C-type natriuretic peptide-mediated crosstalk of endothelial cells with fibroblasts: a protective role in lung inflammation and fibrosis?

Pulmonary fibrosis (PF) is a progressive, restrictive lung disease driven by fibroblast hyperactivation and increased interstitial collagen deposition. PF can be idiopathic (IPF) or secondary to severe inflammation. For instance, after severe acute coronavirus 2 (SARS-CoV-2) pneumonia, more than a third of patients develop fibrotic abnormalities. Hence there is a tremendous need for a better understanding of the molecular pathways preventing IPF, to find novel tractable targets. Infusion of exogenous synthetic C-type natriuretic peptide (CNP), activating its guanylyl cyclase B (GC-B) receptor in fibroblasts, exerted antifibrotic effects in experimental PF. To elucidate the possible therapeutical relevance, here we studied CNP actions in cultured lung fibroblasts from patients with IPF. Additionally, to dissect possible protective paracrine roles of the endogenous hormone, we generated a novel genetic mouse model with conditional, fibroblast-restricted GC-B deletion for studies of bleomycin-induced inflammatory PF. Last we investigated the potential of a long acting (LA) CNP (LA-CNP) to counteract fibrosis. In vitro exogenous CNP, via GC-B/cyclic GMP signaling, prevented the profibrotic and pro-proliferative effects of growth factors such as transforming growth factor beta (TGF-β) and platelet derived growth factor BB (PDGF-BB) on fibroblasts cultured from human “healthy” lung specimens. Notably, such inhibitory actions of CNP were fully preserved in fibroblasts from IPF patients. In vivo one week after bleomycin instillation, the fibroblast GC-B knockdown mice displayed significant increases in pulmonary plasma leakage and immune cell infiltration as well as augmented albumin and Mac-2 levels in their bronchoalveolar lavage fluid (BALF) in comparison to control littermates, indicating aggravated inflammation. Nevertheless, the extent of subsequent interstitial fibrosis and the decline of lung compliance were not different between the two genotypes. In contrast pretreatment with a single subcutaneous injection of LA-CNP exerted protective effects in bleomycin treated mice, reducing pulmonary plasma leakage, immune cell infiltration, collagen deposition and lung function decline. Together these results indicate that attenuation of CNP/GC-B signaling in fibroblasts participates in the development of inflammatory lung fibrosis and rescuing this pathway has protective effects. This work provides important insights into the potential of CNP, which in form of LA-CNP may add to the therapeutic options, improving the quality of life of patients with lung fibrosis.