T-F Li1, K Yukata2, G Yin3, T Sheu4, T Maruyama5, J H Jonason4, W Hsu5, X Zhang4, G Xiao6, Y T Konttinen7, D Chen6, R J O'Keefe8. 1. Department of Biochemistry, Rush University Medical Center, 1735 W. Harrison St, Chicago, IL 60612, USA; Department of Orthopaedics, Rush University Medical Center, 1611 W. Harrison St, Chicago, IL 60612, USA. Electronic address: tianfang_li@rush.edu. 2. Department of Orthopaedics, University of Tokushima, 3-18-15 Kuramoto, Tokushima 770-8503, Japan; Center for Musculoskeletal Research, Department of Orthopaedics, University of Rochester, 601 Elmwood Ave., NY 14642, USA. 3. Department of Orthopaedics, The First Affiliated Hospital, Nanjing Medical University, 300 Guangzhou Rd., Nanjing, Jiangsu 210029, China. 4. Center for Musculoskeletal Research, Department of Orthopaedics, University of Rochester, 601 Elmwood Ave., NY 14642, USA. 5. Department of Biomedical Genetics, Center for Oral Biology, and James P. Wilmot Cancer Center, University of Rochester, 601 Elmwood Ave., Rochester, NY 14642, USA. 6. Department of Biochemistry, Rush University Medical Center, 1735 W. Harrison St, Chicago, IL 60612, USA. 7. Department of Medicine, Institute of Clinical Medicine, University of Helsinki, PO Box 700 (Haartmaninkatu 8, Biomedicum 1), 00029 HUS, Finland. 8. Center for Musculoskeletal Research, Department of Orthopaedics, University of Rochester, 601 Elmwood Ave., NY 14642, USA. Electronic address: Regis_OKeefe@urmc.rochester.edu.
Abstract
OBJECTIVE: Bone morphogenic protein (BMP)-2 is approved for fracture non-union and spine fusion. We aimed to further dissect its downstream signaling events in chondrocytes with the ultimate goal to develop novel therapeutics that can mimic BMP-2 effect but have less complications. METHODS: BMP-2 effect on cyclooxygenase (COX)-2 expression was examined using Real time quantitative PCR (RT-PCR) and Western blot analysis. Genetic approach was used to identify the signaling pathway mediating the BMP-2 effect. Similarly, the pathway transducing the PGE2 effect on ATF4 was investigated. Immunoprecipitation (IP) was performed to assess the complex formation after PGE2 binding. RESULTS: BMP-2 increased COX-2 expression in primary mouse costosternal chondrocytes (PMCSC). The results from the C9 Tet-off system demonstrated that endogenous BMP-2 also upregulated COX-2 expression. Genetic approaches using PMCSC from ALK2(fx/fx), ALK3(fx/fx), ALK6(-/-), and Smad1(fx/fx) mice established that BMP-2 regulated COX-2 through activation of ALK3-Smad1 signaling. PGE-2 EIA showed that BMP-2 increased PGE2 production in PMCSC. ATF4 is a transcription factor that regulates bone formation. While PGE2 did not have significant effect on ATF4 expression, it induced ATF4 phosphorylation. In addition to stimulating COX-2 expression, BMP-2 also induced phosphorylation of ATF4. Using COX-2 deficient chondrocytes, we demonstrated that the BMP-2 effect on ATF4 was COX-2-dependent. Tibial fracture samples from COX-2(-/-) mice showed reduced phospho-ATF4 immunoreactivity compared to wild type (WT) ones. PGE2 mediated ATF4 phosphorylation involved signaling primarily through the EP2 and EP4 receptors and PGE2 induced an EP4-ERK1/2-RSK2 complex formation. CONCLUSIONS: BMP-2 regulates COX-2 expression through ALK3-Smad1 signaling, and PGE2 induces ATF4 phosphorylation via EP4-ERK1/2-RSK2 axis.
OBJECTIVE: Bone morphogenic protein (BMP)-2 is approved for fracture non-union and spine fusion. We aimed to further dissect its downstream signaling events in chondrocytes with the ultimate goal to develop novel therapeutics that can mimic BMP-2 effect but have less complications. METHODS: BMP-2 effect on cyclooxygenase (COX)-2 expression was examined using Real time quantitative PCR (RT-PCR) and Western blot analysis. Genetic approach was used to identify the signaling pathway mediating the BMP-2 effect. Similarly, the pathway transducing the PGE2 effect on ATF4 was investigated. Immunoprecipitation (IP) was performed to assess the complex formation after PGE2 binding. RESULTS: BMP-2 increased COX-2 expression in primary mouse costosternal chondrocytes (PMCSC). The results from the C9 Tet-off system demonstrated that endogenous BMP-2 also upregulated COX-2 expression. Genetic approaches using PMCSC from ALK2(fx/fx), ALK3(fx/fx), ALK6(-/-), and Smad1(fx/fx) mice established that BMP-2 regulated COX-2 through activation of ALK3-Smad1 signaling. PGE-2 EIA showed that BMP-2 increased PGE2 production in PMCSC. ATF4 is a transcription factor that regulates bone formation. While PGE2 did not have significant effect on ATF4 expression, it induced ATF4 phosphorylation. In addition to stimulating COX-2 expression, BMP-2 also induced phosphorylation of ATF4. Using COX-2 deficient chondrocytes, we demonstrated that the BMP-2 effect on ATF4 was COX-2-dependent. Tibial fracture samples from COX-2(-/-) mice showed reduced phospho-ATF4 immunoreactivity compared to wild type (WT) ones. PGE2 mediated ATF4 phosphorylation involved signaling primarily through the EP2 and EP4 receptors and PGE2 induced an EP4-ERK1/2-RSK2 complex formation. CONCLUSIONS: BMP-2 regulates COX-2 expression through ALK3-Smad1 signaling, and PGE2 induces ATF4 phosphorylation via EP4-ERK1/2-RSK2 axis.
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