INTRODUCTION:

Besides plants many fungal species are known to possess medicinal values and some are already being used for such purposes and mushrooms are used in folk medicines throughout the world since ancient times as “the ultimate health food”^[1] . As per Dictionary of the Fungi total 97,330 discovered species of fungi includes slime molds, chromistan fungi, chytridiaceous fungi, lichen-forming fungi, yeasts and molds including mushroom producing filamentous fungi^[2]. Mushrooms being neither plant nor animal have been placed in a kingdom called Myceteae. Among fungi, mushroom fungus is more prominent because it is a macrofungus with a distinctive fruiting body which can be hypogenous or epigenous, large enough to be seen with the naked eyesand to be picked by hand ^[3].

Edible mushrooms have been utilized as human food for centuries and appreciated for texture and flavor as well as some medicinal and tonic attributes. They are also considered as healthy food because they are low in calories and fat but rich in proteins and dietary fibers ^{[4, 5]}. Mushrooms are relatively a good source of nutrients also, like phosphorus, iron and vitamins, including thiamine, riboflavin, ascorbic acid, ergo sterol, and niacin ^[6].

Mushrooms include 14,000 to 22,000 species and there are over 2000 species of mushrooms that are edible, however, a dozen of them are commercially cultivated, and a few of them are poisonous if consumed. Poisonous mushrooms mainly belong to genus Amanita, Inocybe, Panaeolus and Russulaceae. Toxic effects of mushrooms are due to their secondary metabolites ^{[7, 8]}.

For a common man mushrooms are still considered as one of the curiosities of nature and many of them are widely consumed for their flavor and aroma. Their nutritive and medicinal values were known as early as 1500 BC based upon many ancient literatures. In the world of medicinal mushrooms, Ganoderma is number one and has been considered as king of medicinal mushrooms followed by Lentinula and others including Pleurotus, the later produces oyster mushroom ^{[9, 10]}.

Pleurotus ostreatus, the oyster mushroom is increasingly being recognized with a significant role in human health and nutrition and is well documented for its probiotic properties. For the first time in 1985, Yoshoka et al reported the antitumor activity and later, the hypoglycemic effects were also reported in experimentally diabetic induced rats, and since then a number of studies have indicated their therapeutic potentialities. Thus, the present review gives an updated comprehensive account of therapeutic properties of Pleurotus ostreatus to help the researchers to explore more untapped metabolites from this mushroom fungus.

Therapeutic properties of Pleurotus ostreatus Antiviral:

Santoyo et al in 2012 reported the antiviral activity of water and methanolic extract of Pleurotus ostreatus, together with Lentinus edodes, Boletus edulis against herpes simplex virus type 1 (HSV-1). Pretreatment of vero cells with 75 mg/ml water extracts before virus addition, inhibited 60% of virus infection and its addition during the virus adsorption period inhibited it up to 80%, whereas the methanolic extracts exhibited a lower antiviral activity. The activity was found to be due to presence of glucans present in the polysaccharides fractions ^[11].

Antioxidant:

Fruiting bodies of Pleurotus possessed higher concentration of antioxidants than other commercial mushrooms ^{[12,
13, 14]}. This activity was mainly due to the presence of polysaccharide pleuran (β-glucan) that has been isolated from Pleurotus ostreatus showing a positive effect on rat colon with pre-cancerous lesions ^[15]. Pleurotus ostreatus increased the activities of important antioxidant enzyme (superoxide dismutase, catalse and peroxidase) thereby reducing oxidative damage in humans ^{[14, 16]}.

Venkatakrishnan et al in 2010 have shown that extracts from Pleurotus ostreatus inhibited the growth of HL-60 cells by cell cycle arrest i.e. by the induction of apoptosis by their experiments due to the presence of flavanoid (quercetin equivalent) and phenolics components (catechin) in fruiting bodies ^[17].

In 2006, Jayakumar et al reported that when rats treated with CCl₄ induced hepatotoxicity and were kept in doses with the extracts of Pleurotus ostreatus, the serum SGOT, SGPT and SALP levels reverted to near normal, while hepatic concentration of GSH, CAT, SOD and GPx were significantly increased and that of MDA significantly lowered, when compared with CCl₄ exposed untreated rats. Thus, it was concluded that Pleurotus ostreatus significantly alleviate the hepatotoxicity induced by CCl₄in rats ^[18].

Patjaraporn and Apissada in 2012 reported that when more concentration of Pleurotus ostreatus extracts used, the less antioxidant activities obtained. Thus, the results indicated that the optimal concentration in cosmetic products should be 0.2% ^[19].

Antidiabetic:

Ravi et al in 2013 found that consumption of ethanolic extracts of Pleurotus ostreatus showed a significant decrease in serum glucose levels and post-treatment with Pleurotus ostreatus extract reduced serum cholesterol, triglyceride and LDL-cholesterol. Thus, Pleurotus ostreatus showed significant hypoglycemic effect in diabetic mice and it is also capable of improving hyperlipidemia and the impaired kidney function in alloxan induced diabetic mice ^[20].

Jayasuriya et al in 2012 investigated the oral hypoglycemic activity of Pleurotus ostreatus and Pleurotus cystidiosus mushrooms on normal and alloxan induced diabetic wistar rats. Extracts of both the mushrooms posseseed marked and significant oral hypoglycemic activity. It also confers that consumption of Pleurotus ostreatus and Pleurotus cystidiosus brings health benefits to mankind as it showed hypoglycemic activity in rats ^[21].

Anti-inflammatory:

Pleuran isolated from fruiting bodies of oyster mushroom possesses anti-inflammatory activity ^{[15, 22]}. Jedinak et al in 2011 found that anti-inflammatory activity of oyster mushroom concentrate was due to the inhibition of proliferation and secretion of interferon-γ, IL-2 and IL-6 from concanavalin-A stimulated mouse splenocytes, and thus, Pleurotus ostreatus possesses anti-inflammatory activities and could be considered a dietary agent against inflammation ^[23].

Anticancer:

In 1969, Wantanabe detected antineoplastic activity of polysaccharide extrtacted from the fruiting body of Pleurotus ostreatus. Later, polysaccharide extracted from mycelium of of Pleurotus sajor-caju, Pleurotus ostreatus were also shown to have antineoplastic activity. These polysaccharide are components of the cell wall of oyster mushroom, polysaccharide extracted from Pleurotus ostreatus culture broth when injected intraperitoneally (i.p.) in the female swiss albino mice caused 76% reduction in the number of neoplastic cells ^[24].

Akanni et al (2010) investigated the anticancer and antileukaemic potentiality of Pleurotus ostreatus and of Pleurotus pulmonarius using micronucleus assay and found that after prolonged use of many antineoplastic agent causes dreadful side effects, hence development of such anticancer agents that will selectively destroy cancer cells without injury to normal cells is must. Thus, screening of Pleurotus ostreatus and of Pleurotus pulmonarius metabolites reveals that both are not clastogenic but rather enhanced erythropoiesis ^[25].

Hot water soluble fractions of Pleurotus ostreatus were examined using HT-29 colon cancer cells in vitro to study its antiproliferative and pro-apoptotic activities. The proliferative inhibition was due to the apoptotic induction and due to induction the pro-apoptotic molecules Bax and cytosolic cytochrome-c were upregulated ^[26].

In 2006, Shlyakhovenko et al studied the immunomodulatory and biotherapeutical potential of DNA isolated from the fruit body of Pleurotus ostreatus and found that mushroom DNA resulted in augmentation of NK cytotoxic activity and significant increase of the life span of mice with solid Ehrlich carcinoma. The biotherapeutic potential was probably due to the presence of unmethylated CpG motifs ^[27].

Anti-ageing:

Extracts of Pleurotus ostreatus elevated the levels of vitamin C and vitamin E, increased activities of catalase, superoxide dismutase and glutathione peroxidase in aged rats ^[28,
29]. They are potent antioxidant enzymes ^{[15, 16]}. The levels of Malonaldehyde, a polyunsaturated lipid and an electrophilic mutagen, were lowered on administration of mushroom extract to aged rats, that reacted with deoxyadenosine and deoxyguanosine in DNA, forming a DNA adduct. Different extracts (methanol, ethanol, acetone or water extract) of Pleurotus can improve the antioxidant status during ageing leading reducing the occurance of age-associated disorders like stoke, Parkinson’s disease, atherosclerosis, diabetes, cancer and cirrhosis ^[30].

Immunomodulatory activity:

The polysaccharide distilled from Pleurotus ostreatus showed immunomodulatory activity against infectious bursal disease (IBD) when four weeks post hatching broilers were used. Whereas, DNA isolated from Pleurotus ostreatus also possesses immunomodulatory activity and biotherpeutic potential that could be due to the presence of unmethylated CpG motifs. Oncoprotective and immunomodulatory effects of substance from Pleurotus ostreatus have also been demonstrated ^[31-33]. Pleurotus ostreatus diminishes the toxicity of cyclophosphamide in mice was shown by Gerasimenya et al^[34].Similar immunomodulatory properties were reported from DNA of Pleurotus ostreatus in same manner as bacterial DNA or DNA from certain viruses, yeast, nematodes, mollusks and insects, DNA of vertebrates like mammals, fish and frogs as well as plants lacked immunomodulatory activities as they do not possess immunogenic activity ^[35-37].

CONCLUSION:

The major attribute of mushrooms is their medicinal properties which have been the main focus of researchers around the world. The potential therapeutic implications of Pleurotus ostreatus are enormous but, detailed mechanism of the various health benefits to humans still requires intensive investigation, especially with the emergence of new evidence of their health benefit effects. The current comprehensive information made available in this review shows that Pleurotus ostreatus possesses many promising therapeutic properties that requires more pharmacological approaches for their deeper exploration. The development of more high tech approaches will allow the researchers to explore novel metabolites from Pleurotus ostreatus.

ACKNOWLEDGEMENT:

Special acknowledgement to AICTE, as this paper is a part of the work done under the sanctioned ongoing AICTE Research Promotion scheme (RPS) project titled Development of Oral Formulation From Bioactive Macrofungus (Ref. No. 8023/RID/RPS-22/201-12 dated April 4 2012).

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Received on 03.06.2013 Modified on 21.06.2013

Research J. Pharm. and Tech. 6(9): September 2013; Page 937-940