Pharmacological or genetic inhibition of salt-inducible kinases (SIKs) can switch activated macrophages (M?) to an anti-inflammatory state and protect against fibrosis, making SIK an attractive therapeutic target in multiple diseases. However, target validation is challenging as the SIK isoform-specific disease contribution is unclear. Here, we have used gene set variation analysis (GSVA) and novel SIK isoform-specific inhibitors (SIKi) for in vitro and in vivo target validation in models of inflammation and fibrosis. 

The effects of SIKi on fibrotic markers were monitored by imaging and immunoassays both in vitro and in vivo, using idiopathic pulmonary fibrosis (IPF) or normal human lung myofibroblasts and an adenoviral transforming growth factor-beta1 (TGF-?1) mouse fibrosis model. IPF myofibroblasts were also used for global gene expression profiling and subsequent calculation of GSVA enrichment scores in two IPF cohorts. Anti-inflammatory cytokine responses to SIKi were measured in human whole blood and primary human alveolar M?. 

Key genes upregulated by panSIKi in IPF myofibroblasts were significantly lower in IPF vs. healthy subjects. PanSIK, but not SIK2/3, inhibition prevented and reversed fibroblast-to-myofibroblast differentiation in vitro and reduced bronchoalveolar lavage fluid Pro-collagen type 1, SERPINE1 and TGF-?1 in vivo with no effect on pSMAD2. Conversely, SIK2/3 inhibition was sufficient for anti-inflammatory effects as measured by decreased TNF-alpha and increased IL-10 in LPS-stimulated human M? and whole blood. 

Taken together, panSIKi conveys pSMAD2-independent anti-fibrotic effects whereas SIK2/3 inhibitors are sufficient for anti-inflammatory effects.