Research Article 1465Introduction A reciprocal interaction between bone and energy metabolism has been described, whereby a hormone secreted by adipocytes influences bone formation and a factor produced by osteoblasts regulates fat metabolism (Lieben et al., 2009;Rosen, 2008). The crucial factor in this systemic loop is leptin, because its deficiency causes obesity and increases bone formation in the mouse (Ducy et al., 2000). Leptin is produced by white adipocytes and acts via the hypothalamus to regulate appetite and to favor energy expenditure. Bone formation is also negatively regulated by leptin through a second hypothalamic pathway, the -adrenergic sympathetic nervous system (Takeda et al., 2002). This pathway increases ATF-4-dependent expression of Esp (protein tyrosine phosphatase, receptor type, V; Ptprv) in osteoblasts, which leads to an inhibition of osteocalcin bioactivity. By contrast, insulin signaling in osteoblasts promotes the production of bioactive osteocalcin via acidification of the extracellular bone matrix as a consequence of increased bone resorption by osteoclasts (Ferron et al., 2010;Fulzele et al., 2010). In turn, osteocalcin, a hormone secreted by osteoblasts, modulates fat metabolism via the stimulation of pancreatic -cell proliferation and insulin secretion and thus, can indirectly, via adiponectin, lower insulin resistance (Hinoi et al., 2008;Yoshizawa et al., 2009). Thus, a common neuroendocrine systemic co-regulation of bone and adipose mass is established.In addition to this systemic regulation of bone and fat metabolism, a local control of cell fates balancing osteoblast and adipocyte differentiation, which is still poorly understood, must exist to integrate the systemic messages. Indeed, osteoblasts share with adipocytes a common mesenchymal progenitor, the mesenchymal stromal or stem cell (MSC) from which also arise other mesenchymal cell lineages such as chondrocytes, fibroblasts and myoblasts (Caplan, 2007). Mesenchymal cell fate decisions are driven by key transcription factors that confer identity to the cell. The major transcription factors regulating MSC differentiation to osteoblasts are -catenin and Runx2, both of which are required for the differentiation to pre-osteoblasts and osterix that drives osteoblast maturation (Karsenty, 2008;Komori, 2006). Similarly, adipocyte differentiation occurs first by activation of C/EBP and C/EBP, resulting in expression of PPAR2 and C/EBP, which then regulate late stages of adipogenesis (Lefterova and Lazar, 2009). Furthermore, a number of additional factors such as bone morphogenic proteins (BMPs) and signaling, for instance through the WNT pathway, have been described to regulate cell fate decisions between osteoblasts and adipocytes by promoting commitment or differentiation into one lineage at the expense of the other (Takada et al., 2007). All these observations strongly argue in favor of a cell-autonomous locally controlled relationship between osteoblastogenesis and adipogenesis.
SummaryA shift from osteoblastogenesis...