Putting the brakes on tumorigenesis with snake venom toxins: New molecular insights for cancer drug discovery

Emerging anticancer potential and mechanisms of snake venom toxins: A review

Animal-derived venom, like snake venom, has been proven to be valuable natural resources for the drug development. Previously, snake venom was mainly investigated in its pharmacological activities in regulating coagulation, vasodilation, and cardiovascular function, and several marketed cardiovascular drugs were successfully developed from snake venom. In recent years, snake venom fractions have been demonstrated with anticancer properties of inducing apoptotic and autophagic cell death, restraining proliferation, suppressing angiogenesis, inhibiting cell adhesion and migration, improving immunity, and so on. A number of active anticancer enzymes and peptides have been identified from snake venom toxins, such as L-amino acid oxidases (LAAOs), phospholipase A2 (PLA2), metalloproteinases (MPs), three-finger toxins (3FTxs), serine proteinases (SPs), disintegrins, C-type lectin-like proteins (CTLPs), cell-penetrating peptides, cysteine-rich secretory proteins (CRISPs). In this review, we focus on summarizing these snake venom-derived anticancer components on their anticancer activities and underlying mechanisms. We will also discuss their potential to be developed as anticancer drugs in the future.

Snake venom toxins: Potential anticancer therapeutics

Snake venom contains a cocktail of compounds dominated by proteins and peptides, which make up the toxin. The toxin components of snake venom attack several targets in the human body including the neuromuscular system, kidney and blood coagulation system and cause pathologies. As such, the venom toxins can be managed and used for the treatment of these diseases. In this regard, Captopril used in the treatment of cardiovascular diseases was the first animal venom toxin-based drug approved by the US Food and Drug Administration and the European Medicines Agency. Cancers cause morbidity and mortality worldwide. Due to side effects associated with the current cancer treatments including chemotherapy, radiotherapy, immunotherapy, hormonal therapy and surgery, there is a need to improve the efficacy of current treatments and/or develop novel drugs from natural sources including animal toxin-based drugs. There is a long history of earlier and ongoing studies implicating snake venom toxins as potential anticancer therapies. Here, we review the role of crude snake venoms and toxins including phospholipase A2, L-amino acid oxidase, C-type lectin and disintegrin as potential anticancer agents tested in cancer cell lines and animal tumour models in comparison to normal cell lines. Some of the anti-tumour activities of snake venom toxins include induction of cytotoxicity, apoptosis, cell cycle arrest and inhibition of metastasis, angiogenesis and tumour growth. We thus propose the advancement of multidisciplinary approaches to more pre-clinical and clinical studies for enhanced bioavailability and targeted delivery of snake venom toxin-based anticancer drugs.

Snake and arthropod venoms: Search for inflammatory activity in human cells involved in joint diseases

Most anti-inflammatory drugs currently adopted to treat chronic inflammatory joint diseases can alleviate symptoms but they do not lead to remission. Therefore, new and more efficient drugs are needed to block the course of joint inflammatory diseases. Animal venoms, rich in bioactive compounds, can contribute as valuable tools in this field of research. In this study, we first demonstrate the direct action of venoms on cells that constitute the articular joints. We established a platform consisting of cell-based assays to evaluate the release of cytokines (IL-6, IL-8, TNFα, IL-1β, and IL-10) by human chondrocytes, synoviocytes and THP1 macrophages, as well as the release of neuropeptides (substance-P and β-endorphin) by differentiated sensory neuron-like cells, 24 h after stimulation of cells with 21 animal venoms from snake and arthropod species, sourced from different taxonomic families and geographic origins. Results demonstrated that at non-cytotoxic concentrations, the venoms activate at varying degrees the secretion of inflammatory mediators involved in the pathology of articular diseases, such as IL-6, IL-8, and TNF-α by chondrocytes, synoviocytes, and macrophages and of substance P by neuron-like cells. Venoms of the Viperidae snake family were more inflammatory than those of the Elapidae family, while venoms of Arthropods were less inflammatory than snake venoms. Notably, some venoms also induced the release of the anti-inflammatory IL-10 by macrophages. However, the scorpion Buthus occitanus venom induced the release of IL-10 without increasing the release of inflammatory cytokines by macrophages. Since the cell types used in the experiments are crucial elements in joint inflammatory processes, the results of this work may guide future research on the activation of receptors and inflammatory signaling pathways by selected venoms in these particular cells, aiming at discovering new targets for therapeutic intervention.

Anticancer activities of natural antimicrobial peptides from animals

Cancer is the most common cause of human death worldwide, posing a serious threat to human health and having a negative impact on the economy. In the past few decades, significant progress has been made in anticancer therapies, but traditional anticancer therapies, including radiation therapy, surgery, chemotherapy, molecular targeted therapy, immunotherapy and antibody-drug conjugates (ADCs), have serious side effects, low specificity, and the emergence of drug resistance. Therefore, there is an urgent need to develop new treatment methods to improve efficacy and reduce side effects. Antimicrobial peptides (AMPs) exist in the innate immune system of various organisms. As the most promising alternatives to traditional drugs for treating cancers, some AMPs also have been proven to possess anticancer activities, which are defined as anticancer peptides (ACPs). These peptides have the advantages of being able to specifically target cancer cells and have less toxicity to normal tissues. More and more studies have found that marine and terrestrial animals contain a large amount of ACPs. In this article, we introduced the animal derived AMPs with anti-cancer activity, and summarized the types of tumor cells inhibited by ACPs, the mechanisms by which they exert anti-tumor effects and clinical applications of ACPs.

Effects on cell cycle progression and cytoskeleton organization of five Bothrops spp. venoms in cell culture-based assays

Snake envenomation is a neglected tropical disease. In Brazil, the Bothrops genus is responsible for about 86% of snakebite accidents. Despite extensive evidence of the cytotoxicity of snake venoms, the cellular and molecular mechanisms involved are not fully understood, especially regarding the effects on cell cycle progression and cytoskeleton organization. Traditionally, the effectiveness and quality control tests of venoms and antivenoms are assessed by in vivo assays. Despite this, there is a rising effort to develop surrogate in vitro models according to the 3R principle (Replacement, Reduction, and Refinement). In this study, we treated rat liver cells (BRL-3A) with venoms from five Bothrops species (B. jararaca, B. jararacussu, B. moojeni, B. alternatus, and B. neuwiedi) and analyzed cell viability and IC50 by MTT assay, cell cycle phases distribution by flow cytometry, and morphology and cytoskeleton alterations by immunofluorescence. In addition, we evaluated the correlation between IC50 and the enzymatic and biological activities of each venom. Our results indicated that Bothrops spp. venoms decreased the cell viability of rat liver BRL-3A cells. The rank order of potency was B. jararacussu > B. moojeni > B. alternatus > B. jararaca > B. neuwiedi. The mechanisms of cytotoxicity were related to microtubules and actin network disruption, but not to cell cycle arrest. No clear correlation was found between the IC50 and retrieved literature data of in vitro enzymatic and in vivo biological activities. This work contributed to understanding cellular and molecular mechanisms underlying the Bothrops spp. venom cytotoxicity, which can help to improve envenomation treatment, as well as disclose potential therapeutic properties of snake venoms.

Pictolysin-III, a Hemorrhagic Type-III Metalloproteinase Isolated from Bothrops pictus (Serpentes: Viperidae) Venom, Reduces Mitochondrial Respiration and Induces Cytokine Secretion in Epithelial and Stromal Cell Lines

From the venom of the Bothrops pictus snake, an endemic species from Peru, we recently have described toxins that inhibited platelet aggregation and cancer cell migration. In this work, we characterize a novel P-III class snake venom metalloproteinase, called pictolysin-III (Pic-III). It is a 62 kDa proteinase that hydrolyzes dimethyl casein, azocasein, gelatin, fibrinogen, and fibrin. The cations Mg²⁺ and Ca²⁺ enhanced its enzymatic activity, whereas Zn²⁺ inhibited it. In addition, EDTA and marimastat were also effective inhibitors. The amino acid sequence deduced from cDNA shows a multidomain structure that includes a proprotein, metalloproteinase, disintegrin-like, and cysteine-rich domains. Additionally, Pic-III reduces the convulxin- and thrombin-stimulated platelet aggregation and in vivo, it has hemorrhagic activity (DHM = 0.3 µg). In epithelial cell lines (MDA-MB-231 and Caco-2) and RMF-621 fibroblast, it triggers morphological changes that are accompanied by a decrease in mitochondrial respiration, glycolysis, and ATP levels, and an increase in NAD(P)H, mitochondrial ROS, and cytokine secretion. Moreover, Pic-III sensitizes to the cytotoxic BH3 mimetic drug ABT-199 (Venetoclax) in MDA-MB-231 cells. To our knowledge, Pic-III is the first SVMP reported with action on mitochondrial bioenergetics and may offer novel opportunities for promising lead compounds that inhibit platelet aggregation or ECM-cancer-cell interactions.

Neuroepithelial Interactions in Cancer

Nerves not only regulate the homeostasis and energetic metabolism of normal epithelial cells but also are critical for cancer, as cancer recapitulates the biology of neural regulation of epithelial tissues. Cancer cells rarely develop in denervated organs, and denervation affects tumorigenesis, in vivo and in humans. Axonogenesis occurs to supply the new malignant epithelial growth with nerves. Neurogenesis happens later, first in ganglia around organs or the spinal column and subsequently through recruitment of neuroblasts from the central nervous system. The hallmark of this stage is regulation of homeostasis and energetic metabolism. Perineural invasion is the most efficient interaction between cancer cells and nerves. The hallmark of this stage is increased proliferation and decreased apoptosis. Finally, carcinoma cells transdifferentiate into a neuronal profile in search of neural independence. The latter is the last stage in neuroepithelial interactions. Treatments for cancer must address the biology of neural regulation of cancer.

A current perspective on snake venom composition and constituent protein families

Snake venoms are heterogeneous mixtures of proteins and peptides used for prey subjugation. With modern proteomics there has been a rapid expansion in our knowledge of snake venom composition, resulting in the venom proteomes of 30% of vipers and 17% of elapids being characterised. From the reasonably complete proteomic coverage of front-fanged snake venom composition (179 species-68 species of elapids and 111 species of vipers), the venoms of vipers and elapids contained 42 different protein families, although 18 were only reported in < 5% of snake species. Based on the mean abundance and occurrence of the 42 protein families, they can be classified into 4 dominant, 6 secondary, 14 minor, and 18 rare protein families. The dominant, secondary and minor categories account for 96% on average of a snake's venom composition. The four dominant protein families are: phospholipase A₂ (PLA₂), snake venom metalloprotease (SVMP), three-finger toxins (3FTx), and snake venom serine protease (SVSP). The six secondary protein families are: L-amino acid oxidase (LAAO), cysteine-rich secretory protein (CRiSP), C-type lectins (CTL), disintegrins (DIS), kunitz peptides (KUN), and natriuretic peptides (NP). Venom variation occurs at all taxonomic levels, including within populations. The reasons for venom variation are complex, as variation is not always associated with geographical variation in diet. The four dominant protein families appear to be the most important toxin families in human envenomation, being responsible for coagulopathy, neurotoxicity, myotoxicity and cytotoxicity. Proteomic techniques can be used to investigate the toxicological profile of a snake venom and hence identify key protein families for antivenom immunorecognition.

Inflammation and Oxidative Stress in Snakebite Envenomation: A Brief Descriptive Review and Clinical Implications

Snakebite envenoming is a pathological condition which may occur in response to the injection of venom. Snake venoms contain a complex mixture of biologically active molecules which are responsible for a broad spectrum of clinical manifestations, ranging from local tissue injuries to fatal complications. Snake venom administration commonly provokes local tissue injury often associated with systemic effects, including neurotoxic and cardiotoxic manifestations, bleeding, acute kidney injury, and rhabdomyolysis. An important spectrum of pathogenesis of snake envenomation is the generation of reactive oxygen species (ROS), which can directly provoke tissue damage and also potentiate the deleterious consequences of inflammation at the bite site. Snake venom components known to induce oxidative stress include phospholipases A₂, metalloproteinases, three-finger toxins, and L-amino acid oxidase. Clear evidence is mounting suggesting that inflammation and oxidative stress participate in the destructive effects of envenoming, including acute renal failure, tissue necrosis, and unusual susceptibility to bleed (hemorrhage), mostly due to hypocoagulability, neuro/cardio toxicity, and myonecrosis. Impaired regulation of oxidative stress may also set the stage for secondary/long-term complications of snakebite envenomation such as musculoskeletal disabilities. Some aspects of natural antioxidant therapeutic options are discussed in this review.

Bioactive peptides from venoms against glioma progression

Venoms are complex mixtures of different molecules and ions. Among them, bioactive peptides have been found to affect cancer hallmarks, such as cell proliferation, cell invasion, cell migration, and can also modulate the immune response of normal and cancer-bearing organisms. In this article, we review the mechanisms of action on these cancer cell features, focusing on bioactive peptides being developed as potential therapeutics for one of the most aggressive and deadly brain tumors, glioblastoma (GB). Novel therapeutic approaches applying bioactive peptides may contribute to multiple targeting of GB and particularly of GB stem cells. Bioactive peptides selectively target cancer cells without harming normal cells. Various molecular targets related to the effects of bioactive peptides on GB have been proposed, including ion channels, integrins, membrane phospholipids and even immunomodulatory treatment of GB. In addition to therapy, some bioactive peptides, such as disintegrins, can also be used for diagnostics or are used as labels for cytotoxic drugs to specifically target cancer cells. Given the limitations described in the last section, successful application in cancer therapy is rather low, as only 3.4% of such peptides have been included in clinical trials and have passed successfully phases I to III. Combined approaches of added bioactive peptides to standard cancer therapies need to be explored using advanced GB in vitro models such as organoids. On the other hand, new methods are also being developed to improve translation from research to practice and provide new hope for GB patients and their families.

An Emergent Role for Mitochondrial Bioenergetics in the Action of Snake Venom Toxins on Cancer Cells

Beyond the role of mitochondria in apoptosis initiation/execution, some mitochondrial adaptations support the metastasis and chemoresistance of cancer cells. This highlights mitochondria as a promising target for new anticancer strategies. Emergent evidence suggests that some snake venom toxins, both proteins with enzymatic and non-enzymatic activities, act on the mitochondrial metabolism of cancer cells, exhibiting unique and novel mechanisms that are not yet fully understood. Currently, six toxin classes (L-amino acid oxidases, thrombin-like enzymes, secreted phospholipases A2, three-finger toxins, cysteine-rich secreted proteins, and snake C-type lectin) that alter the mitochondrial bioenergetics have been described. These toxins act through Complex IV activity inhibition, OXPHOS uncoupling, ROS-mediated permeabilization of inner mitochondrial membrane (IMM), IMM reorganization by cardiolipin interaction, and mitochondrial fragmentation with selective migrastatic and cytotoxic effects on cancer cells. Notably, selective internalization and direct action of snake venom toxins on tumor mitochondria can be mediated by cell surface proteins overexpressed in cancer cells (e.g. nucleolin and heparan sulfate proteoglycans) or facilitated by the elevated Δψm of cancer cells compared to that non-tumor cells. In this latter case, selective mitochondrial accumulation, in a Δψm-dependent manner, of compounds linked to cationic snake peptides may be explored as a new anti-cancer drug delivery system. This review analyzes the effect of snake venom toxins on mitochondrial bioenergetics of cancer cells, whose mechanisms of action may offer the opportunity to develop new anticancer drugs based on toxin scaffolds.

Venom of Viperidae: A Perspective of its Antibacterial and Antitumor Potential

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The emergence of multi-drug resistant bacteria and limitations on cancer treatment represent two important challenges in modern medicine. Biological compounds have been explored with a particular focus on venoms. Although they can be lethal or cause considerable damage to humans, venom is also a source rich in components with high therapeutic potential.

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Viperidae family is one of the most emblematic venomous snake families and several studies highlighted the antibacterial and antitumor potential of viper toxins. According to the literature, these activities are mainly associated to five protein families - svLAAO, Disintegrins, PLA2, SVMPs and C-type lectins- that act through different mechanisms leading to the inhibition of the growth of bacteria, as well as, cytotoxic effects and inhibition of metastasis process. In this review, we provide an overview of the venom toxins produced by species belonging to the Viperidae family, exploring their roles during the envenoming and their pharmacological properties, in order to demonstrate its antibacterial and antitumor potential.

Extracellular Matrix Signals as Drivers of Mitochondrial Bioenergetics and Metabolic Plasticity of Cancer Cells During Metastasis

The role of metabolism in tumor growth and chemoresistance has received considerable attention, however, the contribution of mitochondrial bioenergetics in migration, invasion, and metastasis is recently being understood. Migrating cancer cells adapt their energy needs to fluctuating changes in the microenvironment, exhibiting high metabolic plasticity. This occurs due to dynamic changes in the contributions of metabolic pathways to promote localized ATP production in lamellipodia and control signaling mediated by mitochondrial reactive oxygen species. Recent evidence has shown that metabolic shifts toward a mitochondrial metabolism based on the reductive carboxylation, glutaminolysis, and phosphocreatine-creatine kinase pathways promote resistance to anoikis, migration, and invasion in cancer cells. The PGC1a-driven metabolic adaptations with increased electron transport chain activity and superoxide levels are essential for metastasis in several cancer models. Notably, these metabolic changes can be determined by the composition and density of the extracellular matrix (ECM). ECM stiffness, integrins, and small Rho GTPases promote mitochondrial fragmentation, mitochondrial localization in focal adhesion complexes, and metabolic plasticity, supporting enhanced migration and metastasis. Here, we discuss the role of ECM in regulating mitochondrial metabolism during migration and metastasis, highlighting the therapeutic potential of compounds affecting mitochondrial function and selectively block cancer cell migration.