C-Myc directly targets an over-expression of pyruvate carboxylase in highly invasive breast cancer

Abstract

Here we showed that the c-Myc oncogene is responsible for overexpression of pyruvate carboxylase (PC) in highly invasive MDA-MB-231 cells. Pharmacological inhibition of c-Myc activity with 10074-G5 compound, resulted in a marked reduction of PC mRNA and protein, concomitant with reduced cell growth, migration and invasion. This growth inhibition but not migration and invasion can be partly restored by overexpression of PC, indicating that PC is a c-Myc-regulated pro-proliferating enzyme. Analysis of chromatin immunoprecipitation sequencing of c-Myc bound promoters revealed that c-Myc binds to two canonical c-Myc binding sites, locating at nucleotides −417 to −407 and −301 to −291 in the P2 promoter of human PC gene. Mutation of either c-Myc binding site in the P2 promoter-luciferase construct resulted in 50–60% decrease in luciferase activity while double mutation of c-Myc binding sites further decreased the luciferase activity in MDA-MB-231 cells. Overexpression of c-Myc in HEK293T cells that have no endogenous c-Myc resulted in 250-fold increase in luciferase activity. Mutation of either E-boxes lowered luciferase activity by 50% and 25%, respectively while double mutation of both sites abolished the c-Myc transactivation response. An electrophoretic mobility shift assay using nuclear proteins from MDA-MB-231 confirmed binding of c-Myc to both c-Myc binding sites in the P2 promoter. Bioinformatic analysis of publicly available transcriptomes from the cancer genome atlas (TCGA) dataset revealed an association between expression of c-Myc and PC in primary breast, as well as in lung and colon cancer tissues, suggesting that overexpression of PC is deregulated by c-Myc in these cancers.