Apoptotic signaling: Beyond cell death

Regeneration following tissue necrosis is mediated by non-apoptotic caspase activity

Tissue necrosis is a devastating complication for many human diseases and injuries. Unfortunately, our understanding of necrosis and how it impacts surrounding healthy tissue - an essential consideration when developing effective methods to treat such injuries - has been limited by a lack of robust genetically tractable models. Our lab previously established a method to study necrosis-induced regeneration in the Drosophila wing imaginal disc, which revealed a unique phenomenon whereby cells at a distance from the injury upregulate caspase activity in a process called Necrosis-induced Apoptosis (NiA) that is vital for regeneration. Here we have further investigated this phenomenon, showing that NiA is predominantly associated with the highly regenerative pouch region of the disc, shaped by genetic factors present in the presumptive hinge. Furthermore, we find that a proportion of NiA fail to undergo apoptosis, instead surviving effector caspase activation to persist within the tissue and stimulate reparative proliferation late in regeneration. This proliferation relies on the initiator caspase Dronc, and occurs independent of JNK, ROS or mitogens associated with the previously characterized Apoptosis-induced Proliferation (AiP) mechanism. These data reveal a new means by which non-apoptotic Dronc signaling promotes regenerative proliferation in response to necrotic damage.

The Dose Rate of Corpuscular Ionizing Radiation Strongly Influences the Severity of DNA Damage, Cell Cycle Progression and Cellular Senescence in Human Epidermoid Carcinoma Cells

Modern radiotherapy utilizes a broad range of sources of ionizing radiation, both low-dose-rate (LDR) and high-dose-rate (HDR). However, the mechanisms underlying specific dose-rate effects remain unclear, especially for corpuscular radiation. To address this issue, we have irradiated human epidermoid carcinoma A431 cells under LDR and HDR regimes. Reducing the dose rate has lower lethality at equal doses with HDR irradiation. The half-lethal dose after HDR irradiation was three times less than after LDR irradiation. The study of mechanisms showed that under HDR irradiation, the radiation-induced halt of mitosis with the accompanying emergence of giant cells was recorded. No such changes were recorded after LDR irradiation. The level of DNA damage is significantly greater after HDR irradiation, which may be the main reason for the different mechanisms of action of HDR and LDR irradiations. Comparing the mechanisms of cell response to LDR and HDR irradiations may shed light on the mechanisms of tumor cell response to ionizing radiation and answer the question of whether different dose rates within the same dose range can cause different clinical effects.

Apoptosis signaling is activated as a transient pulse in neurons

Apoptosis is a fundamental process of all mammalian cells but exactly how it is regulated in different primary cells remains less explored. In most contexts, apoptosis is engaged to eliminate cells. However, postmitotic cells such as neurons must efficiently balance the need for developmental apoptosis versus the physiological needs for their long-term survival. Neurons are capable of reversing the commitment to death even after the point of cytochrome c release. This ability of neurons to recover from an apoptotic signal suggests that activation of the apoptotic pathway in neurons could be much more transient than is currently recognized. Here, we investigated whether the apoptotic pathway in neurons is a persistent signal or a transient pulse in continuous presence of apoptotic stimulus. We have examined this at three key steps in apoptotic signaling: phosphorylation of c-Jun, induction of the BH3-only family proteins and Bax activation. Strikingly, we found all three of these events occur as transient signals following Nerve Growth Factor (NGF) deprivation-induced apoptosis in sympathetic neurons. This transient apoptosis signal would effectively allow neurons to reset and permit recovery if the apoptotic stimulus is reversed. Excitingly, we have also discovered that a neuron's ability to recover from an apoptotic signal is dependent on expression of the anti-apoptotic Bcl-2 family protein Bcl-xL. Bcl-xL-deficient neurons lose the ability to recover from NGF deprivation even if NGF is restored. Additionally, we show that recovery from a previous exposure to NGF deprivation is protective against subsequent deprivation. Together, these results define a novel mechanism by which apoptosis is regulated in neurons where the transient pulse of the apoptotic signaling supports neuronal resilience.

1,2-bis(2,4,6-tribromophenoxy) ethane induces necroptosis via the co-competition of GAS5 and NUAK1 for miR-743a-5p in rat hepatocytes

The brominated flame retardant 1,2-bis(2,4,6-tribromophenoxy) ethane (BTBPE) widely used in manufacturing is inevitably released into the environment, resulting in the exposure of organisms to BTBPE. Therefore, it is particularly important to explore its toxic mechanism. The liver is one of the main accumulating organs of BTBPE, but the mechanism underlying BTBPE hepatotoxicity has not been thoroughly investigated. In our study, BTBPE was administered to Sprague-Dawley (SD) rats and rat hepatocytes (BRL cells) in vivo and in vitro, respectively, and HE staining, AO/EB staining, fluorescent probes, qPCR, immunofluorescence, and dual-luciferase reporter assays were performed. We investigated the mechanism of action of growth arrest-specific 5 (GAS5), miR-743a-5p, and NUAK family kinase 1 (NUAK1) in BTBPE-induced necroptosis from the perspective of competing endogenous RNAs (ceRNAs) using NUAK1 inhibitors, siRNAs, mimics, and overexpression plasmids. Our study showed that exposure to BTBPE caused necroptosis in the liver and BRL cells, accompanied by an oxidation-reduction imbalance and an inflammatory response. It is worth noting that NUAK1 is a newly discovered upstream regulatory target for necroptosis. In addition, miR-743a-5p was shown to inhibit necroptosis by targeting NUAK1 and down-regulating NUAK1. GAS5 upregulates NUAK1 expression by competitively binding to miR-743a-5p, thereby inducing necroptosis. This study demonstrated, for the first time, that the GAS5-miR-743a-5p-NUAK1 axis is involved in the regulation of necroptosis via ceRNAs. Thus, GAS5 and NUAK1 induce necroptosis by competitively binding to miR-743a-5p.

Amitotic Cell Division, Malignancy, and Resistance to Anticancer Agents: A Tribute to Drs. Walen and Rajaraman

Cell division is crucial for the survival of living organisms. Human cells undergo three types of cell division: mitosis, meiosis, and amitosis. The former two types occur in somatic cells and germ cells, respectively. Amitosis involves nuclear budding and occurs in cells that exhibit abnormal nuclear morphology (e.g., polyploidy) with increased cell size. In the early 2000s, Kirsten Walen and Rengaswami Rajaraman and his associates independently reported that polyploid human cells are capable of producing progeny via amitotic cell division, and that a subset of emerging daughter cells proliferate rapidly, exhibit stem cell-like properties, and can contribute to tumorigenesis. Polyploid cells that arise in solid tumors/tumor-derived cell lines are referred to as polyploid giant cancer cells (PGCCs) and are known to contribute to therapy resistance and disease recurrence following anticancer treatment. This commentary provides an update on some of these intriguing discoveries as a tribute to Drs. Walen and Rajaraman.

Overview of BH3 mimetics in ovarian cancer

Ovarian carcinoma is the leading cause of gynecological cancer-related death, still with a dismal five-year prognosis, mainly due to late diagnosis and the emergence of resistance to cytotoxic and targeted agents. Bcl-2 family proteins have a key role in apoptosis and are associated with tumor development/progression and response to therapy in different cancer types, including ovarian carcinoma. In tumors, evasion of apoptosis is a possible mechanism of resistance to therapy. BH3 mimetics are small molecules that occupy the hydrophobic pocket on pro-survival proteins, allowing the induction of apoptosis, and are currently under study as single agents and/or in combination with cytotoxic and targeted agents in solid tumors. Here, we discuss recent advances in targeting anti-apoptotic proteins of the Bcl-2 family for the treatment of ovarian cancer, focusing on BH3 mimetics, and how these approaches could potentially offer an alternative/complementary way to treat patients and overcome or delay resistance to current treatments.

The Impact of HIV and Parasite Single Infection and Coinfection on Telomere Length: A Systematic Review

HIV and parasite infections accelerate biological aging, resulting in immune senescence, apoptosis and cellular damage. Telomere length is considered to be one of the most effective biomarkers of biological aging. HIV and parasite infection have been reported to shorten telomere length in the host. This systematic review aimed to highlight work that explored the influence of HIV and parasite single infections and coinfection on telomere length. Using specific keywords related to the topic of interest, an electronic search of several online databases (Google Scholar, Web of Science, Scopus, Science Direct and PubMed) was conducted to extract eligible articles. The association between HIV infection or parasite infection and telomere length and the association between HIV and parasite coinfection and telomere length were assessed independently. The studies reported were mostly conducted in the European countries. Of the 42 eligible research articles reviewed, HIV and parasite single infections were independently associated with telomere length shortening. Some studies found no association between antiretroviral therapy (ART) and telomere length shortening, while others found an association between ART and telomere length shortening. No studies reported on the association between HIV and parasite coinfection and telomere length. HIV and parasite infections independently accelerate telomere length shortening and biological aging. It is possible that coinfection with HIV and parasites may further accelerate telomere length shortening; however, this is a neglected field of research with no reported studies to date.

Changing the Landscape of Solid Tumor Therapy from Apoptosis-Promoting to Apoptosis-Inhibiting Strategies

The many limitations of implementing anticancer strategies under the term "precision oncology" have been extensively discussed. While some authors propose promising future directions, others are less optimistic and use phrases such as illusion, hype, and false hypotheses. The reality is revealed by practicing clinicians and cancer patients in various online publications, one of which has stated that "in the quest for the next cancer cure, few researchers bother to look back at the graveyard of failed medicines to figure out what went wrong". The message is clear: Novel therapeutic strategies with catchy names (e.g., synthetic "lethality") have not fulfilled their promises despite decades of extensive research and clinical trials. The main purpose of this review is to discuss key challenges in solid tumor therapy that surprisingly continue to be overlooked by the Nomenclature Committee on Cell Death (NCCD) and numerous other authors. These challenges include: The impact of chemotherapy-induced genome chaos (e.g., multinucleation) on resistance and relapse, oncogenic function of caspase 3, cancer cell anastasis (recovery from late stages of apoptosis), and pitfalls of ubiquitously used preclinical chemosensitivity assays (e.g., cell "viability" and tumor growth delay studies in live animals) that score such pro-survival responses as "lethal" events. The studies outlined herein underscore the need for new directions in the management of solid tumors.

Cell Death, by Any Other Name…

Studies trying to understand cell death, this ultimate biological process, can be traced back to a century ago. Yet, unlike many other fashionable research interests, research on cell death is more alive than ever. New modes of cell death are discovered in specific contexts, as are new molecular pathways. But what is "cell death", really? This question has not found a definitive answer yet. Nevertheless, part of the answer is irreversibility, whereby cells can no longer recover from stress or injury. Here, we identify the most distinctive features of different modes of cell death, focusing on the executive final stages. In addition to the final stages, these modes can differ in their triggering stimulus, thus referring to the initial stages. Within this framework, we use a few illustrative examples to examine how intercellular communication factors in the demise of cells. First, we discuss the interplay between cell-cell communication and cell death during a few steps in the early development of multicellular organisms. Next, we will discuss this interplay in a fully developed and functional tissue, the gut, which is among the most rapidly renewing tissues in the body and, therefore, makes extensive use of cell death. Furthermore, we will discuss how the balance between cell death and communication is modified during a pathological condition, i.e., colon tumorigenesis, and how it could shed light on resistance to cancer therapy. Finally, we briefly review data on the role of cell-cell communication modes in the propagation of cell death signals and how this has been considered as a potential therapeutic approach. Far from vainly trying to provide a comprehensive review, we launch an invitation to ponder over the significance of cell death diversity and how it provides multiple opportunities for the contribution of various modes of intercellular communication.