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Regulatory actions of 3',5'-cyclic adenosine monophosphate on osteoclast function: possible roles of Epac-mediated signaling.

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Jeevaratnam, Kamalan 
Salvage, Samantha C 
Li, Mengye 
Huang, Christopher L-H 


Alterations in cellular levels of the second messenger 3',5'-cyclic adenosine monophosphate ([cAMP]i ) regulate a wide range of physiologically important cellular signaling processes in numerous cell types. Osteoclasts are terminally differentiated, multinucleated cells specialized for bone resorption. Their systemic regulator, calcitonin, triggers morphometrically and pharmacologically distinct retraction (R) and quiescence (Q) effects on cell-spread area and protrusion-retraction motility, respectively, paralleling its inhibition of bone resorption. Q effects were reproduced by cholera toxin-mediated Gs -protein activation known to increase [cAMP]i , unaccompanied by the [Ca2+ ]i changes contrastingly associated with R effects. We explore a hypothesis implicating cAMP signaling involving guanine nucleotide-exchange activation of the small GTPase Ras-proximate-1 (Rap1) by exchange proteins directly activated by cAMP (Epac). Rap1 activates integrin clustering, cell adhesion to bone matrix, associated cytoskeletal modifications and signaling processes, and transmembrane transduction functions. Epac activation enhanced, whereas Epac inhibition or shRNA-mediated knockdown compromised, the appearance of markers for osteoclast differentiation and motility following stimulation by receptor activator of nuclear factor kappa-Β ligand (RANKL). Deficiencies in talin and Rap1 compromised in vivo bone resorption, producing osteopetrotic phenotypes in genetically modified murine models. Translational implications of an Epac-Rap1 signaling hypothesis in relationship to N-bisphosphonate actions on prenylation and membrane localization of small GTPases are discussed.



Epac, Rap1, cAMP, integrins, osteoclast, Animals, Bone Density Conservation Agents, Bone Resorption, Calcitonin, Calcium Signaling, Cell Movement, Cyclic AMP, Cyclic AMP-Dependent Protein Kinases, Diphosphonates, Guanine Nucleotide Exchange Factors, Humans, Integrins, Models, Biological, Osteoclasts, Osteoporosis, Second Messenger Systems, Signal Transduction, Translational Research, Biomedical, rap1 GTP-Binding Proteins

Journal Title

Ann N Y Acad Sci

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Medical Research Council (MR/M001288/1)
British Heart Foundation (None)
Medical Research Council