MAPK (mitogen-activated protein kinase) signaling pathways regulate a variety of biological processes through multiple cellular mechanisms. In most of these processes, such as apoptosis, MAPKs have a dual role since they can act as activators or inhibitors, depending on the cell type and the stimulus. In this review, we present the main pro- and anti-apoptotic mechanisms regulated by MAPKs, as well as the crosstalk observed between some MAPKs. We also describe the basic signaling properties of MAPKs (ultrasensitivity, hysteresis, digital response), and the presence of different positive feedback loops in apoptosis. We provide a simple guide to predict MAPKs’ behavior, based on the intensity and duration of the stimulus. Finally, we consider the role of MAPKs in osmostress-induced apoptosis by using Xenopus oocytes as a cell model. As we will see, apoptosis is plagued with multiple positive feedback loops. We hope this review will help to understand how MAPK signaling pathways engage irreversible cellular decisions.

https://www.mdpi.com/1422-0067/21/7/2346/pdf

Understanding MAPK Signaling Pathways in Apoptosis.

Inflammation is a healthy response to infection or danger and should be rapid, specific and terminated once the threat has passed. Inflammatory diseases, where this regulation fails, cause considerable human suffering. Treatments have successfully targeted pro-inflammatory cytokines, such as tumor-necrosis factor (TNF), that directly induce genes encoding inflammatory products. Inflammatory signals, including TNF, may also directly induce caspase-independent cell death (necroptosis), which can also elicit inflammation. Necroptosis was originally defined as being dependent on the kinase RIPK1 but is now known to be dependent on RIPK3 and the pseudo-kinase MLKL. Therefore, RIPK1, RIPK3 and MLKL are potential therapeutic targets. RIPK1 and RIPK3 also directly regulate inflammatory signaling, which complicates interpretation of their function but might alter their therapeutic utility. This Review examines the role of cell death, particularly necroptosis, in inflammation, in the context of recent insights into the roles of the key necroptosis effector molecules RIPK1, RIPK3 and MLKL.

The diverse role of RIP kinases in necroptosis and inflammation (vol 16, pg 930, 2016)

Caspase-6 Is a Key Regulator of Innate Immunity, Inflammasome Activation, and Host Defense

Cell suicide pathways, termed regulated cell death (RCD), play a critical role in organismal development, homeostasis, and pathogenesis. Here, we provide an overview of key RCD modalities, namely apoptosis, entosis, necroptosis, pyroptosis, and ferroptosis. We explore how various RCD modules serve as a defense mechanism against the emergence of cancer as well as the manner in which they can be exploited to drive oncogenesis. Furthermore, we outline current therapeutic agents that activate RCD and consider novel RCD-based strategies for tumor elimination. SIGNIFICANCE: A variety of antitumor therapeutics eliminate cancer cells by harnessing the devastating potential of cellular suicide pathways, emphasizing the critical importance of RCD in battling cancer. This review supplies a mechanistic perspective of distinct RCD modalities and explores the important role they play in tumorigenesis. We discuss how RCD modules serve as a double-edged sword as well as novel approaches aimed at selectively manipulating RCD for tumor eradication.

Modes of Regulated Cell Death in Cancer

mTOR is a highly conserved serine/threonine protein kinase that serves as a central regulator of cell growth, survival and autophagy. Deregulation of the PI3K/Akt/mTOR signaling pathway occurs commonly in cancer and numerous inhibitors targeting the ATP-binding site of these kinases are currently undergoing clinical evaluation. Here we report the characterization of Torin2, a second generation ATP-competitive inhibitor that is potent and selective for mTOR with a superior pharmacokinetic profile to previous inhibitors. Torin2 inhibited mTORC1-dependent T389 phosphorylation on S6K (RPS6KB1) with an EC50 of 250 pM with approximately 800-fold selectivity for cellular mTOR versus PI3K. Torin2 also exhibited potent biochemical and cellular activity against PIKK family kinases including ATM (EC50 28 nM), ATR (EC50 35 nM) and DNA-PK (EC50 118 nM) (PRKDC), the inhibition of which sensitized cells to Irradiation. Similar to the earlier generation compound Torin1 and in contrast to other reported mTOR inhibitors, Torin2 inhibited mTOR kinase and mTORC1 signaling activities in a sustained manner suggestive of a slow dissociation from the kinase. Cancer cell treatment with Torin2 for 24 hours resulted in a prolonged block in negative feedback and consequent T308 phosphorylation on Akt. These effects were associated with strong growth inhibition in vitro. Single agent treatment with Torin2 in vivo did not yield significant efficacy against KRAS-driven lung tumors, but the combination of Torin2 with MEK inhibitor AZD6244 yielded a significant growth inhibition. Taken together, our findings establish Torin2 as a strong candidate for clinical evaluation in a broad number of oncological settings where mTOR signaling has a pathogenic role.

Characterization of Torin2, an ATP-Competitive Inhibitor of mTOR, ATM, and ATR

Apoptosis is a complex multi-step process driven by caspase-dependent proteolytic cleavage cascades. Dysregulation of apoptosis promotes tumorigenesis and limits the efficacy of chemotherapy. To assess the complex interactions among caspases during apoptosis, we disrupted caspase-8, -9, -3, -7, or -6 and combinations thereof, using CRISPR-based genome editing in living human leukemia cells. While loss of apical initiator caspase-8 or -9 partially blocked extrinsic or intrinsic apoptosis, respectively, only combined loss of caspase-3 and -7 fully inhibited both apoptotic pathways, with no discernible effect of caspase-6 deficiency alone or in combination. Caspase-3/7 double knockout cells exhibited almost complete inhibition of caspase-8 or -9 activation. Furthermore, deletion of caspase-3 and -7 decreased mitochondrial depolarization and cytochrome c release upon apoptosis activation. Thus, activation of effector caspase-3 or -7 sets off explosive feedback amplification of upstream apoptotic events, which is a key feature of apoptotic signaling essential for efficient apoptotic cell death.

https://www.zora.uzh.ch/id/eprint/177433/1/eaau9433.full.pdf

Efficient apoptosis requires feedback amplification of upstream apoptotic signals by effector caspase-3 or -7

https://www.zora.uzh.ch/id/eprint/198019/1/Aguade_et_al.pdf

TNFR2 is required for RIP1-dependent cell death in human leukemia.

Despite rapid progress in genomic profiling in acute lymphoblastic leukemia (ALL), identification of actionable targets and prediction of response to drugs remains challenging To identify specific vulnerabilities in ALL, we performed a drug screen using primary human ALL samples cultured in a model of the bone marrow microenvironment combined with high content image analysis Among the 2487 FDA-approved compounds tested, anthelmintic agents of the class of macrocyclic lactones exhibited potent anti-leukemia activity, similar to the already known anti-leukemia agents currently used in induction chemotherapy Ex vivo validation in 55 primary ALL samples of both precursor B cell and T-ALL including refractory relapse cases confirmed strong anti-leukemia activity with IC50 values in the low micromolar range Anthelmintic agents increased intracellular chloride levels in primary leukemia cells, inducing mitochondrial outer membrane depolarization and cell death Supporting the notion that simultaneously targeting cell death machineries at different angles may enhance the cell death response, combination of anthelmintic agents with the BCL-2 antagonist navitoclax or with the chemotherapeutic agent dexamethasone showed synergistic activity in primary ALL These data reveal anti-leukemia activity of anthelmintic agents and support exploiting drug repurposing strategies to identify so far unrecognized anti-cancer agents with potential to eradicate even refractory leukemia

/pdf/repurposing-anthelmintic-agents-to-eradicate-resistant-1lrua65872.pdf

Anna Guinot

Papers

Efficient apoptosis requires feedback amplification of upstream apoptotic signals by effector caspase-3 or -7

TNFR2 is required for RIP1-dependent cell death in human leukemia.

Repurposing anthelmintic agents to eradicate resistant leukemia