Abolition by cycloheximide of caffeine-enhanced lethality of alkylating agents in hamster cells.

Greatly enhanced lethality for Syrian hamster cells (baby hamster kidney cells and polyomavirus-transformed derivative of baby hamster kidney cells) that had been treated with alkylating agents or ultraviolet light is produced by caffeine. Thus, nitrogen mustard [methylbis(beta-chloroethyl)amine] (HN2) at 0.5 microM decreased cell viability by only a few percent in the absence of caffeine. The addition of 2 mM caffeine during the first 24 hr after nitrogen mustard decreased viability more than 95%. Further kinetic studies of the caffeine effects using other alkylating agents indicate the existence of two modes which damaged cells become caffeine sensitive. Treatment with ultraviolet light or nitrogen mustard makes cells immediately sensitive to caffeine; cells treated with methyl methanesulfonate or N-methyl-N'-nitro-N-nitrosoguanidine, on the other hand, express their maximal caffeine sensitivity only about 15 to 20 hr later. A low concentration of cycloheximide completely abolished this caffeine effect. When 0.36 microM cycloheximide was present, loss of viability was negligible even in the presence of caffeine. Based on further experiments involving the timing of caffeine and cycloheximide additions, caffeine does not affect viability by itself but is involved with some substance such as a protein the rapid synthesis of which is prevented by cycloheximide. This substance does not appear to be needed for recovery from damage, but rather for the action of caffeine in fixing the damaging caused by alkylating agents.