MLKL: Functions beyond serving as the Executioner of Necroptosis

Abstract

Recently, necroptosis, as a programmed cell death pathway, has drawn much attention as it has been implicated in multiple pathologies, especially in the field of inflammatory diseases. Pseudokinase mixed lineage kinase domain-like protein (MLKL) serves as a terminal-known obligate effector in the process of necroptosis. To date, the majority of research on MLKL has focused on its role in necroptosis, and the prevailing view has been that the sole function of MLKL is to mediate necroptosis. However, increasing evidence indicates that MLKL can serve as a regulator of many diseases via its non-necroptotic functions. These functions of MLKL shed light on its functional complexity and diversity. In this review, we briefly introduce the current state of knowledge regarding the structure of MLKL, necroptosis signaling, as well as cross-linkages among necroptosis and other regulated cell death pathways, and we particularly highlight recent progress related to newly identified functions and inhibitors of MLKL. These discussions promote a better understanding of the role of MLKL in diseases, which will foster efforts to pharmacologically target this molecule in clinical treatments.

Discovery of Sulforaphane as an Inducer of Ferroptosis in U-937 Leukemia Cells: Expanding Its Anticancer Potential

“…To investigate the involvement of necroptotic cell death in the cytotoxicity of SFN, we analyzed the protein expression of the key regulators of necroptosis. Firstly, we investigated the level of phosphorylated MLKL (p-MLKL), as phosphorylation of MLKL is crucial for the execution of the necroptotic process [ 55 ]. Results obtained indicate that at all tested time treatments (i.e., 9, 16, and 24 h), phosphorylation of MLKL takes place only in U-937 cells treated with a combination of TNF-α/SM-164/Z-VAD-FMK, the gold standard of necroptosis induction [ 55 ] ( Figure 6 a and Figure S11 ).…”

“…Firstly, we investigated the level of phosphorylated MLKL (p-MLKL), as phosphorylation of MLKL is crucial for the execution of the necroptotic process [ 55 ]. Results obtained indicate that at all tested time treatments (i.e., 9, 16, and 24 h), phosphorylation of MLKL takes place only in U-937 cells treated with a combination of TNF-α/SM-164/Z-VAD-FMK, the gold standard of necroptosis induction [ 55 ] ( Figure 6 a and Figure S11 ). To further confirm that necroptosis was not the type of cell death induced by SFN, we analyzed the expression of phosphorylated RIP3 (p-RIP3).…”

Tumor necroptosis is correlated with a favorable immune cell signature and programmed death-ligand 1 expression in cholangiocarcinoma

“…These results demonstrated the differential roles of MLKL and its role in necroptosis activation in CCA patients. Accumulating evidence indicates the newly identified functions of MLKL beyond necroptosis, including regulating the generation of intraluminal and extracellular vesicles 44 , 45 and endothelial cell adhesion gene expression 46 ; however, the functional roles and a deeper mechanistic understanding of non-necroptotic functions of MLKL in cancers, including CCA, require further investigation 47 . Consistent with other studies in these patients, infiltration of CD8+ T cells and CD163+ M2 macrophages were both significantly associated with longer and shorter patient survival in our CCA cohorts, respectively 4 , 5 , 48 – 50 but the FOXp3+ regulatory T cells reported to suppress immune responses against tumor cells and to be correlated with poor clinical outcome has remained an opposing finding compared to other types of cancers 51 .…”

Necroptosis in Pulmonary Diseases: A New Therapeutic Target

“…The endosomal sorting complex required for transport-III can temporarily protect cells undergoing necroptosis to express functional proteins ( Gong et al, 2017 ). Aside from being the executioner of necroptosis, MLKL can indirectly regulate gene expression, block specific processes via affinity for selected molecules ( Zhan et al, 2021 ), affect cancer development and metastasis ( Martens et al, 2021 ), and promote inflammation by activating NLRP3 ( Conos et al, 2017 ).…”