Acute extracellular acidification reduces intracellular pH, 42°C-induction of heat shock proteins and clonal survival of human melanoma cells grown at pH 6.7

Abstract

Two human melanoma cell lines, SK-Mel-28 and DB-1, were used for in vitro studies of the mechanisms underlying heat resistance of human tumour cells adapted to growth in acidic environments. Adaptation to growth at low pH was characterized by resistance to 42°C cytotoxicity and accompanied by an increase in endogenous levels of Hsp70 and/or Hsp27. Acute extracellular acidification to levels below pH 6.5 was required to sensitize the melanoma cells to 42°C. Furthermore, cells grown at low pH were more resistant to sensitization by acute acidification than cells grown at pH 7.3. The intracellular pH (pH_i) of cells grown at pH 6.7 was less than the pH_i of cells grown at pH 7.3 both before and after acute acidification. A pH_i threshold existed for melanoma cells growing at pH 7.3 below which they became sensitized to 42°C. This pH_i threshold differed between the SK-Mel-28 and DB-1 cells. In contrast, a pH_i threshold for heat sensitization did not exist for cells growing at pH 6.7: any reduction in pH_i before heating resulted in increased cell killing. Since cells grown at low pH lack a pH_i threshold for heat sensitization, they are sensitized more to 42°C per unit decrease in pH_i than cells grown at pH 7.3. Acute acidification abrogated the 42°C-induction of Hsp70 and Hsp27 in the melanoma cells. The pH_i thresholds for abrogation of these HSPs are slightly higher than or comparable with the thresholds for cytoxicity for each cell line grown at pH 7.3, but abrogation occurred over a narrower range of pH_i compared with cytotoxicity. Abrogation of heat-induced expression of these HSPs correlates with cytotoxicity in both cell lines with the exception of Hsp27 expression in SK-Mel-28 cells. In conclusion, strategies that reduce pH_i in melanoma cells growing at low pH, such as in acidotic regions of tumours, could selectively sensitize them to hyperthermia because they lack a pH_i threshold for heat sensitization.

• Hyperthermia
• acute intracellular acidification
• heat shock proteins
• human melanoma cells