The role of the death-associated protein Daxx in modulation of apoptosis induced in cardiac myocytes by oxidative stress was studied. Exposure of cultured cardiomyocyte-like cells to oxidative stress or simulated hypoxia increased the level of accumulated ROS and apoptosis. Under conditions of sub-apoptotic stimulation of cardiac myocytes, there was no increase in the level of the Daxx protein, but it translocated from the nucleus to the cytoplasm. Daxx overexpression protected the cells from apoptosis, while they were sensitised to cell death following its down-regulation by siRNA. Moreover, lowering the level of the Daxx protein sensitised cardiac myocytes to spontaneous apoptosis, suggesting that the protein may also have a pro-survival role under physiological conditions. Finally, it was shown that DJ-1, a protein suggested previously to sequester Daxx in the nucleus under conditions of oxidative stress (thereby preventing its cytosolic translocation), was localised solely in the cytoplasm of cardiac myocytes. This indicates that the protein does not modulate the apoptosis regulatory activity of Daxx in cardiac myocytes by its nuclear sequestration. Taken together, Daxx plays a protective role in cultured cardiomyocyte-like cells, at least under the conditions used. 頲008 w. S. Maney and Son Ltd.