Mechanisms of cytoprotective effect of amino acids on local toxicity caused by sodium laurate, a drug absorption enhancer, in intestinal epithelium

Several amino acids, including L-glutamine (L-Gln), were found to protect the intestinal epithelial cells from the local toxicity caused by a drug absorption enhancer, sodium laurate (C12), in our previous study. To develop more efficient and safer formulations for enhancing drug absorption, the mechanisms of cytoprotection by amino acids were studied using rats and Caco-2 cells. Four amino acids, including L-Gln, could generally maintain the absorption-promoting action of C12, although taurine tended to attenuate it. Three amino acids, except for L-Gln, significantly suppressed the decrease in the transepithelial electrical resistance caused by C12. Quercetin, an inhibitor for biosynthesis of heat shock protein 70 (HSP70), masked only the protective effect of L-Gln in both rat large intestine and Caco-2 cells. Western blot analysis indicated clearly that HSP70 is induced extensively only by the addition of L-Gln in both rat large-intestinal cells and Caco-2 cells. C12 was found to increase the intracellular concentration of Ca²⁺ ([Ca²⁺]_i) remarkably, and amino acids, especially L-arginine, L-methionine, and taurine, significantly attenuated the increase in [Ca²⁺]_i caused by C12. Furthermore, although C12 stimulated the release of histamine, an inflammatory mediator, from rat large-intestinal tissue, amino acids were also found to suppress the release of histamine enhanced by C12. The results in the present study showed that an induction of HSP70, a decrease in [Ca²⁺]_i elevated by C12, and a suppression of histamine release stimulated by C12 should be involved in the mechanisms behind the cytoprotective action of amino acids against the local toxicity caused by C12.