NF-kB and not the MAPK signaling pathway regulates GADD45b expression during acute inflammation

The GADD45 ( growth arrest and DNA damage-inducible) family of genes is involved in the regulation of cell cycle progression and apoptosis. To study signaling pathways affecting GADD45b expression and to examine systematically in vivo the GADD45b expression in tissues following various toxic stresses, we created a transgenic mouse by fusing the GADD45b promoter to firefly luciferase (Gadd45b-luc). In vivo GADD45b expression was assessed by measuring the luciferase activity in the Gadd45b-luc transgenic mouse using a noninvasive imaging system (IVIS Imaging System, Xenogen Corporation). We found that a number of agents that induce oxidative stress, such as sodium arsenite, CC14, lipopolysaccharide (LPS), or tumor necrosis factor-a, are able to induce luciferase expression throughout the entire animal. In liver, spleen, lung, intestine, kidney, and heart, we observed an induction of luciferase activity after LPS treatment, which correlates with an increase of GADD45b mRNA in these tissues. Processes that induce DNA damage activate the NF-kB signaling pathway. Several inhibitors of the NF-kB signaling pathway, including dexamethasone, thalidomide, and a proteasome inhibitor, bortezomib, showed inhibitory effects on LPS-induced GADD45b expression as indicated by a decrease of the luciferase activity. Northern blot analysis confirmed a broad inhibitory effect of bortezomib on LPS-induced GADD45b mRNA expression in spleen, lung, and intestine. In liver of bortezomib-treated mice, we observed a reverse correlation between the luciferase activity and the GADD45b mRNA level. We speculate that such a discrepancy could be due to severe liver toxicity caused by bortezomib and LPS co-treatment. MAPK inhibitors had transient and inconsistent effects on LPS-induced luciferase expression. Our data are consistent with the notion that NF-kB, but not the MAPK signaling pathways, is involved in the in vivo regulation of GADD45b expression. Thus, NF-kB signaling involves induction of GADD45b expression, which supports the proposed role of GADD45b in protecting cells against DNA damaged under various stress conditions.