RESUMO
Preconditioning with sublethal ischemia results in natural tolerance to ischemic stress, where multiple mediators of ischemic damage are simultaneously counteracted. Tumor necrosis factor alpha (TNF-alpha) has been implicated in development of ischemic tolerance. Using cellular models of ischemic tolerance, we have demonstrated that an effector of TNF-alpha-induced preconditioning is ceramide, a sphingolipid messenger in TNF-alpha signaling. TNF-alpha/ceramide-induced preconditioning protected cultured neurons against ischemic death and cultured astrocytes against proinflammatory effects of TNF-alpha. TNF-alpha activates a transcription factor NF-kappaB that binds promoters of multiple genes, thus ensuring pleiotropic effects of TNF-alpha. We describe here a mechanism that allows selective suppression of TNF-alpha/NF-kappaB-induced harmful genes in preconditioned cells while preserving cytoprotective responses. We demonstrate that in astrocytes activation of an adhesion molecule ICAM-1 by TNF-alpha is regulated through association of the phosphorylated p65 subunit of NF-kappaB with an adapter protein, p300, and that in preconditioned cells p65 remains unphosphorylated and ICAM-1 transcription is inhibited. However, TNF-alpha-activated transcription of a protective enzyme, MnSOD, does not depend on p300 and does not become inhibited in preconditioned cells. This new understanding of TNF-alpha-induced adaptation to ischemic stress and inflammation could suggest novel avenues for clinical intervention during ischemic and inflammatory diseases.
