Despite many advances in diagnosis and treatment, colorectal cancer (CRC) remains one of the leading causes of cancer-related morbidity and mortality worldwide. Understanding factors in the colonic epithelial that promote colonic adenoma formation and subsequent development of CRC are of critical importance. We have shown that menin, a nuclear scaffold protein that has both tumor suppressor and tumor promoter roles in a highly tissue specific manner, is over-expressed in CRC compared to benign colonic epithelium, and unbiased RNA-Seq revealed that menin inhibition leads to enhanced transcription of target genes of the liver X receptor (LXR), therefore demonstrating that menin represses LXR-mediated gene transcription. The LXRs, including LXR? and LXR?, are nuclear receptors that are critical for maintaining cellular cholesterol homeostasis, and increased LXR-mediated gene transcription can lead to suppression of CRC growth. However, the mechanism whereby menin represses LXR-mediated gene transcription as well as the involvement of menin in progression through the colon adenoma-carcinoma sequence is currently unknown and serves as the focus of this application. Our data shows that LXR target genes are the most significantly upregulated after menin inhibition in CRC cells, and that menin binds to LXR response elements (LXREs) with LXR? and inhibits the recruitment of RNA polymerase II. Furthermore, the increase in LXR-mediated gene transcription induced by menin inhibition has functional consequences, leading to decreased levels of cellular cholesterol in CRC cells and enhancing sensitivity of CRC cells to lipid starvation. As the importance of the interaction between menin and the LXRs in the colonic epithelial is uncertain, our findings lead to a novel hypothesis that menin upregulation in the colonic epithelium inhibits LXR activity, enriches cellular cholesterol, and promotes progression along the adenoma-carcinoma sequence. We will test this hypothesis using innovative methods and novel strategies to define the mechanism whereby menin represses LXR-mediated gene transcription. Furthermore, we will determine whether menin, and its repression of LXR-mediated transcription, are important for promoting progression through the adenoma-carcinoma sequence in the colonic epithelium both in vitro and in vivo. These experiments will provide important insights into the role of menin in both the colonic epithelium and epithelial progression through the adenoma-carcinoma sequence. Moreover, these experiments will serve as a platform for the development of future proposals that will have application to CRC prevention and treatment. Additionally, it is expected that this R03 award will be critical to facilitating future publication generation, a R01 grant application, and the PI's transition to independence.
Colorectal cancer (CRC) remains a leading cause of cancer-related morbidity and mortality worldwide, and understanding factors that promote development of CRC are of critical importance. This study will investigate the mechanisms whereby menin, a nuclear scaffold protein that is over-expressed in CRC, can repress LXR- mediated signaling in the colon, and whether this repression may increase cellular cholesterol and promote adenoma and CRC development in the colon. The proposed experiments will further define the importance of menin in the colonic epithelium, and will inform future prevention and treatment efforts in CRC.
