4.11 The potential of fungi used in traditional Chinese medicine: shiitake (Lentinula edodes) (contributed by Dawn Soo, 2002)

Abstract

Traditional Chinese medicine (TCM) uses many products derived from fungi, including Lentinula edodes (shiitake). Evidence, in the form of published papers, regarding its usefulness has been examined. The review finds that the anti-tumour claims are substantiated, especially those of the lentinan polysaccharide isolated from shiitake. Lentinan is also found to be an immunomodulator, although the medicinal value of these findings are yet to be reliably established. Cholesteremic effects were shown but there was disagreement over the details of the effects. It was concluded that Lentinula edodes does have much potential in medicine but further research is needed.

Introduction

Traditional Chinese medicine uses many products derived from fungi, including Lentinula edodes, or shiitake. Its place in traditional Chinese medicine and the West’s science based medicine has been examined. The Institute for Scientific Information’s (ISI) impact factor (IF), which is measured by the number of citable articles in a journal divided by the number of journals it has been cited in, has been used as a method of critique.

Traditional Chinese Medicine

Traditional Chinese medicine (TCM) encompasses the use of herbal remedies, acupuncture, acupressure, massage, moxibustion [1]; and more holistic disciplines such as dietary therapy, mind and body exercises and meditation [2]. In recent years various aspects of TCM have received attention in the West [3], acupuncture enjoying the earliest wave of popularity, is now the most frequently requested complementary therapy [4].

Now the focus has switched to medicinal herbs, which are becoming increasingly popular and important in the public and scientific communities of the Western world [5]. For example, a recent upsurge in the number of studies investigating traditional Chinese herbs, with regard to the treatment of atopic eczema, has produced exciting successes [6-8]. More specifically, mushrooms, in the context of Chinese herbal medicine, have enjoyed a recent swell of interest [9]. For example, studies have been conducted on the medicinal properties of Lentinula (shiitake), Pleurotus (oyster), Auricularia (mu-er), Flammulina (enokitake), Tremella (yin-er), Hericium, and Grifola (maitake), and of the annual 12 million metric tonnes of cultivated edible mushrooms it is estimated that approximately 50% contain functional "nutraceutical" or medicinal properties [10].

Mushrooms

Mushrooms are defined as the fruiting body of a macrofungus. Their chemical composition is shown in Table 1. Most mushrooms contain vitamins, particularly niacin, thiamine, riboflavin, biotin and vitamin C. Mushrooms also contain a wide variety of bioactive molecules including terpenoids, steroids, phenols, nucleotides and their derivatives, glycoproteins and polysaccharides [11]. Mushrooms are claimed to exhibit antitumour, antiviral, antibiotic, anti-inflammatory, hypoglycaemic, hypocholesterolemic and hypotensive activities [12-14], as well as having therapeutic and nutritional value. Their antitumour activity is the most thoroughly researched medicinal effect, especially that of the shiitake mushroom, the maitake mushroom, Sclerotina sclerotiorum and Schizophyllum commune [11].

Table 1. Nutritional and medicinal value of speciality mushrooms

Component

Percentage by weight

Water

90

Protein

10-40

Fat

2-8

Carbohydrates

3-28

Fibre

3-32

Ash

8-10

Data from ref. 13.

Shiitake

Shiitake, is the common Japanese name for the edible mushroom Lentinula edodes, which is now cultivated and is the second most commonly produced edible mushroom in the world [15, 16]. It grows naturally on fallen wood of broadleaf forests and according to a Chinese physician of the Ming Dynasty (1368-1644), Wu Juei, it preserves health, improves stamina and circulation, cures colds and lowers blood cholesterol [17].

Chinese Medicine is prescribed as a concoction of several herbs, whereas Western medicine employs pure, single compounds, either natural or synthetic. The most highly researched bioactive molecule isolated from shiitake is the pure β(1,3)-D-glucan called lentinan. Chemical details can be found in Table 2.

Table 2. Structure of lentinan

Glycosidic linkage back bone

Side chain

Molecular weight

Degree of branching

Higher structure

Lentinan

β(1,3)- glucan

β(1,6)-glucan

4-8 × 105

2/5

Triple helix

Data from reference 18.

Anti-tumour activity

Research into the possibilities of anti-tumour substances in mushrooms began in Japan in the late 1960’s.11 With a one in four chance of getting cancer, according to the Imperial Cancer Research Fund, a male five-year survival rate of 31%, and a female equivalent of 43%, it’s not surprising the hint of anti-cancer activity stirred interest in shiitake.

Six week old ICR mice treated with N-butyl-N’-butanolnitrosoamine (BBN) received normal feed in the control group and 5% dried and powdered shiitake in the experimental. 100% of the control group developed urinary bladder carcinoma (10/10) while the incidence was reduced to 52.9% in the experimental group (9/17) [19]. The shiitake enriched diet also increased the levels of macrophage activity and mitogenic response of lymphocytes to concanavalin A (con A) to almost normal levels, which were severely suppressed by the BBN treatment. It also increased the cytotoxic activity of lymphokine-activated killer (LAK) cells and natural killer (NK) cells which were depressed by BBN treatment. These accompanying immunomodulating effects may have been the mechanism for the anti-tumour effect.

The study indicates that the powdered shiitake fruit bodies administered in the diet have anti-tumour activities in murine systems. Similarly, 20% shiitake feed showed effects in inhibiting MM-46 carcinoma growth in C3H mice at 79% tumour inhibition; but the effects were less profound (21%) in inhibiting IMC carcinoma growth in CDF1 mice [11]. This suggests that although shiitake is effective, it is strain specific in mice, and raises the question as to how effective it would be in human systems.

The anti-tumour effects of shiitake feed in murine systems has been paralleled by the anti-tumour effects of lentinan, which has been reported to prevent both chemical and viral carcinogenesis. An early study found that intraperitoneal injections of an aqueous extract of shiitake, of which the active substance was lentinan, greatly inhibited (81%) the growth of tumours arising from sarcoma 180 ascites cells implanted in Swiss albino mice [20]. The findings have been supported in many trials since and although it is a dated article, they are often cited in new articles. The study was also published in the highly reputable journal, Nature, which has an impact factor of 25.8, which lends much support to the findings. Moreover lentinan has been shown to potentiate the effects of other drugs such as 5-fluorouracil (5-FU) [21] and cis-diamminedichloroplatinum (II) (CDDP) [22] in cancer treatment.

Lentinan is described as a host mediated anti-cancer drug and has been put through numerous clinical studies in Japan, though none that were placebo controlled and double-blind. In Japan a purified lentinan has been approved for clinical use [11] and it has become “large market item” in Japan [23]. It has proven effective in prolonging the survival of cancer patients, particularly those with gastric and colorectal cancer. In unresectable gastric cancer patients the administration of lentinan, in combination with 2 other chemotherapeutic agents, increased one year survival rate and significantly increased median survival rate, compared with those just receiving the 2 chemotherapeutic agents. Total quality of life (QoL) score was also significantly improved with the administration of lentinan [24]. Although, the latter claim may prove of little value as there is debate over how QoL should be measured and if, indeed, it can be measured.

Nevertheless, work in both human and murine models has shown both shiitake and lentinan to be effective anti-tumour agents and extensive work in Japan has lead to them pioneering the use of lentinan in clinical medicine, although there seems to be little work published in reliable journals. The results are very promising but until more clinical studies are conducted particularly with randomised controlled trials, it seems unlikely that lentinan will be accepted into Western clinical medicine.

Immunomodulatory effects

Oral ingestion of whole mushroom extracts has shown to modulate certain immune functions, as described above, although the oral administration of the polysaccharide lentinan is ineffective [25]. β-glucans lose much of their anti-tumour activity with decreasing molecular weight, so their digestion would account for their lack of oral activity. The differences in effectiveness between shiitake and the lentinan extracted indicates that other constituents of the mushroom may have a bioactive role. The mechanisms are still unclear.

The mechanism of lentinan’s anti-tumour activity is not clear either; lentinan is known as a biological response modifier (BRM) which is often taken as a synonym for an immunomodulator. Its anti-tumour action is host mediated; lentinan is claimed to enhance cell-mediated immune responses in vitro and in vivo which probably plays a part in its anti-tumour activity.

The effect of lentinan on T cells has been thoroughly investigated. Intraperitoneal lentinan administration resulted in complete tumour regression in 7 of 8 BDF1 mice inoculated with FBL-3 erythroleukemia cells. When monoclonal antibodies (mAb) against both CD4 and CD8 were given before the lentinan treatment the tumour growth inhibition stopped [26]. Recent studies add to the evidence for this theory, patients with advanced cancer were demonstrated to have an imbalance between Th1 and Th2 responses, and thus impaired cell-mediated immunity [27]. Flow cytometry of peripheral blood samples found that intravenous administration of lentinan (2 mg, three times per week) cancelled the Th2-dominant condition in patients with digestive cancers. It was also suggested that the proportions of Th1/Th2 cells may have been restored. These are pioneering findings, and are found in a reliable journal, (IF=1.331) therefore the argument has weight and the mechanism should be seriously considered in future investigations.

There have been several other studies supporting the theory that the anti-tumour activity is T-cell mediated including those that propose a different mechanism of action. It has been suggested that there is correlation between the vascular dilation and haemorrhage (VDH) reaction and the anti-tumour effects of lentinan. VDH was induced in mice by lentinan, but those receiving a combination of anti-CD4 and anti-CD8 mAb before lentinan treatment did not show a VDH response, and either mAb subset alone had no effect, showing that VDH is T cell dependent [11]. Across several strains of mice, those which showed a high VDH response after lentinan treatment, also showed high responses to bradykinnin. The bradykinnin induced skin reaction was examined in B10D2 mice bearing S908D2 sarcoma treated with lentinan and fluorouracil either alone or in combination. Separately the agents had no effect on tumour growth; their combination resulted in complete regression and augmented the skin reaction, whereas the former treatments had not [28]. The results support the theory and the assumed mechanism is induction of haemorrhagic necrosis in the tumour. These results have also been found in reliable journals although they are not as recent as the T cell experiments. It is plausible to presume that both approaches may be correct and exert a combined effort, but whichever mechanism or mechanisms the anti-tumour actions occur by the weighty evidence seems to make lentinan’s anti-tumour activity indisputable.

Immunomodulation and anti-bacterial activity

Other significant aspects of the immunomodulatory effects of shiitake are increased host resistance to bacterial and viral infections. Lenthionine, a cyclic organosulfur compound (molecular formula C2H4S5) partly responsible for the flavour of shiitake mushrooms, has antibacterial and antifungal activity and bis[(methylsulfonyl)methyl]disulphide, a derivative of lenthionine, has strong inhibitory effects against Staphylococcus aureus, Bacillus subtilis and Escherichia coli [29]. The chloroform and ethylacetate extracts of the dried mushroom have bactericidal activity against both growing and resting Streptococcus mutans and Prevotella intermedia [30]. Only a few studies have explored antibacterial components of shiitake, and these have concentrated on their potential in terms of bacteria of oral origin. Anti-bacterial activity is an exciting result, with increasing bacterial resistance to antibiotics, improving host immunity may be the way forward in fighting bacterial infection. But, these results should be viewed with caution because the published papers in this area are confined to only one or two journals, and although these have reliable IF scores, this fact does not enhance the credibility of the reports. There is a lack of significant studies in this area; and none that contributed to the anticaries aspect of anti-bacterial activity.

A postulated mechanism for the anti-bacterial activity of lentinan may be by the induction of increased levels of complement C3 and C3b formation [31]. Although, modulation of the non-specific immune system has also been displayed in numerous studies, and may be the potentiator of the anti-bacterial activity of lentinan. Many immunomodulatory effects have been reported, including increase of monocyte function in terms of IL-1 production [32], inhibition of circulating tumour necrosis factor-α (TNFα) [33] and increases in the expression of cytokines: analysis of the cytokine expression profile after lentinan administration revealed a marked increase in the mRNA levels of IL-1a, IL-1b, TNF-a, IFN-g and M-CSF in the peritoneal exudate cells and splenocytes [34]. From the various enquiries there is a vast array of empirical evidence from reliable journals; there is no doubt that lentinan is an immunomodulator. These effects could be helpful in a number of disease states, but the significance of some of these findings, in terms of potential medicinal value, has yet to be established.

Immunomodulation and anti-viral activity

The human immunodeficiency virus (HIV) has shown some sensitivity to shiitake extracts in several experiments. Lentinan in combination with 3’-azido-3’-deoxythymidine (AZT) suppressed the surface expression of HIV antigen more strongly than AZT alone in vitro. It was also shown to enhance the effect of AZT on replication of HIV in various human haematopoietic cell lines in vitro [17]. In another study several fractions of LEM (an aqueous extract of the shiitake and its solid culture medium) caused inhibition of the infectivity and cytopathic effect of HIV [29]. There has not been much further progress with these findings. Most reported experiments have been conducted in vitro and to this date there have been no studies on human participants. Therefore, the possibility of shiitake extracts being used in HIV treatment is not yet established.

Infection with HIV leads to acquired immune deficiency syndrome (AIDS). The CD4 molecule on helper T cells serves as a receptor for HIV infection. The viral RNA is converted into DNA by the host reverse transcriptase and is incorporated into cellular DNA, this results in insufficiency of cell-mediated immunity. The immunomodulating effects of lentinan may be useful drug therapy for AIDS patients, although data on this idea is scarce.

Hypocholesteremic (hypolipidemic)

Cardiovascular (CV) disease is the biggest cause of mortality worldwide and high blood cholesterol levels are an important risk factor in development of CV problems, so any hypocholesteremic effects are of great importance. The ability of shiitake to lower blood cholesterol was first reported in the 1960s. It was found that a diet supplemented with the dried ground sporophores lowered average plasma cholesterol when fed to rats. The main active component was defined and named eritadenine. It supposedly lowers all lipid components of serum lipoproteins in both animals and humans. Orally it was found to be effective ant have low toxicity although only 10% is absorbed from the intestinal tract. Intravenous eritadenine administration has proven ineffective, being cleared rapidly from circulation and excreted in the kidneys.

Several other studies have supported these findings. For example in spontaneously hypertensive rats dried shiitake decreased both the VLDL and HDL cholesterol levels and therefore prevented blood pressure increase in hypertension, and in human testing, serum cholesterol was decreased in groups of women fed fresh, dried or UV-irradiated shiitake.

A recent study has perhaps disproved the early claim that all lipid components of serum lipoproteins are lowered by shiitake [35]. Rats fed with a diet that contained 50g/kg of shiitake fibre (SF) for four weeks showed decreased levels of serum very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL) and low density lipoprotein (LDL) compared to the control which had been fed 50g/kg of cellulose. But no significant changes in high density lipoprotein (HDL) were noted. There were also no significant differences in the liver cholesterol concentration or hepatic LDL receptor mRNA levels although faecal cholesterol concentrations were significantly increased in SF-fed animals. Serum cholesterol lowering effects in rats were evident. Similarly credible research has shown that cholesterol lowering effects can be found in mice and humans, but the effects were perhaps not as all encompassing as first described.

Conclusion

Natural products are a huge resource for medicine as shown with the use of hundreds of plant species in thousands of different pharmaceutical products. Therefore the investigation of the potential medicinal value of shiitake mushrooms has become a matter of great significance. Particularly in preventing or treating serious health conditions such as cancer, acquired immune deficiency syndrome (AIDS), and hypercholesterolemia.

Epidemiological data relating to the prophylactic (or other) effects of mushroom intake on, for example, the development of spontaneous tumours is lacking. Instead, much research has been conduced on isolated purified mushroom constituents, particularly lentinan. There is a lot of evidence which appears to support the anti-tumour claims made for lentinan. A number of valuable studies have been conducted on the consequence of lentinan administration, and its acceptance into clinical medicine in Japan should perhaps highlight its efficacy.

The researchers in Japan have covered much ground in the area of neutraceuticals from mushrooms and have unearthed the potential of Lentinula edodes. At the moment there is not enough credible information to warrant the marketing of lentinan in the UK. Further investigationwould be helpful, particularly into the anti-tumour potential of dietary shiitake consumption; anti-tumour effects of shiitake administration have been claimed in murine systems. Similarly, further research is needed to establish content and bioactivity of the many compounds present in shiitake extracts and the effect of preparation and consumption differences on their medicinal activity. TCM employs composite formulae rather than isolated elements and some attention should be given to the evaluation their pharmacological properties.

Various claims have been made about the immunomodulatory effects of lentinan, although many of the claims have yet been carried through to any significance in a therapeutic context. Suggestions that these account for the anti-bacterial and anti-viral action demonstrated is an exciting prospect, especially in the face of a post-antibiotic era and increasing incidence of AIDS respectively. Although studies and information on both anti-bacterial and anti-viral activity is restricted. So while the immunomodulatory effects of lentinan are unquestionable their implications in terms of anti-bacterial and anti-viral effects are dubious.

Research into cholesteremic effects of shiitake has revealed some exciting results. Lowering of cholesterol levels was displayed in several models, although agreement over what types of lipids are effected and the mechanism of action has not yet been reached.

Overall, many studies have demonstrated potential therapeutic effects although there is still much that is unknown and many more areas need exploration. Yet it would seem that science supports the folklore; functional mushrooms deserve further serious investigation [36].

“...to surrender to the old style is wrong; to abolish or discard is wrong. Our responsibility is to unite those of the old style that can be used and to help stimulate and reform them...” [Chairman Mao, talking about the integration of Western medicine and TCM in China in 1944].

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Updated December 7, 2016