title: Hypoxia suppresses runx2 independent of modeled microgravity.
abstract: Bone loss is a consequence of skeletal unloading as seen in bed rest and space flight. Unloading decreases oxygenation and osteoblast differentiation/function in bone. Previously we demonstrated that simulation of unloading in vitro, by culturing differentiating mouse osteoblasts in a horizontal rotating wall vessel (RWV), results in suppressed expression of runx2, a master transcriptional regulator of osteoblast differentiation. However, the RWV is able to reproduce in a controlled fashion at least two aspects of disuse that are directly linked, model microgravity and hypoxia. Hypoxia in the RWV is indicated by reduced medium oxygen tension and increased expression of GAPDH and VEGF. To uncouple the role of model microgravity from hypoxia in suppressed runx2 expression, we cultured osteoblasts under modeled microgravity (oxygenated, horizontal RWV rotation), hypoxia (vertical RWV rotation), or both conditions (horizontal RWV rotation). The expression, DNA binding activity and promoter activity of runx2, was suppressed under hypoxic but not normoxic modeled microgravity RWV conditions. Consistent with a role for hypoxia in suppression of runx2, direct exposure to hypoxia alone is sufficient to suppress runx2 expression in osteoblasts grown in standard tissue culture plates. Taken together, our findings indicate that hypoxia associated with skeletal unloading could be major suppressor of runx2 expression leading to suppressed osteoblast differentiation and bone formation.
authors.affiliation: Molecular Imaging Research Center, Departments of Physiology and Radiology, Michigan State University, East Lansing, Michigan 48824, USA.
authors.forename: Christopher
authors.initials: C
authors.lastname: Ontiveros
authors.forename: Regina
authors.initials: R
authors.lastname: Irwin
authors.forename: Robert W
authors.initials: RW
authors.lastname: Wiseman
authors.forename: Laura R
authors.initials: LR
authors.lastname: McCabe
mesh_terms: D016475:3T3 Cells; D000818:Animals; D018929:Cell Culture Techniques / Q000379:methods; D015687:Cell Hypoxia; D002999:Clone Cells; D050659:Core Binding Factor Alpha 1 Subunit; D005786:Gene Expression Regulation; D018470:Gravity, Altered; D051379:Mice; D009363:Neoplasm Proteins / Q000235:genetics / Q000378:metabolism*; D010006:Osteoblasts / Q000166:cytology / Q000502:physiology*; D010744:Phosphoric Monoester Hydrolases / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013997:Time Factors; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D015854:Up-Regulation; D042461:Vascular Endothelial Growth Factor A / Q000235:genetics / Q000378:metabolism; D018474:Weightlessness Simulation
chemical_list: D050659:Core Binding Factor Alpha 1 Subunit; D009363:Neoplasm Proteins; D012333:RNA, Messenger; D014157:Transcription Factors; D042461:Vascular Endothelial Growth Factor A; C055686:glyceraldehyde 3-phosphate phosphatase; D010744:Phosphoric Monoester Hydrolases