The Transport Of Tea Polyphenols In Caco-2 Cell Model

It was well known that tea polyphenols have many biological benefits.However,they have a low bioavalibility that hinder them to play healthy founctions.In this study,Caco-2 monolyer model was applied to compare the transport ways of epigallocatechin gallate(EGCG),epicatechin gallate(ECG),and their methylated derivatives EGCG-3”-OMe,ECG-3”-OMe as well as their diemer theaflavin-3,3'-digallate(TFDG).Meanwhile,the co-transportation of TFDG with caffeine,theanine,gluatamate acid and proline were explored respectively.Moreover,the molecular docking using SYBYL software was applied to calculate the binding affinity of the specific compounds with their possible efflux proteins and further tried to explain the transport mechanism of tea polyphenols.The main results in this study are as follows.1.The 1/ER value of EGCG was 0.47 in Caco-2 cell model which indicate that its transport is primarily the active efflux way related to the proteins of MRP(multiresistance protein),Pgp(P-glycoprotein)and MCT1(monocarboxylate transporter)confirmed by specific inhibition assay.Compared with the transport of EGCG,the transport of its methylated derivatives EGCG-3”-OMe is passive diffusion(1/ER=0.78).This may contribute to the higher lipid affinity of the methyl substitution of EGCG and the lower docking affinity to the efflux proteins.2.However,the transports of both ECG and its derivatives ECG-3”-OMe are active efflux with the 1/ER value of 0.40 and 0.41 respectively.The main protein transporters as MRP,Pgp and MCT1 play an important role in the transport of ECG and its methylated form.Compared to EGCG and EGCG-3”-OMe transports,ECG and its derivatives ECG-3”-OMe show a different way.The affinity scores of those two catechins with MRP1 and Pgp in molecular docking were 8~9 that are more than EGCG and EGCG-3”-OMe's scores,which may contribute to their higher rate of active efflux.3.TFDG,a condensed dimer of EGCG and ECG,is also actively effluxed in Caco-2 cell model with a lower 1/ER value at 0.23,compared to the value of of its precursors EGCG and ECG.After being docked with Pgp and MRP1,the binding score is at 8 to 9.It is speculated that the transport process of TFDG is affected by some efflux proteins other than Pgp and MRP1.4.Apart from main polyphenols in tea,caffeine and theanine are passively effluxed in Caco-2 cell model.Their values of 1/ER are 1.38 and 1.05 respectively.When TFDG co-transported with caffeine or glutamate acid,the both efflux way of TFDG change to passive diffusion,not active efflux or active efflux.Those indicate that caffeine or glutamate acid could increase the TFDG bioavailability in Caco-2 cells.Meanwhile,caffeine and TFDG share the H-bond sites completely when binding with MRP1 and PgP respectively,which show the two compounds have competition on combining with these proteins;However,L-glutamate acid use some H-bond sites with TFDG in common,so the effect of increasing the TFDG bioavailability is not obvious as caffeine.While TFDG co-transported with theanine,theanine impedes the transport of TFDG apparently.However,proline doesn't change the active efflux way of TFDG when co-transported with it.All of above results indicate that tea polypnenols like EGCG,ECG,ECG-3”-OMe and TFDG primarily depend on active efflux in the intestine while EGCG-3”-OMe is absorbed and transported through passive diffusion.This suggests that chemical structural modification of polyphenols is a potential approach to enhance their bioavalibility.Moreover,the co-transport findings show that other components in tea could influence the transport of polyphenols,the mechanism needs to be clarified in future.