Endogenous prostaglandin E-2 and insulin-like growth factor 1 can modulatethe levels of parathyroid hormone receptor in human osteoarthritic osteoblasts

Subchondral bone sclerosis may be important for the onset and/or progressio n of cartilage loss/damage in human osteoarthritis (OA). OA osteoblasts are resistant to parathyroid hormone (PTH) stimulation, which could explain bo ne sclerosis via the inhibition of PTH-dependent catabolism. Here, we inves tigated the molecular mechanism(s) responsible for reduced PTH-dependent cy clic adenosine monophosphate (cAMP) synthesis in OA subchondral osteoblasts . Although cholera toxin (CTX) increased basal cAMP formation in these cell s, it failed to stimulate PTH-dependent cAMP synthesis, whereas pertussis t oxin (PTX) did not inhibit basal cAMP, yet diminished PTH-dependent cAMP pr oduction. Binding of I-125-PTH indicated lower PTH receptor levels in OA th an in normal osteoblasts (-50.5 +/- 9.5%). This could be attributed to eith er reduced expression of the PTH receptor (PTH-R) or altered recycling of e xisting pools of receptors. Reverse-transcription 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 situatio n that reflects disease severity. Interestingly, OA osteoblasts produced mo re prostaglandin E-2 (PGE(2)) than normal osteoblasts, and using naproxen, a cyclo-oxygenase inhibitor, increased PTH-dependent cAMP formation to a le vel similar to normal osteoblasts. Because heterologous desensitization can explain a decrease in PTH binding but cannot account for reduced PTH-R exp ression, we looked at the possible effect of insulin-like growth factor 1 ( IGF-1) on this parameter. Blocking IGF-1 signaling with a neutralizing rece ptor antibody increased I-125-PTH binding in both normal and OA osteoblasts . Conversely, treatments with IGF-1 receptor (IGF-1R) antibody only slightl y increased the levels of PTH-R mRNA whereas the addition of IGF-1 signific antly reduced PTH-R mRNA levels (-24.1 +/- 7.1%), yet neither PGE, nor napr oxen modified PTH-R levels. These results suggest that both IGF-1 signaling and PGE, formation repress PTH-dependent response in OA osteoblasts, a sit uation that can contribute to abnormal bone remodeling and bone sclerosis i n OA.