Re-induction of hsp70 synthesis: an assay for thermotolerance

Of the many heat shock proteins (HSPs), hsp70 appears to correlate best with heat resistance, either permanent or transient. We have investigated various approaches to quantify the concentration of hsp 70, and examined the relationship between hsp70 and cells' thermal sensitivity during the development and decay of thermotolerance in model systems. Here, experiments were performed to determine the possibility of using the rate of synthesis of hsp70 after a second test heat shock to predict the kinetics of thermotolerance. Specifically, we studied the relationship between the retained thermotolerance in a murine tumor cell line SQ-1 and a human tumor cell line, HCT-8, after fractionated heat doses and the cells' ability to re-initiate synthesis of hsp70 in response to an additional test heat dose in vitro. Monolayers of cells were exposed to a first heat treatment (e.g., 41 degrees C, 4 h) and then incubated at 37 degrees C for 0-72 h. At various times after the first heat treatment, cells were either challenged with a 45 degrees C, 45 min heat shock to assess the residual thermotolerance by colony formation, or labelled with [35S]methionine before or after an additional test heat dose (e.g. 43.5 degrees C, 15 min). We found that the cells' ability to re-initiate hsp70 synthesis in response to the test heat shock inversely correlated with retained thermotolerance. Our data suggest the level of hsp70 in thermotolerant cells regulates the rate of synthesis of additional hsp70 in response to the subsequent heat challenge. Furthermore, the results showed that the rate of re-induction of hsp70 synthesis after a test shock can be used as a rapid measure of retained thermotolerance. This study suggests an approach for quantifying the level of retained thermotolerance during a course of fractionated hyperthermia.