Pdcd4 knockdown up-regulates MAP4K1 expression and activation of AP-1 dependent transcription through c-Myc.

Programmed cell death 4 (Pdcd4) is a novel tumor suppressor, whose expression is frequently down-regulated in several types of cancers. In the present study, we demonstrated that Pdcd4 knockdown up-regulates MAP kinase kinase kinase kinase 1 (MAP4K1) expression and increases phosphorylation of c-Jun. Over-expression of c-Myc in HEK293 cells increases the levels of MAP4K1, MAP4K1 promoter activity, and phospho-c-Jun. Mutation analysis showed that the c-Myc binding site at -536bp (relative to the initiation ATG) of map4k1 promoter responds to c-Myc regulation. In addition, chromatin immunoprecipitation demonstrated that c-Myc directly binds to map4k1 promoter at this site. Down-regulation of c-Myc reverses MAP4K1 expression and AP-1 activation in Pdcd4 knockdown cells. Moreover, over-expression of dominant negative Tcf4 decreases expression of c-Myc and MAP4K1, JNK activation, and AP-1 dependent transcription. Thus, activation of β-catenin/Tcf dependent transcription in Pdcd4 knockdown cells up-regulates MAP4K1 expression and AP-1 activity via c-Myc. The study presented here further reveals in detail the mechanism of how Pdcd4 inhibits tumor cell invasion and provides a functional connection between β-catenin/Tcf and AP-1 dependent transcription.