title: Small GTPase Ras and Rho expression in rat osteoblasts during spaceflight.
abstract: Rat osteoblasts were cultured for 4 and 5 days aboard a space shuttle and solubilized after a 24-h treatment with 1alpha,25 dihydroxyvitamin D(3). The quantitative RT-PCR determined the mRNA levels of signaling molecules upstream and downstream Ras. The small GTPase is activated by guanine nucleotide exchange protein (GEF) and deactivated by GTPase-activating protein (GAP). When external stimuli are transduced into intracellular signals, various pathways are recruited: focal adhesion kinase (FAK) is associated with integrin-beta, and directs tyrosine phosphorylation of downstream substrates, including phospholipase C-gamma (PLC-gamma) and son of sevenless (SOS, a Ras GEF). The mRNA levels of FAK and PLC-gamma1 and -gamma2 in the flight cultures were increased 150% and 250% of the ground controls. The SOS mRNA levels in the flight cultures were increased 520% and 320% of the ground controls. Signals via G protein-coupled receptors are transmitted through PLC-beta and Ras GRF (another Ras GEF). Activated Ras then stimulates Raf, mitogen-activated protein kinase (MAPK) cascades. The mRNA levels of Raf, extracellular signal-regulated protein kinase of MAPK family (ERK-1 and -2), and PLC-beta were increased during spaceflight. Rho GAP expression in the flight cultures was increased twofold of the ground controls. Since Rho GAP deactivates Rho, microgravity may suppress Rho signals, regulating actin filament rearrangement. Microgravity signals may involve two pathways (G protein-coupled receptor-mediated pathway and tyrosine phosphorylation-mediated pathway) that activate Ras, Raf, and MAPK cascades in rat osteoblasts.
authors.affiliation: Biochemistry, Department of Hard Tissue Engineering, Graduate School of Tokyo Medical and Dental University, Tokyo 113-8549, Japan. kumei.bch@tmd.ac.jp
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Kumei
authors.forename: Hitoyata
authors.initials: H
authors.lastname: Shimokawa
authors.forename: Kei'ichi
authors.initials: K
authors.lastname: Ohya
authors.forename: Hisako
authors.initials: H
authors.lastname: Katano
authors.forename: Hideo
authors.initials: H
authors.lastname: Akiyama
authors.forename: Masahiko
authors.initials: M
authors.lastname: Hirano
authors.forename: Sadao
authors.initials: S
authors.lastname: Morita
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D010006:Osteoblasts / Q000201:enzymology*; D051381:Rats; D015398:Signal Transduction / Q000502:physiology; D013026:Space Flight; D014893:Weightlessness; D018631:ras Proteins / Q000096:biosynthesis* / Q000235:genetics*; D020741:rho GTP-Binding Proteins / Q000096:biosynthesis* / Q000235:genetics*
chemical_list: D018631:ras Proteins; D020741:rho GTP-Binding Proteins