Abstract 1984: A regulatory triad consisting of BMI1, let-7i miRNA and ERK3 kinase in controlling cancer cell invasiveness

Extracellular signal-regulated kinase 3 (ERK3) is an atypical mitogen-activated protein kinase, whose biological activity is tightly regulated by its cellular abundance. ERK3 expression is upregulated in multiple cancer types, including head and neck cancers. Importantly, our recent finding revealed a critical role for ERK3 in promoting cancer cell invasiveness. However, little is known about the molecular regulation of ERK3 level in cancers. Here we identify a pathway which regulates ERK3 expression and cell invasiveness of head and neck cancers, comprising the oncogenic polycomb group protein BMI1 and the tumor suppressive miRNA let-7i. To study the regulation of ERK3, we performed several loss-of-function and gain-of-function assays in head and neck cancer cell lines, including OECM1 and Fadu. Stable knockdown of BMI1 greatly reduced ERK3 protein level, whereas overexpression of BMI1 significantly increased ERK3 level, demonstrating a positive regulation of ERK3 by BMI1. Interestingly, we found that this regulation is mediated by the miRNA let-7i, whose transcription is repressed by BMI1, a component of the polycomb repressive complex 1. ERK3 mRNA is directly targeted by let-7i miRNA, which was demonstrated by the assay using a luciferase reporter containing the 3’UTR of ERK3 mRNA downstream of luciferase gene. Mutation of the let-7i binding site in ERK3 3’UTR abolished the negative regulation of let-7i on ERK3. Further, let-7i mimic decreased ERK3 expression and let-7i inhibitor increased ERK3 expression in head and neck cancer cells. To determine the functional significance of this regulation, we investigated their role in cancer cell invasiveness by performing trans-well migration/invasion assays. Importantly, ERK3 depletion abolished BMI1-induced increase in cancer cell migration and invasion, so was by let-7i mimic. Finally, we examined by immunohistochemistry the expression of both BMI1 and ERK3 in head and neck tumor specimens and found that ERK3 expression is positively correlated with BMI1. All together, we unravel a novel regulatory mechanism for ERK3 by the BMI1/let-7i axis, which together controls head and neck cancer cell invasiveness. Citation Format: Lobna Elkhadragy, Minyi Chen, Weiwen Long. A regulatory triad consisting of BMI1, let-7i miRNA and ERK3 kinase in controlling cancer cell invasiveness. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Phi