PP2A inhibition is a druggable MEK inhibitor resistance mechanism in KRAS-mutant lung cancer cells.

Pharmacological PP2A activation is a druggable approach to overcome MEK inhibitor resistance. An oncogene's enemy is our friend: Inhibitors of oncogenic kinases such as MEK are becoming increasingly common as an approach to treating cancer, but these drugs' effectiveness is often short-lived, as tumors develop resistance. Phosphatases, a class of proteins whose activity counteracts that of kinases, are not routinely targeted by cancer therapies but may offer an alternative approach to treatment in some cases. In particular, Kauko et al. determined that the loss of a phosphatase called PP2A can play a major role in resistance to MEK inhibition in lung cancer. The authors also selected a compound that activates PP2A and demonstrated that it can effectively combine with a MEK inhibitor and overcome drug resistance in multiple mouse models of human lung cancer. Kinase inhibitor resistance constitutes a major unresolved clinical challenge in cancer. Furthermore, the role of serine/threonine phosphatase deregulation as a potential cause for resistance to kinase inhibitors has not been thoroughly addressed. We characterize protein phosphatase 2A (PP2A) activity as a global determinant of KRAS-mutant lung cancer cell resistance across a library of >200 kinase inhibitors. The results show that PP2A activity modulation alters cancer cell sensitivities to a large number of kinase inhibitors. Specifically, PP2A inhibition ablated mitogen-activated protein kinase kinase (MEK) inhibitor response through the collateral activation of AKT/mammalian target of rapamycin (mTOR) signaling. Combination of mTOR and MEK inhibitors induced cytotoxicity in PP2A-inhibited cells, but even this drug combination could not abrogate MYC up-regulation in PP2A-inhibited cells. Treatment with an orally bioavailable small-molecule activator of PP2A DT-061, in combination with the MEK inhibitor AZD6244, resulted in suppression of both p-AKT and MYC, as well as tumor regression in two KRAS-driven lung cancer mouse models. DT-061 therapy also abrogated MYC-driven tumorigenesis. These data demonstrate that PP2A deregulation drives MEK inhibitor resistance in KRAS-mutant cells. These results emphasize the need for better understanding of phosphatases as key modulators of cancer therapy responses. [ABSTRACT FROM AUTHOR]