Hypoxia modulates the phenotype of osteoblasts isolated from knee osteoarthritis patients leading to under-mineralized bone nodule formation

Objective: To investigate the role of hypoxia in the pathology of OA bone by exploring its effect on the phenotype of isolated primary osteoblasts from patients with knee OA.

Methods: OA bone samples were collected from donors with knee and hip OA following elective joint replacement surgery. Normal OA bone samples were collected from post-mortem donors. Primary osteoblasts were isolated from knee OA bone chips and cultured in normoxic or hypoxic (2% O2) conditions. Alkaline phosphatase activity was quantified using an enzymatic assay, osteopontin and PGE2 production were assayed by ELISA. Total RNA was extracted from bone and osteoblasts and gene expression profiled by qRT-PCR.

Results: Human OA bone tissue sections stained positively for CAIX, a biomarker of hypoxia, and exhibited differential expression of genes that mediate the vasculature and blood coagulation compared to normal bone. Culturing of primary osteoblasts isolated from knee OA bone under hypoxic conditions profoundly affected the osteoblast phenotype including the expression of genes that mediate bone matrix, bone remodelling and bone vasculature. Hypoxia also increased COX2 expression and the production of PGE2 from OA osteoblasts. The osteoblast expression of Col2A1, ANGPTL4 and IGFBP1 was shown to be mediated by HIF-1α levels. Chronic hypoxia reduced osteoblast mineralised bone nodule formation.

Conclusion: These findings demonstrate that hypoxia can induce pathological changes in osteoblast functionality consistent with an OA phenotype, providing evidence that hypoxia is a key driver of OA pathology. © 2014 American College of Rheumatology.