UTP UV
CHAPTER 2 Literature Review
2.21 FUMONISIN-INDUCED DISRUPTION OF SPHINGOLIPID METABOLISM IN VIVO
2.21 FUMONISIN-INDUCED DISRUPTION OF SPHINGOLIPID
2.21.2 Fumonisins as cancer initiators and promoters in rats, mice and rabbits
Fumonisin BI appears to be non-genotoxic based on in vitro assays (Gelderblom and Snyman, 1991; Norred et aI., 1992a), and is a complete carcinogen (Gelderblom et aI., 1991) in that it effects the different stages of cancer development in rat liver, including cancer initiation, promotion and progression. Studies with other carcinogens have uncovered a basic pattern that is characteristic for initiation and promotion in rat liver. This pattern represents induction of rare "resistant" hepatocytes (resistant to negative regulation or sensitive to growth stimuli), the outgrowth of which can be selectively stimulated during the process of cancer promotion, to generate hepatocyte nodules. A subset of nodules, the persistent or late nodules, is one site of origin for hepatocellular carcinoma (Farber and Sarma, 1987; Farber et aI., 1989).
Fumonisins are slow initiators because prolonged exposure (up to 14 days) at a relatively high dosage is required to effect cancer initiation (Gelderblom et al., 1992; 1994). Creation of an environment conducive to expansion of initiated cells by inhibition of normal hepatocyte proliferation IS often observed with genotoxic hepatocarcinogens (Farber, 1990). Selective proliferation of resistant hepatocytes results in clonal expansion of these initiated cells. It is not known whether the characteristic enzyme phenotype induced by genotoxic carcinogens is also induced by fumonisins (Roomi et aI., 1985;
Riley et aI., 1994a; 1994b).
Gelderblom et al. (1994) showed that prolonged exposure of rats to FBI or even a single gavage dosage, inhibited compensatory hepatocyte proliferation following partial hepatectomy. In this regard, FBI resembles many other carcinogens that are potent inhibitors of normal proliferation, as typically seen even after hepatectomy and chemically induced necrosis, is sometimes a pre-requisite for cancer initiation (Cayama et aI., 1978).
The inhibitory effect of FBI could explain why FBI is a slow cancer initiator (Gelderblom et aI., 1992; 1994). It can also be reasoned that a threshold level for cancer initiation by fumonisins exists in rat liver which, as a function of time, will be determined by the dosage used, induced effects, and inhibitory effects on the resultant compensatory cell proliferation (Gelderblom et aI., 1994). The rate-limiting step for cancer induction may be the induction of cell death and resultant compensatory cell proliferation. Superimposed on this is the inhibitory effect on cell proliferation, which could further delay or inhibit
cancer initiation, and therefore the apparent carcinogenic potential of this mycotoxin (Riley et al., 1994a; 1994b).
Fumonisins also serve as promoters. The cancer promoting activity by F. moniliforme culture material and pure FBI in DEN-initiated rats is associated with its hepatotoxic effects (Gelderblom et al., 1988a; 1988b; 1994). Long-term feeding studies have indicated that hepatotoxicity of FB I plays a determining role in carcinogenicity of this mycotoxin in rat liver. In the absence of a promoting regimen, FBI is a promoter and is continuously present during long term feeding studies (Gelderblom et al., 1992). When considering these properties of FBI, it would appear that cancer promotion by this mycotoxin proceeds via the induction of a growth differential because it has the potential to serve both to increase the growth rate of abnormal (pre-cancerous) cells and arrest growth (and be toxic) for normal cells. This data suggests that the fumonisins are complete carcinogens because they are potent selecting agents.
2.21.3 Fumonisin-induced disruption of sphingolipid metabolism and cancer
Given the close correlation between the elevation in free sphingoid bases and the severity and extent of liver and kidney ultrastructural alterations in rats fed pure FBI for four weeks (Riley et al., 1994b), it seems likely that disruption of sphingolipid metabolism occurred in animals fed fumonisins for longer periods to produce cancer. In vivo, fumonisin acting as a mitogen could increase the probability, or irreversibly lock in a spontaneous genomic error.
Mitogenesis increases cancer risk by increasing the probability of DNA damage being converted to mutations, making DNA more sensitive to being damaged, increasing gross chromosomal alterations, and increasing the expression of oncogenes (Ames and Gold, 1990). Cancer is a proliferative disorder, thus the fact that different cell types might respond in a differential fashion to elevated intracellular free sphingoid bases is not consistent with either the multistage model of carcinogenesis or the genotoxic/non- genotoxic classification scheme of Cohen and Ellwein (1990). Cohen and Ellwein (1990) hypothesised that non-genotoxic carcinogens act by increasing cell proliferation by direct mitogenic stimuli, toxicity and consequent regeneration, or through interruption of physiological processes.
Livers from rats fed F. moniliforme culture material containing fumonisins at concentrations which caused focal necrosis and hepatocellular hyperplasia, also contained free sphingoid bases and elevated free Sa:So ratios (Voss et aI., 1990). Focal necrosis and hepatocellular hyperplasia is indicative of ongoing repair and regeneration. The observation of mitotic figures in these tissues indicate that the presence of high concentrations of free sphingoid bases in these livers do not preclude cell division, and thus tissue repair processes may have been suppressed. Alternatively, clonal selection may have selected for cells sensitive to the mitogenic effects of elevated free sphingoid bases.
Changes in pool size of specific sphingolipids (e.g., SM and CER) that are associated with cellular signalling systems could alter the way extracellular messages are transduced.
Systems which are important control points for cellular growth and differentiation could be disrupted during the interval when repair processes and thus mitogenesis are rapidly proceeding. The mitogenic, cytostatic and cytotoxic potential of fumonisin induced disruption of sphingolipid metabolism, may all play some role in the increased cancer risk in rats (Riley et aI., 1994a; 1994b).
In short-term studies with rats, rabbits and mice, disruption of sphingolipid metabolism occurred at, or below the fumonisin dosages that cause liver or kidney lesions (Riley et aI., 1994b; Martinova and Merrill, 1995; LaBorde et aI., 1997;
Tsunoda et al., 1998; Voss et al., 1998). In rats and rabbits, the fumonisin concentration that causes nephrotoxicity and an increase in kidney free Sa concentration is lower than the fumonisin dose that causes hepatotoxicity (Voss et aI., 1993; 1996b; 1998;
Gumprecht et al., 1995; LaBorde et aI., 1997).
2.21.4 Short and long term cancer initiation studies
Short-term studies in a cancer initiation/promotion model in rat liver provide important information about the possible mechanisms involved during the initial stages of cancer development by this apparently non-genotoxic mycotoxin. The short-term assays have mainly used rat liver nodules as the end-point, assessed either using microscopy or histochemical analysis of different enzyme activities such as gamma-glutamyl transferase (GGT) or placental glutathione S-transferase (PGST). In these assays, different stages of the multistage carcinogenesis model were investigated by combining FBI treatment with classical promotion assays. In these studies, FBI or a Fusarium extract was administered alone, before a promoting treatment, or after an initiating treatment. These studies
indicated that a cytotoxic/proliferative response is required for cancer induction, and that a no-effect threshold exists for cancer induction. The mechanisms proposed for cancer induction include the possible role of oxidative damage during initiation, the disruption of lipid metabolism, integrity of cellular membranes, and altered growth-regulatory responses as important events during promotion (Gelderblom et al., 2001).
Short term in vivo studies have shown that FBI mimics genotoxic carcinogens with respect to the induction of resistant hepatocytes in rat liver (Gelderblom et al., 1992; 1994). This was substantiated by observations that FBI induces GGT and PGST, which are accepted histological markers for putative pre-neoplastic lesions initiated by genotoxic carcinogens.
It is unclear whether the characteristic enzyme phenotype that is associated with the fumonisins in rats indicated an increase in cell proliferation is also likely to playa role in the induction of the "resistant phenotype" as hepatotoxicity, and resultant regenerative cell proliferation is a prerequisite for initiation (Gelderblom et al., 1994). The only difference noticed thus far in the induction of the resistant phenotype between the fumonisins and other genotoxic carcinogens, lies in the kinetics of the cancer initiation step.
2.22 ANIMAL DISEASES ASSOCIA TED WITH CONSUMPTION OF