Changes in c-myc and c-fos Expression in a Human Tumor Cell Line following Exposure to Bifunctional Alkylating Agents

This study was initiated to determine if DNA-damaging chemothera-peutic agents can suppress the expression of oncogenes. The effects of three structurally related bifunctional alkylating agents on the steady state mRN A levels of c-myc, c-fos, N-ras, and 0-actin in the human colon carcinoma cell line Colo320HSR were examined. Colo320HSR has an amplified c-myc oncogene, which is highly overexpressed, and is assumed to be one of the transforming genes of this cell line. Two concentrations of mechlorethamine, L-phenylalanine mustard, and 4-hydroperoxycyclo-phosphamide, which produced 1 or 3 log cell kills were used to examine the effects of drug exposure on the expression of specific genes. Steady state mRNA levels were measured by Northern blot analysis. Following a 1-h drug exposure, RNA was isolated from cells at 0, 6, 12, and 24 h following drug removal. The agents used produced changes in the expression of specific genes, and all three did so in a similar fashion. Immediately following drug removal, the steady state expression of c-myc in treated cells was increased 2- to Mold compared to control. At 6 and 12 h following drug removal, c-myc levels were depressed 2.5- to 5-fold. By 24 h, c-myc expression approached, but remained below, control levels. Immediately following drug removal, c-fos levels were increased 3- to 4-fold, and from 6 to 24 h following drug removal, c-fos levels gradually returned to, or fell below low basal levels. During the 24-h time course, drug treatment had little or no effect on the steady state levels of N-ras or 0-actin. These data support the hypothesis that alkylating agents may suppress the expression of specific transforming genes.