title: Mechanical stimulation of polycystin-1 induces human osteoblastic gene expression via potentiation of the calcineurin/NFAT signaling axis.
abstract: Mechanical forces trigger biological responses in bone cells that ultimately control osteoblastogenesis and bone program. Although several mechanosensors have been postulated, the precise mechanotransduction pathway remains obscure as the initial mechanosensing event has not yet been identified. Studies in kidney cells have shown that polycystin-1 (PC1), via its extracellular N-terminal part, may function as an "antenna-like" protein providing a linkage between environmental cues and their conversion into biochemical responses that regulate various cellular processes via the calcineurin/NFAT pathway. Here we explored the involvement of PC1 in mechanical load (stretching)-induced signaling cascades that control osteoblastogenesis/bone formation. FACS and TransAM Transcription Factor ELISA analyses employing extracts from primary human osteoblast-like, PC1 expressing cells subjected to mechanical stretching (0-6 h) revealed that unphosphorylated/DNA-binding competent NFATc1 increased at 0.5-1 h and returned to normal at 6 h. In accordance with the activation mechanism of NFATc1, stretching of cultured cells pre-treated with cyclosporin A (CsA, a specific inhibitor of the calcineurin/NFAT pathway) abrogated the observed decrease in the abundance of the cytoplasmic pNFATc1 (phosphorylated/inactive) species. Furthermore, stretching of osteoblastic cells pre-treated with an antibody against the mechanosensing N-terminal part of PC1 also abrogated the observed decrease in the cytoplasmic levels of the inactive pNFATc1 species. Importantly, under similar conditions (pre-incubation of stretched cells with the inhibitory anti-PC1 antibody), the expression of the key osteoblastic, NFATc1-target gene runx2 decreased compared to untreated cells. Therefore, PC1 acts as chief mechanosensing molecule that modulates osteoblastic gene transcription and hence bone-cell differentiation through the calcineurin/NFAT signaling cascade.
authors.affiliation: Department of Biological Chemistry, University of Athens Medical School, 11527 Athens, Greece.
authors.forename: Georgia
authors.initials: G
authors.lastname: Dalagiorgou
authors.affiliation: Department of Biological Chemistry, University of Athens Medical School, 11527 Athens, Greece.
authors.forename: Christina
authors.initials: C
authors.lastname: Piperi
authors.affiliation: Molecular Virology Laboratory, Hellenic Pasteur Institute, 11527 Athens, Greece.
authors.forename: Urania
authors.initials: U
authors.lastname: Georgopoulou
authors.affiliation: Department of Biological Chemistry, University of Athens Medical School, 11527 Athens, Greece.
authors.forename: Christos
authors.initials: C
authors.lastname: Adamopoulos
authors.affiliation: Department of Biological Chemistry, University of Athens Medical School, 11527 Athens, Greece.
authors.forename: Efthimia K
authors.initials: EK
authors.lastname: Basdra
authors.affiliation: Department of Biological Chemistry, University of Athens Medical School, 11527 Athens, Greece.
authors.forename: Athanasios G
authors.initials: AG
authors.lastname: Papavassiliou
mesh_terms: D019703:Calcineurin / Q000378:metabolism*; D002478:Cells, Cultured; D050659:Core Binding Factor Alpha 1 Subunit / Q000235:genetics / Q000378:metabolism; D005786:Gene Expression Regulation; D006801:Humans; D040542:Mechanotransduction, Cellular / Q000502:physiology*; D050778:NFATC Transcription Factors / Q000378:metabolism*; D010006:Osteoblasts / Q000378:metabolism / Q000502:physiology*; D010012:Osteogenesis / Q000502:physiology; D010812:Physical Stimulation; D012333:RNA, Messenger / Q000378:metabolism; D050396:TRPP Cation Channels / Q000378:metabolism / Q000502:physiology*
chemical_list: D019703:Calcineurin; D050659:Core Binding Factor Alpha 1 Subunit; D050778:NFATC Transcription Factors; D012333:RNA, Messenger; D050396:TRPP Cation Channels; C493709:RUNX2 protein, human