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Chapter 1
Introduction
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1. Introduction
Mushrooms are in use as food and medicine by human since time immemorial, and
have been recorded in ancient Chinese manuscripts (Bensky & Gamble, 1993; Wasser,
2002). Among these the wild edible mushrooms are accepted as an important source of
food/medicine and income in both developing and developed countries (Wang et al.,
2006; Boa, 2004). An increasing scientific and medical research in recent years
confirms the medicinal efficacy and efforts to identify such bioactive molecules are in
progress (Wasser & Weis, 1999; Ooi & Liu, 2000; Hobbs, 2000). Mushrooms are
classified in the independent kingdom Fungi. They contain true nucleus, reproduce by
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spores and obtain essential nutrients by breaking down of organic substances
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(heterotrophic). The fungal body is composed of either a single cell or a threadlike
structure (hypha) and large number of branching hyphae giving rise to structures called
mycelia. In most types of fungi, spore-producing cells (basidia, asci, conidiophores)
form a part of special structure made of hyphal tissue and called the fruit body
(sporocarp). In accordance to Breene (1990) the gross composition of mushrooms is
water (90%), and dry matter such as protein (10%–40%), fat (2%–8%), carbohydrates
(3%–28%), fiber (3%–32%) and ash (8%–10%) (Ash percentage is the fraction of dry
matter that remains after incineration of the organic material in a sample and is mainly
composed of salts, metals and so forth). The different mushroom species on earth is
estimated at 140,000. Approximately 14,000 species that are known today (roughly
10%), of which about 50% are considered to possess varying degrees of edibility. More
than 2000 are safe and about 700 species are known to possess significant
pharmacological properties. Of the vast number of available mushrooms about 35
species have been cultivated commercially and 20 are cultivated on an industrial scale
(Somasundaram et al., 1998; Ogundana et al., 1982 and Kavishree et al., 2008). The
most cultivated mushrooms worldwide are Agaricus bisporus (button mushroom),
followed by Lentinus edodes (shiitake), Pleurotus spp (oyster mushrooms), Auricula
auricula (wood ear mushroom), Flamulina velutipes (winter mushroom) and
Volvariella volvacea (straw mushroom) (Aida et al., 2009). The market value of
mushroom dietary supplement products worldwide is about US$5–6 billion per year
(Lull et al., 2005).
Historically, various preparations from mushrooms are used as therapeutics;
especially hot water-soluble fractions (decoctions and essences) from mushrooms were
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used as medicine in the Far East, where knowledge and practice of mushroom use
primarily originated. These metabolites and bioactive compounds are increasingly
utilized for cure of a wide variety of diseases, as they can be added to the diet and used
orally. The possible uses of these molecules for treatment of diseases like allergic
asthma, food allergy, atopic dermatitis, inflammation, autoimmune joint inflammation
such as rheumatoid arthritis, atherosclerosis, hyperglycemia, thrombosis, human
immunodeficiency virus (HIV) infection, listeriosis, tuberculosis, septic shock and
cancer are being investigated. The bioactive compounds mainly polysaccharides,
polysaccharopeptides (PSP), polysaccharide proteins and proteins, subsequently small
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molecules such as triterpenes, lipids, and phenols, have been identified and
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characterized in mushrooms with proven medicinal properties. Among the various
therapeutic values, anticancer and immunomodulating properties are being investigated
in a large scale (Lull et al., 2005).
1.1 Mushrooms as source of bioactive compounds
Edible mushroom derived glucan/proteoglucans recognized as biological response
modifier (BRMs) are being studied and tried for therapeutic protocols. These stimulate
host immune system by increasing effector cells counts, producing one or more soluble
mediators, decrease host-suppressor mechanisms. BRMs are not only important in
immunomodulation but they also help in maintaining homeostasis and improving
quality of life, thereby mounting anti tumor immunity and alleviating the disease
associated pathologies. Mushroom derived β-D glucans, heteropolysaccharides,
glycoproteins and proteoglycans are well investigated as BRMs in cancer therapy.
About 651 species representing 182 genera of hetero- and homobasidiomycetes
mushrooms are reported to contain anti-tumor or immunomodulating metabolites
(Tzianabos, 2000; Chen and Seviour, 2007). The metabolites/ compounds derived from
mushroom modulate the biologic response of immune system either positively or
negatively. Compounds having stimulatory properties are being employed for the
treatment of cancer, immunodeficiency diseases or for generalized immunosuppression
following drug treatment (Jong et al., 1992). Compounds with immunosuppressive
potential are utilized in treatment of autoimmune (abnormal immune response against
self-antigens) or certain gastro-intestinal tract diseases (e.g. Crohns) (Badger, 1983).
The activity of compounds towards stimulation or suppression of immune responses
depends on various factors such as dose, route of administration, and timing of
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administration of the compound, mechanism of action, and site of activity (Tzianabos,
2000). Immunomodulating compounds act on immune effecter cells such as
hematopoietic stem cells, lymphocytes, macrophages, T cells, dendritic cells and natural
killer cells involved in the innate and adaptive immunity. Therefore the present
approach has been to isolate, characterize, and administer the pure active constituents
for their biological activities. The responses of different polysaccharides are likely to be
mediated by different cell surface receptors, which may be present only on specific
subsets of cells and may trigger distinct downstream responses. A combination of such
responses involving different cell subsets could conceivably provide greater tumor
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1.2 Polysaccharides from mushroom as bioactive compounds
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inhibition than could be induced by a single polysaccharide (Moradali et al., 2007).
Hot water soluble fractions (decoctions and essences) from medicinal mushrooms
containing mostly polysaccharides, were used as medicine (Hobbs, 1995; 2000). Some
of the early studies reported the anti-tumor activity of extracts from mushrooms
belonging to family polyporaceae. Extensive research has established that Ganoderma
lucidum polysaccharide (GLPS) fractions have immunomodulating properties (Ooi et
al., 2002). Polysaccharides belong to a structurally diverse class of macromolecules,
polymers of monosaccharide residues linked to each other by glycosidic linkages and
have high degree of structural variability. This enormous potential variability in
polysaccharide structure gives the necessary flexibility to the precise regulatory
mechanisms of various cell-cell interactions in higher organisms (Zhang et al., 2007).
Mushroom polysaccharides are present mostly as glucans with different types of
glycosidic linkages, such as (1-3), (1-6)-β-glucans and (1-3)-α-glucans, but some are
true heteroglycans (Wasser, 2002). The main source of anti-tumor polysaccharides
appears to be fungal cell walls that consist of polysaccharides. On the other hand, chitin
and chitosan (fungal chitin) have no anti-tumor activity (Mizuno et al., 1995). β-Dglucan is a polysaccharide yielding exclusively D-glucose upon acid hydrolysis
(Mizuno, 1996, 1999). Acidic glucuronoxylomannan isolated from the fruit body of
Tremella fuciformis was also demonstrated as having a left-handed, threefold helical
backbone conformation (Yui et al., 1995). Besides the well known anti-tumor (1-3)-βglucans, a wide range of biologically active glucans with other structures have been
described. These polysaccharides have linear or branched molecules in a backbone
composed of α- or β-linked glucose units and they contain side chains that are attached
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in different ways. Heteroglucan side chains contain glucuronic acid, xylose, galactose,
mannose, arabinose, or ribose as a main component or in different combinations.
Glycans, in general, are polysaccharides containing units other than glucose in their
backbone. They are classified as galactans, fucans, xylans and mannans by the
individual sugar components in the backbone. Heteroglycan side chains contain
arabinose, mannose, fucose, galactose, xylose, glucuronic acid and glucose as a main
component in different combinations. All of these preparations are chemically β-Dglucans in nature or β-D-glucans linked to proteins. The biological activity of β-Dglucans is influenced by their solubility in water (Ishibashi et al., 2004), molecular
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weight (Ozaki et al., 1995; Mueller et al., 2000), branching rate (Kataoka et al., 2002),
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triple helical solution conformation (Mueller et al. 2000; Falch et al., 2000), and β-(1-6)
bonding system in the β-(1-3) major chain (Cleary et al., 1999). β-D-glucans induce
biological responses by binding to a membrane receptor. The β-glucan receptor was
first idendified as a β-glucan inhibitable receptor for particulate activators of the
alternative complement pathway (Czop and Austen, 1985). More recently, another
receptor, Dectin-1 was characterized as a β-glucan receptor that mediates this activity
(Adachi et al., 2004; Brown and Gordon, 2003). Biologically active polysaccharides are
widespread among higher Basidiomycetes mushrooms, and most of them have unique
structures in different species. Moreover, different strains of one Basidiomycetes
species can produce polysaccharides with different properties.
1.3 Anti-cancer and immunomodulatory activity of mushroom polysachharide
Mushroom polysaccharides exert their anti-tumor action mostly via activation of the
immune response of the host organism. The induction of cellular responses by
mushroom and other β-glucans are likely to involve their specific interaction with one
or more cell surface receptors. In 1985, β-glucan receptors were first identified on the
surface of monocytes by Czop and Austen (1985) as opsonin-independent receptors for
particulate activators of the alternative complement activation pathway. To date, several
β-glucan receptors have been identified as candidates mediating these activities (Brown
and Gordon, 2003), namely, complement receptor 3 (CR3, αMβ2 integrin, or
CD11b/CD18) (Ross et al., 1987), lactosylceramide (Zimmerman et al., 1998),
scavengers receptors (Rice et al., 2002), dectin-1 (Brown and Gordon, 2001), and tolllike receptors TLR-2 and TLR-4 (Shao et al, 2004). Adaptive immunity uses
somatically generated receptors that recognize antigenic patterns to which the host has
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been previously exposed. In contrast, innate immunity relies on genetically
predetermined pattern recognition receptors (PRRs) that recognize carbohydrates,
lipids, and proteins that are unique to microorganisms and are not produced by the host.
These macromolecular structures, usually found in the cell wall, are referred to as
pathogen-associated molecular patterns (PAMPs). Glucans may be fungal recognition
molecules (PAMPs) for the innate-immune system of the host. Also, the anti-tumor
activity of lentinan and other polysaccharides is inhibited by pretreatment with
antimacrophage agents (such as carrageenan). Thus, the various effects of
polysaccharides are thought to be due to potentiation of the response of precursor T
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cells and macrophages to cytokines produced by lymphocytes after specific recognition
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of tumor cells. In addition, the induction of a marked increase in cytokines by
polysaccharides results in maturation, differentiation, and proliferation of the
immunocompetent cells for host defense mechanisms (Hamuro and Chihara, 1985).
Mushroom polysaccharides are known to stimulate natural killer cells, T-cells, B-cells,
and macrophage-dependent immune system responses. Lentinan is known to restore the
suppressed activity of helper T-cells in the tumor-bearing host to their normal state,
leading to complete restoration of humoral immune responses (Ooi and Liu, 2000). The
same effect is true for PSK, while it has no substantial effect on immune responses of
the host under normal conditions.
1.4. Astraeus hygrometricus as an un-explored source of bioactive compounds
Astraeus hygrometricus is a non cultivated wild edible mushroom, which grows on
nutrient poor soil in association with mycorrhiza primarily during rainy season. And
Astraeus is one of the most common gasteromycete genera in temperate and tropical
ecosystems (Lloyd, 1902). Morgan (1889) was the first to recognise Astraeus as a
distinct genus with A. hygrometricus (Pers.) Morgan as the type and only species.
Whereas, Phosri et al. (2007) described four species of this genus on the basis of its
rDNA region. Although this species of mushroom is wide spread however very few
studies with respect to biological activity are conducted. Earlier studies have reported a
lectin with no agglutination potential from this species. Chakraborty et al. (2004)
reported a polysaccharide from this species with Splenocyte proliferating potential in
vivo. In another report, Maiti et al. (2008) observed a protein fraction isolated by
cibacron blue affinity chromatography to inhibit the growth of several tumor cell lines,
and it had a stimulatory effect on the growth of splenocytes, thymocytes, and bone
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marrow cells from mice and stimulation of mouse NK cells and Macrophage. The local
availability of this mushroom and its potential of bioactive component make it a
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suitable candidate for investigation for bioactive components.
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