title: Increased bone formation in mice lacking apolipoprotein E.
abstract: ApoE is a plasma protein that plays a major role in lipoprotein metabolism. Here we describe that ApoE expression is strongly induced on mineralization of primary osteoblast cultures. ApoE-deficient mice display an increased bone formation rate compared with wildtype controls, thereby showing that ApoE has a physiologic function in bone remodeling.
  
  
  BACKGROUND
  Apolipoprotein E (ApoE) is a protein component of lipoproteins and facilitates their clearance from the circulation. This is confirmed by the phenotype of ApoE-deficient mice that have high plasma cholesterol levels and spontaneously develop atherosclerotic lesions. The bone phenotype of these mice has not been analyzed to date, although an association between certain ApoE alleles and BMD has been reported.
  
  
  METHODS
  Primary osteoblasts were isolated from newborn mouse calvariae and mineralized ex vivo. A genome-wide expression analysis was performed during the course of differentiation using the Affymetrix gene chip system. Bones from ApoE-deficient mice and wildtype controls were analyzed using radiography, micro CT imaging, and undecalcified histology. Cellular activities were assessed using dynamic histomorphometry and by measuring urinary collagen degradation products. Lipoprotein uptake assays were performed with (125)I-labeled triglyceride-rich lipoprotein-remnants (TRL-R) using primary osteoblasts from wildtype and ApoE-deficient mice. Serum concentrations of osteocalcin were determined by radioimmunoassay after hydroxyapatite chromatography.
  
  
  RESULTS
  ApoE expression is strongly induced on mineralization of primary osteoblast cultures ex vivo. Mice lacking ApoE display a high bone mass phenotype that is caused by an increased bone formation rate, whereas bone resorption is not affected. This phenotype may be explained by a decreased uptake of triglyceride-rich lipoproteins by osteoblasts, resulting in elevated levels of undercarboxylated osteocalcin in the serum of ApoE-deficient mice.
  
  
  CONCLUSIONS
  The specific induction of ApoE gene expression during osteoblast differentiation along with the increased bone formation rate observed in ApoE-deficient mice shows that ApoE has a physiologic role as a regulator of osteoblast function.
authors.affiliation: Department of Trauma, Hand, and Reconstructive Surgery, Hamburg University School of Medicine, Hamburg, Germany.
authors.forename: Arndt F
authors.initials: AF
authors.lastname: Schilling
authors.forename: Thorsten
authors.initials: T
authors.lastname: Schinke
authors.forename: Christian
authors.initials: C
authors.lastname: Münch
authors.forename: Matthias
authors.initials: M
authors.lastname: Gebauer
authors.forename: Andreas
authors.initials: A
authors.lastname: Niemeier
authors.forename: Matthias
authors.initials: M
authors.lastname: Priemel
authors.forename: Thomas
authors.initials: T
authors.lastname: Streichert
authors.forename: Johannes M
authors.initials: JM
authors.lastname: Rueger
authors.forename: Michael
authors.initials: M
authors.lastname: Amling
mesh_terms: D000483:Alleles; D000818:Animals; D000831:Animals, Newborn; D001057:Apolipoproteins E / Q000235:genetics* / Q000502:physiology*; D015519:Bone Density; D001846:Bone Development; D001862:Bone Resorption; D001842:Bone and Bones / Q000378:metabolism*; D002784:Cholesterol / Q000097:blood; D003094:Collagen / Q000652:urine; D017886:Durapatite / Q000737:chemistry; D016678:Genome; D008074:Lipoproteins / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D020411:Oligonucleotide Array Sequence Analysis; D010006:Osteoblasts / Q000378:metabolism*; D015675:Osteocalcin / Q000097:blood / Q000378:metabolism; D010641:Phenotype; D012313:RNA / Q000378:metabolism; D011863:Radioimmunoassay; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013997:Time Factors; D014057:Tomography, X-Ray Computed
chemical_list: D001057:Apolipoproteins E; D008074:Lipoproteins; D015675:Osteocalcin; D012313:RNA; D003094:Collagen; D017886:Durapatite; D002784:Cholesterol