The role of the heat shock response in the cytoprotection of the intestinal epithelium

Under normal conditions, the intestinal epithelial cells produce constitutive amount of heat shock proteins (Hsps) that are elevated following stressful stimuli. As the intestine is constantly exposed to variety of agents like diet, normal flora, infectious microorganisms, chemicals, and immune mediators, the expression of Hsps by the intestinal epithelial cells is therefore, multifactorial and the protective mechanisms are variable. The latter include restoration of unfolded or aggregated polypeptides to their native conformations, proteolysis of proteins too damaged to refold, assembly of proteins, translocation of proteins across membranes, stabilization of cell cytoskeleton and subsequent inhibition of pathogenic invasion, inactivation of intracellular parasites and prevention of persistent production of inflammatory cytokines that would otherwise lead to tissue damage. The aim of this study was to explore the various protective mechanisms of Hsps to the intestinal epithelium using Caco-2 cell model. The general experimental set up involved induction of Hsps production followed by a challenge with infectious Salmonella enteritidis. Hsps were mainly induced by thermal stress (42°C for 1 hr followed by 6 hr of recovery) and measured by immunostaining of Western blots. Protection against Salmonella infection was studied by assessing the levels of interleukin (IL) 8 that was measured by ELISA. In some experiments, Hsps were induced by fermentation products of gut flora such as butyrate and spent culture supernatants of lactobacilli. Results showed that the induced Hsps in various conditions did protect the cells against Salmonella enteritidis by suppressing the production of IL-8, which if it persists, causes tissue damage. It was interesting to note in one of the experiments that Hsps could also be induced following Salmonella enteritidis infection but not after exposure of the cells to its endotoxin. This phenomenon was linked with the stabilization of cell cytoskeleton and subsequent inhibition of invasion. However, it was observed that the induced Hsps could not benefit cells against Salmonella invasion. In addition to the production of Hsps, the fermentation products of gut flora (formate, propionate, and butyrate) were found to selectively and differentially modulate the growth characteristics, cellular metabolism, and differentiation of enterocyte-like Caco-2 cells in a concentration- and carbon atom-related fashion. Butyrate, with the highest number of carbon atom, was the most potent to induce markers of differentiation namely the transepithelial resistance and sucrase isomaltase. While it is known that cells may become considerably resistant to pathogenic invasion following differentiation, especially after induction of transepithelial resistance, in our study, the induced transepithelial resistance could not protect cells against Salmonella enteritidis invasion. It is concluded that the gut flora and their fermentation products play an important role in the protection of the intestinal epithelium. The protection is, at least in part, mediated by the production of Hsps. The mechanisms of Hsps in this protection include suppression of persistent production of inflammatory cytokines like IL-8 that would otherwise cause tissue damage and stabilization of barrier integrity and the subsequent inhibition of pathogenic invasion