The shunt from the cyclooxygenase to lipoxygenase pathway in human osteoarthritic subchondral osteoblasts is linked with a variable expression of the 5-lipoxygenase-activating protein

Osteoarthritis (OA) is characterized by articular cartilage degradation and hypertrophic bone changes with osteophyte formation and abnormal bone remodeling. Two groups of OA patients were identified via the production of variable and opposite levels of prostaglandin E-2 (PGE(2)) or leukotriene B-4 (LTB4) by subchondral osteoblasts, PGE(2) levels discriminating between low and high subgroups. We studied whether the expression of 5-lipoxygenase (5-LO) or 5-LO-activating protein (FLAP) is responsible for the shunt from prostaglandins to leukotrienes. FLAP mRNA levels varied in low and high OA groups compared with normal, whereas mRNA levels of 5-LO were similar in all osteoblasts. Selective inhibition of cyclooxygenase-2 (COX-2) with NS-398-stimulated FLAP expression in the high OA osteoblasts subgroup, whereas it was without effect in the low OA osteoblasts subgroup. The addition of PGE(2) to the low OA osteoblasts subgroup decreased FLAP expression but failed to affect it in the high OA osteoblasts subgroup. LTB4 levels in OA osteoblasts were stimulated about twofold by 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) plus transforming growth factor-beta (TGF-beta), a situation corresponding to their effect on FLAP mRNA levels. Treatments with 1,25(OH)(2)D-3 and TGF-beta also modulated PGE(2) production. TGF-alpha stimulated PGE(2) production in both OA osteoblast groups, whereas 1,25(OH)(2)D-3 alone had a limited effect but decreased the effect of TGF-beta in the low OA osteoblasts subgroup. This modulation of PGE(2) production was mirrored by the synthesis of COX-2. IL-18 levels were only slightly increased in a subgroup of OA osteoblasts compared with normal; however, no relationship was observed overall between IL-18 and PGE(2) levels in normal and OA osteoblasts. These results suggest that the shunt from the production of PGE(2) to LTB4 is through regulation of the expression of FLAP, not 5-LO, in OA osteoblasts. The expression of FLAP in OA osteoblasts is also modulated differently by 1,25(OH)(2)D-3 and TGF-beta depending on their endogenous low and high PGE(2) levels.