title: RhoA GTPase interacts with beta-catenin signaling in clinorotated osteoblasts.
abstract: Bone is a dynamic tissue under constant remodeling in response to various signals including mechanical loading. A lack of proper mechanical loading induces disuse osteoporosis that reduces bone mass and structural integrity. The β-catenin signaling together with a network of GTPases is known to play a primary role in load-driven bone formation, but little is known about potential interactions of β-catenin signaling and GTPases in bone loss. In this study, we addressed a question: Does unloading suppress an activation level of RhoA GTPase and β-catenin signaling in osteoblasts? If yes, what is the role of RhoA GTPase and actin filaments in osteoblasts in regulating β-catenin signaling? Using a fluorescence resonance energy transfer (FRET) technique with a biosensor for RhoA together with a fluorescent T cell factor/lymphoid enhancer factor (TCF/LEF) reporter, we examined the effects of clinostat-driven simulated unloading. The results revealed that both RhoA activity and TCF/LEF activity were downregulated by unloading. Reduction in RhoA activity was correlated to a decrease in cytoskeletal organization of actin filaments. Inhibition of β-catenin signaling blocked unloading-induced RhoA suppression, and dominant negative RhoA inhibited TCF/LEF suppression. On the other hand, a constitutively active RhoA enhanced unloading-induced reduction of TCF/LEF activity. The TCF/LEF suppression by unloading was enhanced by co-culture with osteocytes, but it was independent on the organization of actin filaments, myosin II activity, or a myosin light chain kinase. Collectively, the results suggest that β-catenin signaling is required for unloading-driven regulation of RhoA, and RhoA, but not actin cytoskeleton or intracellular tension, mediates the responsiveness of β-catenin signaling to unloading.
authors.affiliation: Department of Biomedical Engineering, Indiana University-Purdue University Indianapolis, 723 West Michigan Street, SL220G, Indianapolis, IN, 46202, USA.
authors.forename: Qiaoqiao
authors.initials: Q
authors.lastname: Wan
authors.forename: Eunhye
authors.initials: E
authors.lastname: Cho
authors.forename: Hiroki
authors.initials: H
authors.lastname: Yokota
authors.forename: Sungsoo
authors.initials: S
authors.lastname: Na
mesh_terms: D000818:Animals; D002460:Cell Line; D031541:Fluorescence Resonance Energy Transfer; D040542:Mechanotransduction, Cellular / Q000502:physiology; D051379:Mice; D010006:Osteoblasts / Q000378:metabolism* / Q000502:physiology; D033081:Shear Strength / Q000502:physiology; D015398:Signal Transduction; D051176:beta Catenin / Q000378:metabolism*; D020742:rhoA GTP-Binding Protein / Q000378:metabolism*
chemical_list: D051176:beta Catenin; D020742:rhoA GTP-Binding Protein