The early growth response 1 (EGR-1) gene is induced after mitogenic stimulation of diverse cell types. The present work has examined the effects of okadaic acid, an inhibitor of protein phosphatases 1 and 2A, on EGR-1 expression during monocytic differentiation of U-937 myeloid leukemia cells. Treatment of U-937 cells with okadaic acid was associated with transient increases in EGR-1 mRNA levels. These increases were maximal at 6 h and occurred in the absence of de novo protein synthesis. Nuclear run-on assays demonstrated that although EGR-1 transcription is detectable in untreated U-937 cells, this rate is increased 6-fold by okadaic acid. Sequences responsive to okadaic acid-induced signals were determined by deletion analysis of the EGR-1 promoter. The results demonstrate that okadaic acid-induced EGR-1 transcription is dependent on the presence of CC (A/T)6 GG (CArG) motifs. The EGR-1 promoter contains six CArG boxes. However, only the 5'-most distal (first) CArG sequence conferred okadaic acid inducibility. A 40-base pair oligomer corresponding to the first CArG element also conferred okadaic acid inducibility of the minimal thymidine kinase gene promoter. In contrast, there was no inducibility using a similar oligomer containing a mutated CArG box. Finally, binding of nuclear proteins to the first CArG sequence was similar for control and okadaic acid-treated cells. Taken together, these results suggest that okadaic acid activates EGR-1 transcription and that this event is mediated at least in part by a single CArG element.

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