Mutations in codon 12 of K-ras occur in a high proportion of pancreatic cancer cases. Although there is evidence that p53 mutations also occur in this tumor, few studies have been reported to date and no comparison has been made of K-ras and p53 mutations in the same tissues. Single-strand conformation polymorphism and sequencing of the PCR products were used to determine mutations in p53 gene; to detect mutations in K-ras genes, the artificial restriction fragment length polymorphism (RFLP) approach was used. Eight out of 30 tissues from primary pancreas cancer and 3 of 4 samples from metastases showed p53 mutations. Fifteen out of 17 pancreatic cancer cell lines had p53 mutations. In 2 cases, the same p53 mutation was identified in the original tumor and in a tumor-derived cell line. The majority of p53 mutations were present in exons 5-9 of the gene. Mutations at codon 12 of the K-ras gene were identified in 23/32 pancreas cancer tissues and in 14/17 cell lines. There was no relationship between the types of mutation observed in the 2 genes. In conclusion, mutations in K-ras and p53 genes are common in pancreatic cancer. p53 mutations may occur more frequently in metastatic lesions than in primary tumors, although further work is necessary to investigate this point.

The 21-kD proteins encoded by ras oncogenes (p21Ras) are modified covalently by a palmitate attached to a cysteine residue near the carboxyl terminus. Changing cysteine at position 186 to serine in oncogenic forms produces a nonpalmitylated protein that fails to associate with membranes and does not transform NIH 3T3 cells. Nonpalmitylated p21Ras derivatives were constructed that contained myristic acid at their amino termini to determine if a different form of lipid modification could restore either membrane association or transforming activity. An activated p21Ras, altered in this way, exhibited both efficient membrane association and full transforming activity. Surprisingly, myristylated forms of normal cellular Ras were also transforming. This demonstrates that Ras must bind to membranes in order to transmit a signal for transformation, but that either myristate or palmitate can perform this role. However, the normal function of cellular Ras is diverted to transformation by myristate and therefore must be regulated ordinarily by some unique property of palmitate that myristate does not mimic. Myristylation thus represents a novel mechanism by which Ras can become transforming.

p21ras is palmitoylated on a cysteine residue near the C-terminus. Changing Cys-186 to Ser in oncogenic forms produces a non-palmitoylated protein that fails to associate with membranes and does not transform NIH 3T3 cells. To examine whether palmitate acts in a general way to increase ras protein hydrophobicity, or is involved in more specific interactions between p21ras and membranes, we constructed genes that encode non-palmitoylated ras proteins containing myristic acid at their N-termini. Myristoylated, activated ras, without palmitate (61Leu/186Ser) exhibited both efficient membrane association and full transforming activity. Unexpectedly, we found that myristoylated forms of normal cellular ras were also potently transforming. Myristoylated c-ras retained the high GTP binding and GTPase characteristic of the cellular protein and, moreover, bound predominantly GDP in vivo. This implied that it continued to interact with GAP (GTPase-activating protein). While the membrane binding induced by myristate permitted transformation, only palmitate produced a normal (non-transforming) association of ras with membranes and must therefore regulate ras function by some unique property that myristate does not mimic. Myristoylation thus represents a novel mechanism by which the ras proto-oncogene protein can become transforming.