title: Lipocalin 2: a new mechanoresponding gene regulating bone homeostasis.
abstract: Mechanical loading represents a crucial factor in the regulation of skeletal homeostasis. Its reduction causes loss of bone mass, eventually leading to osteoporosis. In a previous global transcriptome analysis performed in mouse calvarial osteoblasts subjected to simulated microgravity, the most upregulated gene compared to unit gravity condition was Lcn2, encoding the adipokine Lipocalin 2 (LCN2), whose function in bone metabolism is poorly known. To investigate the mechanoresponding properties of LCN2, we evaluated LCN2 levels in sera of healthy volunteers subjected to bed rest, and found a significant time-dependent increase of this adipokine compared to time 0. We then evaluated the in vivo LCN2 regulation in mice subjected to experimentally-induced mechanical unloading by (1) tail suspension, (2) muscle paralysis by botulin toxin A (Botox), or (3) genetically-induced muscular dystrophy (MDX mice), and observed that Lcn2 expression was upregulated in the long bones of all of them, whereas physical exercise counteracted this increase. Mechanistically, in primary osteoblasts transfected with LCN2-expression-vector (OBs-Lcn2) we observed that Runx2 and its downstream genes, Osterix and Alp, were transcriptionally downregulated, and alkaline phosphatase (ALP) activity was less prominent versus empty-vector transduced osteoblasts (OBs-empty). OBs-Lcn2 also exhibited an increase of the Rankl/Opg ratio and IL-6 mRNA, suggesting that LCN2 could link poor differentiation of osteoblasts to enhanced osteoclast stimulation. In fact, incubation of purified mouse bone marrow mononuclear cells with conditioned media from OBs-Lcn2 cultures, or their coculture with OBs-Lcn2, improved osteoclastogenesis compared to OBs-empty, whereas treatment with recombinant LCN2 had no effect. In conclusion, our data indicate that LCN2 is a novel osteoblast mechanoresponding gene and that its regulation could be central to the pathological response of the bone tissue to low mechanical forces.
authors.affiliation: Department of Biotechnological and Applied Clinical Sciences, University of L'Aquila, L'Aquila, Italy.
authors.forename: Nadia
authors.initials: N
authors.lastname: Rucci
authors.forename: Mattia
authors.initials: M
authors.lastname: Capulli
authors.forename: Sara Gemini
authors.initials: SG
authors.lastname: Piperni
authors.forename: Alfredo
authors.initials: A
authors.lastname: Cappariello
authors.forename: Patrick
authors.initials: P
authors.lastname: Lau
authors.forename: Petra
authors.initials: P
authors.lastname: Frings-Meuthen
authors.forename: Martina
authors.initials: M
authors.lastname: Heer
authors.forename: Anna
authors.initials: A
authors.lastname: Teti
mesh_terms: D000209:Acute-Phase Proteins / Q000235:genetics*; D000328:Adult; D000818:Animals; D001510:Bed Rest; D001842:Bone and Bones / Q000503:physiopathology*; D002454:Cell Differentiation; D004195:Disease Models, Animal; D005786:Gene Expression Regulation; D057809:HEK293 Cells; D019417:Hindlimb Suspension; D006706:Homeostasis / Q000235:genetics*; D006801:Humans; D000071068:Lipocalin-2; D054834:Lipocalins / Q000097:blood / Q000235:genetics*; D008297:Male; D040542:Mechanotransduction, Cellular / Q000235:genetics*; D008810:Mice, Inbred C57BL; D015513:Oncogene Proteins / Q000235:genetics*; D010006:Osteoblasts / Q000378:metabolism / Q000473:pathology; D010010:Osteoclasts / Q000378:metabolism / Q000473:pathology; D010012:Osteogenesis; D010243:Paralysis / Q000473:pathology / Q000503:physiopathology; D010641:Phenotype; D011518:Proto-Oncogene Proteins / Q000097:blood / Q000235:genetics*
chemical_list: D000209:Acute-Phase Proteins; C489506:LCN2 protein, human; D000071068:Lipocalin-2; D054834:Lipocalins; D015513:Oncogene Proteins; D011518:Proto-Oncogene Proteins; C071332:Lcn2 protein, mouse
keywords: ADIPOKINE; LCN2; MECHANICAL FORCES; OSTEOBLASTS; OSTEOCLASTS; UNLOADING