Abstract
BACKGROUND: Misdirected apoptosis in endothelial cells participates in the development of pathological conditions such as atherosclerosis. Tight regulation of apoptosis is necessary to ensure normal cell function. The rate of cell turnover is increased at sites prone to lesion development. Laminar shear stress is protective against atherosclerosis, and helps suppress apoptosis induced by cytokines, oxidative stress, and serum depletion. Current Studies have shown that the pro-apoptotic DAPK expression and function to be regulated in part by shear stress, and that shearing cells already treated with cytokine tumor necrosis factor (TNF) α significantly reduced apoptosis. We investigate further the suppression of endothelial apoptosis by shear stress with other apoptotic triggers, and the involvement of DAPK and caspase 3/7. RESULTS: We have shown that exposure to shear stress (12 dynes/cm(2) for 6 hrs) suppressed endothelial apoptosis triggered by cytokine (TNFα), oxidative stress (H2O2), and serum depletion, either before or after a long term (18 hr) induction. This is correlated with a parallel decrease of DAPK expression and caspase activity compared to non-sheared cells. We found similar modulation of DAPK and apoptosis by shear stress with other pro-apoptotic signals. Changes in DAPK and caspase 3/7 are directly correlated to changes in apoptosis. Interestingly, shear stress applied to cells prior to induction with apoptosis agents resulted in a higher suppression of apoptosis and DAPK and caspase activity, compared to applying shear stress post induction. This is correlated with a higher expression and activation of DAPK in cells sheared at the end of 24-hr experiment. Also, shear stress alone also induced higher apoptosis and DAPK expression, and the effect is sustained even after 18 hrs incubation in static condition, compared to non-sheared cells. CONCLUSIONS: Overall, we show that laminar shear stress inhibits various apoptosis pathways by modulating DAPK activity, as well as caspase activation, in a time-dependent manner. Shear stress could target DAPK as a converging point to exert its effects of suppressing endothelial apoptosis. The temporal shear stress stimulation of DAPK and its role in different apoptosis pathways may help identify key mechanisms of the endothelial mechanotransduction pathway.