title: Static and dynamic compressive strains influence nitric oxide production and chondrocyte bioactivity when encapsulated in PEG hydrogels of different crosslinking densities.
abstract: OBJECTIVE
  Mechanical loading is an important regulator of chondrocytes; however, many of the mechanisms involved in chondrocyte mechanotransduction still remain unclear. Here, poly(ethylene glycol) (PEG) hydrogels are proposed as a model system to elucidate chondrocyte response due to cell deformation, which is controlled by gel crosslinking (rho(x)).
  
  
  METHODS
  Bovine articular chondrocytes (50 x 10(6)cells/mL) were encapsulated in gels with three rho(x)s and subjected to static (15% strain) or dynamic (0.3 Hz or 1 Hz, 15% amplitude strain) loading for 48 h. Cell deformation was examined by confocal microscopy. Cell response was assessed by total nitric oxide (NO) production, proteoglycan (PG) synthesis ((35)SO(4)(2-)-incorporation) and cell proliferation (CP) ([(3)H]-thymidine incorporation). Oxygen consumption was assessed using an oxygen biosensor.
  
  
  RESULTS
  An increase in rho(x) led to lower water contents, higher compressive moduli, and higher cell deformations. Chondrocyte response was dependent on both loading regime and rho(x). For example, under a static strain, NO was not affected, while CP and PG synthesis were inhibited in low rho(x) and stimulated in high rho(x). Dynamic loading resulted in either no effect or an inhibitory effect on NO, CP, and PG synthesis. Overall, our results showed correlations between NO and CP and/or PG synthesis under static and dynamic (0.3 Hz) loading. This finding was attributed to the hypoxic environment that resulted from the high cell-seeding density.
  
  
  CONCLUSIONS
  This study demonstrates gel rho(x) and loading condition influence NO, CP, and PG synthesis. Under a hypoxic environment and certain loading conditions, NO appears to have a positive effect on chondrocyte bioactivity.
authors.affiliation: Department of Chemical Engineering, University of Colorado, Campus Box 424, Engineering Center, ECCH 111, Boulder, CO 80309-0424, USA.
authors.forename: I
authors.initials: I
authors.lastname: Villanueva
authors.forename: D S
authors.initials: DS
authors.lastname: Hauschulz
authors.forename: D
authors.initials: D
authors.lastname: Mejic
authors.forename: S J
authors.initials: SJ
authors.lastname: Bryant
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D002417:Cattle; D002478:Cells, Cultured; D019902:Chondrocytes / Q000378:metabolism*; D019245:Compressive Strength / Q000502:physiology; D020100:Hydrogels / Q000737:chemistry*; D009569:Nitric Oxide / Q000096:biosynthesis*; D011092:Polyethylene Glycols / Q000737:chemistry*; D013314:Stress, Mechanical; D016474:Weight-Bearing / Q000502:physiology
chemical_list: D020100:Hydrogels; D009569:Nitric Oxide; D011092:Polyethylene Glycols