Contribution of integrin [alpha]6[bêta]4 to colon cancer
Integrin [alpha]6[bêta]4 is a specific laminin binding protein expressed at the base of epithelia. This integrin has been previously analyzed in colon cancers by indirect immunofluorescence using various antibodies by distinct groups, but their results have raised conflicting data. In the current study, [bêta]4 subunit expression was analyzed at the protein level in 22 primary tumors and corresponding resection margins; and at transcript level in 8 primary tumors and corresponding resection margins as well as in 6 adenocarcinoma cell lines. For immunodetection, the antibodies recognizing both the truncated and wild type intestinal forms of [bêta]4 (Basora et al., 1999) were used. In the normal colonic mucosa, both forms were detected at the base of the epithelium in the bottom and at the surface of the glands, respectively. In colon cancers, [bêta]4 staining intensity was found to be increased in 19/22 specimens as compared with their normal counterparts. Analysis of integrin [bêta]4 expression at the protein level in primary carcinomas and in the adenocarcinoma cell lines showed the presence of the [bêta]4 wild type form. The levels of transcription of the integrin [bêta]4 and [bêta]1 subunits, the transcription factors c-Myc, N-Myc and AP-1 (c-Fos, Fra-1, Fra-2, c-Jun) and the housekeeping genes S14 and [bêta]-actin were determined by RT-PCR in the 8 primary tumor paired samples. The results showed that the relative amounts of the [bêta]4 integrin and c-Myc transcripts were significantly upregulated in primary carcinomas in comparison to the resection margins (p < 0.01). Interestingly, a significant correlation was noted between integrin [bêta]4 and c-Myc transcript levels in the various samples (p < 0.005). Co-transfection studies with a c-Myc expression vector and the integrin [bêta]4 promoter linked to the luciferase reporter gene in Caco-2/15 cells showed that the integrin [bêta]4 promoter is activated by c-Myc (p < 0.05). c-Fos mRNA was reduced in colon cancers (p < 0.05), while integrin [bêta]1, Fra-1, Fra-2 and c-Jun transcript levels remained unchanged. In conclusion: (1) in vivo , the level of integrin [bêta]4A subunit expression increased in colon carcinomas relative to their normal counterparts, (2) the increased expression of integrin [bêta]4A is closely correlated with an elevated level of c-Myc as judged by RT-PCR, (3) over-expression of c-Myc activates the integrin [bêta]4 promoter in Caco2/15 cells, (4) the [bêta]4Actd-form, which was detected in the proliferative/undifferentiated compartment of the normal colonic mucosa, is lost in colon cancer cells in both primary tumors and adenocarcinoma cell lines. These finding indicate that the full length form of integrin [bêta]4A is involved in colon cancer cell proliferation and progression.