The differential effects of cAMP mobilizing agents on TGF-β-induced extracellular matrix in human lung-derived fibroblasts: Insights into therapeutic targets for lung fibrosis

Background: Injury-repair responses typically induce tissue or organ scarring. Generally, cAMP-mobilizing agents inhibit the activation of fibroblasts and deposition of extracellular matrix. Some but not all cAMP-mobilizing agents inhibit fibrosis. Evidence suggests that inhaled treprostinil, a prostacyclin (IP) analog, increases intracellular cAMP levels [cAMP] _I , and slows the decline in pulmonary function in patients with idiopathic pulmonary fibrosis (IPF). However, the molecular mechanisms by which cAMP-mobilizing agents, including treprostinil, alter the expression of matrix proteins in human lung fibroblasts (HLF) remain unclear. Unlike other G _αs -coupled receptors, we posit that the antifibrotic properties of treprostinil are driven by cAMP-mobilizing-dependent and -independent responses mediated by the IP receptor activation. Methods: As a model of lung fibrosis, primary HLF derived from non-IPF and IPF donors were stimulated with TGF-β; collagen 1A1 and plasminogen activator inhibitor-1 (PAI) expression were then measured in the presence and absence of cAMP mobilizing agents. The necessity of receptor activation for inhibiting TGF-β-induced markers of fibrosis was determined by using soluble receptor inhibitors and decreasing receptor expression with siRNA. Results: Treprostinil decreased TGF-β-induced extracellular matrix production by HLF, and the magnitude of the inhibition was greater than that of other cAMP-mobilizing GPCR agonists despite these agents comparably increasing cAMP levels. There was no difference in the sensitivity and magnitude of the treprostinil inhibition in HLF derived from non-fibrosis and lung fibrosis donors. Treprostinil inhibition of TGF-β-induced collagen 1A1and PAI-1 was mediated through the activation of the IP receptor. The activation of the EP2 receptor, in part, inhibited TGF-β-induced collagen 1A1 expression by treprostinil or prostaglandin E2. β2 agonists had little effect on TGF-β-induced expression of collagen 1A1 and PAI-1. The inhibitory effects of treprostinil on TGF-β-induced collagen 1A1 expression required G _αs activation, while G _αs only partially mediated treprostinil inhibition of PAI-1. Conclusion: The anti-fibrotic properties of treprostinil are primarily mediated by the IP receptor, acting through both G _αs -dependent and -independent pathways. Understanding the differential effects of cAMP-mobilizing pathways on HLF fibrotic signatures can provide insight into developing novel targets to manage IPF.