Abstract 3079: Identification of the translational targets of tumor suppressor Pdcd4
Authors: Yoshikawa Noriko , Young Matthew R. , Hegamyer Glenn , Baker Alyson R. , Santhanam Arti N. , Wei Qiou , Colburn Nancy H. 
Published: June 2012

Programmed-cell-death-4 (Pdcd4) is a novel protein that functions as a transformation suppressor. Pdcd4 deficiency renders mice susceptible to increased tumor multiplicity in initiation-promotion skin carcinogenesis. On the contrary transgenic expression of Pdcd4 in mice inhibits DMBA-TPA induced skin tumorigenesis and tumor progression. Pdcd4 interacts with translation initiation factors eIF4A and eIF4G to inhibit translation in an mRNA-specific fashion and consequently blocks pro-oncogenic events such as activation of activator protein-1 (AP-1)-dependent transcription, anchorage-independent transformation and invasion. Pdcd4 expression decreases during carcinogenesis in several human cancer sites including colon, esophagus, lung, pancreas and brain and its activation appears to be important in the mechanism by which some cancer therapeutic drugs work. While Pdcd4 is not mutationally inactivated, its expression is post translationally regulated. Pdcd4 expression is decreased during carcinogenesis by mechanisms involving microRNA-21 (miR21) inhibition of translation and by S6 kinase-dependent proteasome degradation. Loss of Pdcd4 has been shown to affect transcription of TIMP-2, MAP4K1, LOX and u-PAR.

Although the tumor suppressor Pdcd4 acts directly on the translation initiation complex, translational targets of Pdcd4 have yet to be identified. In order to identify and characterize functionally significant translational targets of tumor suppressor Pdcd4 when it inhibits translation during tumor promotion, tumorigenesis and invasion of malignant tumors, we knocked down Pdcd4 in T47D breast cancer cells. Loss of Pdcd4 increases cell migration and invasion as well as activity of luciferase reporters of AP-1-dependent transcription and translation initiation. To identify translational targets of Pdcd4, translationally active mRNA species were fractionated by sucrose gradient. Polyribosome bound mRNA fractions were pooled and analyzed for mRNA levels by microarray. Knockdown of Pdcd4 produced more than 2 fold increases in 14 mRNAs in the translationally active polyribosomal fractions compared to the control. The polysome shifts in Pdcd4 translational targets are being confirmed by quantitative RT-PCR and Western blot analysis of protein. Pdcd4 targets recently confirmed by quantitative RT-PCR are PPARGC1B and delta p73, both known to be oncogenic, as well as zinc finger protein ZNF281. The functional significance of these targets is being determined. Discovery of the mRNAs selectively targeted by Pdcd4 will facilitate the elucidation of the mechanism by which Pdcd4 suppresses tumorigenesis and may provide additional targets for intervention.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3079. doi:10.1158/1538-7445.AM2011-3079