Analysis of the p53/HIF-1A/miR-34a pathway in colorectal cancer

点击翻译按钮获取中文摘要

Tumor-associated hypoxia, i.e. decreased availability of oxygen, is a hallmark of the tumor microenvironment which results in a poor clinical outcome since it selects for therapy-resistant cells and promotes metastasis. The p53 status has previously been identified as a critical determinant of the effect of hypoxia on tumor cells. However, the down-stream pathways providing a selective advantage to hypoxic cells with loss of p53 function have not been characterized yet. Hypoxia is also central for metastasis of colorectal cancer (CRC) since it mediates epithelial-mesenchymal transition/EMT, which provides primary tumor cells with invasive, migratory and anti-apoptotic properties, as well as increased stemness and chemo-resistance. Recently, we reported that under hypoxia p53-deficient cells display EMT, whereas p53-proficient cells undergo MET. In addition, we could show that miR-34a is repressed by HIF-1A during hypoxia in p53-deficient cells. This down-regulation of miR-34a was necessary for hypoxia-induced EMT, invasion and migration as well as STAT3 activation of CRC cell lines. To fully understand the function of the p53/miR-34a pathway in tumor suppression, it is necessary to identify and characterize the miR-34a targets, which are up-regulated under hypoxic conditions and mediate the effects of miR-34a down-regulation. We recently identified XBP-1 as a putative miR-34a target. Importantly, XBP-1 is a known effector of endoplasmatic reticulum/ER-stress elicited by hypoxia and its deregulation has been implicated in the progression of colitis-associated and sporadic colorectal cancer. In addition, we found that IRE1, a regulator of XBP1 splicing, may also be under negative control of p53-induced miRNAs. In this proposal we plan to validate XBP-1 as a direct target of miR-34a and determine the functional implications of these regulations. Furthermore, we will determine the role of the p53/HIF1A/miRNA/IRE1A/XBP-1 connection in hypoxia-induced EMT by using xenograft mouse models of CRC metastasis. To evaluate the in vivo relevance of the proposed regulatory loop for tumor progression we will analyse it in miR-34a and p53 knock-out mice in the context of models of hereditary and sporadic colorectal cancer. In addition, we will analyse this signalling pathway in hypoxia-induced EMT and chemo-resistance using CRC-derived tumoroids. Finally, we will analyze the expression of the loop components in cohorts of human colorectal cancer samples available at the Institute of Pathology to evaluate the conservation of these regulations in humans and the prognostic value of its deregulation. Taken together, this study may offer a mechanistic explanation for the frequent inactivation of p53 and/or miR-34a in tumors and reveal how these alterations contribute to metastasis. In addition, the components of this new signaling pathway represent attractive prognostic markers and potential intervention points for tumor therapeutic approaches to treat CRCs.