Effects of changes in oxygen tension, pH, and glucose concentration on the response to CCNU of EMT6 mouse tumor monolayer cells and multicellular spheroids

Abstract

We have studied the effect of individually changing medium glucose content, pH and oxygen tension upon the response to CCNU (1-(2-chloroethyl)3-cyclohexyl-l-nitrosourea) of EMT6/Ca/VJAC cells grown as early plateau phase monolayer cultures or small (200 um diameter) spheroids. The effect of changing all three factors together has also been studied in the spheroid model. All the changes in medium conditions (except for 4 hr hypoxia) were maintained for 24 hr prior to drug exposure. Plating efficiency (PE) of monolayer cells was decreased by reduced medium pH (below 6.5) or oxygen tension while no change in PE was brought about by reduced medium glucose content. In small spheroids reduction in PE caused by low pH was similar to that seen in monolayer, there was again no effect of reduced glucose, and the effect of hypoxia was clearly less than in monolayer. Combination treatment of spheroids (pH 6.5, 120 mg/l glucose and hypoxia) reduced the PE of spheroid cells to 50% of control. Reducing medium glucose content from 920 to 0 mg/l, or oxygen tension from 20% to near zero (for either 4 or 24 hr) reduced the sensitivity of monolayer cells to CCNU. A similar pattern was seen for reducing medium pH from 7.2 to 6.1 during the 24 hr pre-incubation period and 1 hr drug exposure period. A reverse trend was, however, seen if medium pH during the drug exposure period was maintained at 7.2 following reduced pH pre-incubation. Reduced sensitivity to CCNU was seen for cells within small spheroids pre-incubated in medium at low pH (for both schedules) or under hypoxia (for either 4 or 24 hr) whereas reduced medium glucose content appeared to have no such effect. Cells in small spheroids after 24 hr combination treatment were also less sensitive to CCNU than cells from control spheroids.