Decreased levels of heat shock proteins in gut epithelial cells after exposure to plant lectins

Abstract

Background - The enterocytes of the intestinal epithelium are regularly exposed to potentially harmful substances of dietary origin, such as lectins. Expression of heat shock proteins (HSPs) by this epithelium may be part of a protective mechanism developed by intestinal epithelial cells to deal with noxious components in the intestinal lumen. Aim - To investigate if the lectins PHA, a lectin from kidney beans (Phaseolus vulgaris) and WGA, a lectin from wheat germ (Triticum aestivum) could modify the heat shock response in gut epithelial cells and to establish the extent of this effect. Methods - Jejunal tissue sections from PHA and WGA fed rats were screened for expression of HSP70, HSP72, and HSP90 using monoclonal antibodies. Differentiated Caco-2 cells, the in vitro counterpart of villus enterocytes, were exposed to 100 μg/ml of PHA-E4 or WGA for 48 hours and investigated for changes in DNA and protein synthesis by double labelling with [2- 14C]thymidine and L-[methyl-3H]methionine. The relative concentrations of HSP60, HSP70, HSP72, and HSP90 and binding protein (BiP) in these cells exposed to lectins were analysed by polyacrylamide gel electrophoresis and immunoblotting. To establish if lectin exposed differentiated Caco-2 cells were still capable of producing a heat shock response, these cells received a heat shock (40°C, 41°C, and 42°C) for one hour and were allowed to recover for six hours at 37°C. During heat shock and recovery periods, lectin exposure was continued. Results - Constitutive levels of HSPs were measured in the intestinal cells of lactalbumin fed (control) rats, as may be expected from the function of this tissue. However, in PHA and WGA fed rats a marked decline in the binding of antibodies against several HSPs to the intestinal epithelium was found. These results were confirmed by in vitro experiments using differentiated Caco-2 cells exposed to PHA-E4 and WGA. However, after exposure to lectins, these cells were still capable of heat induced heat shock protein synthesis, and total protein synthesis was not impaired indicating specific inhibition of HSP synthesis in non-stressed cells. Conclusions - We conclude that PHA and WGA decrease levels of stress proteins in rat gut and enterocyte-like Caco-2 cells, leaving these cells less well protected against the potentially harmful content of the gut lumen.