Kinases in bone homeostasis: Studies on the roles of AMPKalpha2 and PI3Kgamma in bone homeostasis

AMPKα2 is a catalytic subunit of AMPK, which is a key regulator for cellular and whole body energy homeostasis. Bone is a dynamic organ and susceptible to metabolic changes. To assess the physiological role of AMPKα2 in bone homeostasis, we characterized bone phenotype of AMPKα2 KO mice. We found that AMPKα2 KO mice have lower bone mass than WT littermates and it was accounted for by the increased osteoclast development. The increased osteoclastogenesis of AMPKα2deficient macrophages in vitro was associated with the up-regulated expression of osteoclast-associated marker genes. To further elucidate how AMPKα2 modulates RANKL-mediated osteoclast formation, we examined global gene expression profiles of BMM-OCL via microarray analysis. Surprisingly, many of the genes that were up-regulated by AMPKα2 deficiency were associated with bone marrow stromal cells. Smooth muscle α-actin (SMAA), which was among the up-regulated genes in AMPKα2 KO, was utilized for our further investigation on the role of AMPKα2-deficient stromal cells in the increased osteoclastogenesis of AMPKα2-deficient BMM-OCL. Experiments with SMAA-GFP;AMPKα2 KO mice exhibited that the increased growth rate of AMPKα2-deficient stromal cells was a contributing factor for the elevated osteoclastogenesis in AMPKα2-deficent BMM-OCL. ^ G protein-coupled receptor-regulated PI3Kγ is abundantly expressed in myeloid cells and became a promising drug target to treat various inflammatory diseases. However, its role in bone homeostasis has not been documented. We therefore characterized bone phenotype of PI3Kγ-deficient mice and found that PI3Kγ-deficient mice had higher bone mass than WT littermates. Our analyses further revealed that PI3Kγ deficiency did not affect the bone formation because no significant changes in osteoblast number and bone formation rate were observed. Instead, the lack of PI3Kγ was associated with decreased bone resorption as evidenced by decreased osteoclast number in vivo and impaired osteoclast formation in vitro. The decreased osteoclast formation was accompanied by down-regulated expression of osteoclastogenic genes, compromised chemokine receptor signaling, and an increase in apoptosis during osteoclast differentiation. These data suggest that PI3Kγ regulates bone homeostasis by modulating osteoclastogenesis. Our study also suggests that inhibition of PI3Kγ, which is being considered as a potential therapeutic strategy for treating chronic inflammatory disorders, may result in an increase in bone mass.