Treating human MSCs with PTH inhibited adipocyte development induced by a specific cocktail of adipogenic inducers through the cAMP pathway. The control of MSC differentiation into adipocytes and osteoblasts is important in maintaining a normal physiologic bone mass, but the signaling pathways and molecular mechanisms that regulate this process are not well understood.Ĭyclic AMP/protein kinase A (PKA) signaling plays a prominent, but ambiguous, role in mesenchymal cell fate decision. Mice haploinsufficient for PPARγ or Alox15, the lipoxygenase that generates oxidized lipid ligands of PPARγ, have high bone mass phenotype. The number of osteoblasts is partially determined by the lineage specification process. Lineage specification is achieved by the expression of transcription factors Cbfa1/Runx2 and Osterix in the case of osteogenesis, and PPARγ in the case of adipogenesis. Mesenchymal stem cells are a pluripotent cell type that can differentiate into a variety of cell types, including adipocytes and osteoblasts. PTH-induced signaling via G s/cAMP pathway plays a particularly important role in skeletal function. PTH (1–34) was also able to regulate osteogenic development via cAMP signaling in a BMP-dependent mesenchymal differentiation system. Although PTH activates multiple signaling pathways, several lines of evidence indicate that PTH peptide fragments that specifically stimulate G s signaling coupled to PTH1R could increase bone formation. This receptor is coupled to multiple G proteins, including G s, G q, and possibly G i, resulting in the activation of diverse downstream effectors. The best-studied GPCR that regulates skeletal function is parathyroid hormone 1 receptor or PTH1R. Several extracellular regulators of skeletal function are known to exert their actions through GPCRs, which are located in the osteoblast membrane. Alteration of G s and G i signallings in osteoblast lineage cells was able to generate striking, but opposite effects on skeletal tissues. Furthermore, expression of an engineered constitutively active G s-coupled receptor under the control of the osteoblast-specific Col1α1 2.3-kb promoter fragment in mice produced cellular features similar to that of the polyostotic fibrous dysplasia of McCune–Albright’s syndrome. Targeted expression in osteoblasts of constitutively active G s-coupled G protein-coupled receptors (GPCRs) resulted in markedly increased trabecular bone mass, whereas expression of a constitutively-active G i-coupled receptor produced trabecular osteopenia. Conditional deletion of the α subunit of G s in osteoblast lineage cells resulted in reduced trabecular bone formation. The role of the G s/cAMP pathway in bone formation is further highlighted by several findings. Examination of the fibrotic area revealed an excess of cells with features of pre-osteoblasts, and the abnormal cellular features of fibrous dysplasia could be reproduced in vitro by the addition of excess exogenous cAMP to human osteogenic cells or by stable expression of constitutively active form of G sα in human BMSCs. In fibrous dysplasia, the expression of G s protein and its transcript is upregulated during the maturation of precursor osteogenic cells to mature osteoblasts. These mutations have been demonstrated in various tissues, including bones and samples from the monostotic form of fibrous dysplasia. Patients with McCune–Albright syndrome have activating missense mutations in GNAS, the gene encoding the α subunit of G s. Thus, activation of cAMP signaling alters the lineage commitment of MSCs, favoring osteogenesis at the expense of adipogenesis.įibrous dysplasia (FD, OMIM 174800) of bone is one of the manifestations of the McCune–Albright syndrome. PTH treatment of BMSCs also enhanced subsequent osteogenesis, but promoted an increased adipogenesis as well. Strikingly, forskolin activation of cAMP signaling in BMSCs conditioned mesenchymal stem cells (MSCs) to undergo increased osteogenic differentiation and decreased adipogenic differentiation. Exposure of differentiating osteoblasts to forskolin markedly inhibited progression to the late stages of osteoblast differentiation, and this effect was replicated by continuous exposure to PTH. To investigate this, we treated primary mouse bone marrow stromal cells (BMSCs) with forskolin to stimulate cAMP signaling and determined the effect on osteoblast and adipocyte differentiation. However, limited information is available on the role of the cAMP signaling in regulating the differentiation of mesenchymal stem cells into mature osteoblasts and adipocytes. Stimulatory G protein-mediated cAMP signaling is intimately involved in skeletal homeostasis.
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