Dl-Methionine or its corresponding hydroxy analogue, dl-2-hydroxy-(4-methylthio) butanoic acid (DLHMB), are commonly added to commercial animal diets to satisfy the TSAA requirement. The utilization of DLHMB as a supplementary source of Met begins with its conversion to l-Met via a 2-step mediated process. l-Methionine can then be transsulfurated to l-Cys, which, in turn, can be catabolized to taurine (TAU). In the present study, the capacity of the chicken small intestine to convert DLHMB to l-Met and to use this amino acid as a source for l-Cys and TAU production was evaluated. The appearance of Met in the serosal compartment of everted sacs incubated with DLHMB is higher in the presence of an H⁺ gradient (mucosal pH 5.5 vs. 7.4). Serosal Cys and TAU concentration was compared in everted sacs incubated at a mucosal pH of 5.5 with DLHMB or l-Met, and the results show significantly higher values after incubation with the hydroxy analogue. Regional comparisons indicate no significant differences in the appearance of serosal Met and Cys, although lower values were obtained for TAU in the duodenum than in the jejunum and ileum. The profile of non-S amino acids was also determined and revealed no significant differences between DLHMB- and l-Met-incubated sacs. In conclusion, Cys and TAU content in chicken enterocytes is higher when DLHMB is used as a Met source.

The small intestinal epithelium is a highly dynamic system continuously renewed by a process involving cell proliferation and differentiation. The intestinal epithelium constitutes a permeability barrier regulating the vectorial transport of ions, water, and solutes. Morphological changes during cell differentiation, as well as changes in the activity of brush-border enzymes and the expression of transport proteins, are well established. However, little is known about the arachidonic acid (AA) cascade underlying epithelial cell differentiation or its role in the development of epithelial barrier function. The main purpose of this study was to examine the activity of the high-molecular-weight phospholipases A₂ (PLA₂) and cyclooxygenase (COX) pathway during differentiation, with particular emphasis on paracellular permeability. PLA₂ activity, AA release, COX-2 expression, prostaglandin E₂ (PGE₂) production, and paracellular permeability were studied in preconfluent, confluent, and differentiated Caco-2 cell cultures. Our results show that Caco-2 differentiation induces a decrease in both calcium-independent PLA₂ activity and COX-2 expression and, consequently, a decrease in AA release and PGE₂ synthesis in parallel with a reduction in paracellular permeability. Moreover, the addition of PGE₂ to differentiated cells, at concentrations similar to those detected in nondifferentiated cultures, induces the disruption of epithelial barrier function. These results suggest that AA release by calcium-independent PLA₂, COX-2 expression, and subsequent PGE₂ release are important for the maintenance of paracellular permeability in differentiated Caco-2 cells.

We recently demonstrated that PGE₂induces the disruption of the intestinal epithelial barrier function. In the present study, our objectives were to study the role of PGE₂receptors (EP₁–EP₄) and the signaling pathways involved in this event. Paracellular permeability (PP) was assessed in differentiated Caco-2 cell cultures from d-mannitol fluxes and transepithelial electrical resistance (TER) in the presence of different PGE₂receptor agonists (carbacyclin, sulprostone, butaprost, ONO-AE1-259, ONO-AE-248, GR63799, and ONO-AE1-329) and antagonists (ONO-8711, SC-19220, AH-6809, ONO-AE3-240, ONO-AE3-208, and AH-23848). The results indicate that EP₁and EP₄but not EP₂and EP₃might be involved in PP regulation. These effects were mediated through PLC-inositol trisphosphate (IP₃)-Ca²⁺and cAMP-PKA signaling pathways, respectively. We also observed an increase in intracellular Ca²⁺concentration ([Ca²⁺]_i) strengthened by cAMP formation indicating a cross talk interaction of these two pathways. Moreover, the participation of a conventional PKC isoform was shown. The results also indicate that the increase in PP may be correlated with the redistribution of occludin, zona occludens 1 (ZO-1), and the perijunctional actin ring together with an increase in myosin light chain kinase activity. Although the disruption of epithelial barrier function observed in inflammatory bowel disease (IBD) patients has been traditionally attributed to cytokines, the present study focused on the role of PGE₂in PP regulation, as mucosal levels of this eicosanoid are also increased in these inflammatory processes.

Cyclooxygenases (COXs) and lipoxygenases (LOXs) are important enzymes that metabolize arachidonic and linoleic acids. Various metabolites generated by the arachidonic acid cascade regulate cell proliferation, apoptosis, differentiation, and senescence. Hydroxyoctadecadienoic acids (HODEs) are synthesized from linoleic acid, giving two enantiomeric forms for each metabolite. The aim was to investigate the effect of 13-HODE enantiomers on nondifferentiated Caco-2 cell growth/apoptosis. Our results indicate that 13(S)-HODE decreases cell growth and DNA synthesis of nondifferentiated Caco-2 cells cultured with 10% fetal bovine serum (FBS). Moreover, 13(S)-HODE showed an apoptotic effect that was reduced in the presence of a specific peroxisome proliferator-activated receptor-γ (PPARγ) antagonist. In addition, we observed that 13(S)-HODE but not 13(R)-HODE is a ligand to PPARγ, confirming the implication of this nuclear receptor in 13(S)-HODE actions. In contrast, 13(R)-HODE increases cell growth and DNA synthesis in the absence of FBS. 13(R)-HODE interaction with BLT receptors activates ERK and CREB signaling pathways, as well as PGE₂synthesis. These results suggest that the proliferative effect of 13(R)-HODE could be due, at least in part, to COX pathway activation. Thus both enantiomers use different receptors and have contrary effects. We also found these differential effects of 9-HODE enantiomers on cell growth/apoptosis. Therefore, the balance between (R)-HODEs and (S)-HODEs in the intestinal epithelium could be important to its cell growth/apoptosis homeostasis.