Molecular basis for Poria cocos mushroom polysaccharide used as an antitumour drug in China

1Poria cocos (Figure 1), known as “Fuling” in Chinese, is an edible medicinal mushroom belonging to dry sclerotium of polyporaceae fungi. It has more than 2000 years of medical application history for its remarkable pharmaceutical effect.1 The bioactive components in Poria cocos include polysaccharides, triterpenoids, fatty acids, sterols and enzymes. Poria cocos polysaccharide (PCP) accounts for 84% by weight among all constituents in the dried sclerotium.2 PCP is also the main bioactive component in Poria cocos.

2PCP is extracted from the sclerotium of Poria cocos. Different solvent extraction methods can obtain different polysaccharide fractions, such as WPS (NaOH‐HAc), PAP (1 mol/L NaOH), PCP2 (hot water), PCP1 (0.9% NaCI), PCP3‐I and PCP3‐II (0.5 mol/L NaOH), PCP4‐I and PCP4‐II (88% formic acid).3, 4 Thus, PCP is a mixture of different types of polysaccharides with the molecular weight ranged from 4.1 × 104 to 5 × 106 Da.5 Glucose, fucose, arabinose, xylose, mannose and galactose are detected in PCP. β‐Glucan is the major PCP with β‐(1→3)‐linked glucose backbone and β‐(1→6)‐linked glucose side chains as shown in Figure 2.3, 6 The β‐glucan from Poria cocos has poor water solubility but decent anticancer activity.7 Chihara et al removed the β‐(1→6) glucose in the β‐glucan of PCP by periodate oxidation and Smith degradation. The derivative is named “pachymaran,” which exhibits better anti‐S‐180 tumour activities.8 Hamuro et al further improved the water solubility issue of pachymaran by chemical carboxymethylation. The carboxymethylated pachymaran (CMP) has enhanced antitumour activity compared to that of pachymaran.9 Subsequently, different chemical modifications, such as sulfation,10 carboxymethylation plus sulfation,11 methylation, hydroxyethylation and hydroxpropylation, have been conducted and different types of modified pachymarans are reported.12 In general, these chemical modified pachymaran derivatives are water‐soluble and show improved bioactivities.

3PCP‐based product named “compound polysaccharide oral solution” was developed in the 1970s and was very popular with consumers as health supplement products. In 2006, “Polysaccharidum of Poria cocos oral solution” was developed by Hunan Butian pharmaceutical company of China and was granted a Chinese patent (200610163425‐X). The major component (95%) in the patented product is CMP. The water solubility of CMP allows 98% of the components to be absorbed through the human digestive track. In 2015, “Polysaccharidum of Poria cocos oral solution” was approved by Chinese Food and Drug Administration with a certified drug number B20050015 for treating multiple types of cancers, hepatitis and other diseases alone or during chemo‐ or radiation therapy for patients with cancer.

4In this article, a total of 72 publications related to different kinds of Poria cocos polysaccharides (PCPs), pachymaran, the derivatives and the biochemical/preclinical/clinical studies up to date were identified through searching PubMed, VIP (Chongqing VIP Chinese Scientific Journals Database), CNKI (China National Knowledge Infrastructure) and Wanfang database. The reported biological and pharmacological activities of PCP are classified based on these publications and shown in Figures 3 and 4. The pharmacological and other biological functions and possible molecular mechanisms shown in Figures 3 and 4 will be the major topics discussed in this article. Moreover, the data that show PCPs can overcome immunosuppression and adverse reactions associated with radiation therapy and chemotherapy13 will also be presented and discussed.

5Parallel to other reported polysaccharides from fungi,14, 15, 16 PCP and its derivatives have more impressive anticancer cell proliferation activities in vivo than in vitro when the same PCP samples are tested both in cancer cell lines and in cancer cell‐injected animal models. The in vitro anticancer cell proliferation effects of PCPs are summarized in Table 1, whereas the in vivo antitumour growth effects of PCPs are summarized in Table 2 from 15 independent studies.17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30

6PCPs possess decent anticancer cell proliferation effect in vitro, which is measured in cell cultures where different tumour cell lines derived from human or mouse tumours have been tested, such as SGC‐7901 (human), Bcap‐37 (human), HepG2 (human) and K562 (human) cells (Table 2). For example, the inhibition rate for SGC‐7901 cells is 96% compared to that of 88% for Bcap‐37 cells.17 As shown in Table 2, the inhibition rates of PCP or its derivatives on the proliferation of cancer cells are largely concentration‐dependent, which resembles the control drug 5‐fluoro‐2,4(1H,3H) pyrimidinedione or 5‐Fu.

7In contrast, in cancer cell‐injected animal models (Table 3), the inhibition rates of PCP or its derivatives on tumour growth are only partially concentration‐dependent in that within a certain range, the higher the concentrations, the higher the inhibition rates are. But beyond the range, the inhibition rates will drop. For example, the experiment conducted by Cheng et al18 showed that the inhibition rates of cancer cell growth are 0, 69%, 87%, 92% and 89%, respectively, with increasing CMP concentrations.

8The inhibition of tumour growth in vivo is measured either by the reduced tumour weight or by ultrasound compared to controls in different animal models and presented as tumour inhibition rates (%). The controls include blank and positive controls where the chemotherapeutic drug, such as 5‐fluoro‐2, 4 (1H, 3H) pyrimidinedione (5‐Fu), is used. PCPs have potent antitumour activities in different animal tumour models when compared to blank or 5‐Fu controls (Table 3). When PCP is modified with different chemical groups, such as carboxymethyl, sulphate, methyl, hydroxypropyl and hydroxyethyl, the derivatives exhibit better inhibition rates. For example, Wang et al extracted six polysaccharides from the fresh sclerotium of Poria cocos using different solvents sequentially, including PCP1 (0.9% NaCl), PCP2 (hot water), PCP3‐I and PCP3‐II (0.5 mol/L NaOH), PCP4‐I and PCP4‐II (88% formic acid). They then modified PCP3‐II chemically and obtained five different PCP3‐II derivatives.3 They showed19 that all the derivatives inhibit tumour growth better than PCP3‐II with the inhibiting rates increased by 36% and 35% for sulphated and carboxymethyled derivatives, respectively, and by 24%, 15% and 20% for methylated, hydroxypropylated and hydroxyethylated derivatives, respectively. They further demonstrated that the increased degree of chemical modifications and the increased molecular weight in the derivatives correlate with better inhibiting rates in vivo.

9Furthermore, the curing effects can be further enhanced when PCPs are combined with chemotherapeutic drugs in different animal models. In addition, the combined therapy also reduces the adverse effects associated with chemotherapeutic drugs (Table 4). For example, Liu et al31 Tang et al32 and Chen et al29 reported that PCPs exhibit better tumour inhibition rates when it is used with other chemotherapeutic drugs. For instance, inhibition rates were 46% when PCP is used with 5‐Fu compared with the inhibition rates of 41% when 5‐Fu is used alone.29

10PCPs exert their antitumour activity via assisting the host to overcome adverse biological stresses, to increase immunity against the tumours and to promote the apoptosis of tumour cells directly. The possible mechanisms reported so far are summarized in Table 5A‐D and Figure 5, 33, 34, 35, 36, 37, 38 and discussed below.

11Polysaccharides could activate effector immune cells, such as macrophages, lymphocytes and natural killer (NK) cells to activate the innate immune system to exert antitumour activity by accelerating the host's defence mechanisms.33 It is reported that the ratio of lymphocytes could increase to 62% in the treatment group compared with control group (47%)34 and phagocytic ability of macrophages could reach 59% compared with control group of 27%.36

12TNF (tumour necrosis factor) secreted by macrophages and lymphocytes is used as a drug for tumour biotherapy. IL‐6 (interleukin‐6) secreted by T lymphocytes and fibroblasts improves the killing ability of NK cells. Miu et al and Chen et al found that carboxymethyl pachymaran could improve the levels of IFN‐γ, IL‐2, TNF and IL‐6.35, 38

13Bcl‐2 and Bax are members of the Bcl‐2 family, which are important for the regulation of apoptosis. The Bax/Bcl‐2 ratio determines the survival of cells. Meng reported that sulphated pachymaran could enhance the expression of apoptosis‐related genes Fas and Bax and reduce the expression of Bcl‐2 gene. The increased Bax/Bcl‐2 ratio is responsible for the apoptosis of S180 tumour cells in the mouse model.33 Zhang et al showed that WPS could inhibit the growth of human breast carcinoma MCF‐7 cells by inducing G1 arrest of the cell cycle and by elevating the Bax/Bcl‐2 ratio.39

14TPK (tyrosine protein kinase) and PTPP (phosphotyrosine protein phosphatase) are two important enzymes controlling the growth, proliferation and differentiation of cells. Several studies reported that the activities of TPK and PTPP have changed significantly when the cells become cancerous, but PCP normalizes the activities of TPK and PTPP in cancer cells and slowdowns cancer cell growth.37

15Studies showed that PCP could enhance host immune function and activate the immune response.26, 36, 40, 41, 42, 43, 44, 45, 46, 47 It is reported that PCPs could modulate their specific immune response via the activation of T cells.40 Pachymaran strongly enhances the generation of alloreactive cytotoxic T lymphocytes in vivo.41 Generally speaking, the strength of the immune function can be judged through three aspects: the capability of macrophage's phagocytosis, the cellular immune function by detecting the index of immune organs42, 43 and the humoural immunity function by measuring the production of antibodies and serum haemolysins. As shown in Table 5C,D, Zhang et al and Xu et al found that the capability of macrophage's phagocytosis, thymus index and spleen index have significantly improved by PCP.26, 42 Zhang et al and Peng et al reported that the levels of IgA, IgG and IgM in serum are increased and the levels of IFN‐γ and IL‐4 are also enhanced by PCPs.46, 47, 48

16Free radicals refer to the dissociative molecules, atoms or ions with unpaired electron that reacting rapidly with other substances. Under certain range, free radicals can help to eliminate microorganisms intruding into body or abnormal cells. However, if free radicals are excessively produced, they would attack the normal cells and tissues. Therefore, human body constantly produces and removes free radicals to maintain a dynamic balance. Studies showed that PCP has antioxidant activity by scavenging free radicals. As shown in Table 6,49, 50, 51, 52, 53 Li et al and Zhang et al found that PCP2 and FP could clear free radicals (O2 −, ·OH and DPPH·) and the clearance rate of DPPH· could reach 93%.51, 52 Wang et al also found that carboxymethylated PCP has DPPH, O2 − and ·OH radical‐scavenging activity in vitro.54, 55 Superoxide dismutase (SOD) is an important antioxidant enzyme in organisms and Chen et al showed that CMP could increase the expression of SOD and reduce the amount of MAD (malondialdehyde).53

17PCPs have other biological effects, such as anti‐inflammatory,56 anti‐ageing,57 antihepatitis,58, 59 antidiabetic,60 anti‐ALL (acute lymphoblastic leukaemia),61 anti‐nephritic62 and antihypertensive effects.63 These pharmacological activities are summarized in Table 7A,B. Hou et al observed that PCP reduces the size of granuloma.56 As an anti‐ageing reagent, PCPs enhance the activities of both T‐SOD and Cu‐SOD and reduce MAD and MAO (monoamine oxidase) activities.57 To understand the antihepatitis effects of CMP, it is found that CMP reduces the expression of HBsAg and HBeAg in a concentration‐dependent manner.58 Zheng et al reported that PCP could reduce blood glucose level and increase the weight of mice in a diabetic mouse model.60 Meanwhile, CMP significantly improves the survival rates of mice suffering acute lymphoblastic leukaemia or ALL.61

18Seven clinical studies on PCPs were found through literature search.59, 64, 65, 66, 67, 68 The clinical data are summarized in Table 8. Most of studies are related to the antitumour effects of CMP where these studies use IL‐2 or IFN‐α obtained from human peripheral blood lymphocytes induced by CMP in combination with chemotherapy or radiotherapy. Sheng et al reported that the total effective rate could reach 97% when CMP‐induced IL‐2 is combined with chemotherapy during cancer treatment compared to that of 27% by chemotherapy alone.65 The effectiveness is defined as improving the symptoms of the disease by increasing appetite, elevating the levels of cAMP in blood circulation, regulating the ratio of cAMP/cGMP, protecting and restoring damaged liver and reducing the side effects of chemotherapy.66 In treating epidemic haemorrhagic fever, cure rate could reach 100% when CMP‐induced IFN‐α is combined with normal therapy compared to that of 63% with normal therapy alone.64 Chen et al also found that the total effective rate could reach 90% during hepatitis treatment.66 The changes of immune functions are measured with Et (total erythrocyte rosettle test), Ea (active erythrocyte rosettle test) and LCT (lymphocytes transformation) during PCP treatment. These values are significantly increased compared to the control groups when PCP is combined with chemotherapy67 (Table 8).

19CMP has very low toxicity. It is reported by Chen et al69 that the mice moves freely, no abnormal reaction is observed when CMP is used during standard acute toxicity test. No teratogenic effects are detected on rats when CMP is used in the teratogenic tests. No toxic reaction is witnessed when the dogs are continuously injected by intravenous injection reported by Ye et al70 They further demonstrated that blood pressure, heart rate, electrocardiogram and breathy of dogs are not affected after intravenously injecting either 400 mg/kg or 800 mg/kg CMP. In consistent, Chai et al71 reported that CMP has no adverse effects on mice as well. Wang et al showed that CMP could enhance the tumour inhibition rate of 5‐FU and decrease the liver injuries simultaneously caused by 5‐FU in CT26 tumour‐bearing mice.72

20In this article, the structure, pharmacological effects, clinical efficacy, immunobalancing molecular mechanism and toxicity of PCPs are summarized in Figures 1, 2, 3, 4, 5 and Tables 1, 2, 3, 4, 5, 6, 7, 8. The broad spectrum of therapeutic properties, relatively low toxicity and low costs make PCPs attractive immune therapeutics for treating not only different types of cancers but also hepatitis B and other diseases.

21Both advantages and disadvantages of PCPs as drugs rely on their complicated polysaccharide structure‐dependent immune regulatory functions. Thus, there is a great need for clarifying the active ingredients in PCPs besides β‐glucan/pachymaran and their molecular targets responsible for their drug effects. In addition, how to standardize the quality of PCPs, especially the degree of chemical modifications of pachymaran derivatives and how to perform reliable pharmacokinetic studies of PCPs are some of important issues to be solved in near future to make use of PCPs for treating patients with cancer worldwide.

22The authors declare no conflict of interests. All grants and funding agencies play no role in the study design; in the collection, analysis and interpretation of data; in the writing of the manuscript; and in the decision to submit the manuscript for publication.