Differential display of mRNA analysis identified Pdcd4 (Cmarik et al., PNAS 1999) as a novel suppressor of transformation. Antisense expression of the novel pdcd4 gene converts transformation resistant (P-) to sensitive (P+) cells and pdcd4 sense expression (Yang et al Oncogene 2001) converts P+ to P- cells, thus establishing a causal relationship to prevention of tumor promoter induced transformation. Furthermore pdcd4 expression suppresses tumor phenotype in transformed JB6 cells (Yang et al Oncogene 2003). A surprising discovery is that Pdcd4 expression in human cancer cell lines is predictive for sensitivity to tamoxifen and geldanamycin. Moreover, expression of Pdcd4 actually confers sensitivity to these drugs (Jansen et al Molec Cancer Ther 2004). Examination of the possible inhibitory effect of pdcd4 on molecular events known to be required for tumor promotion revealed that pdcd4 expression inhibited the activation of transcription factor AP-1 but not of NFkappa B or of ornithine decarboxylase(Yang et al Oncogene 2001). The AP-1 inhibiting activity of Pdcd4 appears to be attributable to blocking the transactivation of cJun and cFos (Yang et al Oncogene 2003). Although expression of Pdcd4 protein blocks AP-1 activation, Pdcd4 does not interact directly with Jun or Fos proteins. Analysis of Pdcd4 binding partners by a yeast two-hybrid assay and co-immunoprecipitation revealed the translation initiation factors RNA helicase eIF4A and scaffold eIF4G as major binding partners (Yang et al Molec Cell Biol 2003, MCB 2004). Binding of Pdcd4 to eIF4A is required for Pdcd4 to inhibit eIF4A's RNA helicase, to inhibit translation, and to inhibit the activation of AP-1 dependent transcription required for neoplastic transformation. Mutational analysis defines two helical MA-3 domains as required for binding to eIF4A and for inhibiting translation (Yang et al Molec Cell Biol 2004). Residues on eIF4A required for binding Pdcd4 have also been characterized (Zakowicz RNA 2005). In collaboration with the laboratory of Alexander Wlodawer, the crystal structure of the C-terminal MA3 domain has been solved (LaRonde-LeBlanc, Santhanam et al MCB in press 2006). Analysis of the crystal structure predicts a mechanism by which Pdcd4 acts to inhibit translation initiation, a prediction that has been experimentally confirmed. Pdcd4 expression is downregulated in a number of human cancers. Pdcd4 overexpression inhibits invasion by human cancer cells. The mechanism involves targeting expression of a kinase upstream of cJun N-terminal kinase to consequently inhibit AP-1 dependent transcription (Yang et al MCB 2006).Recent investigation of the mechanism by which tumor suppressor Pdcd4 is down regulated during carcinogenesis revealed tumor promoter induced destabilization of the protein by a mechanism involving signaling through Akt, S6kinase and MEK/ERK (Schmid, Jansen et al, submitted). In collaboration with Michele Pagano at NYU, Pdcd4 has emerged as a target for degradation by the ubiquitin ligase betaTRCP (Dorrello et al Science in press). Current research is focused on identifying specific transformation relevant mRNAs whose translation is inhibited by Pdcd4 expression. Tools for drug discovery targeting translation initiation are currently being generated in collaboration with Bruce Shapiro, Stuart LeGrice, Nahum Sonenberg (McGill Univ) and Jim McMahon.
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