Toxicity of mycotoxins to insects and underlying molecular and biochemical mechanisms

Mycotoxins are synthesized by fungi and released as secondary metabolites toxic to many animal species. The structurally diverse mycotoxins can be carcinogenic, mutagenic, teratogenic, estrogenic, hemorrhagic, immunotoxic, nephrotoxic, hepatotoxic, dermotoxic, and neurotoxic to humans and other mammals. To minimize the harmful effects of mycotoxins on humans, strict regulations have been established worldwide. Economic losses due to mycotoxin contamination are enormous and estimated at millions of dollars annually in the United States alone Insects inevitably encounter mycotoxins in nature when they feed on unharvested (mummy) fruits in orchard situations. Insects can be vectors of fungal spores and also facilitate fungal access to their host plants. Insects in turn may take advantage of fungi for protection against their natural enemies and for processing refractory plant constituents to increase their digestibility. This interaction is manifested in many crops as an association between insect damage and increase in mycotoxin contamination. Control of insects can thus reduce fungal toxin levels in crops. Understanding how insects deal with mycotoxins is critical in control of both insect pests and mycotoxin contamination. In this study, three insects, including Helicoverpa zea (corn earworm), Amyelois transitella (navel orangeworm) and Apis mellifera (honey bee) varying in their exposure to mycotoxins were selected to study toxicity of mycotoxins to insects and to elucidate their underlying mechanisms of toxicity. To compare toxicity of mycotoxins to H. zea with that to A. transitella, I measured the developmental delay caused by aflatoxin B1 (AFB1), and found that the LC50 (defined as the concentration preventing 50% of newly hatched larvae from entering the second instar within 48 hr) for AFB1 is 100 times greater for A. transitella than that for H. zea. Similarly, A. transitella first instars display substantially higher tolerance of ochratoxin A (OTA) than do H. zea. Honey bees and their resource-rich nests are hosts to a wide range of saprophytic fungi, including species that produce mycotoxins. Bioassays showed that the honey bee workers can tolerate relatively high levels of aflatoxins (1-2.5μg/g AFB1) and ochratoxins (1μg/g OTA). Cytochrome P450 monooxygenases (P450s) are critical in detoxification and activation of mycotoxins. In this study, I found that midgut homogenates isolated from H. zea larvae consuming diets supplemented with phytochemicals (coumarin and xanthotoxin) showed significant AFB1 disappearance and generated two metabolites, with the primary one identified as aflatoxin P1 (AFP1), an O-demethylated less toxic product of AFB1.Three P450 proteins from H. zea including CYP6B8, CYP6B27 and CYP321A1 were co-expressed with house fly reductase in insect cells and only the expressed CYP321A1 can metabolize AFB1, producing the same two metabolites as the midgut homogenates. RT-PCR gel blots indicated that the magnitude of CYP321A1 transcript induction by these chemicals is associated with the magnitude of increase in the metabolic activities of induced midgut enzymes (coumarin>xanthotoxin>indole 3-carbinol). These results indicate that induction of P450s, such as CYP321A1, plays an important role in reducing AFB1 toxicity to H. zea. Docking of AFB1 in the molecular models of CYP321A1 and CYP6B8 highlights differences in their proximal catalytic site volumes that allow only CYP321A1 to generate the AFP1 metabolite. Enhancement of the toxicity of AFB1 by piperonyl butoxide, a P450 inhibitor, indicates a role for P450s in AFB1 detoxification in honey bees. Extracts of propolis, a complex mixture of plant-derived chemicals that contains many flavonoids and other phenolic compounds, similarly ameliorated aflatoxin toxicity and delayed the onset of mortality. Collectively, these results suggest that tolerance of AB1 by honey bees may be due to P450-mediated metabolic detoxification. To understand molecular mechanisms underlying detoxification of mycotoxins by navel orangeworms, three full-length P450 cDNAs, including CYP6AB11, CYP321C1 and CYP6B44, were isolated from larval midguts using Rapid Amplification of cDNA Ends PCR. These P450s were co-expressed with house fly reductase and fruit fly cytochrome b5 in Sf9 insect cells infected with recombinant baculoviruses. Assays conducted with 16 compounds, including AFB1, showed that CYP6AB11 can efficiently metabolize imperatorin (0.88pmol/min/pmol) and slowly metabolize PBO (0.11 pmol/min/pmol). In view of all of results, it is clear that similar metabolic pathways of detoxification of mycotoxins similar to those found in mammals exist in insects. However, some insects, such as navel orangeworm, may have evolved in response to specializing on food containing mycotoxin-releasing fungi such as navel orangeworm. A potentially safe and sustainable approach for managing this serious fungus vector in orchards may be to use natural essential oil synergists. My results show that myristicin, a natural essential oil compound, synergized the toxicity of α-cypermethrin, a pyrethroid insecticide, over time and slightly increased toxicity of a plant toxin, xanthotoxin, after seven days. Myristicin should be explored further as a field treatment to reduce survival of this pest species and to prevent aflatoxin contamination in orchard situations