Down-regulation of E-cadherin is an essential event in activating β-catenin/Tcf dependent transcription and expression of its target genes in Pdcd4 knock-down cells

We previously reported that knock-down of tumor suppressor Pdcd4 (Programmed Cell Death 4) down-regulates E-cadherin expression and activates β-catenin/Tcf (T cell factor) dependent transcription in colon tumor cells. However, the underlying mechanism of these observations remains unknown. In this study, we demonstrated that knock-down of Pdcd4 down-regulates E-cadherin expression through elevated protein level of Snail. Over-expression of Pdcd4 up-regulates E-cadherin expression and inhibits β-catenin/Tcf dependent transcription. We then showed that knock-down of E-cadherin activates β-catenin/Tcf dependent transcription. Conversely, over-expression of E-cadherin in Pdcd4 knock-down cells inhibits β-catenin/Tcf dependent transcription. In addition, Pdcd4 knock-down stimulates u-PAR and c-Myc expression, while u-PAR and c-Myc expression can be reversed by over-expressing E-cadherin in Pdcd4 knock-down cells. Using chromatin immunoprecipitation, we demonstrated that β-catenin/Tcf4 directly binds to the promoters of u-PAR and c-myc in Pdcd4 knock-down cells. Futhermore, knock-down of u-PAR or c-Myc inhibits invasion in Pdcd4 knock-down cells, suggesting that both u-PAR and c-Myc contribute to invasion induced by Pdcd4 knock-down. Taken together, our data demonstrated that elevated Snail expression by Pdcd4 knock-down leads to down-regulation of E-cadherin resulting in activating β-catenin/Tcf dependent transcription and stimulating the expression of c-Myc and u-PAR, thus providing molecular explanation of how Pdcd4 suppresses tumor invasion.