TAK-ing aim at chemoresistance: The emerging role of MAP3K7 as a target for cancer therapy

Cellular drug resistance remains the main obstacle to the clinical efficacy of cancer chemotherapy. Alterations in key pathways regulating cell cycle checkpoints, apoptosis and Epithelial to Mesenchymal Transition (EMT), such as the Mitogen-activated protein kinase (MAPK) pathway, appear to be closely associated to cancer chemoresistance. Transforming growth factor-β (TGF-β)- activated kinase 1 (TAK1, also known as MAP3K7) is a serine/threonine kinase in the mitogen-activated protein kinase (MAP3K) family. It represents the cellular hub to which IL1, TGF-β and Wnt signaling pathways converge. By regulating the phosphorylation status and activities of transcription factors including Activated Protein-1 (AP-1) and nuclear factor κ-B (NF-κB), TAK1 mediates inflammatory and pro-survival responses. The interest towards the therapeutic targeting of TAK1 is due to its identification as one of the main mediators of both chemoresistance and EMT in several types of tumors, and as the possible target for a subset of treatment-refractory colon cancers exhibiting mutated KRAS or activated WNT pathways. For these reasons, many efforts have been made to design inhibitors of TAK1 kinase activity, which could be used to reverse TAK1-mediated chemoresistance. The activity of these inhibitors, in combination with the most commonly used chemotherapeutic drugs, has been tested in preclinical studies, proving the efficacy of TAK1 inhibition in reducing tumor growth and survival following chemotherapy administration. In the first part of this review, we describe the mechanisms underlying TAK1 regulation such as phosphorylation, ubiquitination and targeting by microRNAs. We then focus on the development of therapeutic small molecule inhibitors of TAK1 kinase activity, as well as preclinical studies supporting the role of TAK1 as a potential target for enhancing the response of tumors to anticancer therapies.