Cleavage of CAD by caspase-3 determines the cancer cell fate during chemotherapy

Metabolic heterogeneity resulting from the intrinsic heterogeneity of human tumors has been shown to cause a myriad of adverse outcomes of tumor therapy, including resistance to chemotherapy, but the mechanisms inside remain largely unknown. Here, we found that the de novo pyrimidine synthesis pathway, which we previously identified as crucial for the proliferation of gastric and colon cancer cells, determines the chemosensitivity of these cancer types. In the chemosensitive cells, chemotherapeutic drugs such as 5-FU promoted the degradation of CAD, an enzyme that is rate-limiting for pyrimidine synthesis, leading to the apoptosis of these cells. We also found that CAD needed to be cleaved, by activated caspase-3 on its Asp1371 residue, before its degradation. Upregulation of CAD, either by overexpression or by a mutation on the Asp1371 to block the caspase-3 cleavage, conferred tumor chemoresistance in both mouse xenograft models and the Cldn18-ATK mouse gastric tumor models. Importantly, mutations related to Asp1371 of CAD were found in gastric tumor samples of patients who underwent unsuccessful neoadjuvant chemotherapy. By searching for a compound that capable for degradating the CAD-Asp1371 mutant, we discovered that a compound called RMY-186 can be used to treat chemoresistant tumors related to CAD mutations, resulting in a significant improvement in the effectiveness of chemotherapy. We have, therefore, revealed the vulnerability of de novo pyrimidine synthesis during chemotherapy, which can be targeted to overcome chemotherapeutic resistance and augment the antitumor efficacy of genotoxic chemotherapy agents.