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CHAPTER 2 Literature Review
2.26 EFFECTS OF FUMONISINS ON THE BRAIN AND BRAIN CELLS
2.27.2 T-2 TOXIN
T-2 toxin belongs to the trichothecenes type A group that are produced by Fusarium
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species of fungi (Figure 2.9). The trichothecenes share a tricyclic nucleus and usually contain an epoxide at C-12 and C-13. This epoxide is essential for toxicity. Trichothecenes are in general very stable compounds, both during storage/milling and the processing/cooking of food and do not degrade at high temperatures (Desjardins, 1993).
Figure 2.9: Chemical structure of T -2 toxin.
Trichothecenes are sesquiterpene epoxides that inhibit eukaryotic protein synthesis and thereby impair human and animal health (Desjardins et ai., 1993). T -2 toxin has been implicated in two outbreaks of acute human mycotoxicoses. The first occurred in Siberia, during the Second World War, producing a disease known as "alimentary toxic aleukia"
(A T A) where thousands of people, who were forced to eat grain that had over-wintered in the field were affected. The symptoms of AT A included fever, vomiting, and acute inflammation of the alimentary tract, anaemia, circulatory failure and convulsions. T-2 toxin poisoning also occurred in Kashmir in India in 1987, and was attributed to the consumption of bread made from mouldy flour from which T2 was isolated.
The major symptom was abdominal pain together with inflammation of the throat, diarrhoea, bloody stools and vomiting. This mycotoxin has also been implicated with the occurrence of haemorrhagic toxicoses in farm animals, especially in poultry. T -2 toxin is a potent skin-damaging agent and is absorbed slowly (12-24 hours) via intact skin, but rapidly through abraded skin. The rapid appearance of symptoms after respiratory exposure in humans, along with the results of animal inhalation studies, indicates rapid absorption and high retention of aerosolised T2, with the respiratory tree retaining small amounts (Augers on, 2000). The many mechanisms by which T2 produces toxicity are varied and their relative importance in producing illness is not fully understood. These mechanisms
include inhibition of protein, DNA and mitochondrial protein synthesis, as well as induction of single strand breaks in DNA (European Commission, 2000b).
Dietary levels ofT-2 toxin as low as O.Sppm were found to cause a reduction in feed intake in pigs (Rafai et aI., 1995). T -2 toxicoses are due to elevation of tryptophan in the brain.
Tryptophan is a precursor of serotonin, a mediator of appetite (Smith and Seddon, 1998).
The relative toxicities of the type A (isovaleryl, hydrogen or hydroxyl at the C8 position,
(e.g., T2, Figure 2.9) and the type B (carbonyl group at the C8 position e.g., DON,
Figure 2.10) trichothecenes have been studied in a number of cell and animal systems.
While the ranking of toxicity is similar, the range of toxicity varies dramatically with the system used. The difference in toxicities between the most (T2 toxin) and the least (3-acetyl DON) toxic trichothecenes was 1300-fold for Vero cells (Thompson and Wannemacher, 1986), 23 000 fold for MIN-GL 1 cells, 2000 fold for KS62 cells (Visconti et aI., 1991), and lS00 for BHK-21 cells (Rotter et aI., 1993).
Holt and Deloach (1988) found that bovine cells were the most sensitive to T2 (SO% inhibition at 2.2ng.mr1) while hamster cells were the least sensitive (SO% inhibition at 26.2ng.mr1). Murine cells exhibited intermediate sensitivity (SO% inhibition at 109ng.mr1). Lymphocytes were three fold more sensitive to T2 inhibition than comparable tissue culture cell lines. Mirocha et al. (1992) found that T -2 toxin was toxic to five rat hepatoma cell lines (H4TG, H4-II-E, H4-II-E-C3, Fao and MH1C1) and one dog kidney cell line (MDCK). The IC50 values for T2 were between 1.S-Sng.mr1 for the rat hepatoma lines and O.Sng.mrl for the dog kidney cell line.
2.27.3 Deoxyn ivalenol
Deoxynivalenol is produced by F. culmorum and F. graminearum, which are abundant in cereal crops (wheat, maize, barley, oats, and rye) and processed grains (malt, beer and bread) (Figure 2.10). Chemically, it belongs to the class of tricothecenes. In contaminated cereals 3- and IS-acetyl DON can occur in significant amounts (10-20%) concomitantly with DON. Deoxynivalenol is a very stable compound, both during storage/milling and the processing/cooking of food (Rotter et aI., 1996; Ehling et aI., 1997; Eriksen and Alexander, 1998).
· ·
·
OH
H ••• OH
Figure 2.10: Chemical structure of 4-deoxynivalenol (DON; vomitoxin, dehydronivalenol, RD-toxin).
The IARC (1993a) classified DON in Category 3, i.e., not classifiable as to its carcinogenicity to humans. At that time however, the negative chronic study in mice was not available (Iverson et al., 1995). Deoxynivalenol inhibits the synthesis of DNA and RNA and protein synthesis at the ribosomal level, and has a haemolytic effect. An acute dose of DON can induce vomiting in pigs, whereas at lower concentrations in the diet it reduces growth and feed consumption. Both effects, which are also seen with other trichothecene toxins, are possibly mediated by modifYing the serotonergic activity in the CNS or via peripheral actions on serotonin receptors (Rotter et al., 1996;
Eriksen and Alexander, 1998).
Deoxynivalenol showed neither mutagenic activity III Ames tests with Salmonella typhimurium, both with and without S-9 activation systems, nor in an in vitro unscheduled DNA synthesis test using rat primary hepatocytes. Deoxynivalenol enhanced cell transformation in mouse embryo cells in vitro, and induced clastogenic effects and inhibited gap-junctional intercellular communication in Chinese hamster V79 cells.
Deoxynivalenol inhibited the protein synthesis in CHO cells in vitro in the same dose range as that inducing clastogenic effects. There are indications for a suppression of humoral and cellular immunity, resulting in an increased susceptibility for infectious diseases (Hsia et al., 1988; IARC, 1993a; 1993b; Leatherman and Middlebrook, 1993;
Eriksen and Alexander, 1998). Regarding this increased susceptibility for infectious diseases, a NOAEL of 0.25 mg.kg-1 body weight per day (Tryphonas et al., 1986) and a lowest-effect level of 0.22 mg.kg-1 body weight per day were reported in studies with male Swiss-Webster and male Balb/C mice, respectively (Baars et al., 1999). Studies are needed to cover the trichothecenes group as a whole, to confmn that there are no neurotoxic effects at doses below those causing effects on growth and body weight, focusing on the known target for tricothecenes, the CNS serotonergic system.
2.27.4 Fusaric acid
Fusaric acid (5-butylpicolinic acid, FA) (Figure 2.11) is a phytotoxin produced mainly by F. moniliforme. Fusaric acid selectively inhibits the enzyme dopamine ~-hydroxylase, both in vitro and in vivo (Nagatsu et al., 1970) leading to an increased concentration of dopamine (DA) and its metabolites with concomitant reduction in brain noradrenalin (NA) concentrations. Dopamine-~ hydroxylase catalyses the final step in the biosynthesis of norepinephrine. Fusaric acid does not influence the activity of tyrosine hydroxylase, monoamine oxidase or aldehyde dehydrogenase (Nagatsu et al., 1970) nor does it affect the release or uptake of catecholamines. In man, FA was shown to increase CSF DA metabolites, while reducing NA metabolites (Sack and Goodwin, 1974). Disease symptoms observed includes loss of appetite, vomiting, gastrointestinal lesions, lethargy, immunosuppression and loss of muscle co-ordination.
COOH
Figure 2.11 : Chemical structure of fusaric acid (5-butylpicolinic acid, FA).
Fusaric acid is mildly toxic to mice and has several important pharmacological properties in that both brain and pineal neurotransmitters and metabolites are affected (Porter et al., 1995). Fusaric acid was growth inhibitory and cytotoxic in vitro (Fernandez-Pol et al., 1993; Vesonder et al., 1993) and inhibited DNA synthesis in WI-38 fibroblasts (Fernandez-Pol et al., 1993). It is a hypotensive agent and by inhibiting
~-hydroxylases (Hidaka et al., 1969), alters tissue levels of catecholamines and indolamines in rats (Porter et ai., 1995; Rimando and Porter, 1997). Fusaric acid was not overtly toxic to rats (Voss et al., 1999) and enhanced vomiting and feed refusal in pigs given trichothecenes (Smith and MacDonald, 1991; Smith et al., 1997). Bacon et al. (1995) found enhanced toxicity when F A and FB, were administered simultaneously to developing chicks in ovo, providing direct evidence of FA and FBI synergism.
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Swine are particularly sensitive to the presence of Fusarium mycotoxins In feed stuffs_
Deoxynivalenol and FA act in a synergistic manner to reduce growth rates and production efficiency in swine, Fusarium mycotoxicoses are known to affect appetite regulation in swine, with suppression of appetite being an economically significant factor. It has been proposed that the loss of appetite is associated with elevated brain tryptophan concentrations, Large doses of tryptophan can cause vomiting in swine, Fusaric acid has a relatively low toxicity in animals, however it has been demonstrated that acute doses of FA administered to rats can elevate brain tryptophan and serotonin levels, Seddon et al. (1997) have shown that orally dosing swine with FA can trigger vomiting and lethargy with accompanying increases in brain tryptophan and serotonin. Deoxynivalenol triggers similar responses in swine. The ability of tryptophan to be taken up by the brain across the BBB is a limiting factor in determining brain tryptophan concentrations. Large neutral amino acids such as leucine, isoleucine, valine, phenylalanine and tyrosine compete with tryptophan for uptake into the brain. Therefore, dietary supplementation of these amino acids or the feeding of feedstuffs rich in these amino acids might overcome the feed refusal due to . Fusarium mycotoxins and would facilitate increased use with Fusarium-contaminated grains in swine production,
2.27.5 Moniliformin
Moniliformin is formed in cereals by a number of Fusarium specles that include F. avenaceum, F. subgiutinans, and F. proliferatum and occurs as the sodium or potassium
salt of I-hydroxycyc1obut-l-ene-3, 4-dione.
Q I . .
: O ' -!Na
',,'-,"': .· ···
. 'Figure 2.12: Chemical structure of moniliformin.
There is a limited amount of data on the effects ofMON on mammalian species. It is a potent inhibitor of mitochondrial pyruvate and ketoglutarate oxidation. In a few studies MON caused chromosomal aberrations, In humans, MON has been linked to Keshan disease which is endemic to certain areas of China.