Decreased polyphenol transport across cultured intestinal cells by a salivary proline-rich protein

Tannins are polyphenols commonly found in plant-derived foods. When ingested they can have various harmful effects, but salivary proline-rich proteins (PRPs) may provide protection against dietary tannins. The aim of this study was to investigate whether basic PRPs, a major family of salivary proteins, can prevent intestinal absorption of tannin. To do so it was necessary first to characterize transport of pentagalloyl glucose (5GG), a hydrolysable tannin, across cultured epithelial cells. Using human intestinal epithelial cells (Caco-2 cells) it was found that a partial degradation of 5GG occurred during transepithelial transport resulting in the presence of 5GG as well as tetra- and trigalloyl glucose and glucose in the receiving compartment. The sodium-dependent glucose transporter SGLT1 played a role in apical (mucosal) to basolateral (serosal) transport and transport in the opposite direction was dependent on the multidrug resistance-associated protein MRP2. An increased uptake from the apical compartment was seen when the basolateral receiving solution was human serum rather than a balanced salt solution. Transport both in apical–basolateral and basolateral–apical directions was reduced when 1B4, a human basic PRP, was added to the 5GG-containing medium. This decrease closely paralleled the formation of insoluble 5GG–1B4 complexes. It appears that the formation of insoluble tannin–protein complexes diminishes the uptake of 5GG and its metabolites. There is little evidence of other biological activities of basic PRPs so in contrast to other salivary proteins they may exert a biological function in the intestines.