Elevated protein expression of cyclin D1 and Fra‐1 but decreased expression of c‐Myc in human colorectal adenocarcinomas overexpressing β‐catenin

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Abstract
Mutations of the adenomatous polyposis coli tumor suppressor gene, or its downstream target β‐catenin, have been implicated in the initiation of most sporadic human colorectal epithelial neoplasms. These mutations, in turn, lead to aberrant nuclear accumulation of β‐catenin and subsequent activation of the β‐catenin/Tcf transcription factor complex. In vitro studies utilizing cultured human colon cancer cell lines have identified c‐myc, cyclin D1 and fra‐1 as target genes of β‐catenin/Tcf signaling. In our study, 12 cases of human colorectal adenocarcinomas were examined by Western immunoblotting analysis and immunohistochemical staining to specifically investigate whether the protein expression of these target genes was indeed altered in vivo by β‐catenin dysregulation. The results show that the protein level of β‐catenin was significantly increased in all 12 tumors (3.4 ± 1.0‐fold increase compared to the control normal mucosa by Western immunoblotting, p < 0.05), and this increase was associated with positive nuclear staining by immunohistochemistry in 10 cases. Increased levels of expression of cyclin D1 and Fra‐1 proteins were also demonstrated in every tumor (9.0 ± 2.7 and 3.3 ± 0.9‐fold increases compared to normal mucosa, respectively). Surprisingly, the protein level of c‐Myc was significantly decreased in all tumors examined by 49 ± 19% (p < 0.05), but the c‐myc mRNA level was increased in 8 of 12 tumors when compared to that in normal mucosa by RT‐PCR. Immunohistochemical staining performed on these carcinomas and additional 27 colorectal carcinomas further demonstrated that the protein expression level of c‐Myc and β‐catenin nuclear localization were not correlated. Moreover, 15 of 20 colorectal adenomas exhibited positive nuclear β‐catenin immunostaining, among which 11 also exhibited increased c‐Myc protein expression. These data thus support the notion that upregulation of cyclin D1 and Fra‐1 in human colorectal adenocarcinomas is driven by abnormally expressed β‐catenin. However, the regulation of c‐myc expression in colorectal tumors appears to be more complex. While dysregulated β‐catenin may cause a transcriptional upregulation of the c‐myc gene, the c‐Myc protein expression appears to be further regulated by a posttranscriptional mechanism(s) during the process of neoplastic progression.