New insights into the pathogenesis of cystic fibrosis: pivotal role of glutathione system dysfunction and implications for therapy.
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The cystic fibrosis transmembrane regulator (CFTR) should no longer be viewed primarily as a 'chloride channel' but recognized as a channel that also controls the efflux of other physiologically important anions, such as glutathione (GSH) and bicarbonate. More effective approaches to cystic fibrosis treatment may result from this reconceptualization of the CFTR by researchers and clinicians. For example, oxidant damage in cystic fibrosis has been assumed to be a significant part of the pathophysiology of the disease. Generally speaking, antioxidant status in cystic fibrosis is compromised. However, until recently this was seen as secondary to the excessive chemoattraction of neutrophils in this disease caused by mutation of the CFTR protein, leading to a high oxidant burden. New findings suggest that the cystic fibrosis mutations in fact cause a primary dysfunction in the system of one of the body's most important antioxidant and immune-signaling substances: the reduced GSH system. Cystic fibrosis mutations significantly decrease GSH efflux from cells without redundant channels to the CFTR; this leads to deficiency of GSH in the epithelial lining fluid of the lung, as well as in other compartments, including immune system cells and the gastrointestinal tract. This deficiency is exaggerated over time as the higher-than-normal oxidant burden of cystic fibrosis leads to successively larger decrements in GSH without the normal opportunity to fully recover physiologic levels. This GSH system dysfunction may be the trigger for initial depletion of other antioxidants and may also play a role in initiating the over-inflammation characteristic of cystic fibrosis. Proper GSH system functioning also affects immune system competence and mucus viscosity, both of relevance to cystic fibrosis pathophysiology. In a way, cystic fibrosis may be thought of as the first identified disease with GSH system dysfunction.This overview provides a review of the most pertinent recent research findings in this area. Exogenous augmentation of GSH in the lung epithelial lining fluid is possible, and therapeutic approaches include administration of aerosolized buffered GSH, intravenous GSH, and oral GSH. However, it is important to remember that the pathophysiology of cystic fibrosis is multifactorial, and rectification of GSH system dysfunction in patients with cystic fibrosis will not eliminate all harmful effects of the disease. The promising results of two clinical trials of aerosolized buffered GSH in cystic fibrosis patients have been published or accepted for publication at the time of this writing. GSH depletion in lung epithelial lining fluid has also been noted in other respiratory diseases such as COPD, idiopathic pulmonary fibrosis, and adult respiratory distress syndrome, and therapies to augment GSH may also be contemplated in these diseases.
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