Abstract
Osteoporosis is an inflammatory disease characterised by low bone mass and quality, resulting in weaker bone strength and fragility fractures. Periostin is a matricellular protein expressed in the periosteum of bone by osteoblasts. It regulates cell recruitment and differentiation in response to fracture and contributes to extracellular matrix (ECM) formation. The aim of the following study was to determine the splice variants of Periostin expressed in human osteoblasts and Periostin's function in the pathophysiology of osteoporosis. Osteoblasts isolated from femoral heads from 29 patients with or without osteoporosis were utilised. Periostin splice variants were compared by quantitative real-time polymerase chain reaction (qPCR). Furthermore, the effect of Periostin inhibition on osteoblast differentiation was investigated using alizarin red S staining. Lastly, the interaction of IL-6 and Periostin and their effect on osteoprotegerin (OPG) secretion were analysed with the implantation of enzyme-linked immunosorbent assays (ELISAs). It could be demonstrated that human osteoblasts preferentially express Periostin isoform 4, even if splice variant expression was not altered in osteoporosis conditions, indicating that Periostin's functions in bone are primarily attributable to this isoform. The inhibition of Periostin resulted in significantly reduced osteoblast differentiation. However, Periostin was secreted in significantly higher amounts in osteoblasts from patients with osteoporosis. Additionally, Periostin significantly reduces OPG secretion and, thereby, rather promotes bone resorption. Furthermore, it could be determined that Periostin and IL-6 induce each other, and both significantly decrease OPG secretion. A positive feedback loop exacerbates the dysregulation found in human osteoblasts from patients with osteoporosis, thereby contributing to bone loss.