Date Published: January 27, 2017
Publisher: Public Library of Science
Author(s): Li Huang, Cuicui Duan, Yujuan Zhao, Lei Gao, Chunhua Niu, Jingbo Xu, Shengyu Li, Salah A Sheweita.
In this study, we investigated the potential of Lactobacillus plantarum isolated from Chinese traditional fermented foods to reduce the toxicity of aflatoxin B1 (AFB1), and its subsequent detoxification mechanism. Among all the investigated L. plantarum strains, L. plantarum C88 showed the strongest AFB1 binding capacity in vitro, and was orally administered to mice with liver oxidative damage induced by AFB1. In the therapy groups, the mice that received L. plantarum C88, especially heat-killed L. plantarum C88, after a single dose of AFB1 exposure, showed an increase in unabsorbed AFB1 in the feces. Moreover, the effects of L. plantarum C88 on the enzymes and non-enzymes antioxidant abilities in serum and liver, histological alterations of liver were assayed. The results indicated that compared to the control group, L. plantarum C88 alone administration induced significant increase of antioxidant capacity, but did not induce any significant changes in the histological picture. Compared to the mice that received AFB1 only, L. plantarum C88 treatment could weaken oxidative stress by enhancing the activity of antioxidant enzymes and elevating the expression of Glutathione S-transferase (GST) A3 through Nuclear factor erythroid (derived factor 2) related factor 2 (Nrf2) pathway. Furthermore, cytochrome P450 (CYP 450) 1A2 and CYP 3A4 expression was inhibited by L. plantarum C88, and urinary aflatoxin B1-N7-guanine (AFB-N7-guanine), a AFB1 metabolite formed by CYP 1A2 and CYP 3A4, was significantly reduced by the presence of viable L. plantarum C88. Meanwhile, the significant improvements were showed in histological pictures of the liver tissues in mice orally administered with viable L. plantarum C88. Collectively, L. plantarum C88 may alleviate AFB1 toxicity by increasing fecal AFB1 excretion, reversing deficits in antioxidant defense systems and regulating the metabolism of AFB1.
Aflatoxin B1 (AFB1) is considered to possess the highest toxicity among various types of secondary metabolites produced by a larger number of Aspergillus spp, and classified as a Group I carcinogen for humans by the International Agency for Research on Cancer . Many foods such as grains (corn, sorghum, and millet), peanuts, beans, and nuts (almonds, pistachios, etc.) may support the growth of Aspergillus, and may be contaminated with aflatoxins. It has been reported that AFB1 could induce growth retardation, hepatocellular carcinoma, and immunosuppression [2,3]. Relevant studies indicated that AFB1 was predominantly metabolized by cytochrome P450 (CYP 450) enzyme systems after being absorbed in the intestinal tract. Subsequently, under the action of CYP 450, including cytochrome P450 (CYP 450) 1A2 and CYP 3A4, AFB1 was transformed to exo-AFB1-8,9-epoxide (AFBO), which could bind to DNA, proteins, and other critical cellular macromolecules to exert its carcinogenic effect . However, AFB1 also could be converted to aflatoxin Q1 (AFQ1) by CYP 3A4, or aflatoxin M1 (AFM1) by CYP 1A2, which would be considered one way of detoxification. It was also found that glutathione conjugation could eliminate AFBO through the catalytic action of glutathione S-transferase (GST), which was activated by Nrf2-Antioxidant Response Element (ARE) response . Furthermore, GST A3 appeared to be the critical factor involved in AFB1 detoxification in mice .
Previous studies have shown that lactobacilli have the ability to efficiently remove AFB1, but the specific detoxification mechanism is poorly understood [9–11]. This study may be the first to explore the protective mechanisms of the oral administration of lactobacilli against AFB1 chronic toxicity in mice. In this study, L. plantarum C88 was selected, which presented the highest binding ability with AFB1 using AFB1 binding assay in vitro compared with other strains. Furthermore, both viable and heat-killed L. plantarum C88 showed strong AFB1 binding activity, which guaranteed that L. plantarum C88 could exert better effects regardless of lower pH or high levels of bile salts. The presence of bile salts and intestinal tract pH affected the AFB1 binding ability of viable bacteria . This is not inexplicable; a previous study pointed out that the binding activities of heat-killed bacteria were not drastically changed compared with those of viable bacteria because heat treatment might change the original binding site of the viable bacteria but expose new binding sites .
L. plantarum C88, a selected lactobacillus with good AFB1-binding ability in vitro, can increase fecal AFB1 excretion, reduce lipid peroxidation, and reverse deficits in antioxidant defense systems to alleviate AFB1 toxicity. L. plantarum C88 might play a role in the suppression of CYP 1A2 and CYP 3A4 expression to decrease the production of AFBO and activate GST A3 through Nrf2 signaling pathways to improve glutathione-conjugating activity and hence induce detoxification (Fig 6).