Resumen:
Aflatoxins and fumonisins are synthesized by fungi Aspergillus and
Fusarium, respectively. Aflatoxin B1 (AFB1) and fumonisin B1 (FB1) are
the most important in their groups due to these are prevalent forms and
their potential toxic effects, including generation of oxidative stress,
hepatotoxicity, immunotoxicity and carcinogenicity in humans and
animals. Co-exposure to AFB1 and FB1 is a situation that occurs
frequently in nature, and has been associated with a high incidence of
human hepatocellular carcinoma. However, little is known about the
effects induced by both-toxins mixtures. Apoptosis mediated by the
FAS/FASL
and TNFR/TNF-alpha pathways is an essential mechanism to maintain
homeostasis of immune response and tissue, such as liver. This study
aims to determinate the effects of AFB1, FB1 and both-mycotoxins mixture
on death receptors and their ligands expression on hepatocytes and
immune cells from rats, and also to evaluate the involvement of
oxidative stress in these effects. FAS expression on the surface of
hepatocytes were detected by immunofluorescence, FASL expression on the
surface of hepatocytes and immune cells was determined by flow
cytometry, and the TNF-alpha production was assessed in immune cells
supernatants by ELISA. BRL-3A rat liver cell line was exposed to 20 uM
AFB1, 30 uM FB1 and MIX (20 uM AFB1 + 30 uM FB1), and rat spleen
mononuclear cells (SMC) were incubated with 20 uM AFB1, 10 uM FB1 and
MIX (20 uM AFB1 + 10 uM FB1) for 24 or 48 h. The results of these
experiments demonstrate that all mycotoxins treatments increased FAS
expression on hepatocytes, being the MIX which produced the least
effect. Furthermore, FASL expression was reduced by MIX on hepatocytes
and by AFB1 alone or in combination with FB1 on SMC, and the TNF-alpha
production was decreased by MIX in SMC. Moreover, antioxidants prevented
the alterations in FAS, FASL and TNF-alpha levels induced by AFB1 and
MIX, indicating that oxidative stress is involved in the mechanism used
by these treatments. These results suggest that AFB1 and FB1 could
induce hepatotoxicity through the up-regulation of FAS expression that
may lead to increased apoptosis of hepatocytes. However, MIX may induce
dysregulation of the FAS/FASL pathway, since although produces a small
increase in the expression of FAS, it induces decrease in FASL
expression on hepatocytes and immune cells, which would produce a
reduction in apoptosis induced by this pathway. Besides, MIX could
decrease the apoptosis of hepatocytes reducing the TNF-alpha production,
since this cytokine is able to sensitize hepatocytes to FASL-induced
apoptosis, addition that it can interact with TNFR. The death receptors
pathways are used by immune surveillance in eliminating nascent
transformed cells. Therefore, defects in these apoptosis-inducing
pathways can eventually lead to the escape of tumour cells, and this
mechanism may be involved in the hepatocarcinogenicity induced by the
both-mycotoxins mixture.