Pathophysiological relevance of PDE inhibition in lung fibroblasts for the treatment of pulmonary diseases

Abstract

Lung fibroblast proliferation and differentiation into myofibroblast are pathological key events during fibrotic alterations occuring in pulmonary diseases such as asthma, idiopathic pulmonary fibrosis or chronic obstructive pulmonary disease. Cyclic nucleotide signaling is described as a negative modulator of these cellular processes, and cyclic AMP degrading type 4 phosphodiesterases (PDE4) are important regulators of these pathways. Together with various cAMP-generating isoforms of adenylyl cyclases and effectors, PDE4 subtypes constitute a network that regulates the signaling via numerous mediators, e.g. growth factors or cytokines. Several studies using selective PDE4 inhibitors under clinical development have shown that the induced increase of cAMP can inhibit lung fibroblast functions and might be able to mitigate lung tissue remodeling. The role of PDE4 subtypes in these functions remains unknown. This thesis aims to analyze mechanisms of cytokine-induced functions of human lung fibroblasts, such as proliferation and differentiation, and to determine the role of PDE4 subtypes within these processes. This study demonstrates, that bFGF-induced proliferation of human lung fibroblasts is either potentiated by the presence of the inflammatory cytokine IL-1β or inhibited depending on the IL-1β concentration. The antiproliferative activity of PDE4 inhibitors (but not of PDE3 inhibitors) is due to the increase of PGE2 by IL-1β via the COX-2/PGE2-synthase pathway. Moreover, this IL-1β triggered signaling pathway is further potentiated by bFGF by increasing IL-1β induced COX-2 expression. In order to assess the role of PDE4 subtypes, their expression in human lung fibroblasts was down regulated by using an optimized liposome mediated transfection method and specific siRNA´s. With this technology, different PDE4 subtypes were assigned to distinct functions of human lung fibroblast. The knockdown of PDE4A and PDE4B inhibited cytokine-induced proliferation, whereas knockdown of PDE4D did not. In contrast, TGF-β induced differentiation of lung fibroblasts to myofibroblasts was only prevented by knockdown of PDE4B and PDE4D, but not of PDE4A. The identification of the predominant role of PDE4B in controlling pathophysiological relevant functions of human lung fibroblasts provides now a rationale for pharmaceutical development of subtype specific PDE4 inhibitors with higher efficacy and a broader therapeutic window.