title: Acute-phase protein serum amyloid A3 is a novel paracrine coupling factor that controls bone homeostasis.
abstract: Serum amyloid A (A-SAA/Saa3) was shown before to affect osteoblastic metabolism. Here, using RT-quantitative PCR and/or immunoblotting, we show that expression of mouse Saa3 and human SAA1 and SAA2 positively correlates with increased cellular maturation toward the osteocyte phenotype. Expression is not detected in C3H10T1/2 embryonic fibroblasts but is successively higher in preosteoblastic MC3T3-E1 cells, late osteoblastic MLO-A5 cells, and MLO-Y4 osteocytes, consistent with findings using primary bone cells from newborn mouse calvaria. Recombinant Saa3 protein functionally inhibits osteoblast differentiation as reflected by reductions in the expression of osteoblast markers and decreased mineralization in newborn mouse calvaria. Yet, Saa3 protein enhances osteoclastogenesis in mouse macrophages/monocytes based on the number of multinucleated and tartrate-resistant alkaline phosphatase-positive cells and Calcr mRNA expression. Depletion of Saa3 in MLO osteocytes results in the loss of the mature osteocyte phenotype. Recombinant osteocalcin, which is reciprocally regulated with Saa3 at the osteoblast/osteocyte transition, attenuates Saa3 expression in MLO-Y4 osteocytes. Mechanistically, Saa3 produced by MLO-Y4 osteocytes is integrated into the extracellular matrix of MC3T3-E1 osteoblasts, where it associates with the P2 purinergic receptor P2rx7 to stimulate Mmp13 expression via the P2rx7/MAPK/ERK/activator protein 1 axis. Our data suggest that Saa3 may function as an important coupling factor in bone development and homeostasis.
authors.affiliation: *Ludwig Boltzmann Institute of Osteology, Wiener Gebietskrankenkasse and Allgemeine Unfallversicherungsanstalt, Trauma Center Meidling, 1st Medical Department, Hanusch Hospital, Vienna, Austria; and Departments of Orthopedic Surgery and Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota, USA.
authors.forename: Roman
authors.initials: R
authors.lastname: Thaler
authors.affiliation: *Ludwig Boltzmann Institute of Osteology, Wiener Gebietskrankenkasse and Allgemeine Unfallversicherungsanstalt, Trauma Center Meidling, 1st Medical Department, Hanusch Hospital, Vienna, Austria; and Departments of Orthopedic Surgery and Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota, USA.
authors.forename: Ines
authors.initials: I
authors.lastname: Sturmlechner
authors.affiliation: *Ludwig Boltzmann Institute of Osteology, Wiener Gebietskrankenkasse and Allgemeine Unfallversicherungsanstalt, Trauma Center Meidling, 1st Medical Department, Hanusch Hospital, Vienna, Austria; and Departments of Orthopedic Surgery and Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota, USA.
authors.forename: Silvia
authors.initials: S
authors.lastname: Spitzer
authors.affiliation: *Ludwig Boltzmann Institute of Osteology, Wiener Gebietskrankenkasse and Allgemeine Unfallversicherungsanstalt, Trauma Center Meidling, 1st Medical Department, Hanusch Hospital, Vienna, Austria; and Departments of Orthopedic Surgery and Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota, USA.
authors.forename: Scott M
authors.initials: SM
authors.lastname: Riester
authors.affiliation: *Ludwig Boltzmann Institute of Osteology, Wiener Gebietskrankenkasse and Allgemeine Unfallversicherungsanstalt, Trauma Center Meidling, 1st Medical Department, Hanusch Hospital, Vienna, Austria; and Departments of Orthopedic Surgery and Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota, USA.
authors.forename: Monika
authors.initials: M
authors.lastname: Rumpler
authors.affiliation: *Ludwig Boltzmann Institute of Osteology, Wiener Gebietskrankenkasse and Allgemeine Unfallversicherungsanstalt, Trauma Center Meidling, 1st Medical Department, Hanusch Hospital, Vienna, Austria; and Departments of Orthopedic Surgery and Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota, USA.
authors.forename: Jochen
authors.initials: J
authors.lastname: Zwerina
authors.affiliation: *Ludwig Boltzmann Institute of Osteology, Wiener Gebietskrankenkasse and Allgemeine Unfallversicherungsanstalt, Trauma Center Meidling, 1st Medical Department, Hanusch Hospital, Vienna, Austria; and Departments of Orthopedic Surgery and Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota, USA.
authors.forename: Klaus
authors.initials: K
authors.lastname: Klaushofer
authors.affiliation: *Ludwig Boltzmann Institute of Osteology, Wiener Gebietskrankenkasse and Allgemeine Unfallversicherungsanstalt, Trauma Center Meidling, 1st Medical Department, Hanusch Hospital, Vienna, Austria; and Departments of Orthopedic Surgery and Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota, USA vanwijnen.andre@mayo.edu franz.varga@osteologie.at.
authors.forename: Andre J
authors.initials: AJ
authors.lastname: van Wijnen
authors.affiliation: *Ludwig Boltzmann Institute of Osteology, Wiener Gebietskrankenkasse and Allgemeine Unfallversicherungsanstalt, Trauma Center Meidling, 1st Medical Department, Hanusch Hospital, Vienna, Austria; and Departments of Orthopedic Surgery and Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota, USA vanwijnen.andre@mayo.edu franz.varga@osteologie.at.
authors.forename: Franz
authors.initials: F
authors.lastname: Varga
mesh_terms: D016475:3T3 Cells; D000595:Amino Acid Sequence; D000818:Animals; D000831:Animals, Newborn; D001842:Bone and Bones / Q000166:cytology / Q000378:metabolism*; D002454:Cell Differentiation; D002460:Cell Line; D002478:Cells, Cultured; D006706:Homeostasis; D006801:Humans; D053509:Matrix Metalloproteinase 13 / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D008969:Molecular Sequence Data; D010006:Osteoblasts / Q000166:cytology / Q000378:metabolism; D015675:Osteocalcin / Q000235:genetics / Q000378:metabolism; D010010:Osteoclasts / Q000166:cytology / Q000378:metabolism; D010011:Osteocytes / Q000166:cytology / Q000378:metabolism; D010012:Osteogenesis; D019899:Paracrine Communication; D010802:Phylogeny; D034741:RNA, Small Interfering / Q000235:genetics; D058486:Receptors, Purinergic P2X7 / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D000685:Serum Amyloid A Protein / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D012886:Skull / Q000166:cytology / Q000378:metabolism
chemical_list: C546469:P2rx7 protein, mouse; D034741:RNA, Small Interfering; D058486:Receptors, Purinergic P2X7; C521404:SAA1 protein, human; C083772:SAA2 protein, human; C076498:Saa3 protein, mouse; D000685:Serum Amyloid A Protein; D015675:Osteocalcin; D053509:Matrix Metalloproteinase 13; C507081:Mmp13 protein, mouse
keywords: osteoblast; osteoclast; osteocyte; osteogenesis