Functional characterisation of tumour suppressor PDCD4 reveals a previously undisclosed role in the control of cell adhesion

PDCD4 is a multifunctional RNA binding protein which has tumour suppressor function. To more fully understand how dysregulation of this protein contributes to carcinogenesis we have carried out a comprehensive analysis of the role of PDCD4 in RNA metabolism in untransformed epithelial cells. We show that PDCD4 predominantly localises in the nucleus, where it interacts with proteins involved in a range of different RNA metabolic processes. We find that PDCD4 knockdown is associated with significant changes in either the expression or splicing of a number of transcripts, although it appears to have an indirect role in splicing. We identified the RNA targets of PDCD4 using iCLIP and observed an enrichment in binding to transcripts encoding cell adhesion and structural proteins. Consistent with these data, we show that PDCD4 acts as a general regulator of cell adhesion, which in a tumour setting would increase the metastatic potential of cells, and demonstrate that the nuclear localisation of PDCD4 is crucial in this process. Overall, the information obtained in untransformed cells provides new perspective for the role of PDCD4 as a tumour suppressor. Overall design: We knocked down PDCD4 with 10nM siRNAs for 48 hours (siPDCD4) and compared the effects on gene expression to control cells (siCT)