Glucocorticoids and tumor necrosis factor α increase oxidative stress and suppress Wnt protein signaling in osteoblasts.
Endogenous glucocorticoids (GCs) and inflammatory cytokines contribute to the age-associated loss of bone mass and strength, but the molecular mechanisms responsible for their deleterious effects on the aging skeleton are unclear. Based on evidence that oxidative stress is a causal mechanism of the insulin resistance produced by either one of these two agents, we tested the hypothesis that their adverse skeletal effects also result from increased oxidative stress. We report that administration of prednisolone to mice increased reactive oxygen species (ROS) and the phosphorylation of p66(shc) (an amplifier of H(2)O(2) generation in mitochondria) in bone. Dexamethasone (Dex) and TNFα had a similar effect on osteoblastic cells in vitro. The generation of ROS by Dex and TNFα required PKCβ/p66(shc) signaling and was responsible for the activation of JNK and induction of apoptosis by both agents. The activity of Forkhead box O (FoxO) transcription factors was also increased in response to ROS; however, FoxO activation opposed apoptosis induced by Dex and TNFα. In addition, both agents suppressed Akt phosphorylation as well as Wnt-induced proliferation and osteoblast differentiation. However, the inhibitory actions on Wnt signaling were independent of PKCβ/p66(shc). Instead, they were mediated by inhibition of Akt and stimulation of FoxOs. These results demonstrate that ROS-induced activation of a PKCβ/p66(shc)/JNK signaling cascade is responsible for the pro-apoptotic effects of Dex and TNFα on osteoblastic cells. Moreover, modulation of Akt and FoxOs by GCs and TNFα are cell-autonomous mechanisms of Wnt/β-catenin antagonism contributing to the adverse effects of GC excess and inflammatory cytokines on bone alike.
Division of Endocrinology and Metabolism, Center for Osteoporosis and Metabolic Bone Diseases, University of Arkansas for Medical Sciences and the Central Arkansas Veterans Health Care System, Little Rock, Arkansas 72205, USA.
Maria
M
Almeida
Li
L
Han
Elena
E
Ambrogini
Robert S
RS
Weinstein
Stavros C
SC
Manolagas
D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology; D002454:Cell Differentiation / Q000187:drug effects / Q000502:physiology; D049109:Cell Proliferation / Q000187:drug effects; D003907:Dexamethasone / Q000494:pharmacology*; D005260:Female; D051858:Forkhead Transcription Factors / Q000235:genetics / Q000378:metabolism; D005938:Glucocorticoids / Q000494:pharmacology*; D048670:MAP Kinase Kinase 4 / Q000235:genetics / Q000378:metabolism; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D010006:Osteoblasts / Q000166:cytology / Q000378:metabolism*; D018384:Oxidative Stress / Q000187:drug effects* / Q000502:physiology; D011239:Prednisolone / Q000494:pharmacology*; D051057:Proto-Oncogene Proteins c-akt / Q000235:genetics / Q000378:metabolism; D017382:Reactive Oxygen Species / Q000378:metabolism; D055765:Shc Signaling Adaptor Proteins / Q000235:genetics / Q000378:metabolism; D000071425:Src Homology 2 Domain-Containing, Transforming Protein 1; D014409:Tumor Necrosis Factor-alpha / Q000235:genetics / Q000378:metabolism*; D051153:Wnt Proteins / Q000235:genetics / Q000378:metabolism; D060449:Wnt Signaling Pathway / Q000187:drug effects* / Q000502:physiology; D051176:beta Catenin / Q000235:genetics / Q000378:metabolism
D051858:Forkhead Transcription Factors; D005938:Glucocorticoids; D017382:Reactive Oxygen Species; D055765:Shc Signaling Adaptor Proteins; C528034:Shc1 protein, mouse; D000071425:Src Homology 2 Domain-Containing, Transforming Protein 1; D014409:Tumor Necrosis Factor-alpha; D051153:Wnt Proteins; D051176:beta Catenin; D003907:Dexamethasone; D011239:Prednisolone; D051057:Proto-Oncogene Proteins c-akt; D048670:MAP Kinase Kinase 4