Histone/protein deacetylase inhibitor therapy for enhancement of Foxp3+ T-regulatory cell function posttransplantation

Abstract

T-regulatory (Treg) cells are like other cells present throughout the body in being subject to biochemical modifications in response to extracellular signals. An important component of these responses involves changes in posttranslational modifications (PTMs) of histones and many nonhistone proteins, including phosphorylation/dephosphorylation, ubiquitination/deubiquitination, and acetylation/deacetylation. Foxp3, the key transcription factor of Tregs, is constantly being rapidly turned over, and a number of these PTMs determine its level of expression and activity. Of interest in the transplant setting, modulation of the acetylation or deacetylation of key lysine residues in Foxp3 can promote the stability and function, leading to increased Treg production and increased Treg suppressive activity. This mini-review focuses on recent data concerning the roles that histone/protein deacetylases (HDACs) play in control of Treg function, and how small molecule HDAC inhibitors can be used to promote Treg-dependent allograft survival in experimental models. These data are discussed in the light of increasing interest in the identification and clinical evaluation of isoform-selective HDAC inhibitors, and their potential application as tools to modulate Foxp3+ Treg cell numbers and function in transplant recipients.

Keywords: T cell biology; basic (laboratory) research/science; cellular biology; immunobiology; immunosuppressant - other; immunosuppression/immune modulation; tolerance; translational research/science.

Figure 1. HDAC reaction and Foxp3

(A) Lysine deacetylation renders the Foxp3 transcriptionally inactive and prone to poly-ubiquitination and proteasomal degradation. (B) Lysine deacetylation reaction by Zn²⁺ and NAD⁺ dependent HDACs and acetylation by HATs. The deacetylation reaction removes acetate from the ε-amino group, changing the polarity of the lysine side chain from neutral to a positive charge, and altering the properties of the client protein. HATs catalyze the reverse of this reaction and acetylate the ε-amino group. Abbreviations: NAD, nicotinamide adenine dinucleotide; NAM, nicotinamide; ADPR, adenosine diphosphate riboside; Ub, ubiquitin; Ac, acetate or acetyl-group; CoA, Coenzyme A; HAT, histone acetyl-transferase.

Figure 2. Role of HDACs in Treg function and their therapeutic potential in transplantation

Deletion and pharmacologic targeting of HDAC2, HDAC7, HDAC9, HDAC6, HDAC10, HDAC11, as well as Sirtuin-1 (Sirt1) improves Treg function. Of those, HDAC6 is the most promising candidate since HDAC6 knockout mice are normal, and HDAC6 selective pharmacologic inhibitors are being tested in clinical trials for other indications. Domain color codes: Red, classic Zn²⁺ dependent HDAC catalytic domain; Pink, class IIa HDAC catalytic domain (histidine to tyrosine substitution compared to class I, IIb, and IV HDAC); orange, NAD⁺ dependent class III HDAC (Sirtuin) catalytic domain; black, inactive leucine rich domain (HDAC10); purple, nuclear localization sequence; brown, mitochondrial targeting sequence; yellow, MEF2 binding site. Abbreviations: N, nucleus; C, cytoplasm; M, mitochondria.

Figure 3. HDAC6 inhibitor design

Schematic representation of how HDAC6 deacetylates Foxp3 and how this process can be inhibited with an HDAC6 inhibitor, e.g. Tubacin. The aliphatic lysine sidechain fits into a tunnel, leading to a catalytic chamber containing a zinc ion that is essential for the deacetylation reaction. HDAC inhibitors take advantage of this principle and are designed to mimic the aliphatic lysine side chain to fit into the tunnel. Instead of an acetyl group, the HDAC6 inhibitor has a zinc binding group, chelating the Zn²⁺ ion and blocking the deacetylation reaction. HDAC-isoform specificity is achieved through the ‘cap region’ of the molecule fitting to the surrounding amino acid side chains specific to HDAC6.