Sustained hyposmotic stress induces cell death: apoptosis by defeat

We describe sustained hyposmotic stress as a novel type of environmental co ndition enforcing apoptosis. In a dose- and time-dependent fashion, hyposmo tic stress leads to a delayed type of apoptosis with considerable variation s in constitutive sensitivity among different cell types. For example, afte r 48 h at 84 mosmol/l, the death rate ranged from 10.8 +/- 0.7% in AsPc1 hu man pancreatic carcinoma cells to 72.0 +/- 1.6% in HK-2 human kidney tubule cells. Caspase inhibitors rendered cells more resistant to hyposmolar stre ss; the caspase 3 inhibitor Ac-Asp-Glu-Val-aspartic acid aldehyde was the m ost efficient. After 24 h of stress, HT-29 colon carcinoma and HK-2 cells h ad increased their mitochondrial mass. This went along with an increase in mitochondrial membrane potential in HT-29 cells but with a decrease in HK-2 cells. Starting at 2 h of stress, we detected transient CD95L transcriptio n followed by surface expression of CD95L in HT-29 but not in HK-2 cells. I nhibitory CD95L antibody partially inhibited specific death in HT-29 but no t in HK-2 cells. Thus, as in other types of stress-induced apoptosis, the C D95/CD95L system is one of the different routes to suicide optionally used by hyposmotically stressed cells. Our findings may have clinical implicatio ns for the prevention and treatment of tissue damage caused by severe hypos molar states.