A Subset of Osteoblasts Expressing High Endogenous Levels of PPARγ Switches Fate to Adipocytes in the Rat Calvaria Cell Culture Model

Abstract

BackgroundUnderstanding fate choice and fate switching between the osteoblast lineage (ObL) and adipocyte lineage (AdL) is important to understand both the developmental inter-relationships between osteoblasts and adipocytes and the impact of changes in fate allocation between the two lineages in normal aging and certain diseases. The goal of this study was to determine when during lineage progression ObL cells are susceptible to an AdL fate switch by activation of endogenous peroxisome proliferator-activated receptor (PPAR)γ.Methodology/Principal FindingsMultiple rat calvaria cells within the ObL developmental hierarchy were isolated by either fractionation on the basis of expression of alkaline phosphatase or retrospective identification of single cell-derived colonies, and treated with BRL-49653 (BRL), a synthetic ligand for PPARγ. About 30% of the total single cell-derived colonies expressed adipogenic potential (defined cytochemically) when BRL was present. Profiling of ObL and AdL markers by qRT-PCR on amplified cRNA from over 160 colonies revealed that BRL-dependent adipogenic potential correlated with endogenous PPARγ mRNA levels. Unexpectedly, a significant subset of relatively mature ObL cells exhibited osteo-adipogenic bipotentiality. Western blotting and immunocytochemistry confirmed that ObL cells co-expressed multiple mesenchymal lineage determinants (runt-related transcription factor 2 (Runx2), PPARγ, Sox9 and MyoD which localized in the cytoplasm initially, and only Runx2 translocated to the nucleus during ObL progression. Notably, however, some cells exhibited both PPARγ and Runx2 nuclear labeling with concomitant upregulation of expression of their target genes with BRL treatment.Conclusions/SignificanceWe conclude that not only immature but a subset of relatively mature ObL cells characterized by relatively high levels of endogenous PPARγ expression can be switched to the AdL. The fact that some ObL cells maintain capacity for adipogenic fate selection even at relatively mature developmental stages implies an unexpected plasticity with important implications in normal and pathological bone development.

Ectonucleotide Pyrophosphatase/Phosphodiesterase-1 (ENPP1) Protein Regulates Osteoblast Differentiation

“…Mouse embryonic fibroblasts from ENPP1 Ϫ/Ϫ mice also show enhanced expression of adipocyte marker gene expression. Osteoblasts and adipocytes derive from the same mesenchymal cell lineage, and some mature osteoblasts may even maintain adipocytic potential (53). That ENPP1 functions in mesenchymal precursor cells to control adipocytic versus osteoblastic differentiation is an intriguing idea.…”

The Role of Bone Marrow Microenvironment in Governing the Balance between Osteoblastogenesis and Adipogenesis

“…Upon ligand activation, co-repressors are degraded while co-activators are recruited to PPARγ, which then induces target gene expression and shifts the differentiation program to favor adipocytes [103]. Furthermore, activation of PPARγ during osteoblastogenesis inhibits the transcriptional activity of Runx2 and turns osteoblasts into adipocytes [104][105][106]. However, these observations were only made in vitro.…”

Osteoblast versus Adipocyte: Bone Marrow Microenvironment-Guided Epigenetic Control

“…Upon ligand activation, corepressors are degraded while coactivators are recruited to PPARγ, which then induces target gene expression through binding to PPAR response elements as a heterodimer with retinoid X receptor (RXR) [104]. Activation of PPARγ during osteoblastogenesis inhibits the transcriptional activity of Runx2 and turns osteoblasts into adipocytes [107-109] by mechanisms that might include epigenetic modulation. Like ERs, most PPARγ cofactors belong to epigenetic modification enzymes.…”