Idiopathic Pulmonary Fibrosis (IPF) is a progressive interstitial lung disease of unknown etiology associated with high morbidity and mortality, and further characterized by abnormal alveolar epithelial and fibro-proliferative responses, excessive extra-cellular matrix deposition, patchy inflammatory infiltrations and progressive loss of normal lung structure. At present there is no demonstrably effective therapy for blocking or reversing the progression of the disease. This situation demands a better understanding of the molecular and cellular mechanisms involved in the pathogenesis of IPF.
Current evidence suggests a role of cyclic guanosine monophosphate phosphodiesterases (cGMP-PDEs) in the pathogenesis of various proliferative lung diseases, including IPF. Lung PDE6 expression and function has received little or no attention. The present study aimed to characterize (i) cGMP PDEs profile in IPF, (ii) PDE6 subunits expression in human lung, (iii) PDE6 subunits expression and alteration in IPF and (iv) functionality of the specific PDE6D subunit. The experiments were carried out with human lung samples from donors and IPF patients. RT-PCR analyses from donor and IPF human lungs revealed up-regulation of PDE1A, PDE10A and PDE11A in the IPF lungs and expression of PDE6 subunits mRNA transcripts in human lungs. Westernblot analysis showed 2-fold up-regulation of PDE6A and PDE6B, and 2-fold down-regulation of PDE6D and membrane localization of PDE6G in the IPF lungs as compared to the donor lungs. Immunohistochemical analysis showed alveolar epithelial localization of the PDE6 subunits. RT-PCR analysis from donor and IPF-derived human primary alveolar type (AT) II cells confirmed the cellular localization of the PDE6 subunits and the down-regulation pattern of PDE6D. Further, siRNA-mediated PDE6D knockdown and an ectopic PDE6D expression in A549 cells demonstrated the modulatory effects of PDE6D on alveolar epithelial cell (AEC) proliferation. Additionally, we showed that these effects specifically involve ERK phosphorylation. Collectively, we report (i) mRNA expression profile of cGMP PDEs in IPF, (ii) previously unrecognized PDE6 expression in human lungs, (iii) pronounced alterations of PDE6 subunits in IPF-derived lungs and (iv) characterize the functional role of PDE6D in AEC proliferation.
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