Die Rolle der Transkriptionsfaktoren “runt-related transcriptionfactor-2“ (RUNX2) und Osterix in humanen Osteoblasten

Abstract

Introduction: Osterix (OSX) and “runt-related transcription factor-2 (RUNX2) are transcription factors which are essential for osteoblast differentiation in-vivo during the process of bone formation. Whether their role in-vivo is reflected during human osteoblastic differentiation in-vitro as well has not been studied. Methods: Primary human osteoblasts (pHOB) cultures were established using bone chips from bone material generated during hip or knee replacement therapy. We used different models of osteoblastic differentiation to reproduce the sequence of gene expression shown in-vivo. Using RUNX2 knock down the influence of this factor on bone differentiation in-vitro was studied. Finally we investigated the gene expression of OSX and RUNX2 and its association with clinical parameters using a group of patients with metabolic bone diseases. Results: While RUNX2 gene expression was detectable in mesenchymal stem cells following osteoblastic induction, OSX was only faintly expressed. In cells growing out from bone chips the expression of RUNX2 was followed by OSX expression which decreased sharply in fully differentiated cells. In contrast, the expression of RUNX2 persisted during the entire osteoblastic differentiation sequence. Knock down of RUNX2 reduced osteocalcin expression by 87% in control cells whereas there was lower effect on OC expression in cells under osteogenic conditions. The mRNA levels of OSX used in the clinical analysis of patients with metabolic bone diseases were highly correlated to clinical and histomorphometrical parameters, whereas RUNX2 didn't correlate to any clinical parameter. Conclusion: The sequence of early osteoblast development can be reproduced in-vitro using human osteoblast models. RUNX2 down regulation in human osteoblasts was sufficient for interfering with osteoblast differentiation when grown under basal conditions. The transcription factor OSX was highly correlated to clinical and histomorphometrical factors suggesting an important role also for clinical use.