Aaron P. Jansen

Programmed cell death 4 (Pdcd4) is a novel repressor of in vitro transformation. Pdcd4 directly inhibits the helicase activity of eukaryotic translation initiation factor 4A, a component of the translation initiation complex. To ascertain whether Pdcd4 suppresses tumor development in vivo, we have generated transgenic mice that overexpress Pdcd4 in the epidermis (K14-Pdcd4). K14-regulated Pdcd4 expression caused a neonatal short-hair phenotype due to early catagen entry compared with matched wild-type siblings. In response to the 7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) mouse skin carcinogenesis protocol, K14-Pdcd4 mice showed significant reductions in papilloma formation, carcinoma incidence, and papilloma-to-carcinoma conversion frequency compared with wild-type mice. The translational efficiency of an mRNA engineered to form a structured 5′ untranslated region (UTR) was attenuated in primary keratinocytes when Pdcd4 was overexpressed. Pdcd4 inhibited by 46% TPA-induced activator protein-1 (AP-1)–dependent transcription, an event required for tumorigenesis. CDK4 and ornithine decarboxylase (ODC) are candidates for Pdcd4-regulated translation as their mRNAs contain 5′structured UTRs. In K14-Pdcd4 primary keratinocytes expressing activated Ha-Ras to mimic DMBA-initiated epidermis, ODC and CDK4 protein levels were decreased by 40% and 46%, respectively. Expression of a protein encoded by 5′ unstructured mRNA showed no change. These results extend to an in vivo model the observations that Pdcd4 inhibits both translation initiation and AP-1 activation while decreasing benign tumor development and malignant progression. The K14-Pdcd4 mice seem to validate translation initiation as a novel target for cancer prevention.

Programmed cell death 4 (Pdcd4), originally identified as an inhibitor of murine cellular transformation, inhibits protein synthesis by directly interacting with eukaryotic initiation factor 4A (eIF4A) of the translation initiation complex. The relevance of Pdcd4 to a broad range of human cancers derived from multiple tissue sites is unknown. Protein expression patterns from the National Cancer Institute drug-screening panel of 60 human cancer cells (NCI60) were analyzed by Western blot methods and revealed frequent reduction of Pdcd4 protein levels in renal-, lung-, and glia-derived tumors. Greater than mean Pdcd4 protein levels correlated with the antitumor activity of geldanamycin and tamoxifen. Stable expression of antisense PDCD4 significantly reduced the sensitivity of MCF-7 breast cancer cells to geldanamycin and to tamoxifen. Sensitivity to geldanamycin significantly increased in UO-31 renal cancer cells expressing sense PDCD4 cDNA. Increased geldanamycin sensitivity was accompanied by enhanced cell cycle arrest and apoptosis. One primary mode of inactivation of Pdcd4 in human cancers appears to involve down-regulated expression, and this down-regulation causes a decreased sensitivity to geldanamycin cytotoxicity. Thus, up-regulating Pdcd4 expression may be promising for geldanamycin-based combination therapy.