These results indicate that IL-6 and PGE(2) production by subchondral Ob can discriminate two subgroups of osteoarthritic patients that cannot otherwise be separated by their expression of cell markers, and that endogenous PGE(2) levels influence IL-6 synthesis in osteoarthritic Ob.

No abstract

Objective. To compare the effect of licofelone, NS-398 (an inhibitor of cyclooxygenase 2 [COX-2]), andBayX-1005 (an inhibitor of 5-lipoxygenase activating protein) on the production of leukotriene B 4 (LTB 4 ) and prostaglandin E 2 (PGE 2 ), and on cell biomarkers by human osteoarthritis (OA) subchondral osteoblasts.Methods. Primary in vitro osteoblasts were prepared from subchondral bone specimens obtained from OA patients and autopsy subjects. LTB 4 and PGE 2 levels were measured by enzyme-linked immunosorbent assay in conditioned media of osteoblasts incubated in the presence or absence of licofelone, NS-398, or BayX-1005. The effect of these drugs or of the addition of LTB 4 on alkaline phosphatase (AP) activity and osteocalcin release by OA and normal osteoblasts was determined. The presence of LTB 4 receptors in normal and OA osteoblasts was evaluated by Western blot analysis.Results. OA osteoblasts produced variable levels of PGE 2 and LTB 4 compared with normal osteoblasts. Licofelone, at the maximal dose used, inhibited production of PGE 2 and LTB 4 by OA osteoblasts by a mean ؎ SEM of 61.2 ؎ 6.4% and 67.0 ؎ 7.6%, respectively. NS-398 reduced PGE 2 production by 75.8 ؎ 5.3%. BayX-1005 inhibited LTB 4 production in OA osteoblasts by 38.7 ؎ 14.5% and marginally affected PGE 2 levels (reduction of 14.8 ؎ 5.3%). Licofelone dose-dependently stimulated 1,25-dihydroxyvitamin D-induced AP activity while inhibiting osteocalcin release. BayX-1005 partly reproduced these effects, but NS-398 failed to affect them. LTB 4 dose-dependently inhibited AP activity in OA osteoblasts, while its effect on osteocalcin depended on endogenous LTB 4 levels in these cells. In normal osteoblasts, LTB 4 dosedependently stimulated osteocalcin, whereas it failed to influence AP. LTB 4 receptors BLT1 and BLT2 were present in normal and OA osteoblasts.Conclusion.

Subchondral bone sclerosis may be important for the onset and/or progression of cartilage loss/damage in human osteoarthritis (OA). OA osteoblasts are resistant to parathyroid hormone (PTH) stimulation, which could explain bone sclerosis via the inhibition of PTH-dependent catabolism. Here, we investigated the molecular mechanism(s) responsible for reduced PTH-dependent cyclic adenosine monophosphate (cAMP) synthesis in OA subchondral osteoblasts. Although cholera toxin (CTX) increased basal cAMP formation in these cells, it failed to stimulate PTH-dependent cAMP synthesis, whereas pertussis toxin (PTX) did not inhibit basal cAMP, yet diminished PTH-dependent cAMP production. Binding of 125 I-PTH indicated lower PTH receptor levels in OA than in normal osteoblasts (؊50.5 ؎ 9.5%). This could be attributed to either reduced expression of the PTH receptor (PTH-R) or altered recycling of existing pools of receptors. Reversetranscription polymerase chain reaction (RT-PCR) analysis indicated decreased PTH-R messenger RNA (mRNA) levels in OA cells that were highly variable (ranging from ؊10% to ؊60%), a situation that reflects disease severity. Interestingly, OA osteoblasts produced more prostaglandin E 2 (PGE 2 ) than normal osteoblasts, and using naproxen, a cyclo-oxygenase inhibitor, increased PTH-dependent cAMP formation to a level similar to normal osteoblasts. Because heterologous desensitization can explain a decrease in PTH binding but cannot account for reduced PTH-R expression, we looked at the possible effect of insulin-like growth factor 1 (IGF-1) on this parameter. Blocking IGF-1 signaling with a neutralizing receptor antibody increased 125 I-PTH binding in both normal and OA osteoblasts. Conversely, treatments with IGF-1 receptor (IGF-1R) antibody only slightly increased the levels of PTH-R mRNA whereas the addition of IGF-1 significantly reduced PTH-R mRNA levels (؊24.1 ؎ 7.1%), yet neither PGE 2 nor naproxen modified PTH-R levels. These results suggest that both IGF-1 signaling and PGE 2 formation repress PTH-dependent response in OA osteoblasts, a situation that can contribute to abnormal bone remodeling and bone sclerosis in OA.