Efficacy of short‚ÄövÑv´chain quinones in colitis : future therapeutic options

Ulcerative colitis (UC) is a long-lasting disorder of the gastrointestinal tract characterised by inflammation, irritation and ulcerations in the inner lining of the colon and rectum. If it is left untreated, it can lead to serious life-threatening complications such as colorectal cancer. Although the exact aetiology of UC is unknown, mitochondrial dysfunction, oxidative stress, endoplasmic reticulum (ER) stress, dysregulated immune response and defective barrier integrity are all involved in the pathogenesis of UC. Current evidence suggests that all these pathophysiological factors are interlinked, making it extremely difficult to identify a therapeutic target. Current treatment regimens involve anti-inflammatory agents, corticosteroids, biologicals and immunomodulators, or a combination of all these drugs. However, these treatments are associated with various adverse effects and are not curative. For many patients, this leaves only surgical interventions as a final therapeutic choice. Therefore, there is an urgent need to develop novel, effective treatments that simultaneously target several pathophysiological factors associated with UC. Quinones are organic compounds consisting of the polycyclic aromatic ring in their structure and are known to be involved in many biological activities. They control many cellular redox reactions such as in blood coagulation and are also reported to protect mitochondrial function. At present, only idebenone, a benzoquinone that belongs to the class of short-chain quinones (SCQs), is approved in Europe to treat a rare mitochondrial disease. Although the exact mode of action of idebenone has not been confirmed, previous studies suggested it is a potent antioxidant that is bioactivated by the detoxifying enzyme NADPH quinone oxidoreductase 1 (NQO-1). Idebenone is also known to restore cellular energy levels under certain conditions by feeding electrons into complex III of the mitochondrial electron transport chain. It is thought that both its mitochondrial electron donor and antioxidant properties are responsible for the mito-protective effect of idebenone. Apart from these two properties, idebenone has been reported to possess numerous associated pharmacological activities including anti-inflammatory activity. In addition to idebenone, another mito-protective SCQ, UTA77, was recently developed at the University of Tasmania. UTA77 showed better cytoprotection and increased metabolic stability compared to idebenone in vitro. At present, only one study investigated the in vivo therapeutic potential of UTA77 in a rat model of diabetic retinopathy, where it was highly effective in restoring vision in diabetic animals. Based on these encouraging data, the current study aimed to investigate the efficacy of both mito-protective agents, idebenone and UTA77, and their potential mode(s) of action in experimental mouse models of acute and chronic colitis. In two mouse models of acute and chronic colitis, idebenone substantially reduced the disease activity index (DAI), improved body weight and reduced histopathological damage in both proximal and distal colon as compared to untreated animals. Idebenone significantly improved colon length, increased mucus staining and effectively suppressed the levels of pro-inflammatory cytokines confirming its anti-inflammatory effect. In the acute colitis model, idebenone preserved intestinal barrier integrity by increasing the expression of TJ proteins occludin and zona-occludens 1 (ZO-1). Moreover, idebenone markedly increased the levels of the detoxifying enzyme NQO-1, increased superoxide dismutase (SOD) activity, reduced nitric oxide levels (NO) and reduced lipid peroxidation, demonstrating its broad antioxidant effects. In contrast, in the model of chronic colitis, which was not associated with oxidative stress, idebenone disrupted the progression of chronic inflammation by simultaneously suppressing ER stress markers and reducing the inflammatory response. Similar to idebenone, UTA77 showed a very similar therapeutic profile in vivo. It also alleviated DAI, improved colon length, reduced histopathology and strongly suppressed the secretion of several pro-inflammatory cytokines in both acute and chronic colitis models. Under conditions of acute colitis, UTA77 also increased the expression of TJ proteins, thereby maintaining intestinal barrier integrity. For the first time, this study demonstrated the anti-inflammatory effect of the new mito-protective SCQ UTA77 in vivo, which effectively attenuated colonic inflammation. Overall, this study has generated the experimental evidence to conclude that SCQs are potent mito-protective agents that can effectively ameliorate colonic inflammation by simultaneously targeting multiple pathophysiologies of UC. These pathogenic factors include oxidative stress, dysregulated immune response, altered barrier integrity and ER stress. It can be postulated that the synergism between the molecular activities of SCQs is likely responsible for reducing the severity of colonic inflammation in both acute and chronic colitis. It can also be speculated that both SCQs, by protecting mitochondrial function in colitic mice, can disrupt the vicious cycle of initiation of inflammation, oxidative damage, ER stress and prolonged inflammatory response at multiple levels. The knowledge attained from this study suggests that SCQs could be developed as a potent new therapeutic alternative to treat UC patients.