Background: Colorectal cancer is thought to originate in the expansion of colonic crypt cells as a result of aberrant gene expression caused by transcription factors of the T-cell factor (TCF)/beta-catenin family. Survivin is a bifunctional regulator of cell death and cell proliferation expressed during embryonic development but undetectable in healthy adult tissues and re-expressed in many cancers, including colorectal cancer.
Methods: We investigated gene expression by promoter analysis, mutagenesis, and electrophoretic mobility shift assay in colorectal cancer cells. Survivin expression in human and mouse embryonic intestine was determined by in-situ hybridisation and immunohistochemistry. Changes in apoptosis were monitored in cell lines engineered to express stabilising mutations in beta catenin.
Findings: TCF/beta catenin stimulated a six-fold to 12-fold increased expression of the survivin gene in colorectal cancer cells. Three TCF-binding elements (TBE) in the survivin promoter were occupied by nuclear factors in colorectal cancer cells, and mutagenesis of the two proximal TBE sites abolished survivin gene expression by 75-79%. Strongly expressed at the bottom of human and mouse embryonic intestinal crypts, expression of survivin was lost in TCF-4 knockout animals, and a TCF-4 dominant negative mutant blocked survivin gene transcription in colorectal cancer cells. Expression of non-destructible beta catenin mutants increased survivin expression and protected against ultraviolet-B-induced apoptosis.
Interpretation: Stimulation of survivin expression by TCF/beta catenin might impose a stem cell-like phenotype to colonic crypt epithelium coupling enhanced cell proliferation with resistance to apoptosis, and contribute to the molecular pathogenesis of colorectal cancer.