Objectives Stimulators of soluble guanylate cyclase (sGC) are currently investigated in clinical trials for the treatment of fibrosis in systemic sclerosis (SSc). In this study, we aim to investigate the role of protein kinases G (PKG) as downstream mediators of sGC–cyclic guanosine monophosphate (cGMP) in SSc.

Methods Mice with combined knockout of PKG1 and 2 were challenged with bleomycin and treated with the sGC stimulator BAY 41-2272. Fibroblasts were treated with BAY 41-2272 and with the PKG inhibitor KT 5823.

Results PKG1 and 2 are upregulated in SSc in a transforming growth factor-β1 (TGFβ1)-dependent manner, as an attempt to compensate for the decreased signalling through the sGC–cGMP–PKG pathway. Inhibition or knockout of PKG1 and 2 abrogates the inhibitory effects of sGC stimulation on fibroblast activation in a SMAD-independent, but extracellular signal-regulated kinase (ERK)-dependent manner. In vivo, sGC stimulation fails to prevent bleomycin-induced fibrosis in PKG1 and 2 knockout mice.

Conclusions Our data provide evidence that PKGs are essential mediators of the antifibrotic effects of sGC stimulators through interfering with non-canonical TGFβ signalling. TGFβ1 promotes its profibrotic effects through inhibition of sGC–cGMP–PKG signalling, sGC stimulation exerts its antifibrotic effects by inhibition of TGFβ1-induced ERK phosphorylation.