Heat shock response by cells treated with azetidine-2-carboxylic acid

The purpose of this study is to reinvestigate the heat shock response in cells treated with the antimetabolite azetidine-2-carboxylic acid (azetidine), an analogue of proline. Previous studies could not clearly discriminate between the progressive thermosensitization caused by amino acid analogues and a parallel induction of thermotolerance by heat shock. Incubation of H35 cells with 2.5 mm azetidine causes an increasing thermosensitization which achieves a maximum after approximately 18-22 h. At this point, azetidine does not prevent the development of acute thermotolerance following a heat shock at 42.5 degrees C, or the simultaneous induction of chronic thermotolerance during mild hyperthermia at 38-41 degrees C. However, for both the acute and chronic heating conditions thermotolerance levels are reduced in proportion with azetidine-thermosensitization. Incorporation of azetidine causes an apparent downward temperature shift of approximately 1 degree C relative to the time-temperature relationships for normal, or following heat shocks, for thermotolerant cells. After 18 h of incubation with azetidine, protein synthesis is reduced by a factor of 4 and cells show a preferential synthesis of heat shock proteins (hsp). A heat shock then, although inducing thermotolerance, is not followed by any noticeable effect on the synthesis of hsps. It is shown that the combination of prolonged azetidine treatment and heat shock causes a persistent inhibition of protein synthesis. This is hypothesized to result in the development of hsp synthesis independent thermotolerance. Additional treatment following heat shock in azetidine-treated cells with the protein synthesis inhibitor cycloheximide does not affect the induction of thermotolerance. In contrast to the heat shock response, no thermotolerance induction is observed in azetidine-treated cells after an exposure to sodium arsenite.