VEGF and its receptor family including VEGFR-1(Flt-1) are well known to be a crucial regulatory system for normal development and pathological angiogenesis. Rheumatoid arthritis(RA) is a chronic systemic disease characterized by an inflammatory erosive synovicitis, which show marked neovascularization, inflammatory cell infiltration and synovial hyperplasia, then produce a pannus of inflammatory vascular tissue and lead to irreversible cartilage and bone destruction. We have already shown VEGFR-1 is expressed not only in vascular endothelial cells but also in inflammatory cells, especially in monocyte/macrophage. A recent report suggests the involvement of VEGFR-1 in RA by using collagen induced RA mouse model. To examine whether the signaling from VEGFR-1 is important for the pathological process of RA, we used VEGFR-1 tyrosine kinase(−/−) mice which cannot generate the signaling from this receptor, and an arthritis mouse model system carrying Human T-cell leukemia virus(HTLV-1) pX transgene. VEGFR-1 TK(−/−) mice with pX gene clearly showed a reduction in the incidence and the degree of clinical symptom of arthritis. Furthermore, the heterozygote VEGFR-1 TK(+/−) with pX transgene showed a partial decrease in the degree of clinical as well as pathological scores. To explain the reason of reduction of clinical symptoms, we investigated involvement of VEGFR-1 TK signal in lineage of bone marrow hematopoietic stem cell(HSC) to monocyte/macrophage proliferation and differentiation and their immunity. VEGFR-1 TK activities are not associated in number of HSC in bone marrow. However, VEGFR-1 TK(−/−) HSC toward multi-lineage proliferation is suppressed in colony-formation. In addition, failures of monocyte/macrophage faculties are observed in immunological reaction, phagocytosis, cytokine secretion(IL-6, VEGF) and migration. Furthermore, expressions of hematopoiesis and inflammation related genes in VEGFR-1 TK(−/−) macrophage are downregulated by microarray analysis. Next we treated with small molecule inhibitors of VEGF receptor(VEGFR-1 and VEGFR-2) of tyrosine kinase, KRN951, in RA model(pX transgenic model and type II collagen Ab cocktail model) for treatment. Treatment with KRN951 strongly attenuated the disease symptom through inhibiting recruitment of BM hematopoietic cells into peripheral inflammatory cells. These observations indicate that VEGFR-1 signals play an important role in both RA mouse model. The tyrosine kinase activity and the signaling of VEGFR-1 enhances hematopoiesis, proliferation/differentiation and immunity of monocyte/macrophage from bone marrow HSC, and promotes rheumatoid arthritis, which may be a new possibilities for the treatment of RA in humans.

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