Abstract
BackgroundWhile regulated WNT activity is required for normal development and stem cell maintenance, mutations that lead to constitutive activation of the WNT pathway cause cellular transformation and drive colorectal cancer. Activation of the WNT pathway ultimately leads to the nuclear translocation of β-catenin which, in complex with TCF/LEF factors, promotes the transcription of genes necessary for growth. The proto-oncogene MYC is one of the most critical genes activated downstream the WNT pathway in colon cancer. Here, we investigate the converse regulation of the WNT pathway by MYC.MethodsWe performed RNA-seq analyses to identify genes regulated in cells expressing MYC. We validated the regulation of genes in the WNT pathway including LEF1 by MYC using RT-qPCR, Western blotting, and ChIP-seq. We investigated the importance of LEF1 for the viability of MYC-expressing cells in in fibroblasts, epithelial cells, and colon cells. Bioinformatic analyses were utilized to define the expression of MYC-regulated genes in human colon cancer and metabolomics analyses were used to identify pathways regulated by LEF1 in MYC expressing cells.ResultsMYC regulates the levels of numerous WNT-related genes, including the β-catenin co-transcription factor LEF1. MYC activates the transcription of LEF1 and is required for LEF1 expression in colon cancer cells and in primary colonic cells transformed by APC loss of function, a common mutation in colon cancer patients. LEF1 caused the retention of β-catenin in the nucleus, leading to the activation of the WNT pathway in MYC-expressing cells. Consequently, MYC-expressing cells were sensitive to LEF1 inhibition. Moreover, we describe two examples of genes induced in MYC-expressing cells that require LEF1 activity: the peroxisome proliferator activated receptor delta (PPARδ) and the Acyl CoA dehydrogenase 9 (ACAD9).ConclusionsWe demonstrated that MYC is a transcriptional regulator of LEF1 in colonic cells. Our work proposes a novel pathway by which MYC regulates proliferation through activating LEF1 expression which in turn activates the WNT pathway.Graphical
Highlights
While regulated WNT activity is required for normal development and stem cell maintenance, mutations that lead to constitutive activation of the WNT pathway cause cellular transformation and drive colorectal cancer
We found that Frizzled 2 (FZD2), LRP6, Lymphoid enhancer binding factor 1 (LEF1), and TCF3 were induced by MYC, while the levels of FZD1 and Leukemia inhibitory factor (LIF) were repressed in MYC-expressing fibroblasts, confirming our RNAseq results (Fig. 1 b, c, Additional file 1: Figure S1A-B)
LEF1 is regulated by MYC in colon cancer cells Because the activity of both MYC and the WNT pathway are profoundly involved in colon cancer [8, 15], we examined the 41 pairs of normal/tumor colonic tissues deposited in the TCGA database for the expression of LEF1 and other genes upregulated by MYC in our RNAseq dataset (Fig. 1a)
Summary
While regulated WNT activity is required for normal development and stem cell maintenance, mutations that lead to constitutive activation of the WNT pathway cause cellular transformation and drive colorectal cancer. Activation of the WNT pathway leads to the nuclear translocation of β-catenin which, in complex with TCF/LEF factors, promotes the transcription of genes necessary for growth. Colorectal cancer results from the accumulation of mutations in specific genes, including adenomatous polyposis coli (APC), K-RAS, and P53 [1]. These mutations drive the transition from normal colonic epithelia to dysplastic adenoma and colorectal carcinoma [2]. TCF/LEF proteins mediate WNT signaling in adult tissues, especially in tissues derived from stem cell populations [8]
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