Gliadin Peptides Induce Tissue Transglutaminase Activation and ER-Stress through Ca2+ Mobilization in Caco-2 Cells.

Background: Celiac disease (CD) is an intestinal inflammatory condition that develops in genetically susceptible individuals after exposure to dietary wheat gliadin. The role of post-translational modifications of gliadin catalyzed by tissue transglutaminase (tTG) seems to play a crucial role in CD. However, it remains to be established how and where tTG is activated in vivo. We have investigated whether gliadin peptides modulate intracellular Ca²⁺ homeostasis and tTG activity. Methods/Principal Findings: We studied Ca²⁺ homeostasis in Caco-2 cells by single cell microfluorimetry. Under our conditions, A-gliadin peptides 31-43 and 57-68 rapidly mobilized Ca²⁺ from intracellular stores. Specifically, peptide 31-43 mobilized Ca²⁺ from the endoplasmic reticulum (ER) and mitochondria, whereas peptide 57-68 mobilized Ca²⁺ only from mitochondria. We also found that gliadin peptide-induced Ca²⁺ mobilization activates the enzymatic function of intracellular tTG as revealed by in situ tTG activity using the tTG substrate pentylamine-biotin. Moreover, we demonstrate that peptide 31-43, but not peptide 57-68, induces an increase of tTG expression. Finally, we monitored the expression of glucose- regulated protein-78 and of CCAAT/enhancer binding protein-homologous protein, which are two biochemical markers of ER-stress, by real-time RT-PCR and western blot. We found that chronic administration of peptide 31-43, but not of peptide 57-68, induces the expression of both genes. Conclusions: By inducing Ca²⁺ mobilization from the ER, peptide 31-43 could promote an ER-stress pathway that may be relevant in CD pathogenesis. Furthermore, peptides 31-43 and 57-68, by activating intracellular tTG, could alter inflammatory key regulators, and induce deamidation of immunogenic peptides and gliadin-tTG crosslinking in enterocytes and specialized antigen-presenting cells. [ABSTRACT FROM AUTHOR]