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Prostaglandin E2‐Dependent Phosphorylation of RAS Inhibition 1 (RIN1) at Ser 291 and 292 Inhibits Transforming Growth Factor‐β‐Induced RAS Activation Pathway in Human Synovial Fibroblasts: Role in Cell Migration

Prostaglandin E2 (PGE2)‐stimulated G‐protein–coupled receptor (GPCR) activation inhibits pro‐fibrotic TGFβ‐dependent stimulation of human fibroblast to myofibroblast transition (FMT), though the precise molecular mechanisms are not fully understood. In the present study, we describe the PGE2‐dependent suppression and reversal of TGFβ‐induced events such as α‐sma expression, stress fiber formation, and Ras/Raf/ERK/MAPK pathway‐dependent activation of myofibroblast migration. In order to elucidate post‐ligand‐receptor signaling pathways, we identified a predominant PKA phosphorylation motif profile in human primary fibroblasts after treatment with exogenous PGE2 (EC50 30 nM, Vmax 100 nM), mimicked by the adenyl cyclase activator forskolin (EC50 5 μM, Vmax 10 μM). We used a global phosphoproteomic approach to identify a 2.5‐fold difference in PGE2‐induced phosphorylation of proteins containing the PKA motif. Deducing the signaling pathway of our migration data, we identified Ras inhibitor 1 (RIN1) as a substrate, whereby PGE2 induced its phosphorylation at Ser291 and at Ser292 by a 5.4‐ and 4.8‐fold increase, respectively. In a series of transient and stable over expression studies in HEK293T and HeLa cells using wild‐type (wt) and mutant RIN1 (Ser291/292Ala) or Ras constructs and siRNA knock‐down experiments, we showed that PGE2‐dependent phosphorylation of RIN1 resulted in the abrogation of TGFβ‐induced Ras/Raf signaling activation and subsequent downstream blockade of cellular migration, emphasizing the importance of such phosphosites in PGE2 suppression of wound closure. Overexpression experiments in tandem with pull‐down assays indicated that specific Ser291/292 phosphorylation of RIN1 favored binding to activated Ras. In principal, understanding PGE2‐GPCR activated signaling pathways mitigating TGFβ‐induced fibrosis may lead to more evidence‐based treatments against the disease. J. Cell. Physiol. 232: 202–215, 2017. © 2016 Wiley Periodicals, Inc.