Zinc Supplementation, via GPR39, Upregulates PKCζ to Protect Intestinal Barrier Integrity in Caco-2 Cells Challenged by Salmonella enterica Serovar Typhimurium

Background: Zinc has been shown to improve intestinal barrier function against Salmonella enterica serovar Typhimurium (S. typhimurium) infection, but the mechanisms involved in this process remain undefined. Objective: We aimed to explore the roles of G protein-coupled receptor (GPR) 39 and protein kinase C zeta (PKC zeta) in the regulation by zinc of intestinal barrier function. Methods: A Transwell Caco-2 monolayer was pretreated with 0, 50, or 100 mu M Zn and then incubated with S. typhimurium for 0-6 h. Afterward, cells silenced by the small interfering RNA for GPR39 or PKC zeta were pretreated with 100 mM Zn and incubated with S. typhimurium for 3 h. Finally, transepithelial electrical resistance (TEER), permeability, tight junction (TJ) proteins, and signaling molecules GPR39 and PKC zeta were measured. Results: Compared with controls, S. typhimurium decreased TEER by 62.3-96.2% at 4-6 h (P < 0.001), increased (P < 0.001) permeability at 6 h, and downregulated (P < 0.05) TJ protein zonula occludens (ZO)-1 and occludin by 104-123%, as well as Toll-like receptor 2 and PKC zeta by 35.1% and 75.2%, respectively. Compared with S. typhimurium-challenged cells, 50 and 100 mu MZn improved TEER by 26.3-60.9% at 4-6 h (P < 0.001) and decreased (P < 0.001) permeability and bacterial invasion at 6 h. A total of 100 mMZn increased ZO-1, occludin, GPR39, and PKC zeta 0.72-to 1.34-fold (P < 0.05); however, 50 mMZn did not affect ZO-1 or occludin (P > 0.1). Silencing GPR39 decreased (P < 0.05) zinc-activated PKC zeta and blocked (P < 0.05) the promotion of zinc on epithelial integrity. Furthermore, silencing PKC zeta counteracted the protective effect of zinc on epithelial integrity but did not inhibit GPR39 (P = 0.138). Conclusion: We demonstrated that zinc upregulates PKC zeta by activating GPR39 to enhance the abundance of ZO-1, thereby improving epithelial integrity in S. typhimurium-infected Caco-2 cells.