Bisphosphonates and estrogens inhibit osteocyte apoptosis via distinct molecular mechanisms downstream of extracellular signal-regulated kinase activation.
Both estrogens and bisphosphonates attenuate osteocyte apoptosis by activating the extracellular signal-regulated kinases (ERKs). However, whereas estrogens activate ERKs via an extranuclear function of the estrogen receptor, bisphosphonates do so by opening connexin 43 hemichannels. Here, we demonstrated that the signaling events downstream of ERKs induced by these two stimuli are also distinct. Inhibition of osteocyte apoptosis by estrogens requires nuclear accumulation of ERKs and activation of downstream transcription factors. On the other hand, anti-apoptosis induced by bisphosphonates requires neither transcription nor ERK-dependent transcription factors. Instead, the effect of bisphosphonates is abolished when ERKs are restricted to the nucleus by blocking CRM1/exportin1-mediated nuclear protein export or by expressing nuclear-anchored ERKs, but it is unaffected in cells expressing cytoplasmic-anchored ERKs. Connexin 43/ERK-mediated anti-apoptosis induced by bisphosphonates requires the kinase activity of the cytoplasmic target of ERKs, p90(RSK), which in turn phosphorylates the pro-apoptotic protein BAD and C/EBPbeta. Phosphorylation of BAD renders it inactive, whereas phosphorylation of C/EBPbeta leads to binding of pro-caspases, thus inhibiting apoptosis independently of the transcriptional activity of this transcription factor. Consistent with the evidence that estrogens and bisphosphonates phosphorylate diverse targets of ERKs, probably resulting from activation of spatially distinct pools of these kinases, the two agents had additive effects on osteocyte survival.
Plotkin
Lilian I
LI
Division of Endocrinology and Metabolism, the Center for Osteoporosis and Metabolic Bone Diseases, the Central Arkansas Veterans Affairs Healthcare System, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205-7199, USA.
Aguirre
J Ignacio
JI
Kousteni
Stavroula
S
Manolagas
Stavros C
SC
Bellido
Teresita
T
D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D001842:Bone and Bones / Q000166:cytology; D002460:Cell Line; D018031:Connexin 43 / Q000378:metabolism; D004164:Diphosphonates / Q000494:pharmacology*; D004357:Drug Synergism; D004967:Estrogens / Q000494:pharmacology*; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism*; D051379:Mice; D010011:Osteocytes / Q000166:cytology / Q000187:drug effects*; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000378:metabolism; D015398:Signal Transduction; D014157:Transcription Factors / Q000378:metabolism
D018031:Connexin 43; D004164:Diphosphonates; D004967:Estrogens; D014157:Transcription Factors; D038744:Ribosomal Protein S6 Kinases, 90-kDa; D048049:Extracellular Signal-Regulated MAP Kinases