Translocation of cytoplasmic HSP70 onto the surface of EL-4 cells during apoptosis.

Heat shock proteins (HSPs) are involved in a variety of intracellular processes and can have both pro- and anti-apoptotic action. However, little is known about the role of HSPs in the progression of apoptosis. Translocation of HSPs to the surface of apoptotic cells is a previously observed phenomenon demonstrating participation of these proteins in execution of the terminal stages of apoptosis. In a previous study we showed that development of EL-4 lymphoma cell apoptosis in vitro is accompanied by elevation of surface HSP expression. In this study we used this model to analyse the relationship of HSP70 expression and its translocation to the cell surface during apoptosis with some key intracellular events. Our data demonstrate a synchronization of surface and intracellular HSP70 expression with bcl-2 expression, intracellular reactive oxygen species (ROS) concentration and caspase-3 activity. A maximum level of surface and intracellular HSP70 expression was observed at an irreversible phase of EL-4 cell apoptosis after mitochondrial potential depolarization. In addition, an enhancement of the relative level of cytoplasmic HSP70 translocation to the cell surface was concomitant with EL-4 cell apoptosis. However, the size of surface and intracellular pools of HSP70, increasing for initial and intermediate stages of cell death, decreased at the terminal phase of apoptosis. Western blot analysis of HSP70 in conditioned supernatant obtained from EL-4 cell tissue showed that the observed decrease of HSP70 cell content might be due to surface HSP70 shedding into the intercellular space.