A feedback loop between miRNA-155, Programmed cell death 4 and Activation Protein-1 modulates the expression of miR-155 and tumorigenesis in tongue cancer

ABSTRACT miR-155 is an oncomir, generated as a non-coding RNA from BIC gene whose promoter activity is mainly controlled via AP-1 and NF-κB transcription factors. We found that the expression levels of miR-155 and Pdcd4 exhibit inverse relationship in tongue cancer cells (SAS and AWL) and tumor tissues compared to normal FBM cells and normal tongue tissues, respectively. Insilco and In-vitro studies with 3’UTR of Pdcd4 via luciferase reporter assays, qPCR and western blots show that miR-155 directly targets Pdcd4 mRNA and blocks its expression. Ectopic expression of Pdcd4 or knockdown of miR-155 in tongue cancer cells predominantly reduces AP-1 dependent transcriptional activity of BIC promoter and decreases miR-155 expression. In this study, we demonstrate that miR-155 expression is modulated by a feedback loop between Pdcd4, AP-1 and miR-155 which results in enhanced expression of miR-155 with a consequent progression of tongue tumorigenesis. Further, miR-155 knockdown increases apoptosis, arrests cell cycle, regresses tumor size in xenograft nude mice and reduces cell viability and colony formation in soft agar and clonogenic assays. Thus, the restoration of Pdcd4 levels by the use of molecular manipulation such as using miR-155 sponge have important role in the therapeutic intervention of cancers, including tongue cancer.