Arsenic has been effectively used to treat acute promyelocytic leukemia, and can induce cell cycle arrest or apoptosis in human solid tumors. Previously, we have demonstrated that As2O3 can induce p21 WAF1/CIP1 (p21) expression in A431 cells and then due to cellular cytotoxicity. Presently, we have clarified these signaling events and compared them with EGF. Using reporter assay, RT-PCR and Western blotting, we show that c-Src activation might be a prerequisite for As2O3-induced EGFR/Ras/Raf/ERK signaling. Furthermore, with the aids of 5′-deletion and site-directed mutagenesis, we demonstrate that Sp1 binding sites, ranging from - 64 to - 84 bp, are essential for As2O3- or EGF-regulated p21 expression. Finally, our experiments utilizing cycloheximide prompt the suggestion that the stability of mRNA or protein also contributes to As 2O3- or EGF-induced p21 expression. Taken together, we conclude that the Sp1 binding sites are required for As2O 3-induced p21 gene transcription through c-Src/EGFR/Ras/Raf/ERK pathway. Furthermore, post-transcriptional or post-translational stabilization mechanism is also essential for As2O3-induced p21 expression. EGF-induced p21 expression may involve similar mechanisms as those that operate in the As2O3-mediated reactions in A431 cells.
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