Activation of Peroxisome Proliferator-activated Receptor-γ Inhibits the Runx2-mediated Transcription of Osteocalcin in Osteoblasts

Abstract

Mesenchymal cells are able to differentiate into several distinct cell types, including osteoblasts and adipocytes. The commitment to a particular lineage may be regulated by specific transcription factors. Peroxisome proliferator-activated receptor-␥ (PPAR␥), acting in conjunction with CCAAT/enhancer-binding protein-␣, has been suggested as a key regulator of adipogenic differentiation. Previous studies have shown that the activation of PPAR␥ in osteoblasts suppresses osteoblast differentiation and the expression of osteocalcin, an osteoblast-specific protein. However, the mechanism of this inhibition remains unclear. We investigated the effect of PPAR␥ activation on the expression of osteocalcin and analyzed the molecular mechanism. Mouse osteoblastic MC3T3-E1 cells expressed PPAR␥, which was transcriptionally active, whereas rat osteosarcoma ROS 17/2.8 cells did not. Treatment of MC3T3-E1 osteoblasts and ROS 17/2.8 cells stably transfected with PPAR␥2 with the PPAR␥ activator 15-deoxy-⌬ 12,14 -prostaglandin J 2 inhibited the mRNA expression of osteocalcin and Runx2, the latter of which is a key transcription factor in osteoblast differentiation. This decreased expression of osteocalcin and Runx2 was partly explained by the decreased level of Runx2 resulting from the suppressed transcription from the Runx2 promoter. However, in addition to this indirect effect, the activation of PPAR␥ by 15-deoxy-⌬ 12,14 -prostaglandin J 2 directly suppressed the Runx2-mediated induction of the activities of the osteocalcin promoter and the artificial promoter p6OSE2, which contains six tandem copies of osteoblast-specific element-2, the Runx2-binding promoter sequence. This inhibition was mediated by a physical interaction between PPAR␥ and Runx2 and the subsequent repression of the transcriptional activity at the osteoblast-specific element-2 sequence. Thus, this study demonstrates that the activation of PPAR␥ inhibits osteocalcin expression both by suppressing the expression of Runx2 and by interfering with the transactivation ability of Runx2.Mesenchymal cells are able to differentiate into several distinct cell types, including osteoblasts and adipocytes (1-3). The mechanisms directing the cells along a particular lineage and the suppression of alternative pathways are not well established, although signals derived from the extracellular environment and several key transcription factors have been identified (4 -8).Peroxisome proliferator-activated receptors (PPARs) 1 are a family of ligand-activated transcription factors that belong to the nuclear hormone receptor superfamily (9, 10). PPAR␥ is abundantly expressed in both white and brown adipose tissue and has been known to play a critical role in the regulation of adipocyte differentiation (10). The transfection of fibroblastic cells with PPAR␥2 and its subsequent activation with ligand have been shown to be sufficient to initiate adipogenesis (11). Moreover, determined myoblasts with no inherent adipogenic potential can be induced to transdifferentiate into mature adipo...

Osteoblast-targeted overexpression of PPARγ inhibited bone mass gain in male mice and accelerated ovariectomy-induced bone loss in female mice

“…Although we have not corroborated the molecular mechanisms by which PPARg inhibits bone formation, the reduced expression of Runx2 and its target genes by PPARg supports our previous report that PPARg interacts with Runx2, thereby inhibiting its transcriptional activity as well as reducing the abundance of Runx2. (21) In addition, recent reports added a possibility that PPARg downregulates the expression of COX-2 and iNOS, which also can contribute to suppressed osteogenesis. (50) Although PPARg has been known to exert negative effects on bone formation with reciprocal enhancement of adipogenesis, we were not able to demonstrate an increase in bone marrow adiposity in Col.1-PPARg mice.…”

The Free Fatty Acid Receptor G Protein-coupled Receptor 40 (GPR40) Protects from Bone Loss through Inhibition of Osteoclast Differentiation*

“…Diascro et al were the first to demonstrate that a mixture of palmitic, oleic, and linoleic acids activates PPARs to drive adipocyte-like differentiation of both ROS17/2.8 and SaOS-2/B10 preosteoblast cell lines (7). At the cellular and molecular levels, PPAR␥ activation was shown to reduce expression and func-tion of Runx2, a transcription factor inducing osteoblastogenesis (8,9). These data support the notion that fatty acid intake may have a strong impact on bone metabolism.…”

A novel PPARγ2 modulator sLZIP controls the balance between adipogenesis and osteogenesis during mesenchymal stem cell differentiation

“…2 Runt-related transcription factor 2 (Runx2) enhances expression of osteogenic genes during osteoblast differentiation; however, PPARg 2 suppresses osteogenesis via inhibition of Runx2 transcriptional activity. 3 Therefore, characterization of the regulatory mechanism of transcription factors involved in MSC differentiation is important for understanding the developmental processes of adipocytes and osteoblasts.The nuclear receptor PPARg is a master regulator in adipogenesis, lipid biosynthesis, inflammation, and glucose metabolism. 2 Binding of PPARg to specific DNA sequences, including the PPAR response element (PPRE), requires heterodimerization with retinoic X receptor (RXR).…”

“…2 Runt-related transcription factor 2 (Runx2) enhances expression of osteogenic genes during osteoblast differentiation; however, PPARg 2 suppresses osteogenesis via inhibition of Runx2 transcriptional activity. 3 Therefore, characterization of the regulatory mechanism of transcription factors involved in MSC differentiation is important for understanding the developmental processes of adipocytes and osteoblasts.…”

“…17 Runx2 is expressed at the site of bone formation and is a key transcription factor of osteoblast differentiation. 3 Activation of PPARg 2 inhibits Runx2-mediated transcription, whereas PPARg 2 -deficient mice show increased bone mass via stimulation of osteoblastogenesis. 24 Clinical surveys indicate that the long-term use of the drug TZDs by diabetes patients leads to an increase in the fat ratio and to bone loss, and MSCs from osteoporosis patients are more differentiated into adipocytes than osteoblasts, compared with patients with normal bone mass.…”