c-myc/p53 interaction determines sensitivity of human colon carcinoma cells to 5-fluorouracil in vitro and in vivo.

Colon carcinoma cells overexpress c-myc due to defective Wnt signaling, but only patients whose tumors have an amplified c-myc gene show improved disease-free and overall survival in response to 5-fluoruracil (5FU). Here we show that in two colon carcinoma cell lines that do not have an amplified c-myc gene but differ in their p53 status, high c-myc levels can be further elevated by introducing a c-myc expression vector. Whereas sensitivity to low serum-induced apoptosis was imposed on the parental lines independent of p53 status and was unaffected by further elevation of c-myc, sensitivity to 5FU-induced apoptosis was dependent on both the higher c-myc levels due to the expression vector and wild-type p53 function. The elevated c-myc levels led to higher c-myc transactivation activity in the p53 wild-type cell line, but not in the mutant p53 cell line. The requirement for both elevated c-myc and p53 for 5FU sensitivity was confirmed using antisense c-myc and pifithrin-alpha, a specific inhibitor of p53. Finally, the in vitro data predicted that only patients with both amplified c-myc and wild-type p53 in their primary tumors would be responsive to 5FU-based therapy, which was borne out by analysis of tumors from 135 patients entered into a Phase III clinical trial of 5FU-based adjuvant therapy. The data provide significant insight into mechanisms that establish colon tumor cell sensitivity to 5FU, clearly demonstrate the necessity of exercising caution in considering combining novel strategies that target elevated c-myc with standard 5FU-based therapy, and suggest alternative therapeutic strategies that target c-myc and/or p53 mutations in the treatment of colon cancer.

Short-chain fatty acid metabolism, apoptosis, and Apc-initiated tumorigenesis in the mouse gastrointestinal mucosa.

Short-chain fatty acids (SCFAs) are physiological regulators of growth and differentiation in the gastrointestinal tract, and we have previously shown that apoptosis induced in colonic cell lines by these compounds is dependent on their metabolism by B-oxidation in the mitochondria (B. G. Heerdt et al., J. Biol. Chem., 266: 19120-19126, 1991; Cancer Res., 54: 3288-3293, 1994). Because tumors initiated by an inherited Apc mutation have been reported to be linked to decreases in apoptosis in the flat mucosa of the gastrointestinal tract, the aims were to determine whether elimination of efficient metabolism of SCFAs affected apoptosis in the gastrointestinal mucosa of the mouse, and whether this altered tumorigenesis initiated by an inherited Apc mutation. We, therefore, generated mice that have a chain-terminating mutation in the Apc gene and that were either wild-type for SCFA metabolism, or deficient, due to homozygous deletion of the gene (Scad) that encodes the enzyme short-chain acyl dehydrogenase, which catalyzes the first step in SCFA B-oxidation. Scad+/+ mice maintained on a wheat bran-fiber-supplemented diet gained significantly more weight than mice maintained on AIN76A, but this was eliminated by the Scad mutation, demonstrating that uptake and metabolism of SCFAs in the gastrointestinal tract can be a significant energy source. As predicted, on either AIN76A or wheat bran diet, the Scad mutation almost completely eliminated apoptosis in the flat mucosa of the proximal colon and reduced apoptosis by 50% in the distal colon compared with littermates that were wild-type for Scad. The mutation also reduced apoptosis by approximately 50% in the duodenum in AIN76A-fed mice. These reductions in apoptosis had no effect on incidence, frequency, or site specificity of tumors initiated by the Apc mutation. Therefore, the metabolism of SCFAs by the gastrointestinal mucosa plays a role in modulating apoptosis, but a general decrease in apoptosis in the mucosa of the gastrointestinal tract is not linked to gastrointestinal tumorigenesis initiated by an inherited Apc mutation.

Low-level c-myc amplification in human colonic carcinoma cell lines and tumors: a frequent, p53-independent mutation associated with improved outcome in a randomized multi-institutional trial.

Human colonic cancer is associated with multiple genetic deletions, mutations, and alterations in gene expression; in contrast, gene amplification has not been recognized as a prominent characteristic of human colonic tumors. Although the c-myc gene is overexpressed in approximately 70% of human colonic cancers, previous studies have not detected frequent gene amplification or rearrangement of c-myc in these tumors, although such amplification has been reported in chemically induced rodent colon cancer and quantitative analysis of gene copy number has shown the gene to be amplified at a low level in mucinous and poorly differentiated human colon carcinomas. Using rigorously controlled blot methodology, we have established that the c-myc gene, located at 8q21, exhibited amplification of 87% to 35-fold in 7 of 10 human colonic carcinoma cell lines. This was highly significant even at a low level of amplification in HT29 cells (P < 0.0001). Cytogenetic analysis by G-banding did not detect aneuploidy involving chromsome 8q, suggesting that the amplification for the c-myc gene on 8q was relatively specific, and this was consistent with a lack of amplification detected for the c-mos gene on 8q24, which was assayed similarly. The same methodology then revealed amplification of c-myc from 1.5-fold to 5-fold in 32% of tumors from 149 patients entered into a multi-institutional Phase III study of adjuvant therapy for colon cancer. c-myc status was not related to time to recurrence or death, but low levels of c-myc amplification identified a subset of patients who showed a statistically significant increase in disease-free survival, and a corresponding trend to longer overall survival, in response to adjuvant therapy with 5-fluorouracil plus levamisole. Presence of c-myc amplification was not related to incidence of p53 mutations.

Biotinated probe containing a long-terminal repeat hybridized to a mouse colon tumor and normal tissue.

The cloned complementary DNA pMCT-1, which contains an intracisternal A particle long-terminal repeat, is more highly expressed in a mouse colon tumor than in the normal mouse colon. In situ hybridization of biotin-substituted pMCT-1 to fixed frozen sections shows that expression of pMCT-1 is seen throughout the tumor and is highly heterogeneous on a cellular basis, while expression is undetectable in any cell in the normal colonic mucosa.