INDUCTION OF HEAT-SHOCK-PROTEIN-70 PROTECTS THYMOCYTES AGAINST RADIATION-INDUCED APOPTOSIS

Objectives: To determine if induction of heat shock protein 70 (HSP 70 ), a stress protein that plays a cytoprotective role and inhibits cell death in response to various stimuli, will protect thymocytes and T-c ell clones from radiation-induced apoptosis, and to define the mechani sm of such protection. Design: Thymocytes from BALB/c mice or T-lympho cyte clones were incubated at 43 degrees C for 1 hour to induce HSP 70 , then irradiated. Control cells were irradiated but not heated. Fragm entation of DNA was quantitated, and p53, bax, and bcl-2 expression wa s analyzed at various times by the Western blot method.Results: Only h eated cells expressed HSP 70. The induction of HSP 70 increased basal apoptosis but significantly decreased radiation-induced apoptosis. Fur thermore, introduction of an HSP 70 antisense oligomer prior to heatin g reversed the protective effect of HSP 70. Induction of HSP 70 in T-c ell clones with sodium arsenite had a similar protective effect agains t radiation-induced apoptosis. Irradiation induced p53 and markedly up regulated bax. The expression of p53 peaked at 4 hours and preceded ma ximal bar induction. Induction of HSP 70 prior to irradiation suppress ed p53 and significantly decreased bax levels. Levels of bcl-2 were un affected. Conclusions: Our data show that HSP 70 induction protects th ymocytes from radiation-induced apoptosis by down-regulating p53 and b ax expression. The induction of HSP 70 may represent a novel mechanism by which the immunosuppressive effects and the associated infectious complications of radiation therapy can be minimized.