Mechanical induction of PGE2 in osteocytes blocks glucocorticoid-induced apoptosis through both the β-catenin and PKA pathways.
Glucocorticoids are known to induce osteocyte apoptosis, whereas mechanical loading has been shown to sustain osteocyte viability. Here we show that mechanical loading in the form of fluid-flow shear stress blocks dexamethasone-induced apoptosis of osteocyte-like cells (MLO-Y4). Prostaglandin E(2) (PGE(2) ), a rapidly induced signaling molecule produced by osteocytes, was shown to be protective against dexamethasone-induced apoptosis, whereas indomethacin reversed the antiapoptotic effects of shear stress. This protective effect of shear stress was mediated through EP2 and EP4 receptors, leading to activation of the cAMP/protein kinase A signaling pathway. Activation of phosphatidylinositol 3-kinase, an inhibitor of glycogen synthesis kinase 3, also occurred, leading to the nuclear translocation of β-catenin, an important signal transducer of the Wnt signaling pathway. Both shear stress and prostaglandin increased the phosphorylation of glycogen synthesis kinase 3 α/β. Lithium chloride, an activator of the Wnt pathway, also was protective against glucocorticoid-induced apoptosis. Whereas it is known that mechanical loading increases cyclooxygenase-2 and EP2 receptor expression and prostaglandin production, dexamethasone was shown to inhibit expression of these components of the prostaglandin pathway and to reduce β-catenin protein expression. β-catenin siRNA knockdown experiments abrogated the protective effects of PGE(2), confirming the central role of β-catenin in mediating the protection against dexamethasone-induced cell death. Our data support a central role for PGE(2) acting through the cAMP/PKA and β-catenin signaling pathways in the protection of osteocyte apoptosis by fluid-flow shear stress.
Kitase
Yukiko
Y
Department of Oral Biology, School of Dentistry, University of Missouri at Kansas City, Kansas City, MO, USA.
Barragan
Leonardo
L
Qing
Hai
H
Kondoh
Shino
S
Jiang
Jean X
JX
Johnson
Mark L
ML
Bonewald
Lynda F
LF
D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D017077:Culture Media, Conditioned / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D003907:Dexamethasone / Q000494:pharmacology; D015232:Dinoprostone / Q000378:metabolism*; D005938:Glucocorticoids / Q000494:pharmacology*; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism; D000071679:Glycogen Synthase Kinase 3 beta; D007213:Indomethacin / Q000494:pharmacology; D018021:Lithium Chloride / Q000494:pharmacology; D051379:Mice; D008954:Models, Biological; D010011:Osteocytes / Q000166:cytology* / Q000187:drug effects / Q000201:enzymology; D058539:Phosphatidylinositol 3-Kinase; D020011:Protective Agents / Q000494:pharmacology; D055550:Protein Stability / Q000187:drug effects; D021381:Protein Transport / Q000187:drug effects; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D058307:Receptors, Prostaglandin E, EP2 Subtype / Q000378:metabolism; D058309:Receptors, Prostaglandin E, EP4 Subtype / Q000378:metabolism; D012212:Rheology / Q000187:drug effects; D015398:Signal Transduction / Q000187:drug effects; D013314:Stress, Mechanical; D051176:beta Catenin / Q000378:metabolism*
D017077:Culture Media, Conditioned; D005938:Glucocorticoids; D020011:Protective Agents; D058307:Receptors, Prostaglandin E, EP2 Subtype; D058309:Receptors, Prostaglandin E, EP4 Subtype; D051176:beta Catenin; D003907:Dexamethasone; D058539:Phosphatidylinositol 3-Kinase; D000071679:Glycogen Synthase Kinase 3 beta; D051057:Proto-Oncogene Proteins c-akt; D017868:Cyclic AMP-Dependent Protein Kinases; D038362:Glycogen Synthase Kinase 3; D018021:Lithium Chloride; D015232:Dinoprostone; D007213:Indomethacin