Transcriptional and post-transcriptional regulation of Nm23-H1 in breast cancer metastasis

Abstract

Tumor metastasis remains a huge obstacle in cancer treatment and is responsible for most cancer-related deaths. Nm23-H1 was discovered as one of the first metastasis suppressor proteins with the ability to inhibit the metastatic process in many cancer types including breast, colon, and liver cancer. Although the loss of Nm23-H1 is frequently observed in aggressive forms of cancer and correlated with metastatic potential, surprisingly little is known regarding the mechanisms that regulate Nm23-H1. Here, we aim to identify regulatory mechanisms that define Nm23-H1 expression in highly aggressive and less invasive breast cancers, which express different levels of Nm23-H1 contributing to distinct phenotypes. Analysis of the Nm23-H1 promoter revealed a proximal promoter and a minimal promoter region, providing binding sites for many transcription factors including CTCF and EGR1. Overexpression of CTCF and EGR1 induced Nm23-H1 transcription and reduced invasiveness of aggressive MDA-MB-231 cells. Moreover, the endogenous expression of CTCF and EGR1 was correlated with Nm23-H1 expression in less invasive MCF-7 cells. To utilize these mechanisms therapeutically, the PKC inhibitor Ro32 was identified from a small panel of drugs with the ability to induce Nm23-H1 expression. Treatment of MDA-MB-231 cells with Ro32 decreased cell invasion, concomitant with its ability to upregulate EGR1 and Nm23-H1. Epigenetic features including promoter methylation, enhancers, and miRNAs, represent plausible mechanisms that may contribute to the downregulation of Nm23-H1. However, potential miRNAs could not be identified by either prediction algorithms or miRNA pull-down assays. The activity of a neighboring enhancer region was also shown irrelevant in reporter studies, while several transcription factor binding regions displayed similar methylation profiles between MDA-MB-231 and MCF-7 cells. Collectively, this study indicates that the loss of Nm23-H1 in aggressive breast cancer is apparently caused by reduced activity of transcriptional pathways, which can potentially be exploited through modulation of EGR1 upon PKC inhibition.

Date of Award	2020
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology