Understanding the role of Aflatoxin B1 in modulating the Type I Interferon Response

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Prolonged dietary exposure to Aflatoxin B1 (AFB1) is one of the major risk factors for the development of primary liver cancer or hepatocellular carcinoma (HCC). AFB1 contamination of some Ghanaian staples has been reported to be very high. AFB1 causes mutations in the tumour suppressor gene TP53. These mutations are known to be involved in the pathogenesis of HCC. However, how AFB1 affects other anti-cancer pathways such as the type I interferon (IFN) pathway is largely unknown. The aim of the study was to test the hypothesis that AFB1 inhibits the type I IFN response in human hepatoma cell lines (HepG2 cells) by directly interfering with key signaling proteins and thus increase the risk of HCC. In this study, the effect of AFB1 on cell viability was investigated by MTS-based assay. The type I IFN response in HepG2 cells was induced using recombinant (r) IFN-α and measured by IFN stimulated response element (ISRE) luciferase reporter gene assay. In addition, the effects of AFB1 on the mRNA levels of JAK1, STAT1 and OAS3 in HepG2 cells stimulated with rIFN-α were determined by RT-qPCR and confirmed by western blotting assay. Exposure of HepG2 cells to AFB1 up to 3200 µM for 24 hours followed by maintenance of cells in AFB1 free media for 24, 48 and 72 hours decreased cell viability in a dose dependent manner. AFB1 also suppressed/inhibited the IFN-α driven activation of a reporter promoter that contained the ISRE. When HepG2 cells which had been stimulated with rIFN-α were treated with AFB1 the mRNA synthesis of JAK1, STAT1 and OAS3 was suppressed/inhibited. AFB1 also inhibited the protein accumulation of STAT1. The biological significance of the inhibition/suppression of the type I IFN pathway by AFB1 was investigated by employing the replication of chandipura virus in HepG2 cells stimulated with or without rIFN-α and treated with or without AFB1. Chandipura virus was titred by plaque assay on L929 cells. Paradoxically when chandipura virus was exposed to AFB1 for 24 hours, the replication of the virus was reduced as measured by a decrease in the virus titer. Results from this study suggest that AFB1 may also induce HCC by inhibiting the type I IFN response pathway which is known to have anti-cancer property. Since AFB1 rather kills the IFN sensitive chandipura virus rather than rescuing them, it is speculated that AFB1 and viruses might not co-operate directly to amplify HCC development and that further studies are required to determine how viruses co-exist with AFB1 to catalyse carcinogenesis caused by AFB1 in human liver.
A thesis submitted to the Department of Clinical Microbiology, Kwame Nkrumah University of Science and Technology in partial fulfillment for the degree of Doctor of Philosophy (PhD),