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

Abstract

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°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°C), 12.5 ± 1.1 to 22.2 ± 1.9 min (43.5°C) and 9.4 ± 1.0 to 15.6 ± 1.5 min (44.5°C). Cells from all lines developed thermotolerance during protracted treatments at 42.5°C. Thermotolerance was also studied by giving the cells a priming treatment of 43.5°C for 90 min and then, after different fractionation intervals at 37°C, second graded treatments at 43.5°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°C. Nevertheless, the lines which were most resistant before thermotolerance developed were also most resistant at maximum thermotolerance.

Key Words:

• heat sensitivity
• thermotolerance
• heterogeneity
• xenograft