A NTITUMOR –I MMUNOMODULATORY E FFECTS

5.4.1 A NTITUMOR –I MMUNOMODULATORY E FFECTS

Most of the research on bioactive microbial polysaccharides so far is centered on potential antitumor properties and has resulted in the commercialization of several immunoactive biopolymers. There are several direct and indirect ways of anticancer activity of microbial polysaccharides, the most important of which are (1) prevention

Microbial Polysaccharides

of oncogenesis by oral consumption, (2) direct antitumor action against existing tumors, (3) immunopotentiation against tumors in conjunction with chemotherapy or radiotherapy, and (4) prevention of metastasis. Selected examples of this activity will be given below.

Schizophyllan and lentinan are two of the most common immunotherapeutic glucans and have been used clinically for cancer treatment since 1986, usually in conjunction with chemotherapy or radiotherapy. In general, they have low toxicity even at high doses (lethal dose of lentinan for mice, rats, dogs, and monkeys was

above 100 mg/kg). 2 Clinical studies showed that when schizophyllan was adminis- tered along with antineoplastic drugs, it prolonged the lifespan of patients suffering from lung or gastric cancer. It has also been used for the therapy of stage II or III cervical cancer. 5,172 Moreover, schizophyllan was shown to possess antiradiation properties, restoring mitosis of bone marrow cells previously suppressed by antitu- mor drugs. 173 Lentinan is also used successfully in the treatment of patients with gastric, colorectal, or breast cancer, and the prevention of metastasis, without having any toxic side effects in humans. 174,175 For both schizophyllan and lentinan, early administration, i.e., before or simultaneously with initiation of chemotherapy or irradiation, improves their effectiveness.

The antitumor effects of lentinan and other bioactive glucans are due to their mitogenic activity, which causes several immune responses, such as natural killer (NK) cell activation and T-cell-mediated cytotoxicity, stimulation of monocytes, increased production of cytokines — e.g., interferons (IFNs), interleukines (ILs), and tumor necrosis factor- α (TNF-α) — activation of peripheral mononuclear cells (PMNCs), and stimulation of phagocytosis by neutrophils. 5,50,52,176,177 In other words, the cells, proteins, or hormones of the natural immune system are stimu- lated. For example, proliferation of NK of peripheral mononuclear cells above normal levels or enhancement of immunoglobulin production by PMNCs after intake of lentinan in cancer patients with declined numbers of PMNCs demon- strated the immunopotentiating activity of the glucan. 176–178 Schizophyllan and lentinan were also found to stimulate secretion of TNF- α by human monocytes. 52 In a study with gastric carcinoma patients, injection of a single dose of 2 mg of lentinan led to significantly increased production of cytokines (interleukins 1-α and 1- β and TNF-α) by monocytes of peripheral blood mononuclear cells (PBMCs) 3, 5, and 7 days after the injection. 179 Also, lentinan caused complete regression of Sarcoma 180 in mice at a dose of 1 mg/kg for 10 days. 176 In addition, lentinan can restore the suppressed activity of helper T-cells in tumor-bearing subjects, and thus aid the restoration of immune responses or activate the normal or alternative pathways of the complement system. 176,180

In many cases, the presence of immunocompetent cytotoxic T-cells of the host immune system is necessary for the development of the antitumor activity of lentinan, schizophyllan, and Phytophthora glucan. 181,182 It has been suggested that this may happen because these and other therapeutic polysaccharides are T-cell adjuvants. However, immune responses (to β-D-glucans) not involving T-cells also exist, such as increased numbers of macrophage or activation of neutrophils and NK cells, which can destroy malignant cells.

Functional Food Carbohydrates

Furthermore, the immunoenhancing ability of lentinan seems to be related to its modulation of hormonal factors involved in tumor growth. It was reported that intake of thyroxin or hydrocortisone inhibited the antitumor functions of lentinan. 176 Also, restriction of anticancer activity of lentinan and other polysaccharides occurred after pretreatment of the glucan with antimacrophage agents, such as carrageenans. 182 This is of particular interest for food scientists and the formulation of nutraceuticals, since it indicates that some food additives or compounds should be avoided or removed before the introduction of a bioactive polysaccharide.

Sinofilan, the immunopharmacological form of scleroglucan, is also used clin- ically for the treatment of cancer. The antitumor activity of scleroglucan is believed

to be mediated by the increase in macrophages in the presence of soluble glucan. 50 It has been reported that scleroglucan has a high affinity for human monocytes and

possesses two main biological properties: the stimulation of phagocytic cells, and monocyte, neutrophil, and platelet hemopoietic activity. 50,183 Sclerotinia sclerotiorum glucan (SSG), the pharmacological form of scleroglucan, exhibits antitumor–immu- nomodulating activities when administered either parenterally or orally. 2,184 Oral administration, uncommon among other glucans, is important in eliminating side effects and pain. Also, effective oral administration of a polysacharide may be indicative of the potential effectiveness of consumption of foods containing this biopolymer.

The glucans deriving from S. cerevisiae have various immunostimulating effects. PGG enhances the production of cytokines and monocyte and neutrophil phagocy- tosis. 28 Zymosan containing mainly insoluble β-glucan (70% (w/w)) from S. cere- visiae activates neutrophils (PMNCs) through a trypsine-sensitive recognition mech- anism and exhibits mitogenic activity. 2,170 Interestingly, the β-glucan of the wild type of yeast (named cerevan), exhibited higher mitogenic activity in rats than zymosan. 170 Also, soluble derivatives of S. cerevisiae glucan maintain or increase their antitumor properties. Glucan sulfate, sulfoethyl glucan, carboxymethyl glucan, and oxidized glucan have shown significant antitumor, antimutagenic, and radioprotective activity, e.g., via stimulation of T-cells of lymphocytes, B-cells, and macrophages, and enhancement of hemopoiesis. 2,60,169,170

Another commercialized antitumor glucan, Krestin (or Polysaccharide-K, PSK), has broad immunostimulating and antineoplastic scope. It has been administered orally or intravenously as a clinical medicine in many types of cancer, along with other drugs, and has been found to be especially effective in the prevention of metastasis when injected on a tumor site. It acts either directly on the tumor cells or indirectly by boosting the immune system, e.g., by enhancing phagocytosis by macrophages. 1,173 The various glucans of Agaricus blazei fruit bodies or cultures are also known for their antitumor functions. A soluble acid-treated fraction of proteo- glucan extracted from fruit bodies of the fungi activated natural killer cells with marked tumoricidal activity, which could infiltrate mice tumors. This bioactive proteoglucan inhibited tumor growth and led to the induction of tumor apoptosis

(death). 65 The proteoglucan was also found to be inactive toward normal mouse splenic mononuclear cells, indicating that it is only cytotoxic upon tumor cells. 65 β- Glucans, α-glucans, and a xyloglucan from A. blazei fruit bodies, as well as gluco-

Microbial Polysaccharides

mannans and a proteomannan from cultured mycelia, also possess antitumor prop- erties.

In a study on the immunopharmacological activities of a solubilized (oxidized) form of Candida soluble β-D-glucan (CSBG), Tokunaka et al. 59 concluded that the biopolymer was characterized by the following properties: stimulation of interleukin-6 synthesis of macrophages in vitro, antagonistic effect for zymosan- mediated TNF- α synthesis, induction of lipopolysaccharide-mediated TNF-α syn- thesis, enhancement of hematopoietic activity, restoration of leukocytes after induced leukopenia, antitumor activity against the ascites form of tumor, and high vascular permeability, which is important to the production of cytokines and other immune responses.

Although linear, native curdlan does not provoke immune responses; substituted derivatives of curdlan (etherified, sulfated, sulfoethylated, sulfoalkylated) were

proved to have significant antitumor activity against Sarcoma 180. They were respon- sible for high mitogenic activity, stimulation of macrophages, and induction of

phagocytosis. 2 Other antitumor glucans include Ganoderma lucidum glucan, 15 glom- erellan, 16 and glucans from Auricularia auricula-judae, 19 Cordyceps cicadae , Dic- tyophora indusiata , 93 Pleurotus ostreatus, 23 Pleurotus tuber-regium, 47 and others. Furthermore, the fucogalactan of Sarcodom aspratus was found to exhibit mitogenic functions. The biopolymer elicited the release of TNF- α and nitric oxide (free radical with tumoricidal and microbicidal activities) in microphages of mice. The in vitro stimulation of TNF- α synthesis after a 50 μg/ml fucogalactan dose was higher than that obtained after a 500 μg/ml lentinan dose. 90

Levans of Zymomonas mobilis also exert antitumor and radioprotective activity. This was evident by the generation of mononuclear cells, the increase in peripheral leukocytes and spleen cell antibodies, the stimulation of macrophages, and the

changes in weight and survival of tumor-bearing mice. 87 Alginate is also capable of stimulating the immune system, by inducing the secretion of cytokines, e.g., tumor necrosis factor, TNF- α, interleukin-1, and interleukin-6. 185 The antitumor properties of alginate are strongly connected to macrophage stimulation. 98

Moreover, functional EPSs from lactic acid bacteria are characterized by anti- tumor or immunostimulatory activity in vitro and in vivo. An extracellular phospho- polysaccharide from Lactococcus lactis ssp. cremoris can enhance lymphocyte mito- genicity, macrophage cytostaticity, and cytokine production in macrophages. 79,186 In addition, antigen-specific antibody production was stimulated by the same EPSs at

a dose of 100 to 500 μg/mouse. 84 Similarly, when the extracellular phosphopolysac- charide of Lactobacillus delbrueckii spp. bulgaricus was injected intraperitoneally at a 100 mg/kg dose in mice, the number of intraperitoneal macrophages increased and macrophage phagocytosis was augmented in vivo and in vitro, while the cyto- static activity of thioglycolate-induced macrophages upon tumor cell lines (Sarcoma 180 and P388) was markedly improved in vitro, after a 6-h treatment with phospho-

polysaccharide solution (10 to 100 μg/ml). 82 Mitogenic activity was also exhibited by Bifodobacterium adolescentis M101-4 exopolysaccharide in vitro. 80 Notably, Sreekumar and Hosono 81 reported that skimmed milk fermented with Bifodobacte- rium longum had antimutagenic properties against heterocyclic amines, mediated via the binding of the EPSs to the mutagens. Even though these results were not

Functional Food Carbohydrates

verified in vivo or in clinical studies, they suggest potential prophylactic benefits from the consumption of dairy products fermented with bifidobacteria.