Epigenetics of Tregs appears to be a common denominator for autoimmunity and endometriosis. See the main text (Chapter 3. The association between endometriosis and autoimmune diseases) for details. Ac – acetylation; Foxp3 – forkhead box P3 protein; Mst1 – mammalian sterile 20-like kinase 1; NAM – nicotinamide; SIRT1 – sirtuin 1 (silent mating type information regulation 2 homolog 1); Th1 – T helper 1 cells; Th17 – T helper 17 cells; Tregs – regulatory T cells.