Department of Oral Growth and Developmental Biology, Hiroshima University Graduate School of Biomedical Sciences, Hiroshima, Japan.
PLoS One. 2010 Jul 26;5(7):e11782. doi: 10.1371/journal.pone.0011782.
Understanding 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)gamma.
METHODOLOGY/PRINCIPAL FINDINGS: Multiple 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 PPARgamma. 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 PPARgamma 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), PPARgamma, 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 PPARgamma and Runx2 nuclear labeling with concomitant upregulation of expression of their target genes with BRL treatment.
CONCLUSIONS/SIGNIFICANCE: We conclude that not only immature but a subset of relatively mature ObL cells characterized by relatively high levels of endogenous PPARgamma 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.
了解成骨细胞谱系（ObL）和脂肪细胞谱系（AdL）之间的命运选择和命运转换对于理解成骨细胞和脂肪细胞之间的发育相互关系以及正常衰老和某些疾病中两种谱系之间命运分配的变化的影响非常重要。本研究的目的是确定在谱系进展过程中，ObL 细胞何时通过激活内源性过氧化物酶体增殖物激活受体（PPAR）γ而容易发生 AdL 命运转换。
方法/主要发现：通过碱性磷酸酶表达的基础上的分级分离或单个细胞衍生的菌落的回顾性鉴定，从多个大鼠颅骨细胞中分离出处于 ObL 发育层次结构中的细胞，并使用 BRL-49653（BRL）处理，这是一种 PPARγ的合成配体。当存在 BRL 时，约 30％的总单细胞衍生菌落表达脂肪生成潜能（通过细胞化学定义）。对超过 160 个菌落的扩增 cRNA 进行 ObL 和 AdL 标记的 qRT-PCR 分析显示，BRL 依赖性脂肪生成潜能与内源性 PPARγ mRNA 水平相关。出乎意料的是，相当一部分相对成熟的 ObL 细胞表现出成骨-脂肪多能性。Western blot 和免疫细胞化学证实 ObL 细胞共表达多种间充质谱系决定因子（ runt 相关转录因子 2（Runx2），PPARγ，Sox9 和 MyoD 最初定位于细胞质中，并且仅在 ObL 进展过程中 Runx2 易位到细胞核。值得注意的是，然而，一些细胞表现出 PPARγ和 Runx2 的核标记，同时伴随着 BRL 处理时其靶基因的表达上调。
结论/意义：我们得出结论，不仅是不成熟的，而且是具有相对高水平内源性 PPARγ表达的相对成熟的 ObL 细胞的亚群可以被转换为 AdL。事实上，一些 ObL 细胞即使在相对成熟的发育阶段也保持脂肪生成命运选择的能力，这意味着具有重要意义的正常和病理性骨发育的意外可塑性。
