PPARγ inhibits osteogenesis via the down-regulation of the expression of COX-2 and iNOS in rats Tzu-Hung Lin a,1 , Rong-Sen Yang b,1 , Chih-Hsin Tang c , Chih-Peng Lin d , Wen-Mei Fu a, ⁎ a Department of Pharmacology, College of Medicine, National Taiwan University, Taipei, Taiwan b Department of Orthopaedics, College of Medicine, National Taiwan University, Taipei, Taiwan c Department of Pharmacology, College of Medicine, China Medical University, Taichung, Taiwan d Department of Anesthesiology, National Taiwan University, Taipei, Taiwan Received 13 January 2007; revised 24 May 2007; accepted 11 June 2007 Available online 4 July 2007 Abstract Peroxisome proliferator-activated receptor gamma (PPARγ), a ligand-activated transcription factor, is considered as an anti-osteoblastic factor associated with adiposity and the elderly osteoporosis due to a defect in osteoblastogenesis. We have found that oral administration of PPARγ activator rosiglitazone decreased tibia BMD and serum ALP but left serum calcium and osteoclast marker C-terminal telopeptide unaffected. In addition, we examined the inhibitory mechanisms of PPARγ on the bone formation by using PPARγ activators ciglitazone and 15-deoxy-Δ 12,14 - prostaglandin-J2 (15d-PGJ2). Our data indicated that PPARγ ligands decreased both mineralized bone nodules and alkaline phosphatase (ALP) activities in cultured primary osteoblasts. Reverse transcription polymerase chain reaction (RT-PCR) showed that the expression of bone morphogenetic protein-2 (BMP-2) and osteocalcin (OCN) was inhibited by ciglitizone and 15d-PGJ2. Furthermore, PPARγ ligands inhibited NF-κB associated downstream COX-2 and iNOS osteogenic signaling. The ultrasound (US)-induced elevation of COX-2 and iNOS expression and nitric oxide (NO) production were attenuated in the presence of PPARγ ligands. Furthermore, local administration of PPARγ ligands into the metaphysis of rat tibia decreased the bone volume in secondary spongiosa. These results suggest that the activation of PPARγ inhibits osteoblastic differentiation and the expression of several anabolic mediators involved in bone formation. These data may reflect osteoporosis and less bone formation in the aging people and patients treated with thiazolidinediones. © 2007 Elsevier Inc. All rights reserved. Keywords: PPARγ; Osteoblast maturation; COX-2; Osteocalcin; iNOS Introduction Peroxisome proliferator-activated receptor gamma (PPARγ), a member of the nuclear receptor family of transcription factors, functions as a heterodimer with a retinoid X receptor by binding the PPAR responsive element (PPRE) within the promoter of the target genes. PPARγ is activated by a wide variety of substances including long chain fatty acids, peroxisome proliferators, 15-deoxy-Δ 12,14 -Prostaglandin J2 (15d-PGJ2), and thiazolidinedione (TZD) compounds [1–3]. The actions of PPARγ are initially assumed as the regulation of lipid metabolism and homeostasis, whereas recent studies have emphasized its important role in the regulation of inflammatory responses, cellular proliferation, and differentiation [4], as well as the balance between osteogenesis and adipogenesis [5,6]. Adipocytes and osteoblasts have been shown to share a common progenitor, i.e., mesenchymal stem cells in bone marrow [7,8]. Osteoblasts play a requisite role in bone formation [9], and PPARγ has been shown to be expressed in osteoblasts [10,11]. The imbalance between adipocytes and osteoblasts is associated with osteoporosis, especially in the age-related osteoporosis that is accompanied by an increase in marrow adipocytes [12,13]. Furthermore, in mesenchymal cell lines, activation of PPARγ stimulates adipogenesis and inhibits osteogenesis [14] and over- Bone 41 (2007) 562 – 574 www.elsevier.com/locate/bone ⁎ Corresponding author. Department of Pharmacology, College of Medicine, National Taiwan University, No. 1 Sec. 1 Jen-Ai Road Taipei (100), Taiwan. Fax: +886 2 23417930. E-mail address: wenmei@ntu.edu.tw (W.-M. Fu). 1 Lin T.H. and Yang R.S. have equal contribution. 8756-3282/$ - see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.bone.2007.06.017