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J. Biol. Chem., Vol. 280, Issue 8, 7317-7325, February 25, 2005

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Bisphosphonates and Estrogens Inhibit Osteocyte Apoptosis via Distinct Molecular Mechanisms Downstream of Extracellular Signal-regulated Kinase Activation^*

Lilian I. Plotkin, J. Ignacio Aguirre, Stavroula Kousteni, Stavros C. Manolagas, and Teresita Bellido

From the 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

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/EBP. Phosphorylation of BAD renders it inactive, whereas phosphorylation of C/EBP 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.

Received for publication, November 12, 2004

^* This research was supported by National Institutes of Health Grants KO2-AR02127 (to T. B.) and P01-AG13918 (to S. C. M.) and by the Department of Veterans Affairs Merit and Research Enhancement Award Program awards (to S. C. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed. Tel.: 501-686-8971; Fax: 501-686-8148; E-mail: tmbellido{at}uams.edu.

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