LOW PH SUPPRESSES SYNTHESIS OF HEAT-SHOCK PROTEINS AND THERMOTOLERANCE

Chinese hamster ovary (CHO) cells heated for 10 min at 45 degrees C be came thermotolerant to a second heat exposure at either pH 7.3 or 6.6. However, low pH delayed the development of thermotolerance and suppre ssed the rate of synthesis of all proteins, including heat-shock prote ins (HSPs). Low-pH-tolerant mutant CHO cells (PHV2) in pH 6.6 medium w ere also delayed in both the development of thermotolerance and protei n synthesis, though less than CHO cells in pH 6.6 medium. The rate of synthesis of inducible HSP-70 under the three conditions paralleled th e kinetics of the development of thermotolerance. The intracellular pH (pH(i)) of CHO cells in pH 6.6 medium, whether heated or not, was far lower than CHO cells in pH 7.3 medium, and the pH(i) of PHV2 cells in pH 6.6 medium was close to that of CHO cells in pH 7.3 medium after t he initial heat shock. Amiloride enhanced the effect of low-pH medium on pH(i), HSP synthesis and development of thermotolerance. The concen tration of HSP-70 was also measured by how cytometry. The level of HSP -70 was not altered within the first 4 h after a 10-min 45.0 degrees C initial heat treatment during which the major portion of thermotolera nce developed, though the level of HSP-70 increased rapidly after 4 h. Low pH caused a further delay in the increase in HSP-70. We conclude that low-pH medium may inhibit the synthesis of HSPs in part by loweri ng pH(i), but the delay and suppression of development of thermotolera nce is not primarily due to the inhibition of HSP synthesis.