Activation of phospholipase C-gamma1 in human keratinocytes by hyperosmolar shock without enzyme phosphorylation.

Human keratinocytes are exposed to strong physical changes, and have the potentiality to react to external stimuli by switching on adaptation mechanisms. In hyperosmotically shocked keratinocytes a rapid and strong increase in calcium has been observed. We showed that this increase could not be prevented by growing the cells in medium devoid of calcium and in the presence of EGTA, indicating that the intracellular calcium increase was due to delivery from internal stores. Further, we observed an increased synthesis of dyacylglycerol and inositol trisphosphates after shock, suggesting that phospholipase C mediates both events. Our experiments demonstrated that osmotic shock in human keratinocytes leads to activation of phospholipase C-gamma1, as measured using an in vitro assay system. This activation is independent of protein tyrosine phosphorylation and corresponded to a relocation of the enzyme to perinuclear membranes as shown by immunofluorescence.

Topographic changes of focal adhesion components and modulation of p125FAK activation in stretched human periodontal ligament fibroblasts.

Mechanical stress has been shown in vitro to modulate integrin-beta1-mediated activation of p125FAK/FAK. To test the hypothesis whether this also applies to periodontal ligament fibroblasts (PDLs), we subjected human PDLs to mechanical stretch and analyzed stress-induced changes of p125FAK activation by quantitative immunoprecipitation of p125FAK and changes in the topography of molecules localizing in focal adhesions by indirect immunofluorescence. Generally, all components of focal contacts under study-including detection of phosphotyrosine, i.e., integrin-beta1, p125FAK, and paxillin-revealed a relative co-localization during stretch application. Under stretch, we observed a re-distribution of all components from the cell periphery to the cytoplasm following the main axes. Tyrosine phosphorylation of p125FAK was monitored up to 72 hours under stretch. While the amount of p125FAK remained essentially constant, the activation of p125FAK was clearly modulated. Tyrosine phosphorylation of p125FAK increased from 15 minutes up to 1 hour and declined after stretching periods of 24, 48, and 72 hours. The analysis of our data indicated a stretch-induced redistribution of focal adhesion components and a modulation of p125FAK activation, suggesting alterations in focal adhesions and their associated signal cascade.