Background and aims Ras activation as well as MAP kinase (MAPK) phosphorylation is known in the synovial tissues from patients with rheumatoid arthritis (RA). RasGRP4 is a guanine nucleotide exchange factor for small GTPase Ras and is expressed predominantly in the mast cells, monocytes and neutrophils. We previously identified ectopic expression of RasGRP4 in fibroblast-like synoviocytes (FLS) of a subset of RA patients, inducing proliferation of FLS (Kono M et al. Arthritis Rheumatol. 2015). Farnesyltransferase inhibitors (FTIs), prevent farnesylation of Ras, are known to prevent human tumour cell proliferation but the effect for proliferation of FLS is still unknown.

The aim of this study is to clarify the molecular mechanisms of how RasGRP4 induces proliferation of FLS and evaluate the effect of FTI on the proliferation of FLS.

Methods FLS or HEK293 cells were transfected with expression vector that encodes hRasGRP4. Phosphorylation of Raf, MEK, Erk, JNK and p38MAPK was evaluated in transfected cells using Western blotting. FLS were treated with tipifarnib, one of FTIs and cell proliferation was evaluated using BrdU Assay.

Results In HEK293 cells forced to express RasGRP4, Raf-MEK-Erk pathway as well as p38MAPK was readily phosphorylated at their steady state. FLS decreased RasGRP4 expression during multiple passages. RasGRP4 transfection into such cells recovered MAPK phosphorylations, especially of Erk and p38 MAPK. FLS treated with tipifarnib down-regulated their proliferation.

Conclusions RasGRP4 expression in FLS from RA patients contributes to the activation of Erk and p38MAPK signalling pathway. The inhibition of FLS proliferation by FTI was suggested as an alternative treatment in RA.