Endoplasmic reticulum stress response mediated by the PERK-eIF2(alpha)-ATF4 pathway is involved in osteoblast differentiation induced by BMP2.

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.

Effects of the bisphosphonate risedronate on osteopenia in OASIS-deficient mice.

Endoplasmic reticulum (ER) stress has been reported to be linked to various diseases such as diabetes, neurodegenerative diseases, and osteogenesis imperfecta (OI). Old astrocyte specifically induced substance (OASIS), a novel type of ER stress transducer, is a basic leucine zipper transcription factor belonging to the CREB/ATF family and is markedly expressed in osteoblasts. Recently, we demonstrated that OASIS activates the transcription of the gene for type I collagen, Col1a1, and contributes to the secretion of bone matrix proteins in osteoblasts. OASIS-/- mice exhibit severe osteopenia involving a decrease in type I collagen in the bone matrix and a dysfunction of osteoblasts, which show abnormal expansion of the rough ER. These phenotypic features of osteopenia are similar to those observed in OI type I. In this study, we investigated whether administration of the third-generation bisphosphonate risedronate (RIS) is effective for treating osteopenia in OASIS-/- mice. Histological and histomorphometric analyses revealed that the trabecular bones increased dramatically in OASIS-/- mice treated with RIS, owing to the inhibition of bone resorption. Intriguingly, the abnormal expansion of the rough ER in OASIS-/- osteoblasts was improved by the treatment with RIS. Taken together, we conclude that OASIS-/- mice will be useful as new model mice for evaluating the medicinal effects of osteopenia treatments and developing new drugs for the osteopenia associated with diseases such as OI and osteoporosis.

Signalling mediated by the endoplasmic reticulum stress transducer OASIS is involved in bone formation.

Eukaryotic cells have signalling pathways from the endoplasmic reticulum (ER) to cytosol and nuclei, to avoid excess accumulation of unfolded proteins in the ER. We previously identified a new type of ER stress transducer, OASIS, a bZIP (basic leucine zipper) transcription factor, which is a member of the CREB/ATF family and has a transmembrane domain. OASIS is processed by regulated intramembrane proteolysis (RIP) in response to ER stress, and is highly expressed in osteoblasts. OASIS(-/-) mice exhibited severe osteopenia, involving a decrease in type I collagen in the bone matrix and a decline in the activity of osteoblasts, which showed abnormally expanded rough ER, containing of a large amount of bone matrix proteins. Here we identify the gene for type 1 collagen, Col1a1, as a target of OASIS, and demonstrate that OASIS activates the transcription of Col1a1 through an unfolded protein response element (UPRE)-like sequence in the osteoblast-specific Col1a1 promoter region. Moreover, expression of OASIS in osteoblasts is induced by BMP2 (bone morphogenetic protein 2), the signalling of which is required for bone formation. Additionally, RIP of OASIS is accelerated by BMP2 signalling, which causes mild ER stress. Our studies show that OASIS is critical for bone formation through the transcription of Col1a1 and the secretion of bone matrix proteins, and they reveal a new mechanism by which ER stress-induced signalling mediates bone formation.