MILD STEP-DOWN HEATING CAUSES INCREASED LEVELS OF HSP68 AND OF HSP84 MESSENGER-RNA AND ENHANCES THERMOTOLERANCE

Mammalian cells exhibit increased sensitivity to hyperthermic temperat ures of 38-42-degrees-C after an acute high-temperature heat shock; th is phenomenon is known as thermosensitization or the step-down heating effect. In order to determine whether the increase of heat shock mRNA after heat stress can be thermosensitized, we studied the induction o f the mRNA of HSP68 and of HSP84 after application of step-down heatin g (SDH) in Reuber H35 rat hepatoma cells. SDH consisted of a pretreatm ent of 30 min at 41.5, 42-5 or 43.5-degrees-C, followed by a continuou s incubation at a lower hyperthermic temperature (40 or 41-degrees-C). After mild pretreatment (30 min at 41.5-degrees-C) the mRNA level of HSP68 was increased by subsequent incubation at 40-degrees-C, although incubation at 40-degrees-C alone had no effect. This increase was eve n more pronounced at 41-degrees-C. An increase in the level of HSP84 m RNA was also observed after mild pretreatment (41-5-degrees-C/30min) f ollowed by 41-degrees-C post-incubation. Interestingly, an enhanced oc currence of thermotolerance was also observed upon application of mild step-down heating (42-degrees-C/30 min-40-degrees-C-43.5-degrees-C/60 min). In contrast, cell cultures treated for 30 min at 43.5-degrees-C (a temperature which induces an increase in HSP mRNA levels) showed a n inhibited or delayed synthesis of HSP mRNA when post-treated at 40 o r 41-degrees-C. Under these conditions the development of thermotolera nce did not take place either. With respect to the effect of step-down heating on HSP mRNA levels as well as on thermotolerance development, our data imply that a distinction should be made between 'mild' and ' severe' pretreatment temperature of the step-down heating protocol.