Gliotoxin is a sulfur-containing

mycotoxin A mycotoxin (from the Greek μύκης , "fungus" and τοξίνη , "toxin") is a toxic secondary metabolite produced by organisms of kingdom Fungi and is capable of causing disease and death in both humans and other animals. The term 'mycotoxin' ...

that belongs to a class of naturally occurring 2,5-diketopiperazines produced by several species of

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

, especially those of marine origin. It is the most prominent member of the epipolythiopiperazines, a large class of natural products featuring a

diketopiperazine A diketopiperazine (DKP), also known as a ''dioxopiperazine'' or ''piperazinedione'', is a class of organic compounds related to piperazine but containing two amide linkages. DKP's are the smallest known class of cyclic peptide. Despite their name, ...

with di- or polysulfide linkage. These highly bioactive compounds have been the subject of numerous studies aimed at new therapeutics. Gliotoxin was originally isolated from '' Gliocladium fimbriatum'', and was named accordingly. It is an epipolythiodioxopiperazine metabolite.

Occurrence

The compound is produced by

human pathogen A human pathogen is a pathogen (microbe or microorganism such as a virus, bacterium, prion, or fungus) that causes disease in humans. The human physiological defense against common pathogens (such as ''Pneumocystis'') is mainly the responsibility ...

s such as ''

Aspergillus fumigatus ''Aspergillus fumigatus'' is a species of fungus in the genus ''Aspergillus'', and is one of the most common ''Aspergillus'' species to cause disease in individuals with an immunodeficiency. ''Aspergillus fumigatus'', a saprotroph widespread in ...

'', and also by species of ''

Trichoderma ''Trichoderma'' is a genus of fungi in the family Hypocreaceae that is present in all soils, where they are the most prevalent culturable fungi. Many species in this genus can be characterized as opportunistic avirulent plant symbionts. This ref ...

'' and ''

Penicillium ''Penicillium'' () is a genus of ascomycetous fungi that is part of the mycobiome of many species and is of major importance in the natural environment, in food spoilage, and in food and drug production. Some members of the genus produce pe ...

''. Gliotoxin has also been reported from

yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...

s of the genus '' Candida'', but results from other studies have cast doubt on the production of this metabolite by ''Candida'' fungi.

Mechanism of action

Gliotoxin is suspected to be an important

virulence factor Virulence factors (preferably known as pathogenicity factors or effectors in plant science) are cellular structures, molecules and regulatory systems that enable microbial pathogens (bacteria, viruses, fungi, and protozoa) to achieve the following ...

(aka pathogenicity factor) in ''

Aspergillus ' () is a genus consisting of several hundred mold species found in various climates worldwide. ''Aspergillus'' was first catalogued in 1729 by the Italian priest and biologist Pier Antonio Micheli. Viewing the fungi under a microscope, Miche ...

'' fungus. Gliotoxin possesses

immunosuppressive Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse reacti ...

properties that may suppress and cause

apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...

in certain cells of the

immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...

, including

neutrophil Neutrophils (also known as neutrocytes or heterophils) are the most abundant type of granulocytes and make up 40% to 70% of all white blood cells in humans. They form an essential part of the innate immune system, with their functions varying in ...

eosinophil Eosinophils, sometimes called eosinophiles or, less commonly, acidophils, are a variety of white blood cells (WBCs) and one of the immune system components responsible for combating multicellular parasites and certain infections in vertebrates. A ...

granulocyte Granulocytes are cells in the innate immune system characterized by the presence of specific granules in their cytoplasm. Such granules distinguish them from the various agranulocytes. All myeloblastic granulocytes are polymorphonuclear. They ha ...

macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer cel ...

s, and

thymocyte A Thymocyte is an immune cell present in the thymus, before it undergoes transformation into a T cell. Thymocytes are produced as stem cells in the bone marrow and reach the thymus via the blood. Thymopoiesis describes the process which turns thymo ...

s. Specifically, neutrophils exposed to gliotoxin release less

reactive oxygen species In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. The reduction of molecular oxygen () p ...

(ROS) and complete fewer

phagocytic Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...

activities. Gliotoxin is also believed to interfere with

T-cell A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell rec ...

activation. Additionally, gliotoxin acts as an inhibitor of

farnesyl transferase Farnesyltransferase () is one of the three enzymes in the prenyltransferase group. Farnesyltransferase (FTase) adds a 15-carbon isoprenoid called a farnesol, farnesyl group to proteins bearing a CaaX Sequence motif, motif: a four-amino acid seque ...

. It noncompetitively inhibits the

chymotrypsin Chymotrypsin (, chymotrypsins A and B, alpha-chymar ophth, avazyme, chymar, chymotest, enzeon, quimar, quimotrase, alpha-chymar, alpha-chymotrypsin A, alpha-chymotrypsin) is a digestive enzyme component of pancreatic juice acting in the duodenu ...

-like activity of the

20S proteasome Proteasomes are protein complexes which degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks peptide bonds. Enzymes that help such reactions are called proteases. Proteasomes are part of a major mechanism by whi ...

In vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and ...

gliotoxin displays

anti-inflammatory Anti-inflammatory is the property of a substance or treatment that reduces inflammation or swelling. Anti-inflammatory drugs, also called anti-inflammatories, make up about half of analgesics. These drugs remedy pain by reducing inflammation as o ...

activity. It was investigated as an

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of ...

and

antifungal An antifungal medication, also known as an antimycotic medication, is a pharmaceutical fungicide or fungistatic used to treat and prevent mycosis such as athlete's foot, ringworm, candidiasis (thrush), serious systemic infections such as crypto ...

in the 1940s and as an antiviral agent. Gliotoxin inactivates many different

enzymes Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

, including

nuclear factor-κB Nuclear may refer to: Physics Relating to the nucleus of the atom: *Nuclear engineering *Nuclear physics *Nuclear power *Nuclear reactor *Nuclear weapon *Nuclear medicine *Radiation therapy *Nuclear warfare Mathematics * Nuclear space *Nuclear ...

(NF-κB),

NADPH oxidase NADPH oxidase (nicotinamide adenine dinucleotide phosphate oxidase) is a membrane-bound enzyme complex that faces the extracellular space. It can be found in the plasma membrane as well as in the membranes of phagosomes used by neutrophil white ...

, and

glutaredoxin Glutaredoxins (also known as Thioltransferase) are small redox enzymes of approximately one hundred amino-acid residues that use glutathione as a cofactor. In humans this oxidation repair enzyme is also known to participate in many cellular functi ...

. The

inhibition Inhibitor or inhibition may refer to: In biology * Enzyme inhibitor, a substance that binds to an enzyme and decreases the enzyme's activity * Reuptake inhibitor, a substance that increases neurotransmission by blocking the reuptake of a neurotra ...

of NF-κB leads prevents

cytokine Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in autocrin ...

release and induction of the

inflammatory response Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molecu ...

. The immunosuppressive properties of gliotoxin are due to the

disulfide bridge In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

within its structure. Interactions occur between

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

molecules that make up the disulfide bridge and

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl gro ...

groups contained in

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, sometime ...

residues. Gliotoxin acts by blocking thiol residues in the

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

. Gliotoxin also activates a member of the

Bcl-2 Bcl-2 (B-cell lymphoma 2), encoded in humans by the ''BCL2'' gene, is the founding member of the Bcl-2 family of regulator proteins that regulate cell death (apoptosis), by either inhibiting (anti-apoptotic) or inducing (pro-apoptotic) apoptosis. ...

family called Bak in order to mediate cell apoptosis. Activated Bak then causes the release of ROS, which form

pore Pore may refer to: Biology Animal biology and microbiology * Sweat pore, an anatomical structure of the skin of humans (and other mammals) used for secretion of sweat * Hair follicle, an anatomical structure of the skin of humans (and other m ...

s within the

mitochondrial membrane A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used t ...

. These pores allow the release of

cytochrome C The cytochrome complex, or cyt ''c'', is a small hemeprotein found loosely associated with the inner membrane of the mitochondrion. It belongs to the cytochrome c family of proteins and plays a major role in cell apoptosis. Cytochrome c is hig ...

and AIF, which initiate apoptosis within the cell.

Biosynthesis

In ''

'', the

needed for gliotoxin

biosynthesis Biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecules. ...

are encoded in 13 genes within the gli gene cluster. When this

gene cluster A gene family is a set of homologous genes within one organism. A gene cluster is a group of two or more genes found within an organism's DNA that encode similar polypeptides, or proteins, which collectively share a generalized function and are o ...

is activated, these enzymes mediate the production of gliotoxin from

serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...

and

phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid with the formula . It can be viewed as a benzyl group substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of alanine. This essential amino a ...

residues. Enzymes Involved in Biosynthesis (in order of activity) :GliZ:

transcription factor In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...

that regulates expression of ''gli'' gene cluster :GliP: facilitates formation of cyclo-phenylalanyl-serine intermediate from serine and :phenylalanine residues :GliC: adds

hydroxyl group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy g ...

to the

alpha carbon In the nomenclature of organic chemistry, a locant is a term to indicate the position of a functional group or substituent within a molecule. Numeric locants The International Union of Pure and Applied Chemistry (IUPAC) recommends the use of ...

of the phenylalanine residue in the :cyclo-phenylalanyl-serine intermediate :GliG:

glutathione S-transferase Glutathione ''S''-transferases (GSTs), previously known as ligandins, are a family of eukaryotic and prokaryotic phase II metabolic isozymes best known for their ability to catalyze the conjugation of the reduced form of glutathione (GSH) to x ...

(GST) that adds two

glutathione Glutathione (GSH, ) is an antioxidant in plants, animals, fungi, and some bacteria and archaea. Glutathione is capable of preventing damage to important cellular components caused by sources such as reactive oxygen species, free radicals, pero ...

molecules forming a :bis-glutathionylated intermediate :GliK:

gamma-glutamyl transferase Gamma-glutamyltransferase (also γ-glutamyltransferase, GGT, gamma-GT, gamma-glutamyl transpeptidase; ) is a transferase (a type of enzyme) that catalyzes the transfer of gamma-glutamyl functional groups from molecules such as glutathione to ...

that removes gamma-glutamyl moieties from :glutathione additions :GliN: adds a

methyl group In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in many ...

to nitrogen to form the

dithiol In organic chemistry, a dithiol is a type of organosulfur compound with two thiol () functional groups. Their properties are generally similar to those of monothiols in terms of solubility, odor, and volatility. They can be classified according ...

gliotoxin intermediate :GliT:

oxidoreductase In biochemistry, an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule, the reductant, also called the electron donor, to another, the oxidant, also called the electron acceptor. This group of enzymes usually ut ...

that mediates closure of the disulfide-bridge DithiolGTtoGT2

:GliA:

Major Facilitator Superfamily The major facilitator superfamily (MFS) is a superfamily of membrane transport proteins that facilitate movement of small solutes across cell membranes in response to chemiosmotic gradients. Function The major facilitator superfamily (MFS) are ...

transporter that secretes gliotoxin across cell membrane :Enzymes GliJ, GliI, GliF, and GliH are necessary for biosynthesis, but their exact function is unknown. Regulation of Biosynthesis Some gliotoxin molecules are not secreted by GliA and remain in the cell. This

intracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

gliotoxin activates the

GliZ, facilitating ''gli''

expression, and an enzyme called GtmA. GtmA acts as a negative regulator for gliotoxin biosynthesis by adding methyl groups to the two sulfur residues on the dithiol gliotoxin intermediate. These additions prevent the formation of the disulfide bridge by GliT, inhibiting gliotoxin formation.

Exposure and health effects

Environmental exposure

Exposure to fungal species that secrete gliotoxin is common because airborne ''

'' fungal spores are

ubiquitous Omnipresence or ubiquity is the property of being present anywhere and everywhere. The term omnipresence is most often used in a religious context as an attribute of a deity or supreme being, while the term ubiquity is generally used to describe ...

in many environments. Regular environmental exposure does not typically cause illness, but can cause serious infections in

immunosuppressed Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse reacti ...

individuals or those with chronic respiratory illnesses. Infections caused by ''Aspergillus'' fungus are called

aspergillosis Aspergillosis is a fungal infection of usually the lungs, caused by the genus ''Aspergillus'', a common mould that is breathed in frequently from the air around, but does not usually affect most people. It generally occurs in people with lung dise ...

. There are many types of aspergillosis, but infections typically affect the

lungs The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of th ...

or the

sinuses Paranasal sinuses are a group of four paired air-filled spaces that surround the nasal cavity. The maxillary sinuses are located under the eyes; the frontal sinuses are above the eyes; the ethmoidal sinuses are between the eyes and the sphenoid ...

. Gliotoxin is hypothesized to be an important

in ''Aspergillus fumigatus''. Experiments have demonstrated that gliotoxin is isolated in the highest concentrations from ''Aspergillus fumigatus'' in comparison to other ''Aspergillus'' species. This species of fungi is the most common cause of aspergillosis in humans. Gliotoxin is also the only toxin that has been isolated from the sera of patients with invasive aspergillosis. These results suggest a link between gliotoxin secretion and fungal

pathogenicity In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

. While not enough data exists to definitively tie chronic gliotoxin exposure to the development of cancer, chronic exposure to other

immunosuppressive agents Immunosuppressive drugs, also known as immunosuppressive agents, immunosuppressants and antirejection medications, are drugs that inhibit or prevent activity of the immune system. Classification Immunosuppressive drugs can be classified int ...

has been linked to the development of

lymphomas Lymphoma is a group of blood and lymph tumors that develop from lymphocytes (a type of white blood cell). In current usage the name usually refers to just the cancerous versions rather than all such tumours. Signs and symptoms may include enlar ...

and

mammary tumors A mammary tumor is a neoplasm originating in the mammary gland. It is a common finding in older female dogs and cats that are not spaying and neutering, spayed, but they are found in other animals as well. The mammary glands in dogs and cats are a ...

. Individuals taking immunosuppressive medications or with previous or current exposure to

chemotherapy Chemotherapy (often abbreviated to chemo and sometimes CTX or CTx) is a type of cancer treatment that uses one or more anti-cancer drugs (chemotherapeutic agents or alkylating agents) as part of a standardized chemotherapy regimen. Chemotherap ...

radiation are at higher risk for the development of these tumors.

Clinical exposure

Gliotoxin is

toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...

if swallowed or inhaled, and can cause

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...

and

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

irritation if exposure occurs to these areas. The oral of gliotoxin is 67 mg/kg. Acute symptoms of gliotoxin start rapidly after

ingestion Ingestion is the consumption of a substance by an organism. In animals, it normally is accomplished by taking in a substance through the mouth into the gastrointestinal tract, such as through eating or drinking. In single-celled organisms ingest ...