Endogenous Prostaglandin E2 and Insulin-like Growth Factor 1 Can Modulate the Levels of Parathyroid Hormone Receptor in Human Osteoarthritic Osteoblasts

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 (250.5 6 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 210% to 260%), a situation that reflects disease severity. Interestingly, OA osteoblasts produced more prostaglandin E 2 (PGE2) 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 (224.1 6 7.1%), yet neither PGE2 nor naproxen modified PTH-R levels. These results suggest that both IGF-1 signaling and PGE2 formation repress PTH-dependent response in OA osteoblasts, a situation that can contribute to abnormal bone remodeling and bone sclerosis in OA. (J Bone Miner Res 2001;16:713‐721)