A phosphorylation of RIPK3 kinase initiates an intracellular apoptotic pathway that promotes prostaglandin2α-induced corpus luteum regression

Caspase-8 activity mediates TNFα production and restricts Coxiella burnetii replication during murine macrophage infection

Coxiella burnetii is an obligate intracellular bacteria that causes the global zoonotic disease Q Fever. Treatment options for chronic infection are limited, and the development of novel therapeutic strategies requires a greater understanding of how C. burnetii interacts with immune signaling. Cell death responses are known to be manipulated by C. burnetii, but the role of caspase-8, a central regulator of multiple cell death pathways, has not been investigated. In this research, we studied bacterial manipulation of caspase-8 signaling and the significance of caspase-8 to C. burnetii infection, examining bacterial replication, cell death induction, and cytokine signaling. We measured caspase, RIPK, and MLKL activation in C. burnetii-infected tumor necrosis factor alpha (TNFα)/cycloheximide-treated THP-1 macrophage-like cells and TNFα/ZVAD-treated L929 cells to assess apoptosis and necroptosis signaling. Additionally, we measured C. burnetii replication, cell death, and TNFα induction over 12 days in RIPK1-kinase-dead, RIPK3-kinase-dead, or RIPK3-kinase-dead-caspase-8-/- bone marrow-derived macrophages (BMDMs) to understand the significance of caspase-8 and RIPK1/3 during infection. We found that caspase-8 is inhibited by C. burnetii, coinciding with inhibition of apoptosis and increased susceptibility to necroptosis. Furthermore, C. burnetii replication was increased in BMDMs lacking caspase-8, but not in those lacking RIPK1/3 kinase activity, corresponding with decreased TNFα production and reduced cell death. As TNFα is associated with the control of C. burnetii, this lack of a TNFα response may allow for the unchecked bacterial growth we saw in caspase-8-/- BMDMs. This research identifies and explores caspase-8 as a key regulator of C. burnetii infection, opening novel therapeutic doors.

Protein phosphorylation and kinases: Potential therapeutic targets in necroptosis

Necroptosis is a pivotal contributor to the pathogenesis of various human diseases, including those affecting the nervous system, cardiovascular system, pulmonary system, and kidneys. Extensive investigations have elucidated the mechanisms and physiological ramifications of necroptosis. Among these, protein phosphorylation emerges as a paramount regulatory process, facilitating the activation or inhibition of specific proteins through the addition of phosphate groups to their corresponding amino acid residues. Currently, the targeting of kinases has gained recognition as a firmly established and efficacious therapeutic approach for diverse diseases, notably cancer. In this comprehensive review, we elucidate the intricate role of phosphorylation in governing key molecular players in the necroptotic pathway. Moreover, we provide an in-depth analysis of recent advancements in the development of kinase inhibitors aimed at modulating necroptosis. Lastly, we deliberate on the prospects and challenges associated with the utilization of kinase inhibitors to modulate necroptotic processes.

A matter of new life and cell death: programmed cell death in the mammalian ovary

BACKGROUND

The mammalian ovary is a unique organ that displays a distinctive feature of cyclic changes throughout the entire reproductive period. The estrous/menstrual cycles are associated with drastic functional and morphological rearrangements of ovarian tissue, including follicular development and degeneration, and the formation and subsequent atrophy of the corpus luteum. The flawless execution of these reiterative processes is impossible without the involvement of programmed cell death (PCD).

MAIN TEXT

PCD is crucial for efficient and careful clearance of excessive, depleted, or obsolete ovarian structures for ovarian cycling. Moreover, PCD facilitates selection of high-quality oocytes and formation of the ovarian reserve during embryonic and juvenile development. Disruption of PCD regulation can heavily impact the ovarian functions and is associated with various pathologies, from a moderate decrease in fertility to severe hormonal disturbance, complete loss of reproductive function, and tumorigenesis. This comprehensive review aims to provide updated information on the role of PCD in various processes occurring in normal and pathologic ovaries. Three major events of PCD in the ovary-progenitor germ cell depletion, follicular atresia, and corpus luteum degradation-are described, alongside the detailed information on molecular regulation of these processes, highlighting the contribution of apoptosis, autophagy, necroptosis, and ferroptosis. Ultimately, the current knowledge of PCD aberrations associated with pathologies, such as polycystic ovarian syndrome, premature ovarian insufficiency, and tumors of ovarian origin, is outlined.

CONCLUSION

PCD is an essential element in ovarian development, functions and pathologies. A thorough understanding of molecular mechanisms regulating PCD events is required for future advances in the diagnosis and management of various disorders of the ovary and the female reproductive system in general.

Anti-Necroptotic Effects of Itaconate and its Derivatives

Itaconate is an unsaturated dicarboxylic acid that is derived from the decarboxylation of the Krebs cycle intermediate cis-aconitate and has been shown to exhibit anti-inflammatory and anti-bacterial/viral properties. But the mechanisms underlying itaconate's anti-inflammatory activities are not fully understood. Necroptosis, a lytic form of regulated cell death (RCD), is mediated by receptor-interacting protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like protein (MLKL) signaling. It has been involved in the pathogenesis of organ injury in many inflammatory diseases. In this study, we aimed to explore whether itaconate and its derivatives can inhibit necroptosis in murine macrophages, a mouse MPC-5 cell line and a human HT-29 cell line in response to different necroptotic activators. Our results showed that itaconate and its derivatives dose-dependently inhibited necroptosis, among which dimethyl itaconate (DMI) was the most effective one. Mechanistically, itaconate and its derivatives inhibited necroptosis by suppressing the RIPK1/RIPK3/MLKL signaling and the oligomerization of MLKL. Furthermore, DMI promoted the nuclear translocation of Nrf2 that is a critical regulator of intracellular redox homeostasis, and reduced the levels of intracellular reactive oxygen species (ROS) and mitochondrial superoxide (mtROS) that were induced by necroptotic activators. Consistently, DMI prevented the loss of mitochondrial membrane potential induced by the necroptotic activators. In addition, DMI mitigated caerulein-induced acute pancreatitis in mice accompanied by reduced activation of the necroptotic signaling in vivo. Collectively, our study demonstrates that itaconate and its derivatives can inhibit necroptosis by suppressing the RIPK1/RIPK3/MLKL signaling, highlighting their potential applications for treating necroptosis-associated diseases.

Cell death in cancer chemotherapy using taxanes

Cancer cells evolve to be refractory to the intrinsic programmed cell death mechanisms, which ensure cellular tissue homeostasis in physiological conditions. Chemotherapy using cytotoxic drugs seeks to eliminate cancer cells but spare non-cancerous host cells by exploring a likely subtle difference between malignant and benign cells. Presumably, chemotherapy agents achieve efficacy by triggering programmed cell death machineries in cancer cells. Currently, many major solid tumors are treated with chemotherapy composed of a combination of platinum agents and taxanes. Platinum agents, largely cis-platin, carboplatin, and oxaliplatin, are DNA damaging agents that covalently form DNA addicts, triggering DNA repair response pathways. Taxanes, including paclitaxel, docetaxel, and cabazitaxel, are microtubule stabilizing drugs which are often very effective in purging cancer cells in clinical settings. Generally, it is thought that the stabilization of microtubules by taxanes leads to mitotic arrest, mitotic catastrophe, and the triggering of apoptotic programmed cell death. However, the precise mechanism(s) of how mitotic arrest and catastrophe activate the caspase pathway has not been established. Here, we briefly review literature on the involvement of potential cell death mechanisms in cancer therapy. These include the classical caspase-mediated apoptotic programmed cell death, necroptosis mediated by MLKL, and pore forming mechanisms in immune cells, etc. In particular, we discuss a newly recognized mechanism of cell death in taxane-treatment of cancer cells that involves micronucleation and the irreversible rupture of the nuclear membrane. Since cancer cells are commonly retarded in responding to programmed cell death signaling, stabilized microtubule bundle-induced micronucleation and nuclear membrane rupture, rather than triggering apoptosis, may be a key mechanism accounting for the success of taxanes as anti-cancer agents.

The biochemical pathways of apoptotic, necroptotic, pyroptotic, and ferroptotic cell death

Apoptosis, the first regulated form of cell death discovered in mammalian cells, is executed by caspase-3/7, which are dormant in living cells but become activated by upstream caspase-8 or caspase-9 in responding to extracellular cytokines or intracellular stress signals, respectively. The same cell death-inducing cytokines also cause necroptosis when caspase-8 is inhibited, resulting in the activation of receptor-interacting protein kinase 3 (RIPK3), which phosphorylates pseudokinase MLKL to trigger its oligomerization and membrane-disrupting activity. Caspase-1/4/5/11, known as inflammatory caspases, instead induce pyroptosis by cleaving gasdermin D, whose caspase-cleaved N terminus forms pores on the plasma membrane. The membrane protein NINJ1 amplifies the extent of membrane rupture initiated by gasdermin D. Additionally, disturbance of peroxidation of polyunsaturated fatty acid tails of membrane phospholipids triggers ferroptosis, an iron-dependent and caspases-independent necrotic death. This review will discuss how these regulated cell death pathways act individually and interconnectively in particular cell types to carry out specific physiological and pathological functions.

Dichotomous roles of RIPK3 in regulating the IFN response and NLRP3 inflammasome in human monocytes

Regulation of the profile and magnitude of toll-like receptor (TLR) responses is important for effective host defense against infections while minimizing inflammatory toxicity. The chemokine CXCL4 regulates the TLR8 response to amplify inflammatory gene and inflammasome activation while attenuating the interferon (IFN) response in primary monocytes. In this study, we describe an unexpected role for the kinase RIPK3 in suppressing the CXCL4 + TLR8-induced IFN response and providing signal 2 to activate the NLRP3 inflammasome and interleukin (IL)-1 production in primary human monocytes. RIPK3 also amplifies induction of inflammatory genes such as TNF, IL6, and IL1B while suppressing IL12B. Mechanistically, RIPK3 inhibits STAT1 activation and activates PI3K-Akt-dependent and XBP1- and NRF2-mediated stress responses to regulate downstream genes in a dichotomous manner. These findings identify new functions for RIPK3 in modulating TLR responses and provide potential mechanisms by which RIPK3 plays roles in inflammatory diseases and suggest targeting RIPK3 and XBP1- and NRF2-mediated stress responses as therapeutic strategies to suppress inflammation while preserving the IFN response for host defense.

Salt-inducible kinases regulate androgen synthesis in theca cells by enhancing CREB signaling

Ovulation is the pinnacle of folliculogenesis, a process that requires an interplay between the oocyte, the granulosa cells, and the theca cells (TCs). TCs are the only source of ovarian androgens, which play a vital role in female fertility. However, abnormally elevated androgen levels reduce fertility. Therefore, uncovering novel mechanisms regulating androgen synthesis in TCs is of great significance. We have shown that salt-inducible kinases (SIKs) regulate granulosa cell steroidogenesis. Here, we investigated whether SIKs regulate androgen production in TCs. SIK2 and SIK3 were detected in the TCs of mouse ovaries and isolated TCs. Next, TCs in culture were treated with luteinizing hormone (LH) in the presence or absence of a highly specific SIK inhibitor. SIK inhibition enhanced the stimulatory effect of LH on steroidogenic gene expression and androgen production in a concentration-dependent manner. SIK inhibition alone stimulated the expression of steroidogenic genes and increased androgen production. Activation of adenylyl cyclase with forskolin or emulation of increased intracellular cyclic AMP levels stimulated steroidogenesis, an effect that was enhanced by the inhibition of SIK activity. The stimulatory effect of downstream targets of cyclic AMP was also significantly augmented by SIK inhibition, suggesting that SIKs control targets downstream cyclic AMP. Finally, it is shown that SIK2 knockout mice have higher circulating testosterone than controls. This evidence shows that TCs express SIKs and reveal novel roles for SIKs in the regulation of TC function and androgen production. This information could contribute to uncovering therapeutic targets to treat hyperandrogenic diseases.

Emerging Insights Into Necroptosis: Implications for Renal Health and Diseases

Necroptosis is a regulated form of cell death that has gained increasing attention in recent years. It plays a significant role in various physiological and pathological processes, including renal health and disease. This review article provides an overview of necroptosis as a regulated cell death pathway and explores its implications in renal physiology and renal diseases. The molecular signaling pathways involved in necroptosis, including the key players such as receptor-interacting protein kinases (RIPKs) and mixed lineage kinase domain-like protein (MLKL), are discussed in detail. The crosstalk between necroptosis and other cell death pathways, particularly apoptosis, is explored to understand the interplay between these processes in renal cells. In normal physiological conditions, necroptosis has been found to play a crucial role in renal development and tissue homeostasis. However, dysregulated necroptosis can contribute to tissue damage, inflammation, and fibrosis in renal diseases. The review highlights the involvement of necroptosis in acute kidney injury, chronic kidney disease, and renal transplant rejection, elucidating the underlying pathophysiological mechanisms and consequences. The therapeutic targeting of necroptosis in renal diseases is an emerging area of interest. Current and emerging strategies to modulate necroptosis, including the inhibition of key mediators and regulators, are discussed here. Additionally, the potential therapeutic targets and inhibitors of necroptosis, along with preclinical and clinical studies exploring their efficacy, are reviewed.

Review: Luteal prostaglandins: mechanisms regulating luteal survival and demise in ruminants

The corpus luteum (CL) is critical for establishment and maintenance of pregnancy in all mammals. However, the fate of the CL in ruminants is dependent on the presence of a functional uterus or signals from a developing embryo to modify uterine function to ensure its own survival. The key molecule secreted by the uterus that must be modified is prostaglandin F2alpha (PGF2A). At the same time, there is evidence that mechanisms within the CL may influence the ability of PGF2A to cause luteolysis. This review focuses on prostaglandins and steroidogenic capacity as endogenous modulators of the sensitivity of the CL to exogenous PGF2A. Early luteal development and early pregnancy are two different luteal stages in which sensitivity of the CL to PGF2A renders it incapable, or less capable, respectively, of undergoing luteolysis in response to PGF2A compared to a midcycle CL. An analysis of molecular changes that occur during these two stages provides some novel insight into molecules and pathways worth exploring to explain the regulation of luteolytic capacity in corpora lutea of ruminants.

Role of Receptor Interacting Protein (RIP) kinases in cancer

The Receptor Interacting Protein (RIP) kinase family consists of seven Serine/Threonine kinases, which plays a key signaling role in cell survival and cell death. Each RIP family member contains a conserved kinase domain and other domains that determine the specific kinase function through protein-protein interactions. RIP1 and RIP3 are best known for their critical roles in necroptosis, programmed necrosis and a non-apoptotic inflammatory cell death process. Dysregulation of RIP kinases contributes to a variety of pathogenic conditions such as inflammatory diseases, neurological diseases, and cancer. In cancer cells, alterations of RIP kinases at genetic, epigenetic and expression levels are frequently found, and suggested to promote tumor progression and metastasis, escape of antitumor immune response, and therapeutic resistance. However, RIP kinases can be either pro-tumor or anti-tumor depending on specific tumor types and cellular contexts. Therapeutic agents for targeting RIP kinases have been tested in clinical trials mainly for inflammatory diseases. Deregulated expression of these kinases in different types of cancer suggests that they represent attractive therapeutic targets. The focus of this review is to outline the role of RIP kinases in cancer, highlighting potential opportunities to manipulate these proteins in cancer treatment.

Mechanisms of TNF-independent RIPK3-mediated cell death

Apoptosis and necroptosis regulate many aspects of organismal biology and are involved in various human diseases. TNF is well known to induce both of these forms of cell death and the underlying mechanisms have been elaborately described. However, cells can also engage apoptosis and necroptosis through TNF-independent mechanisms, involving, for example, activation of the pattern recognition receptors Toll-like receptor (TLR)-3 and -4, or zDNA-binding protein 1 (ZBP1). In this context, cell death signaling depends on the presence of receptor-interacting serine/threonine protein kinase 3 (RIPK3). Whereas RIPK3 is required for TNF-induced necroptosis, it mediates both apoptosis and necroptosis upon TLR3/4 and ZBP1 engagement. Here, we review the intricate mechanisms by which TNF-independent cell death is regulated by RIPK3.

Inhibition of Pseudomonas aeruginosa LPS-Induced airway inflammation by RIPK3 in human airway

Although the physiological function of receptor-interacting protein kinase (RIPK) 3 has emerged as a critical mediator of programmed necrosis/necroptosis, the intracellular role it plays as an attenuator in human lungs and human bronchial epithelia remains unclear. Here, we show that the expression of RIPK3 dramatically decreased in the inflamed tissues of human lungs, and moved from the nucleus to the cytoplasm. The overexpression of RIPK3 dramatically increased F-actin formation and decreased the expression of genes for pro-inflammatory cytokines (IL-6 and IL-1β), but not siRNA-RIPK3. Interestingly, whereas RIPK3 was bound to histone 1b without LPS stimulation, the interaction between them was disrupted after 15 min of LPS treatment. Histone methylation could not maintain the binding of RIPK3 and activated movement towards the cytoplasm. In the cytoplasm, overexpressed RIPK3 continuously attenuated pro-inflammatory cytokine gene expression by inhibiting NF-κB activation, preventing the progression of inflammation during Pseudomonas aeruginosa infection. Our data indicated that RIPK3 is critical for the regulation of the LPS-induced inflammatory microenvironment. Therefore, we suggest that RIPK3 is a potential therapeutic candidate for bacterial infection-induced pulmonary inflammation.

Human RIPK3 C-lobe phosphorylation is essential for necroptotic signaling

Necroptosis is a caspase-independent, pro-inflammatory mode of programmed cell death which relies on the activation of the terminal effector, MLKL, by the upstream protein kinase RIPK3. To mediate necroptosis, RIPK3 must stably interact with, and phosphorylate the pseudokinase domain of MLKL, although the precise molecular cues that provoke RIPK3 necroptotic signaling are incompletely understood. The recent finding that RIPK3 S227 phosphorylation and the occurrence of a stable RIPK3:MLKL complex in human cells prior to exposure to a necroptosis stimulus raises the possibility that additional, as-yet-unidentified phosphorylation events activate RIPK3 upon initiation of necroptosis signaling. Here, we sought to identify phosphorylation sites of RIPK3 and dissect their regulatory functions. Phosphoproteomics identified 21 phosphorylation sites in HT29 cells overexpressing human RIPK3. By comparing cells expressing wild-type and kinase-inactive D142N RIPK3, autophosphorylation sites and substrates of other cellular kinases were distinguished. Of these 21 phosphosites, mutational analyses identified only pT224 and pS227 as crucial, synergistic sites for stable interaction with MLKL to promote necroptosis, while the recently reported activation loop phosphorylation at S164/T165 negatively regulate the kinase activity of RIPK3. Despite being able to phosphorylate MLKL to a similar or higher extent than wild-type RIPK3, mutation of T224, S227, or the RHIM in RIPK3 attenuated necroptosis. This finding highlights the stable recruitment of human MLKL by RIPK3 to the necrosome as an essential checkpoint in necroptosis signaling, which is independent from and precedes the phosphorylation of MLKL.