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J. Biol. Chem. 286 (6): 4809-4818

© 2011 by The American Society for Biochemistry and Molecular Biology, Inc.

Endoplasmic Reticulum Stress Response Mediated by the PERK-eIF2α-ATF4 Pathway Is Involved in Osteoblast Differentiation Induced by BMP2*

Atsushi Saito{ddagger}, Kimiko Ochiai§, Shinichi Kondo{ddagger}, Kenji Tsumagari{ddagger}, Tomohiko Murakami§, Douglas R. Cavener, , and Kazunori Imaizumi{ddagger}1

From the {ddagger}Department of Biochemistry, Division of Genome Radiobiology and Medical Science, Graduate School of Biomedical Science, University of Hiroshima, Hiroshima 734-8553, Japan,
the §Division of Molecular and Cellular Biology, Department of Anatomy, Faculty of Medicine, University of Miyazaki, Miyazaki 889-1692, Japan, and
the Department of Biology, Pennsylvania State University, University Park, Pennsylvania 16802

ABSTRACT Back to Top

Abstract: To avoid excess accumulation of unfolded proteins in the endoplasmic reticulum (ER), eukaryotic cells have signaling pathways from the ER to the cytosol or nucleus. These processes are collectively termed the ER stress response. Double stranded RNA activated protein kinase (PKR)-like endoplasmic reticulum kinase (PERK) is a major transducer of the ER stress response and directly phosphorylates eIF2α, resulting in translational attenuation. Phosphorylated eIF2α specifically promotes the translation of the transcription factor ATF4. ATF4 plays important roles in osteoblast differentiation and bone formation. Perk–/– mice are reported to exhibit severe osteopenia, and the phenotypes observed in bone tissues are very similar to those of Atf4–/– mice. However, the involvement of the PERK-eIF2α-ATF4 signaling pathway in osteogenesis is unclear. Phosphorylated eIF2α and ATF4 protein levels were attenuated in Perk–/– calvariae, and the gene expression levels of osteocalcin (Ocn) and bone sialoprotein (Bsp), which are targets for ATF4, were also down-regulated. Treatment of wild-type primary osteoblasts with BMP2, which is required for osteoblast differentiation, induced ER stress, leading to an increase in ATF4 protein expression levels. In contrast, the level of ATF4 in Perk–/– osteoblasts was severely diminished. The results indicate that PERK signaling is required for ATF4 activation during osteoblast differentiation. Perk–/– osteoblasts exhibited decreased alkaline phosphatase activities and delayed mineralized nodule formation relative to wild-type cultures. These abnormalities were almost completely restored by the introduction of ATF4 into Perk–/– osteoblasts. Taken together, ER stress occurs during osteoblast differentiation and activates the PERK-eIF2α-ATF4 signaling pathway followed by the promotion of gene expression essential for osteogenesis, such as Ocn and Bsp.

Key Words: Bone • Differentiation • ER Stress • Transcription Factors • Transcription Target Genes • Osteoblast

Received for publication June 8, 2010. Revision received October 26, 2010.

Author's Choice—Final version full access.

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1 To whom correspondence should be addressed: 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8553, Japan. Tel.: 81-82-257-5130; Fax: 81-82-257-5134; E-mail: imaizumi{at}

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