title: Akt1 in murine chondrocytes controls cartilage calcification during endochondral ossification under physiologic and pathologic conditions.
abstract: OBJECTIVE
  To examine the role of the phosphoinositide-dependent serine/threonine protein kinase Akt1 in chondrocytes during endochondral ossification.
  
  
  METHODS
  Skeletal phenotypes of homozygous Akt1-deficient (Akt1(-/-)) mice and their wild-type littermates were compared in radiologic and histologic analyses. An experimental osteoarthritis (OA) model was created by surgically inducing instability in the knee joints of mice. For functional analyses, we used primary costal and articular chondrocytes from neonatal mice and mouse chondrogenic ATDC5 cells with retroviral overexpression of constitutively active Akt1 or small interfering RNA (siRNA) for Akt1.
  
  
  RESULTS
  Among the Akt isoforms (Akt1, Akt2, and Akt3), Akt1 was the most highly expressed in chondrocytes, and the total level of Akt protein was decreased in Akt1(-/-) chondrocytes, indicating a dominant role of Akt1. Akt1(-/-) mice exhibited dwarfism with normal proliferative and hypertrophic zones but suppressed cartilage calcification in the growth plate compared with their wild-type littermates. In mice with surgically induced OA, calcified osteophyte formation, but not cartilage degradation, was prevented in the Akt1(-/-) joints. Calcification was significantly suppressed in cultures of Akt1(-/-) chondrocytes or ATDC5 cells overexpressing siRNA for Akt1 and was enhanced in ATDC5 cells overexpressing constitutively active Akt1. Neither proliferation nor hypertrophic differentiation was affected by the gain or loss of function of Akt1. The expression of ANK and nucleotide pyrophosphatase/phosphodiesterase 1, which accumulate pyrophosphate, a crucial calcification inhibitor, was enhanced by Akt1 deficiency or siRNA for Akt1 and was suppressed by constitutively active Akt1.
  
  
  CONCLUSIONS
  Our findings indicate that Akt1 in chondrocytes controls cartilage calcification by inhibiting pyrophosphate during endochondral ossification in skeletal growth and during osteophyte formation in OA.
authors.affiliation: University of Tokyo, Tokyo, Japan.
authors.forename: Atsushi
authors.initials: A
authors.lastname: Fukai
authors.forename: Naohiro
authors.initials: N
authors.lastname: Kawamura
authors.forename: Taku
authors.initials: T
authors.lastname: Saito
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Oshima
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Ikeda
authors.forename: Fumitaka
authors.initials: F
authors.lastname: Kugimiya
authors.forename: Akiro
authors.initials: A
authors.lastname: Higashikawa
authors.forename: Fumiko
authors.initials: F
authors.lastname: Yano
authors.forename: Naoshi
authors.initials: N
authors.lastname: Ogata
authors.forename: Kozo
authors.initials: K
authors.lastname: Nakamura
authors.forename: Ung-Il
authors.initials: UI
authors.lastname: Chung
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Kawaguchi
mesh_terms: D000818:Animals; D015153:Blotting, Western; D001846:Bone Development / Q000502:physiology; D002356:Cartilage / Q000473:pathology / Q000502:physiology*; D002478:Cells, Cultured; D019902:Chondrocytes / Q000737:chemistry*; D018450:Disease Progression; D051379:Mice; D009999:Ossification, Heterotopic; D010003:Osteoarthritis / Q000503:physiopathology; D010012:Osteogenesis; D051057:Proto-Oncogene Proteins c-akt / Q000032:analysis / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D051057:Proto-Oncogene Proteins c-akt