title: Knee loading reduces MMP13 activity in the mouse cartilage.
abstract: BACKGROUND
  Moderate loads with knee loading enhance bone formation, but its effects on the maintenance of the knee are not well understood. In this study, we examined the effects of knee loading on the activity of matrix metalloproteinase13 (MMP13) and evaluated the role of p38 MAPK and Rac1 GTPase in the regulation of MMP13.
  
  
  METHODS
  Knee loading (0.5-3 N for 5 min) was applied to the right knee of surgically-induced osteoarthritis (OA) mice as well as normal (non-OA) mice, and MMP13 activity in the femoral cartilage was examined. The sham-loaded knee was used as a non-loading control. We also employed primary non-OA and OA human chondrocytes as well as C28/I2 chondrocyte cells, and examined MMP13 activity and molecular signaling in response to shear at 2-20 dyn/cm².
  
  
  RESULTS
  Daily knee loading at 1 N for 2 weeks suppressed cartilage destruction in the knee of OA mice. Induction of OA elevated MMP13 activity and knee loading at 1 N suppressed this elevation. MMP13 activity was also increased in primary OA chondrocytes, and this increase was attenuated by applying shear at 10 dyn/cm². Load-driven reduction in MMP13 was associated with a decrease in the phosphorylation level of p38 MAPK (p-p38) and NFκB (p-NFκB). Molecular imaging using a fluorescence resonance energy transfer (FRET) technique showed that Rac1 activity was reduced by shear at 10 dyn/cm² and elevated by it at 20 dyn/cm². Silencing Rac1 GTPase significantly reduced MMP13 expression and p-p38 but not p-NFκB. Transfection of a constitutively active Rac1 GTPase mutant increased MMP13 activity, while a dominant negative mutant decreased it.
  
  
  CONCLUSIONS
  Knee loading reduces MMP13 activity at least in part through Rac1-mediated p38 MAPK signaling. This study suggests the possibility of knee loading as a therapy not only for strengthening bone but also preventing tissue degradation of the femoral cartilage.
authors.forename: Kazunori
authors.initials: K
authors.lastname: Hamamura
authors.forename: Ping
authors.initials: P
authors.lastname: Zhang
authors.forename: Liming
authors.initials: L
authors.lastname: Zhao
authors.forename: Joon W
authors.initials: JW
authors.lastname: Shim
authors.forename: Andy
authors.initials: A
authors.lastname: Chen
authors.forename: Todd R
authors.initials: TR
authors.lastname: Dodge
authors.forename: Qiaoqiao
authors.initials: Q
authors.lastname: Wan
authors.forename: Han
authors.initials: H
authors.lastname: Shih
authors.forename: Sungsoo
authors.initials: S
authors.lastname: Na
authors.forename: Chien-Chi
authors.initials: CC
authors.lastname: Lin
authors.forename: Hui Bin
authors.initials: HB
authors.lastname: Sun
authors.affiliation: Department of Biomedical Engineering, Indiana University Purdue University Indianapolis, SL220C, 723 West Michigan Street, Indianapolis, IN 46202, USA. hyokota@iupui.edu.
authors.forename: Hiroki
authors.initials: H
authors.lastname: Yokota
mesh_terms: D000818:Animals; D002358:Cartilage, Articular / Q000201:enzymology* / Q000473:pathology; D002460:Cell Line; D019902:Chondrocytes / Q000201:enzymology; D017364:Collagenases / Q000378:metabolism; D015536:Down-Regulation; D005260:Female; D018093:Gelatinases / Q000378:metabolism; D006801:Humans; D007719:Knee Joint / Q000201:enzymology*; D053509:Matrix Metalloproteinase 13 / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D016328:NF-kappa B / Q000378:metabolism; D020370:Osteoarthritis, Knee / Q000201:enzymology* / Q000473:pathology / Q000517:prevention & control; D034741:RNA, Small Interfering; D013314:Stress, Mechanical; D016474:Weight-Bearing / Q000502:physiology*; D048051:p38 Mitogen-Activated Protein Kinases / Q000378:metabolism; D020830:rac1 GTP-Binding Protein / Q000378:metabolism
chemical_list: D016328:NF-kappa B; D034741:RNA, Small Interfering; D048051:p38 Mitogen-Activated Protein Kinases; D017364:Collagenases; D018093:Gelatinases; C507079:MMP13 protein, human; D053509:Matrix Metalloproteinase 13; C507081:Mmp13 protein, mouse; D020830:rac1 GTP-Binding Protein