Abstract Growth arrest and DNA damage (GADD) 45α is a member of GADD inducible gene family, which can be induced in cell response to oxidative stresses. GADD45α upregulation is able to induce MKK4/JNK activation in some published experimental systems. However, we demonstrated here that the depletion of GADD45α in mouse embryonic fibroblasts (MEFs) (GADD45α-/-) resulted in an increase in the phosphorylation of MKK4/7, MKK3/6 and consequently specific up-regulated the activation of JNK/p38 and their downstream transcription factors, such as c-Jun and ATF2, in comparison to those in GADD45α+/+ MEFs due to nickel exposure. This up-regulation of MKK-JNK/p38 pathway in GADD45α-/- could be rescued by the re-constitutional expression of HA-GADD45α in GADD45α-/-, GADD45α-/- (HA-GADD45α). Subsequent studies indicated that GADD45α deletion repressed expression of PP2Cα, the phosphotase of MKK3/6 and MKK4/7. Collectively, our results demonstrate a novel function and mechanisms responsible for GADD45α regulation of MKK/MAPK pathway, further provides insight into understanding the big picture of GADD45α in the regulation of cellular responses to oxidative stress and environmental carcinogens. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5458. doi:1538-7445.AM2012-5458