Fifty percent of human colon carcinomas contain activating mutations in the K-ras gene. However, whether these alterations in K-ras affect the function of Ras proteins in growth factor (GF) signal transduction is not known. Here we have characterized a previously defined human colon carcinoma cell model system for K-ras gene mutations and for altered levels of Ras protein expression and have examined whether these alterations affect Ras function in GF signal transduction. Sequence analysis of PCR-amplified K-ras gene fragments indicated that among the more aggressive cell lines, four had a normal K-ras sequence, whereas 3 others (isolated from the same human tumor) contained a mutation at codon 13. In contrast, all 7 of the less aggressive cell lines contained a mutation at either codon I 2 or 13. In addition to the presence of a K-ras mutation, one cell line expressed higher levels of the K-Ras protein and displayed elevated Ras-GTP loading (in the absence of GF addition) compared with the other cell lines examined. Despite these alterations, the mitogenic GF combination epidermal growth factor + insulin + transferrin resulted in an activation of Ras and extracellular signal-regulated kinase 2.
Collectively, our results indicate that the malignant phenotype of the cell lines was not correlated with the presence of K-ras mutations or with higher levels of Ras protein expression. Furthermore, K-ras mutations, high levels of K-Ras protein expression, and elevated Ras-GTP loading, as they occur naturally in human colon carcinomas, do not abolish the function of Ras in GF signaling.