Osteoblast proliferation or differentiation is regulated by relative strengths of opposing signaling pathways

Abstract

Skeletal development requires the correct balance of osteoblast proliferation, survival, and differentiation which is modulated by a network of signaling pathways and transcription factors. We have examined the role of the AKT (PKB), and ERK1/2 signaling pathways in the osteoblast response to FGFs, which inhibit differentiation, and to IGF-1 and Wnt signaling, which promote it. Using osteoblastic cell lines as well as primary calvarial osteoblasts, we show that ERK1/2 and AKT have distinct effects in FGF-induced osteoblast proliferation and differentiation. ERK1/2 is a primary mediator of FGF-induced proliferation, but also contributes to osteoblast differentiation, while AKT is important for osteoblast survival. Signaling by IGF-1, that promotes osteoblast differentiation, strongly activates AKT and weakly ERK1/2, while the opposite results are obtained with FGF, which inhibits differentiation. By introducing a constitutively active form of AKT, we found that increased AKT activity drives osteoblasts to differentiation. Increasing the AKT signal in osteoblasts that harbor FGFR2 activating mutations, found in Crouzon (342Y) and Apert (S22W) syndromes, is also able to drive differentiation in these cells, that normally fail to differentiate. Wnt signals, that promotes differentiation, also induce AKT phosphorylation, and cells expressing active AKT have increased levels of stabilized beta-catenin, a central molecule in Wnt signaling. Our results indicate that the relative strengths of ERK and AKT signaling pathways determine whether osteoblasts are driven into proliferation or differentiation, and that the effects of AKT may be due, in part, to synergy with the Wnt pathway as well as with the Runx2 transcription factor.

The antitumor properties of a nontoxic, nitric oxide–modified version of saquinavir are independent of Akt

“…In contrast, treatment of B16 cells with Saq-NO induced only a transient activation of Akt. A similar pattern activation of Akt has been observed during differentiation of endothelial cells, osteoblasts, and myofibroblasts (36)(37)(38). Interestingly, cotreatment with an Akt inhibitor or an inhibitor of upstream PI3K resulted in a further decrease in cell viability.…”

Akt promotes BMP2-mediated osteoblast differentiation and bone development

“…This initiates a series of protein-protein interactions that lead to activation of intracellular signal transduction pathways (Nakae et al, 2001). Although several signaling pathways mediate IGF action in bone, as well as in other tissues (Giustina et al, 2008), a growing literature supports the idea that the PI3-kinase-Akt network is critical for both osteoblast differentiation and bone growth (Fujita et al, 2004;Ghosh-Choudhury et al, 2002;Liu et al, 2007;Osyczka and Leboy, 2005;Peng et al, 2003;Raucci et al, 2008), yet the biochemical or molecular mechanisms through which the IGF-stimulated PI3-kinase-Akt pathway increases osteoblast development and function have not been elucidated. Fujita and colleagues have postulated an interaction with Runx2, because the PI3-kinase inhibitor LY294002 reduced both its DNA-binding activity and its ability to stimulate target gene transcription (Fujita et al, 2004).…”

Mechanotransduction and Osteogenesis

“…FAK interacts with integrins and cytoskeleton components (actin, paxillin, talin and vinculin) -which also interact with the nucleus -and signal cellular mechanical deformation. Hughes-Fulford, 2004;Kawamura et al, 2007;Liedbert et al, 2006;Mitra et al, 2005;Ogasawara et al, 2001;Raucci et al, 2008;Schlaepfer et al, 1998;Tang et al, 2006) (Fig.7 and Fig. 9).…”