Adipocytes decrease Runx2 expression in osteoblastic cells: Roles of PPARγ and adiponectin

Abstract

The mechanisms through which bone marrow adipocytes might influence differentiation and function of osteoblasts are not completely understood. To investigate the direct effects of bone marrow fat cells on osteoblast function, an ex vivo co-culture system was utilized comprising either primary fat cells or differentiated 3T3-L1 adipocytes and osteoblastic cells on transwells. In co-culture, both adipocytes and osteoblastic cells were differentiated into adipocytes or osteoblasts, respectively, before culturing on transwells. Co-culture with either primary fat cells or fully differentiated 3T3-L1 adipocytes significantly decreased mRNA and protein expression of runt-related transcription factor 2 (Runx2) in osteoblastic cells. An increase in mRNA and protein expression of peroxisome proliferator-activated receptor γ (PPARγ) occurred concomitantly with the reduction of Runx2 expression. Adiponectin concentration was increased in the media by co-culture. In addition, co-culture with conditioned media from fat cells increased PPARγ promoter activity and decreased Runx2 promoter activity. Knockdown of PPARγ or adiponectin receptor 1 in osteoblastic cells by siRNA prevented the down-regulation of mRNA expression of Runx2 in osteoblastic cells cultured with fully differentiated 3T3-L1 cells. Furthermore, co-transfection with PPARγ decreased Runx2 promoter activity. A marker of osteogenesis, alkaline phosphatase activity in osteoblastic cells was significantly decreased by co-culture. Annexin V/propidium iodide staining showed that co-culture did not induce apoptosis in osteoblastic cells. Thus, we conclude that adipocytes modulate key metabolic functions of osteoblasts through the release of secretory products. PPARγ plays a key role in mediating the effects of adipocytes on osteoblasts.

Characterization of age-related gene expression profiling in bone marrow and epididymal adipocytes

“…Some studies have suggested that the presence of adipocytes can influence differentiation of mesenchymal stem cells (MSCs) into adipocytes, thereby inhibiting the differentiation into other cell lines [13]. We and others reported that directly co-culturing bone marrow MSCs with fully differentially adipocytes decreased osteoblast differentiation by decreasing RunX2 mRNA expression [14,15], suggesting that these cells are metabolically active but negatively regulate differentiation of MSCs into osteoblasts. Recent studies have suggested that, in addition to adipose, liver and muscle tissue, the osteoblast is also an important target tissue for insulin action [16,17].…”

Cultured Human Periosteum-Derived Cells Can Differentiate into Osteoblasts in a Perioxisome Proliferator-Activated Receptor Gamma-Mediated Fashion via Bone Morphogenetic Protein signaling

“…The reciprocal relationship between bone adiposity and bone mineral density has long been recognized. PPARγ is a prime regulator of adipogenesis and as such, the activation of PPARγ in precursors of nonadipogenic lineage cells triggers their transdifferentiation into adipocytes and shifts the bone marrow cell fate decision towards the adipogenesis 4,26,30. However, the effects of PPARγ in ligands on the differentiation of cultured osteoprogenitor cells remain controversial 10-12.…”

Cultured Human Periosteal-Derived Cells Have Inducible Adipogenic Activity and Can Also Differentiate Into Osteoblasts in a Perioxisome Proliferator-Activated Receptor-Mediated Fashion

“…Recent studies have shown that PPARγ activity directly inhibits osteogenesis by diverting mesenchymal stem cells from the osteogenic phenotype to the adipogenic lineage. This shift in mesenchymal stem cell differentiation to favor the adipocyte lineage over the osteoblast lineage directly contributes to imbalances in bone formation and resorption, and ultimately leads to bone loss 8,10,17,18.…”