Homeobox protein CDX2 reduces Cox-2 transcription by inactivating the DNA-binding capacity of nuclear factor-κB

Background. While cyclooxygenase-2 (COX-2) is not normally expressed by epithelial cells lining the human colon, COX-2 protein is aberrantly overexpressed in premalignant adenomatous polyps and carcinomas of the human colon. On the other hand, Cdx2 has been identified as a colonic tumor-suppressor gene, besides its role in cell differentiation. However, the relationship between CDX2 attenuation and COX-2 overexpression in colorectal carcinoma has not been established. Here, we investigated the mechanistic link between CDX2 downregulation and COX-2 upregulation. Methods. Gene expression was examined by immunoblotting, reverse transcription-polymerase chain reaction, and promoter analysis. Promoter transactivation was quantified by using a luciferase construct. DNA binding of nuclear factor-kappa B (NF-kappa B) was examined by electromobility shift analysis. Results. CDX2 decreased expression of COX-2 mRNA and protein at the transcriptional level in the human colon cancer Caco-2 cell line. Though p50/p65 NF-kappa B translocated into nucleus in the presence of CDX2, CDX2 interacted with p50/p65 NF-kappa B and impeded the formation of an NF-kappa B-DNA complex, required for promotion of Cox-2 transcription. Conclusion. The results indicate that CDX2 inhibits transcription of Cox-2 by interfering with the binding of NF-kappa B on the NF-kappa B binding site.