The c-myc protein is constitutively expressed at elevated levels in colorectal carcinoma cell lines

In this study, we have employed both indirect immunofluorescence and ELISA assays to compare the relative levels of c-myc protein in cell lines derived from normal human colon and colon adenocarcinomas. We show that the levels of protein found in the majority of carcinoma cell lines are consistent with the levels of mRNA expressed, and that both are significantly elevated with respect to the levels found in normal cells. Growing populations of fibroblastic and epithelial cell lines derived from normal colonic mucosa exhibit small numbers of steady-state transcripts and immunofluorescence signals which are weak and confined to the nucleus. The adenocarcinoma cell lines, however, express 5- to 10-fold elevated levels of c-myc mRNA and exhibit correspondingly intense immunofluorescence signals which appear to reside principally in the nucleus. Quantitation of c-myc protein levels in these tumor cell lines by ELISA assay indicates that they are 8- to 37-fold higher than the levels of protein in normal cells. Elevated expression of the c-myc gene at both the mRNA and protein levels occurs constitutively in the colorectal carcinoma cell lines during their growth in culture, in contrast to the transiently elevated levels of expression observed in normal cells which have been subjected to a mitogenic stimulus. The constitutively elevated expression of the c-myc protein in colorectal carcinoma cell lines is not typically accompanied by gross rearrangement or amplification of the gene.