Hyperosmotic stress stimulates tissue plasminogen activator expression by a PKC-independent pathway

Shear, stretch, and the generation of oxygen radicals stimulate increases in tissue plasminogen activator (t-PA) mRNA levels and antigen production, suggesting that environmental stress may regulate t-PA gene expression. We have examined whether t-PA production is also responsive to a hyperosmotic environment. Endothelial and HeLa cells were treated with hyperosmotic medium, and t-PA mRNA and antigen secretion were measured. Endothelial cells incubated in hyperosmotic medium showed a dose-dependent decrease in cell volume and a 1.9 +/- 0.3- and 3.7 +/- 0.9-fold increase in t-PA secretion at 425 and 485 mosmol/kgH2O, respectively. HeLa cells showed a 3.3 +/- 0.6- and 5.1 +/- 1.2-fold increase at the same osmolalities. Increased secretion began between 8 and 16 h and continued through 24 h. Cultures returned to isosmotic medium after 8 h of treatment continued to release 98.1 +/- 7% of the maximum levels of t-PA for the next 16 h, despite the reversal of other responses to hyperosmotic environment. t-PA mRNA levels also increased between 8 and 16 h to five times control levels but returned to baseline by 24 h. No change in intracellular Ca2+ concentration, inositol 1,4,5-trisphosphate, or diacylglycerol content was detected, suggesting that a different intracellular signal pathway may be involved in the response to hyperosmolar stimulus. Thus environmental stress may be a general stimulatory signal through which t-PA production can be induced.