Maria A. Halili

Histone deacetylases as regulators of inflammation and immunity

Histone deacetylases (HDACs) remove an acetyl group from lysine residues of target proteins to regulate cellular processes. Small-molecule inhibitors of HDACs cause cellular growth arrest, differentiation and/or apoptosis, and some are used clinically as anticancer drugs. In animal models, HDAC inhibitors are therapeutic for several inflammatory diseases, but exacerbate atherosclerosis and compromise host defence. Loss of HDAC function has also been linked to chronic lung diseases in humans. These contrasting effects might reflect distinct roles for individual HDACs in immune responses. Here, we review the current understanding of innate and adaptive immune pathways that are regulated by classical HDAC enzymes. The objective is to provide a rationale for targeting (or not targeting) individual HDAC enzymes with inhibitors for future immune-related applications.

Histone deacetylase inhibitors in inflammatory disease.

Lysine acetylation is becoming increasingly appreciated as a key post-translational modification in the endogenous regulation of protein function. The so-called histone acetyl transferases (HATs) and histone deacetylases (HDACs), best known for their roles in controlling chromatin remodeling via histone acetylation/deacetylation, are now known to modify a large number of non-histone proteins to control diverse cell processes. In relation to inflammation, acetylation modulates the activity or function of cytokine receptors, nuclear hormone receptors, intracellular signaling molecules and transcription factors. Small molecule inhibitors of HDACs have been found to trigger both pro- and anti-inflammatory effects in a range of inflammation-relevant cell types. Although their inflammatory profiles have only just begun to be elucidated, some HDAC inhibitors are already showing therapeutic promise in animal models of inflammatory diseases such as arthritis, inflammatory bowel diseases, septic shock, ischemia-reperfusion injury, airways inflammation and asthma, diabetes, age-related macular degeneration, cardiovascular diseases, multiple sclerosis and other CNS and neurodegenerative diseases. This article describes those HDAC inhibitors which have been most examined to date for their potentially beneficial effects on inflammatory cells or in animal models of inflammatory disease.

Differential effects of selective HDAC inhibitors on macrophage inflammatory responses to the Toll‐like receptor 4 agonist LPS

Broad‐spectrum inhibitors of HDACs are therapeutic in many inflammatory disease models but exacerbated disease in a mouse model of atherosclerosis. HDAC inhibitors have anti‐ and proinflammatory effects on macrophages in vitro. We report here that several broad‐spectrum HDAC inhibitors, including TSA and SAHA, suppressed the LPS‐induced mRNA expression of the proinflammatory mediators Edn‐1, Ccl‐7/MCP‐3, and Il‐12p40 but amplified the expression of the proatherogenic factors Cox‐2 and Pai‐1/serpine1 in primary mouse BMM. Similar effects were also apparent in LPS‐stimulated TEPM and HMDM. The pro‐ and anti‐inflammatory effects of TSA were separable over a concentration range, implying that individual HDACs have differential effects on macrophage inflammatory responses. The HDAC1‐selective inhibitor, MS‐275, retained proinflammatory effects (amplification of LPS‐induced expression of Cox‐2 and Pai‐1 in BMM) but suppressed only some inflammatory responses. In contrast, 17a (a reportedly HDAC6‐selective inhibitor) retained anti‐inflammatory but not proinflammatory properties. Despite this, HDAC6−/− macrophages showed normal LPS‐induced expression of HDAC‐dependent inflammatory genes, arguing that the anti‐inflammatory effects of 17a are not a result of inhibition of HDAC6 alone. Thus, 17a provides a tool to identify individual HDACs with proinflammatory properties.

Antifibrotic activity of an inhibitor of histone deacetylases in DOCA-salt hypertensive rats

Background and purpose:  Histone deacetylases (HDACs) silence genes by deacetylating lysine residues in histones and other proteins. HDAC inhibitors represent a new class of compounds with anti‐inflammatory activity. This study investigated whether treatment with a broad spectrum HDAC inhibitor, suberoylanilide hydroxamic acid (SAHA), would prevent cardiac fibrosis, part of the cardiovascular remodelling in deoxycorticosterone acetate (DOCA)‐salt rats.

Modulating human proteinase activated receptor 2 with a novel antagonist (GB88) and agonist (GB110)

BACKGROUND AND PURPOSE Many cells express proteinase activated receptor 2 (PAR2) on their plasma membrane. PAR2 is activated by proteolytic enzymes, such as trypsin and tryptase that cleave the receptor N‐terminus, inititating signalling to intracellular G proteins. Studies on PAR2 have relied heavily upon activating effects of proteases and peptide agonists that lack stability and bioavailability in vivo.

Disarming Burkholderia pseudomallei: structural and functional characterization of a disulfide oxidoreductase (DsbA) required for virulence in vivo

AIMS The intracellular pathogen Burkholderia pseudomallei causes the disease melioidosis, a major source of morbidity and mortality in southeast Asia and northern Australia. The need to develop novel antimicrobials is compounded by the absence of a licensed vaccine and the bacteriums resistance to multiple antibiotics. In a number of clinically relevant Gram-negative pathogens, DsbA is the primary disulfide oxidoreductase responsible for catalyzing the formation of disulfide bonds in secreted and membrane-associated proteins. In this study, a putative B. pseudomallei dsbA gene was evaluated functionally and structurally and its contribution to infection assessed. RESULTS Biochemical studies confirmed the dsbA gene encodes a protein disulfide oxidoreductase. A dsbA deletion strain of B. pseudomallei was attenuated in both macrophages and a BALB/c mouse model of infection and displayed pleiotropic phenotypes that included defects in both secretion and motility. The 1.9 Å resolution crystal structure of BpsDsbA revealed differences from the classic member of this family Escherichia coli DsbA, in particular within the region surrounding the active site disulfide where EcDsbA engages with its partner protein E. coli DsbB, indicating that the interaction of BpsDsbA with its proposed partner BpsDsbB may be distinct from that of EcDsbA-EcDsbB. INNOVATION This study has characterized BpsDsbA biochemically and structurally and determined that it is required for virulence of B. pseudomallei. CONCLUSION These data establish a critical role for BpsDsbA in B. pseudomallei infection, which in combination with our structural characterization of BpsDsbA will facilitate the future development of rationally designed inhibitors against this drug-resistant organism.

Rv2969c, essential for optimal growth in Mycobacterium tuberculosis, is a DsbA-like enzyme that interacts with VKOR-derived peptides and has atypical features of DsbA-like disulfide oxidases.

The gene product of M. tuberculosis Rv2969c is shown to be a disulfide oxidase enzyme that has a canonical DsbA-like fold with novel structural and functional characteristics.

Noncovalent Tripeptidyl Benzyl- and Cyclohexyl-Amine Inhibitors of the Cysteine Protease Caspase-1

Potent and noncovalent inhibitors of caspase-1 were produced by incorporating a secondary amine (reduced amide) isostere in place of the conventional electrophile (e.g., aldehyde) that normally confers high potency to cysteine protease inhibitors. Benzyl- or cyclohexylamines produced potent, reversible, and competitive inhibitors that were selective for caspase-1 (e.g., K(i) = 47 nM) over caspases 3 and 8 with minimal cytotoxicity. Unlike most cysteine protease inhibitors, these compounds do not react covalently and indiscriminately with thiols.

Peptide Inhibitors of the Escherichia coli DsbA Oxidative Machinery Essential for Bacterial Virulence

One approach to address antibiotic resistance is to develop drugs that interfere with bacterial virulence. A master regulator of virulence in Gram-negative bacteria is the oxidative folding machinery comprising DsbA and DsbB. A crystal structure at 2.5 Å resolution is reported here for Escherichia coli DsbA complexed with PFATCDS, a heptapeptide derived from the partner protein Escherichia coli DsbB. Details of the peptide binding mode and binding site provide valuable clues for inhibitor design. Structure-activity relationships for 30 analogues were used to produce short peptides with a cysteine that bind tightly to EcDsbA (Kd = 2.0 ± 0.3 μM) and inhibit its activity (IC50 = 5.1 ± 1.1 μM). The most potent inhibitor does not bind to or inhibit human thioredoxin that shares a similar active site. This finding suggests that small molecule inhibitors can be designed to exploit a key interaction of EcDsbA, as the basis for antivirulence agents with a novel mechanism of action.

Comparative Sequence, Structure and Redox Analyses of Klebsiella pneumoniae DsbA Show That Anti-Virulence Target DsbA Enzymes Fall into Distinct Classes

Bacterial DsbA enzymes catalyze oxidative folding of virulence factors, and have been identified as targets for antivirulence drugs. However, DsbA enzymes characterized to date exhibit a wide spectrum of redox properties and divergent structural features compared to the prototypical DsbA enzyme of Escherichia coli DsbA (EcDsbA). Nonetheless, sequence analysis shows that DsbAs are more highly conserved than their known substrate virulence factors, highlighting the potential to inhibit virulence across a range of organisms by targeting DsbA. For example, Salmonella enterica typhimurium (SeDsbA, 86 % sequence identity to EcDsbA) shares almost identical structural, surface and redox properties. Using comparative sequence and structure analysis we predicted that five other bacterial DsbAs would share these properties. To confirm this, we characterized Klebsiella pneumoniae DsbA (KpDsbA, 81 % identity to EcDsbA). As expected, the redox properties, structure and surface features (from crystal and NMR data) of KpDsbA were almost identical to those of EcDsbA and SeDsbA. Moreover, KpDsbA and EcDsbA bind peptides derived from their respective DsbBs with almost equal affinity, supporting the notion that compounds designed to inhibit EcDsbA will also inhibit KpDsbA. Taken together, our data show that DsbAs fall into different classes; that DsbAs within a class may be predicted by sequence analysis of binding loops; that DsbAs within a class are able to complement one another in vivo and that compounds designed to inhibit EcDsbA are likely to inhibit DsbAs within the same class.