Abstract 4304: Feedback-phosphorylation of MKK4 by MAPKs promotes apoptosis

Abstract Feedback-phosphorylation of MKK4 by MAPKs promotes apoptosis Moriizumi Hisashi, Takanori Nakamura and Mutsuhiro Takekawa Div. of Cell Signaling and Mol. Med., IMS, Tokyo Univ. The mitogen-activated protein (MAP) kinase pathway is an important system for cellular responses to extracellular stimuli. Each of the MAPK pathways is composed of MAPKKK, MAPKK and MAPK. In mammalian cells, at least three subfamilies of MAPKs, namely ERK, p38 and JNK, are present. The ERK pathway is activated in response to mitogenic stimuli and plays a key role in proliferation and differentiation. In contrast, the p38 and JNK pathways [collectively called stress-activated protein kinase (SAPK) pathways] regulate cellular stress responses such as apoptosis and cell cycle arrest. MKK4 is unique among the members of the mammalian SAPKK family (i.e., MKK3/4/6/7) in its ability to phosphorylate and activate both p38 and JNK. Interestingly, inactivating mutations of the MKK4 gene are frequently found in many types of cancer. Although MKK4 has been suggested as a tumor suppressor, the molecular mechanism by which MKK4 inhibits tumorigenesis remains unclear. Here we demonstrate that MKK4 can be phosphorylated at specific serine and/or threonine residues by downstream MAPKs(named the “feedback-phosphorylation”)and thereby regulates stress-induced apoptotic cell death. By generating antibodies specific to these phosphorylation sites, we found that under stress conditions both Thr and Ser residues of MKK4 were phosphorylated by JNK, while only the Ser residue was phosphorylated by ERK in response to mitogenic stimuli. In order to clarify physiological significance of the feedback-phosphorylation, we then generated a phosphorylation-deficient mutant of MKK4 (MKK4-AA) by substituting both Thr and Ser residues with Ala. Although the feedback-phosphorylation did not affect the enzymatic activity, stability and subcellular localization of MKK4, we found that cells expressing MKK4-AA mutant suppressed apoptosis induced by various stresses stimuli such as UV irradiation and anticancer drug treatment. These results suggest that feedback-phosphorylation of MKK4 regulates stress-induced apoptosis. Moreover, we are currently investigating the mechanism of how MKK4 affects apoptotic cell death in response to feedback-phosphorylation. Citation Format: Hisashi Moriizumi. Feedback-phosphorylation of MKK4 by MAPKs promotes apoptosis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4304. doi:10.1158/1538-7445.AM2017-4304