Pulmonary fibrosis is a serious lung disease which mainly affects older people; there are virtually no effective treatments. The disease is characterized by progressive tissue changes which lead to scarring of the lung. However, its causes are largely unknown, and the cellular mechanisms involved in its development remain poorly understood. The term ‘mucociliary clearance’ refers to a self-cleaning mechanism which relies on ciliated cells in the lung epithelium propelling inhaled pathogens and other particles trapped in mucus out of the airways. It is know that changes in the epithelium are associated with the production of excess mucus. They are also associated with the impaired clearance of this mucus and its primary proteins, the ‘mucins’. Working alongside research groups from Heidelberg, researchers from Charité – Universitätsmedizin Berlin have elucidated a molecular pathway involved in the development of pulmonary fibrosis. They were able to show that the protein known as NEDD4-2 plays a key role in lung health and that loss of this crucial regulatory molecule has a significant impact on various mechanisms involved in the development of chronic progressive lung disease. These new insights make it easier to further investigate the precise mechanisms involved in the development and progression of pulmonary fibrosis.
NEDD4-2 is involved in the degradation of a range of other proteins which in turn are responsible for using these processes to regulate lung epithelial function. For example, NEDD4-2 regulates the epithelial Na+ channel (ENaC) by ubiquitination, retrieval from the apical cell surface and targeting for degradation. This ion channel is responsible for the more or less viscosity of the bronchial mucus, by regulating its salinity for a proper mucociliar clearance. NEDD4-2 affects also the amount of lung surfactant C protein (proSP-C), needed for lung compliance.That said, the researchers firstly succeeded in developing a novel animal model of idiopathic pulmonary fibrosis (IPF). As NEDD4-2 is crucial for early development, they only deleted the relevant encoding gene in lung epithelial cells once the animals had reached adulthood. The researchers examined the animals once they had reached a stage roughly comparable to the point at which the disease would be diagnosed in a human patient. Oxygen saturation measurements taken at that stage revealed a level of lung function impairment which is characteristic of the disease. Using tissue sections and CT imaging to examine the lungs, the researchers also found evidence of patchy scarring patterns which is indicative of fibrosis.
The researchers found further evidence of the significance of NEDD4-2 in the pathogenesis of IPF: lung tissue biopsy samples of patients with IPF contained significantly reduced levels of both transcripts and proteins. Using mass spectrometry, the researchers then performed what is known as ‘protein profiling’, an analysis of the complete set of proteins produced in the lungs. This revealed a high degree of overlap between the proteins found to be expressed differently in the lungs of patients with IPF and in the animals used in this study. When studying the underlying disease mechanisms, the researchers discovered that reduced levels of NEDD4-2 in epithelial cells result in epithelial remodeling in the airways. Not only are the different cell types present in different proportions, the cells also produce increased amounts of certain mucins. When combined with changes in epithelial sodium transport and the resultant reduction in the volume of airway surface liquid, this will lead to impaired mucociliary clearance. Lack of NEDD4-2 also causes increased activity of the TGFβ signaling pathway, which promotes the formation of fibrosis. It is like removing the handbrake from a running car.
Anti-fibrotic drugs have been used to treat pulmonary fibrosis for some years. While these drugs usually succeed in slowing the development of scarring, they cannot wholly replace lung transplants as a last resort treatment option. Numerous studies have shown that TGF-β is the most important cytokine in the development of pulmonary fibrosis. Indeed, pirfenidone and nintedanib, the only two IPF therapeutic options work by inhibitoing the TGFbeta-mediated signal transduction and the subsequent fibroblast proliferation. But scientists are trying to implement evolution of these molecules with more potent and/or more specific ones. For example a team of chemists at Sichuan University and Collaborative Innovation Center, developed a new quinolone scaffold that impaired TGF-beta receptor-mediated phosphorylation of Smad3. The new compound is more specific than the original quinolone-like halofuginone, which has antifibrotic properties. The team focused also on coumarin derivatives of nintedanib itself, creating hybrids that had a difference with the original drug: they suppressed collagen synthesis and deposition avoiding to kill fibroblasts, as instead nintedanib does. The best derivative (called 9d), was a good bioavailability and a reasonable long half-lfe (13h), which lead teh researchers i twill be developed as a potential new drug against IPF.
- edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
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