Intracellular acidification is associated with enhanced morphological transformation in Syrian hamster embryo cells

A series of studies has indicated that the frequency of morphological transformation induced by chemical carcinogens in early passage Syrian hamster embryo (SHE) cells is significantly higher when these cells are cultured in medium of reduced bicarbonate concentration and pH (6.70) compared with cells cultured in medium of higher pH. It has also been shown that intercellular gap junctional communication is decreased in these cells when they are cultured at pH 6.70 compared with medium of higher pH. The purpose of the studies reported here was to characterize the effect of changing extracellular pH on intracellular pH in SHE cells. The frequency of morphological transformation induced by benzo(a)pyrene was established at various extracellular pHs and compared with intracellular pH values. Cells cultured in medium of pH ranging from 6.70 to 7.35 were loaded with the pH-sensitive fluorescent dye 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein, and either the steady-state intracellular pH values or the kinetics of change in intracellular pH following refeeding of the cultures with medium of pH ranging from pH 6.70 to pH 7.35 was monitored via image analysis techniques. Results from these studies indicate that, at culture medium pH above 6.95, SHE cells were relatively insensitive to changes in extracellular pH, maintaining an intracellular pH of 7.30 to 7.35 in medium containing 0% serum or pH 7.05 to 7.10 in medium containing 20% fetal bovine serum. At extracellular pHs below 6.95, intracellular pH decreased and, in the presence of serum, equilibrated with extracellular pH. The decrease in intracellular pH was closely associated with an increase in benzo(a)pyrene-induced morphological transformation frequency observed in parallel studies. These results indicate that SHE cells have active intracellular pH regulatory activities and suggest that intracellular acidification plays a role in the increased frequency of transformation observed in SHE cells cultured under acidic conditions.