These findings indicate an altered state of osteoblast differentiation and function in OA driven by several key molecular regulators. In association with this differential gene expression, an altered state of both trabecular bone remodelling and resulting microarchitecture were also observed, further characterising the pathogenesis of primary hip OA.
Previous studies have shown a generalised increase in bone mass in patients with osteoarthritis (OA). Using molecular histomorphometry, this study examined the in vivo expression of mRNA encoding bone anabolic factors and collagen type I genes (COL1A1, COL1A2) in human OA and non-OA bone. Bone samples were obtained from the intertrochanteric (IT) region of the proximal femur, a skeletal site distal to the active site of disease, from individuals with hip OA at joint replacement surgery and from autopsy controls. Semi-quantitative reverse transcription-polymerase chain reaction analysis revealed elevated mRNA expression levels of alkaline phosphatase (p < 0.002), osteocalcin (OCN) (p < 0.0001), osteopontin (p < 0.05), COL1A1 (p < 0.0001), and COL1A2 (p < 0.002) in OA bone compared to control, suggesting possible increases in osteoblastic biosynthetic activity and/or bone turnover at the IT region in OA. Interestingly, the ratio of COL1A1/COL1A2 mRNA was almost twofold greater in OA bone compared to control (p < 0.001), suggesting the potential presence of collagen type I homotrimer at the distal site. Insulin-like growth factor (IGF)-I, IGF-II, and transforming growth factor-β1 mRNA levels were similar between OA and control bone. Bone histomorphometric analysis indicated that OA IT bone had increased surface density of bone (p < 0.0003), increased trabecular number (Tb.N) (p < 0.0003), and decreased trabecular separation (Tb.Sp) (p < 0.0001) compared to control bone. When the molecular and histomorphometric data were plotted, positive associations were observed in the controls for OCN/glyceraldehyde-3-phosphate dehydrogenase (GAPDH) versus bone tissue volume (r = 0.82, p < 0.0007) and OCN/ GAPDH versus Tb.N (r = 0.56, p < 0.05) and a negative association was observed for OCN/GAPDH versus Tb.Sp (r = -0.64, p < 0.02). These relationships were not evident in trabecular bone from patients with OA, suggesting that bone regulatory processes leading to particular trabecular structures may be altered in this disease. The finding of differential gene expression, as well as architectural changes and differences in molecular histomorphometric associations between OA and controls, at a skeletal site distal to the active site of joint degeneration supports the concept of generalised involvement of bone in the pathogenesis of OA.
Pentosan polysulphate sodium (PPS) is a promising therapeutic agent for blocking knee pain in individuals with knee osteoarthritis (KOA). The mode of action of PPS in this context is unknown. We hypothesised that the osteocyte, being the principal cell type in the sub-chondral bone, was capable of expressing the pain mediator Nerve Growth Factor (NGF), and that this may be altered in the presence of PPS. We tested the expression of NGF and the response to PPS in the presence or absence of the proinflammatory cytokine tumour necrosis factor-alpha (TNFα), in human osteocytes. For this we differentiated human primary osteoblasts grown from subchondral bone obtained at primary knee arthroplasty for KOA to an osteocyte-like stage over 28d. We also tested NGF expression in fresh osteocytes obtained by sequential digestion from KOA bone and by immunofluorescence in KOA bone sections. We demonstrate for the first time the production and secretion of NGF/proNGF by this cell type derived from patients with KOA, implicating osteocytes in the pain response in this pathological condition and possibly others. PPS inhibited TNFα-induced levels of proNGF secretion and TNFα induced NGF mRNA expression. Together, this provides evidence that PPS may act to suppress the release of NGF in the subchondral bone to ameliorate pain associated with knee osteoarthritis.
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