Regulation of TNFα and IL1β in rheumatoid arthritis synovial fibroblasts by leukotriene B4

Arachidonic acid metabolites, such as leukotriene B4 (LTB4), are known to play an important role in pathogenesis of rheumatoid arthritis (RA). Apart from leukocytes, RA synovial fibroblasts (RASF) produce a broad array of inflammatory mediators to recruit, retain and activate immune cells and resident mesenchymal cells in the joints to promote ongoing inflammation and tissue destruction. To determine how LTB4 may contribute to this process, RASF was cultured from synovial tissues collected from RA patients undergoing total knee replacement. The level of LTB4 in culture medium was determined using ELISA, and expression of LTB4 receptors (BLT1 and BLT2) by RT-PCR. In the presence of exogenous LTB4, mRNA and protein levels of tumor necrosis factor-alpha (TNF alpha) and interleukin 1 beta (IL1 beta) were determined by real-time PCR and ELISA. Furthermore, we examined the effects of leukotrienes synthesis inhibitors, MK886 and bestatin, on mRNA and protein levels of TNF alpha and IL1 beta in RASF. We found that LTB4 was present at a low concentration in the culture medium of RASF, and the major LTB4 receptor expressed in RASF was BLT2. LTB4 synthesis was activated by treatment with LIT (LPS, ionomycin and thapsogargin), and suppressed by MK886 and bestatin. Exogenous LTB4 remarkably increased the expression of TNF alpha and IL1 beta at both the mRNA level and the protein level. In contrast, MK886 and bestatin significantly inhibited their expression. These data suggested that LTB4 contributed to RA by regulating the expression of TNF alpha and IL1 beta in RASF. BLT2 was probably the major receptor mediating such effects.