Dissecting the signaling of the MAP3 Kinases TAK1 and MEKK3 for the treatment of pancreatic cancer

Pancreatic cancer (PC) remains one of the most lethal and poorly understood human malignancies and will continue to be a major unsolved health problem in the 21st century. The MAP3K pathway is one of most important pathways that regulate the aggressiveness of PC. In particular, MAP3Ks act by regulating NFkB and YAP/TAZ signaling, two of the most well characterized pathways sustaining the chemoresistance and EMT features of this cancer. We focused our attention on two members of the MAP3K pathway, the TGF-β-activated kinase 1 (TAK1) and the Mitogen-Activated protein kinase kinase kinase 3 (MEKK3) with the aim to understand whether and how they could impact on YAP/TAZ. We showed that TAK1 silencing affects the HIPPO pathway by modulating YAP/TAZ protein levels. We reported for the first time that TAK1 can regulate the stability of YAP/TAZ, independently on its kinase activity, by modulating the expression of E3-ubiquitin ligases, such as TRAF6 and ITCH/AIP4. Moreover, based on a recent report showing that the pharmacological inhibition of GSK3 caused a reduction of TAK1 levels, we treated our cells with GSK3 inhibitors and we observed a reduction of both TAK1 and YAP/TAZ proteins, as well as YAP/TAZ regulated genes. Pharmacological silencing of TAK1 impaired YAP/TAZ-regulated features, such as proliferation and migration. As for MEKK3, we knocked out its expression in different cellular models by CRISPR-Cas9 technology. Then, we assessed the impact of MEKK3 knock-out (MEKK3 KO) on the aggressiveness of PC. We observed a decrease of proliferation and colony formation ability in MEKK3 KO cells. Simultaneously, we observed that MEKK3 KO affects the YAP/TAZ target genes expression, without altering YAP/TAZ protein levels or the NFkB pathway. The emerging role of YAP/TAZ in orchestrating the development and the sustainment of PC opens the need for the discovery of drugs to inhibit their activities but, so far, no specific inhibitors of YAP/TAZ have been identified. Our data open the path for targeting the YAP/TAZ pathway through pharmacological inhibition of GSK3/TAK1 and MEKK3.