Osteogenesis is regulated by multiple signaling pathways and kinases play a critical role in several of these signaling pathways. As an effort to uncover novel factors for bone mass regulation, we screened a human kinase siRNA library searching for kinases that are involved in osteogenic differentiation of mesenchymal stem cells. Small interfering RNA-mediated knockdown of alpha2 catalytic subunit of adenosine 5'- monophosphate-activated protein kinase (AMPK) in mesenchymal stem cells significantly increased activity of alkaline phosphatase, an early marker of osteogenic differentiation, suggesting that AMPKa2 may have a role in osteoblasts, probably in bone formation. AMPK is an energy sensor of mammalian cells because it directly responds to the depletion of the fuel molecule ATP. Although the roles of AMPK in various peripheral organs, including muscle, heart, and liver, have been extensively studied, implication of AMPK in bone homeostasis has not been described. Here, we propose to elucidate the role of AMPKa2 in bone homeostasis. First, we will characterize skeletal phenotype of AMPKa2 knockout mice by carrying out X-ray radiography, DXA, bone histomorphometry, and microcomputed tomography. Employing various biochemical and molecular biological approaches, including BrdU labeling, von Kossa staining, quantitative RT-PCR, TUNEL assay, and immunoblotting analyses, the role of AMPK in osteoblast proliferation, maturation, and apoptosis will also be investigated both in vivo and ex vivo. As bone is a dynamic organ that is susceptible to systemic and cellular energy balance, this study will be groundwork for the further elucidation of the association of energy homeostasis with bone metabolism.
Given that AMPK is a regulator of cellular and systemic energy homeostasis and that bone is a dynamic organ susceptible to energy imbalance, this study may offer new perspectives in the coordination of energy balance and bone physiology. Moreover, AMPK is one of the targets of drug discovery for diabetes and cancers. Diabetes is known to be associated with disturbed skeletal homeostasis and an increased risk for fracture. Malignant metastases to bone are a major cause of morbidity and mortality. Thus, elucidation of the role of AMPK in bone may provide important insights into new strategies to address skeletal disease.
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