The effect of mechanical strain on plasminogen activator in keratinocytes

Objectives: Keratinocytes are normally in tension in the skin and oral mucosa but this is lost when breached. One of the subsequent key phases in wound healing is the recovery of the tensional state by contraction of granulation tissue and restoration of an intact epithelial layer. It is known that up regulated proteolysis of the extracellular matrix results in enhanced cell migration and thereby facilitates the epidermal cells coverage of the denuded wound surface. The aim of this study was to investigate the effect of different magnitudes of mechanical strain on the release of urokinase-type plasminogen activator (uPA) activity in keratinocytes in vitro.

Methods: Normal human keratinocytes (NHK) and virally-transduced keratinocytes (UP) were cultured on matrix protein coated flexible-bottomed plates (collagen type I and collagen type IV) and placed in a Flexercell tension plus strain unit. Cells were subjected to a variety of stretch regimes: 10%, 20% and 30% static (continuous) stretch regimes for 24 hours. Stretched bioflex plates were compared to non-stretched control plates. Proteolytic activity was determined by plasminogen activator activity assay to assess the level of uPA.

Results: The effect of mechanical strain on both cell types revealed a significant increase in the level of uPA produced by UP cells (p<0.05) and a significant decrease in the level of uPA produced by NHK cells (p<0.05) on collagen type I and collagen type IV matrix proteins.

Conclusions: NHK cells exhibit different responses to mechanical stimulation as opposed to UP cells, suggesting that normal cells in tension down-regulate proteolytic activity, and that as cells become abnormal this response is impaired or lost completely. The different rates of proteolytic production by NHK and UP cells in response to being cultured on various matrix proteins and different magnitudes of mechanical stress may be of importance in wound healing.