Two Histone/Protein Acetyltransferases, CBP and p300, Are Indispensable for Foxp3⁺ T-Regulatory Cell Development and Function

Abstract

e T-regulatory (Treg) cells are important to immune homeostasis, and Treg cell deficiency or dysfunction leads to autoimmune disease. A histone/protein acetyltransferase (HAT), p300, was recently found to be important for Treg function and stability, but further insights into the mechanisms by which p300 or other HATs affect Treg biology are needed. Here we show that CBP, a p300 paralog, is also important in controlling Treg function and stability. Thus, while mice with Treg-specific deletion of CBP or p300 developed minimal autoimmune disease, the combined deletion of CBP and p300 led to fatal autoimmunity by 3 to 4 weeks of age. The effects of CBP and p300 deletion on Treg development are dose dependent and involve multiple mechanisms. CBP and p300 cooperate with several key Treg transcription factors that act on the Foxp3 promoter to promote Foxp3 production. CBP and p300 also act on the Foxp3 conserved noncoding sequence 2 (CNS2) region to maintain Treg stability in inflammatory environments by regulating pCREB function and GATA3 expression, respectively. Lastly, CBP and p300 regulate the epigenetic status and function of Foxp3. Our findings provide insights into how HATs orchestrate multiple aspects of Treg development and function and identify overlapping but also discrete activities for p300 and CBP in control of Treg cells. Eukaryotic transcription is tightly regulated by histone modifications, including acetylation of lysine residues of histone tails (1). Acetylation is controlled by the competing actions of histone/ protein acetyltransferases (HATs) and histone/protein deacetylases (HDACs), which also regulate the acetylation of many nonhistone proteins. Three main families of HATs are reported: CBP/ p300, GNAT (GCN5/PCAF), and MYST (2). CBP and p300 have high sequence homology (1) but share little sequence similarity with other HAT enzymes (3). CBP (KAT3A) and p300 (KAT3B) function as transcriptional coactivators, acetyltransferases, and ubiquitin ligases and interact with multiple transcription factors (4). Changes resulting in homozygous CBP or p300 knockout mice or CBP/p300 doubly heterozygous mice are invariably lethal (5), consistent with the widespread roles of these molecules in cell growth and development. CBP-and p300-deficient embryos have phenotypes that are partially distinct from those of p300 and CBP heterozygous null mice (5, 6), suggesting they are not interchangeable and have different substrate specificities. For example, CBP and p300 exhibit distinct specificities on acetylating histones (7,8), and they can acetylate the same lysines with different degrees of efficiency (9). Biologically, CBP promotes but p300 inhibits transcription of the antiapoptotic survivin gene (10). Likewise, loss of CBP, but not loss of p300, increases the proportion of CD8 singly positive thymocytes and the likelihood of development of T cell lymphoma (11,12). Lastly, retinoic acid-induced transcriptional upregulation of the p21Cip1 cell cycle inhibitor required p300 but did not require CBP, whereas the rev...

Harnessing the plasticity of CD4+ T cells to treat immune-mediated disease

“…At the Foxp3 locus, DNA demethy lation by the recruitment of TET proteins is necessary for the induction and maintenance of FOXP3 expression 135,139,140 . The recruitment of HATs, such as CBP (also known as CREBBP) and p300, is also crucial for maintaining T Reg cell integrity by keeping the locus open; in their absence, T Reg cells lose FOXP3 expression and gain IL-17 expression 141 . Activating HMTs, such as the tri thorax family member mixed-lineage leukaemia (MLL; also known as KMT2A), which adds methyl groups to histone H3 lysine 4 (H3K4), is especially important for the retention of cytokine production specifically in memory T H 2 cells, but not T H 1 cells 142 .…”

“…In cancer, such targeting strategies could have dual effects by directly impeding the cancer cells and by modulating the antitumour immune response 189 . Inhibiting the activity of HATs globally has selective effects on T Reg cell plasticity, driving T Reg cells to lose FOXP3 expression and gain IL-17 production, and probably accounts for the improved antitumour immune responses observed with this approach 141,190 . Alternatively, blocking the interactions of bromodomain-containing proteins with acetylated histones through the use of BET inhibitors (bromo domain and extraterminal inhibitors) reduces the severity of autoimmune disease in mouse models of multiple sclerosis and type 1 diabetes by broadly skewing immunity towards regulatory phenotypes 191,192 .…”

Regulatory T Cell Modulation by CBP/EP300 Bromodomain Inhibition

“…Loss of function mutations in FOXP3 lead to severe and systemic autoimmunity in both mice and humans (1, 2, 31). Treg-specific deletion of EP300 or CBP in mice results in decreased Treg function and reduced tumor growth in cancer models (21,22), although the specific role for the bromodomain has not been determined. We asked whether CBP/EP300 bromodomains are involved in modulating similar phenotypes in human cells and specifically whether bromodomain inhibition could result in impaired Treg differentiation.…”

“…CBP and EP300 each encode a single bromodomain and HAT activity, along with other protein interaction modules, and function as transcriptional co-activators (19,20). Recent research underscores the importance of CBP/EP300 in Treg biology, because conditional deletion of either EP300 or CBP in mouse Tregs or inhibition of their HAT activity led to impaired Treg suppressive function and reduced tumor growth in murine cancer models (21,22). In addition to evidence for the role of HAT activity, there is growing appreciation for the importance of the bromodomain in transactivation functions of CBP/EP300 (23).…”

p300-Dependent Chromatin Remodelling Regulates Multiple Liver Functions

“…Histone acetyl transferase p300 regulates chromatin structure through interactions with many transcription factors, TFs [19]. Our lab is investigating the role of C/EBP-p300 family complexes on the epigenetic regulation of liver biology.…”