Tumors from rats given 1,2-dimethylhydrazine plus chlorophyllin or indole-3-carbinol contain transcriptional changes in β-catenin that are independent of β-catenin mutation status

Tumors induced in the rat by 1,2-dimethylhydrazine (DMH) contain mutations in β-catenin, but the spectrum of such mutations can be influenced by phytochemicals such as chlorophyllin (CHL) and indole-3-carbinol (I3C). In the present study, we determined the mutation status of β-catenin in more than 50 DMH-induced colon tumors and small intestine tumors, and compared this with the concomitant expression of β-catenin mRNA using quantitative real-time RT-PCR analysis. In total, 19/57 (33%) of the tumors harbored mutations in β-catenin, and 14/19 (74%) of the genetic changes substituted amino acids adjacent to Ser33, a key site for phosphorylation and β-catenin degradation. These tumors were found to express a 10-fold range of β-catenin mRNA levels, independent of the β-catenin mutation status and phytochemical exposure, i.e. CHL or I3C given post-initiation. However, β-catenin mRNA levels were strongly correlated with mRNA levels of c-myc, c-jun and cyclin D1, which are targets of β-catenin/Tcf signaling. Tumors with the highest levels of β-catenin mRNA often had over-expressed β-catenin protein, and those with lower β-catenin mRNA typically had low β-catenin protein expression, but there were exceptions (high β-catenin mRNA/low β-catenin protein, or vice versa). We conclude that DMH-induced mutations stabilize β-catenin protein in tumors, which increase c-myc, c-jun and cyclin D1, but there also can be over-expression of β-catenin itself at the mRNA level, contributing to high β-catenin protein levels. Similar findings have been reported in primary human colon cancers and their liver metastases, compared with matched normal-looking tissue. Thus, further studies are warranted on the mechanisms that upregulate β-catenin at the transcriptional level in human and rodent colon cancers.