Modulation of the Tumor Supressor Programmed Cell Death 4 by the Protein Arginine Methyl Transferase 5: From Biology to Biomarker

Programmed cell death 4 (PDCD4) has been described as a tumor suppressor in multiple cancer cell types. In vitro, exogenous expression of PDCD4 results in decreased anchorage-independent cell growth and invasion. These anticancer phenotypes are attributed to inhibition of the translation initiation factor eIF4A when bound to PDCD4. In this dissertation, I report the discovery of novel interactions with the nuclear pore protein Nup153, the exon junction core protein eIF4AIII, and protein arginine methyltransferase 5 (PRMT5) that may modulate PDCD4 in a cancer context. PDCD4 levels are often suppressed in cancerous cells compared to normal surrounding tissues, and elevated expression in tumors is correlated with better survival outcomes. Despite this, 20-30% of patients with tumors that express high levels of PDCD4 have poor outcomes, indicating that these cancers deactivate PDCD4. Our analyses of transcript expression in breast cancer patients show that simultaneous upregulation of PRMT5 with PDCD4 results in poor survival outcomes. Using an orthotopic tumor model, I demonstrate that simultaneous expression of PDCD4 and PRMT5 in the breast cancer cell line MCF7 causes accelerated tumor growth. This tumor growth phenotype is dependent on PRMT5 enzymatic activity and the PDCD4 Nterminal site that is modified by PRMT5. This demonstrates that PDCD4 tumor suppressor function is radically altered when modified by PRMT5. Furthermore, this provides a mechanism for poor outcomes in patients with tumors that express elevated iv PDCD4. These findings show the utility of tracking both PDCD4 and PRMT5 as biomarkers and reveals PRMT5 as a potential target of chemotherapy. Finally, PDCD4 acts as a tumor suppressor through inhibition of the RNA helicase activity of eIF4A, although the precise mechanism of how this is accomplished has been unknown. In this dissertation, I report that PDCD4 interferes with the ability of eIF4A to interact with RNA, thereby deactivating its RNA helicase function. This provides a clear in vitro mechanism for eIF4A inhibition by PDCD4.