CHIP mediates CAD ubiquitination and degradation to suppress the proliferation and colony formation of glioblastoma cells

Abstract

Cancer cells are characterized as the uncontrolled proliferation, which demands high levels of nucleotides that are the building blocks for DNA replication. CAD is a trifunctional enzyme that initiates the de novo pyrimidine synthesis, which is normally enhanced in cancer cells to preserve the pyrimidine pool for cell division. Glioma, representing the majority of brain cancers, is highly addicted to nucleotides like pyrimidine to sustain the abnormal growth and proliferation of cells. CAD is previously reported to by dysregulated in glioma, but the underlying mechanism remains unclear. Here, we showed that the expression of CAD was upregulated in glioma, which was positively correlated with tumor grade and the survival of glioma patients. Knockdown of CAD robustly inhibited the cell proliferation and colony formation of glioblastoma (GBM) cells, indicating the essential role of CAD in the pathogenesis of GBM. Mechanistically, we firstly identified that CAD was modified by the K29-linked polyubiquitination, and subsequently subjected to degradation. CHIP interacted and ubiquitinated CAD, which accounted for the anti-proliferative role of CHIP in GBM. Specifically, the ectopic expression of CAD dramatically enhanced the colony formation of GBM cells, which was subsequently abolished by CHIP overexpression. To sustain the expression of CAD, CHIP is significantly downregulated, which is correlated with the poor prognosis and survival of GBM patients. Notably, low level of CHIP and high level of CAD predict the short overall survival of GBM patients. These results illustrated the essential role of CAD in glioma and reveal novel therapeutic targets for CAD-positive and CHIP-negative cancer.