Receptors for platelet-derived growth factor (PDGF) are abundantly expressed on synovial fibroblast-like (SFL) cells from patients with rheumatoid arthritis (RA), and stimulation with PDGF enhances both the anchorage-dependent and -independent growth of RA-SFL cells. To elucidate the molecular mechanisms responsible for the excessive growth of RA-SFL cells and to seek a novel molecular-targeting therapy for RA, we examined the expression of adapter proteins and the effect of the specific inhibition of PDGF receptor activation by imatinib mesylate. Cultured SFL cells were used in the present study after 2-5 passages. The anchorage-dependent and -independent growth patterns of the SFL cells were evaluated using a tetrazolium-based assay and colony formation in 0.3% agar, respectively. Adapter proteins Gab1 and Gab2 were expressed in RA-SFL cells, and both proteins were rapidly (< 1 min) tyrosine-phosphorylated after the stimulation of RA-SFL cells with 10 ng/ml of PDGF and, to a lesser extent, after stimulation with 100 ng/ml of epidermal growth factor (EGF). The inhibition of PDGF receptor tyrosine kinase activation by 1 μM or less of imatinib mesylate specifically suppressed the PDGF-dependent, but not EGF-dependent, tyrosine phosphorylation of various proteins. Moreover, imatinib mesylate abolished both the anchorage-dependent and -independent proliferation of RA-SFL cells induced by PDGF stimulation. These results suggest that Gab adapter proteins are expressed and likely to be involved in the growth signalling of rheumatoid synovial cells and that imatinib mesylate, a key drug in the treatment of chronic myeloid leukaemia, may also be effective for the treatment of RA.
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