Mutations in APC, CTNNB1, AXIN1 or AXIN2 cause impairment in the β-catenin degradation pathway and result in accumulation of β-catenin in a wide range of human cancers. Accumulated β-catenin then associates with Tcf/LEF transcription factors and transactivates their target genes. To uncover in detail the role of accumulated β-catenin in colorectal carcinogenesis, we searched for genes involved in the β-catenin/Tcf signaling pathway by cDNA microarray. We identified and characterized a human gene, SP5, that was down-regulated after depletion of β-catenin by transduction of wild-type APC into SW480 cells. SP5 is a member of the Sp transcription factor family, which binds to the GC box or closely related sequences in promoters of many genes and control their expression. Reporter assays and an electromobility-shift assay revealed a DNA fragment between −285 and −279 in the 5' flanking region of this gene to be a target of the β-catenin/Tcf4 complex. Our results indicate that SP5 is a novel direct down-stream target in the Wnt signaling pathway.

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