Mechanical loading regulates protease production by fibroblasts in three-dimensional collagen substrates

Mechanical loading is important in tissue formation and remodelling, notably in wound repair. The aim of this study was to measure the effects of controlled loading on the release of extracellular matrix protease activities by fibroblasts. Fibroblast populated collagen lattices were subjected to external cyclical loads through a computer controlled unit incorporated into a culture system, a tensioning-Culture Force Monitor. Cyclical loading was compared to untensioned and statically loaded gels (tethered endogenous contraction). Overall changes in a range of protease activities were monitored (chiefly by zymography) as measures of the cyto-mechanical response to these loads. Under static load, 2.5- and 13-fold more matrix metalloproteinase-2 was produced than matrix metalloproteinase-9, at 24 and 48 hours. Total matrix metalloproteinase-9 increased 37 fold on cyclical loading. Total matrix metalloproteinase-3 and urokinase plasminogen activator activities were dramatically reduced on cyclical loading while tissue type plasminogen activator activity was increased. Comparison with cell responses on stiffer substrates (collagen sponges) identified similar matrix metalloproteinase responses to load, but at much reduced levels (4-6 fold matrix metalloproteinase-9 stimulation on loading), showing the importance of matrix compliance to this mechano-response. In conclusion, physiological mechanical loading of fibroblasts in three dimensional collagen lattices elicited complex and substantial changes in matrix modifying proteases. These changes suggest that cells switch between expression of comparable protease activities mainly influencing cell-matrix interactions associated with migration or more generalized extracellular matrix remodelling.