Inflammation involves a complex interplay of immunological responses in reaction to an infection from invading pathogens and to tissue injury. However, if the inflammatory stimulus is not eliminated or if there is an aberrant response, chronic inflammation and disease can result. Most inflammatory diseases are targeted by drugs that provide only symptomatic relief by reducing the associated pain and swelling, without combating the underlying cause and progression of the disease. The purpose of this thesis was to investigate potential drug targets in different sub-cellular compartments to learn more about inflammatory pathways and the consequences of inhibiting them using different classes of small-molecule agents.
Chapter 1 is a brief overview of inflammation, outlining differences between acute and chronic inflammation in relation to normal and aberrant immune responses. Major drug classes that are currently in use are briefly discussed, and some potential new inflammatory drug targets in the nucleus, cytoplasm, plasma membrane and serum that are to be investigated in the thesis are described.
Chapter 2 addresses histone deacetylase (HDAC) enzymes that modulate gene transcription, and investigates the regulatory potential of HDACs and their inhibitors in macrophages. Mainly murine macrophages were investigated, but preliminary data on human macrophages is also presented. Four kinds of small-molecule inhibitors of HDAC enzymes are described in relation to their capacity to exhibit pro- or anti- inflammatory gene expression. Broad-spectrum inhibitors, those that do not discriminate between different isoforms of HDACs, were found to induce expression of a number of pro-inflammatory mediators. This was also the case for compounds that selectively inhibit class-I HDACs. On the other hand, compounds that are known to selectively inhibit class-II HDACs did not regulate pro-inflammatory genes and could be classified as anti-inflammatory based on their profiles of gene expression. An inhibitor of class-III HDACs, the sirtuins, was also briefly investigated for anti-inflammatory gene expression. This separation of the pro- and anti-inflammatory responses suggested that specific inhibitors of class II HDACs might be promising for treating inflammatory conditions.
Chapters 3 and 4 discuss a group of extracellular serum proteins that are produced through the complement pathway. Complement proteins circulate in plasma while some are formed on membranes through serine protease action, and effect both innate and adaptive immune responses. Chapter 3 deals with the alternative pathway (AP) of complement activation, in particular with the protease Factor B which plays an important role as part of a protein complex (C3 convertase) in the initiation and propagation of complement. Factor B is an inactive zymogen at pH 7, but is catalytically active at alkaline pH. Substrate-based inhibitors were developed for Factor B using a chromogenic assay previously optimised in our lab. Over 60 peptide aldehydes were evaluated, with two potent inhibitors of Factor B studied further, and were found to inhibit C3 convertase by blocking cleavage of the native substrate C3 as measured by SDS-PAGE.
Chapter 4 examined the properties of a homologue of Factor B found in the classical pathway (CP), the protease C2. C2 was found herein to be an inactive zymogen at pH 7. Under alkaline conditions C2 cleaved both C3 and short chromogenic peptide substrates. Substrates of 7- or 8-residues, corresponding to those in the native substrate C3, were optimally processed. C2 was inhibited by inhibitors identified in Chapter 3, which also inhibited formation of the different C3 convertase formed through the CP, as well as preventing formation of the terminal membrane attack complex (MAC) as measured by an immobilized ELISA and haemolysis assay. The selectivity of small-molecule peptidic inhibitors for different serine proteases was investigated.
Chapter 5 introduces two other unrelated inflammatory proteins which have been incompletely studied herein. Protease-activated receptor 2 (PAR-2) is a cell surface protein that spans the membrane like other G-protein coupled receptors. PAR-2 has been implicated in inflammatory and proliferative diseased states, though its precise roles are still unclear. Here, PAR-2 mRNA expression was measured for different human cancer and immune cells. In macrophages, PAR-2 mRNA expression was inducible by HDACi, indicating that PAR-2 is negatively regulated by HDAC enzymes. The receptor was present and functional on the surface of HDACi-treated macrophages, permitting evaluation of PAR-2 agonists and antagonists. Caspase-1 is a cytosolic cysteine protease responsible for cleaving proIL-1β and proIL-18 into their active forms that are then secreted from cells as pro-inflammatory cytokines. Using monocytic cells, novel inhibitors of caspase-1 were developed and found to dose-dependently decrease levels of IL-1β released by the cell. Inhibition of caspase-1 may be useful for modulating circulating levels of the inflammatory cytokine IL-1β.
The investigation of different prospective inflammatory targets in different locations of the cell has provided a lateral overview of inflammatory mechanisms and drug intervention strategies. Specific inhibitors have also been identified for these targets; HDACs in the nucleus, caspase-1 in the cytosol, PAR-2 and MAC on the cell surface, C2 and Factor B and C3 convertase in plasma. This unconventional study of the inflammatory network provides a different viewpoint of inflammatory proteins as drug targets.