Heterogeneity in heat sensitivity and development of thermotolerance of cloned cell lines derived from a single human melanoma xenograft.

One uncloned and five cloned cell lines were isolated from a single human melanoma xenograft in passage 39 in athymic mice. Cells from passages 7-12 in vitro were heated at 42.5, 43.5 or 44.5 degrees C and the colony forming ability of the cells was assayed in soft agar. The six cell lines showed individual and characteristic responses to heat treatment. The D0 values of the survival curves were in the ranges 76 +/- 5 to 131 +/- 13 min (42.5 degrees C), 12.5 +/- 1.1 to 22.2 +/- 1.9 min (43.5 degrees C) and 9.4 +/- 1.0 to 15.6 +/- 1.5 min (44.5 degrees C). Cells from all lines developed thermotolerance during protracted treatments at 42.5 degrees C. Thermotolerance was also studied by giving the cells a priming treatment of 43.5 degrees C for 90 min and then, after different fractionation intervals at 37 degrees C, second graded treatments at 43.5 degrees C. Thermotolerance ratio (TTR), i.e. the ratio of the slopes of the survival curves for preheated and single-heated cells, was used as a quantitative measure of the thermotolerance. Thermotolerance developed rapidly for all lines, reached a maximum at 12 or 16 h, and then decayed slowly. Maximum TTR varied among the lines from 4.2 +/- 0.5 to 6.0 +/- 0.9, i.e. within a factor of about 1.4. The survival curves and the TTR-curve for the uncloned line were positioned in the midst of those of the cloned lines. A linear correlation between maximum TTR and heat sensitivity was found for the six lines; maximum TTR decreased with increasing D0 value at 43.5 degrees C. Nevertheless, the lines which were most resistant before thermotolerance developed were also most resistant at maximum thermotolerance.