Bioavailability And Gastrointestinal Cytotoxicity Of Cadmium And/or Copper Exposure

Heavy metal pollution poses a major threat to life forms including human in the ecological environment,due to its persistent,nondegradable and bioaccumulative characteristics.Even though the contents of heavy metals(HMs)in environmental media is relatively low,HMs can still pose the exposure risks to human and cause public health problems.HMs is attracting increasing attention.In the real environment,HMs often coexist in various environmental media,and the toxicity of multiple HMs can exert combined effects.Therefore,studies on individual heavy metal may not comprehensive enough.Previous studies have shown that cadmium(Cd)and copper(Cu)often coexist in various environmental media including foods.Many studies have been performed on the biotoxicity and bioavailability after the individual exposure to Cd or Cu.However,few studies have been conducted on Cd and Cu combined exposure(co-exposure)on human.When the people ingest HMs orally,the gastrointestinal tract is not only an important barrier against HMs entering the body,but also the main place for HMs absorption.Given that,we carried out the studies on human gastric epithelial(SGC-7901)cells and human intestinal epithelial(Caco-2)cells after exposure to Cd and/or Cu.The cytotoxic effects of exposure to Cd and/or Cu were measured,and the cytotoxic mechanisms were mainly elucidated by using multiple techniques from multiple perspectives.Moreover,the Caco-2 cell model was established to analyze and compare the bioavailability of Cd and Cu as well as the intracellular contents of various metals before and after exposure.The molecular mechanisms of metal accumulation and transport was further discussed.This M.S.program can provide new data for the toxicologic studies of HMs co-exposure to help people to better understand the health risks posed by HMs exposure.The major results of the current study were as follows:(1)The cytotoxicity of Cd and/or Cu exposure on SGC-7901 cells for 24 h and its association with oxidative damage,cell cycle arrest and apoptosis were investigated.Compared with individual exposure,co-exposure obviously changed cell morphology and reduced cell viability,indicating enhanced cytotoxicity.Further studies found that co-exposure was associated with disturbance of the antioxidant enzyme system,which led to the aggravated oxidative damage.Then,cell cycle was arrested in S phase via altering cell cycle regulatory genes,and apoptosis was eventually induced.Moreover,maintenance of Trx R1 and Nrf2 expression resulted in less cytotoxicity after individual treatment.These results indicated that oxidative stress,cell cycle arrest and apoptosis were important mechanisms of enhanced cytotoxicity induced by co-exposure.We concluded that Cd+Cu co-exposure may increase the risk of gastric epithelial injury in human.(2)After exposure to Cd and/or Cu for 24 h,the toxicity of Caco-2 cells and its association with apoptosis,tight junctions(TJs)and cytoskeleton were investigated.Co-exposure elicited severer cytotoxicity than individual exposure,which was manifested by round and floating cell morphology and inhibition of cell viability.It was related to the increased apoptosis and up-regulation of pro-apoptotic genes.Furthermore,we also found that the down-regulation of CLDN3 and the disruption of cytoskeleton F-actin may be the target of co-exposure to disturb TJs.Therefore,Cd+Cu co-exposure promotes apoptosis and may disturb the stability of TJs,which increases the risk of paracellular permeation of harmful substances.(3)The effects of Cd and/or Cu exposure on the intracellular contents of various metals and the bioavailability of Cd/Cu were investigated.The data showed that neither individual exposure nor co-exposure changed intracellular Ca and Fe.Cd exposure elevated intracellular Cd,while there was no difference in intracellular Cd accumulation between co-exposure and individual exposure.All treatment groups increased intracellular Cu,but there was no significant difference between each other.Intracellular Zn increased after Cd exposure,while Zn content was not significantly changed following co-exposure and Cu exposure.Thus,the increased Zn contents may be the cytoprotection under Cd exposure.Additionally,co-exposure did not alter the bioavailability of Cd/Cu through a study of Caco-2 cell model based on Transwell chamber.Furthermore,changes on transporter gene expression were the possible mechanisms for altered intracellular metal accumulation and unchanged bioavailability.