Sp1-Dependent Regulation of PPARα in Bone Metabolism

Purpose: Successful repair and regeneration in bone tissue engineering vastly depends on proper interaction between the tissue-engineered construct and the recipient's immune system. In clinical application, adverse responses to bioartificial implants may result in chronic inflammation and loss of the implant. It is known that prolonged inflammation linked to NF-kappa B inflammatory pathways inhibits bone-forming activity of osteoblast cells. Contributing to orchestrate inflammatory processes, the ligand-activated transcription factor peroxisome proliferator-activated receptor alpha (PPAR alpha) holds inhibitory effects on NF-kappa B and CEB beta activity. Sp1, a widely expressed transcription factor, has been linked to PPAR pathways, cellular homeostasis, and responsiveness to environmental perturbation. Formerly not being characterized, the role of PPAR alpha in inflammatory-mediated bone loss requires further investigation. The aim of the present study was to identify regulatory transcription factor binding sites (TFBS) on the PPAR alpha promoter and to assess the role of Sp1 and associated proteins in its regulation. Materials and Methods: In a first set of experiments, polymerase chain reaction assessed the presence of PPAR alpha gene expression in isolated murine bone tissue. Deletion mutagenesis was performed on the human PPAR alpha (hPPAR alpha) promoter gene, and the deletion constructs were transiently transfected to murine osteoblasts to identify important TFBS. PPAR alpha promoter-driven reporter gene expression was monitored in response to overexpression and repression of Sp1 to analyze functional transcription factor recruitment to the PPAR alpha promoter. Results: This study could demonstrate that the full-length hPPAR alpha promoter contains inhibiting promoter regions and that hPPAR alpha basal expression can be significantly increased by deletion mutagensis. Sp1 TFBS proved functional in the regulation of PPAR alpha promoter activity, and the first five Sp1 motifs on the PPAR alpha promoter were sufficient to significantly increase PPAR alpha expression. Additional transient co-transfection experiments could not detect any direct effect of NF-kappa B/I kappa B downstream pathway on the regulation of PPAR alpha promoter activity. Taken together, we could demonstrate that Sp1 plays a key role in transcriptional regulation of PPAR alpha promoter activity and gene expression. Conclusion: This study provides further insight on Sp1-dependent PPAR alpha regulatory mechanisms and suggests that Sp1-regulated PPAR alpha expression plays a key role in inflammatory mediated bone loss.