Glutamine induces nuclear degradation of the NF-κB p65 subunit in Caco-2/TC7 cells

In the intestine, NF-κB is the main transcription factor involved in the anti-inflammatory effect of glutamine and we previously demonstrated that glutamine via its conversion to glutamate diminished the p65 protein content in Caco-2/TC7 cell nuclei without affecting the stimulating effect of IL-1β on NF-κB [21]. However, the molecular mechanism by which glutamine acts is not established. We therefore tried to identify such a mechanism. Our results demonstrate that glutamine decreased the intracellular NF-κB through the nuclear ubiquitin-proteasome pathway requiring therefore the nuclear translocation of the factor. Indeed, time-course study revealed that glutamine induced an increase in the nuclear p65 content within the first 15 min of culture, the p65 nuclear and cytosolic content decreasing gradually thereafter to reach 50 % of the control value after 60 min. This translocation was initiated by the phosphorylation of IκBα by the IKKβ subunit inducing its degradation and the p65 translocation. In parallel, glutamine activated the IKKα subunit which in turn phosphorylates p65 at Ser 536 which was responsible for p65 degradation by the nuclear proteasome. We also demonstrate that p38 MAPK lies between glutamine and the NF-κB pathway. In conclusion, this study identified for the first time the signaling pathway by which glutamine may protect against inflammatory conditions.