Suppression of thermotolerance development through cycloheximide-induced negative control of stress protein gene expression.

Expression of a luciferase reporter gene by Chinese hamster ovary cells under the control of the human heat shock protein (hsp) 70 gene promoter was suppressed by incubation at 37 degrees C after treatment with cycloheximide (CHX) during 42 degrees C heat shock exposure. The CHX-induced suppression of hsp gene expression induced no development of thermotolerance. However, 42 degrees C heat shock treatment without CHX followed by CHX inhibition of protein synthesis during recovery incubation at 37 degrees C induced thermotolerance expression by inducing enhanced synthesis of hsps including hsp70 in subsequent heat challenge incubation at 43 degrees C. The results demonstrated a direct correlation between suppression (induction) of stress protein gene expression and non-expression (expression) of thermotolerance. Kinetic analysis showed that the CHX suppression of hsp gene induction was greater than the CHX inhibition of protein synthesis, and that it depended on the severity of heat stress: it decreased with increasing heat stress doses. Moreover, prior feeding of the proline analog L-azetidine 2-carboxylic acid abrogated the CHX-induced suppression of hsp gene expression. In addition, CHX treatment during heat cell-killing at 43 degrees C induced protection of cells. These results were well explained by the proposed model of negative or positive control of stress protein gene expression depending on the level of free hsp70, which may be modulated by both the rate of protein synthesis and the severity of heat stress.