Both epidemiological investigations and animal studies have linked arsenic-contaminated water to cancers, including skin, liver and lung cancers. Besides genotoxicity, arsenic exposure-related pathogenesis of disease is widely considered through epigenetic mechanisms; however, the underlying mechanism remains to be determined. Herein we explore the initial epigenetic changes via acute sodium arsenite (As) exposures of mouse embryonic fibroblast (MEF) cells and histone H3K79 methyltransferase Dot1L knockout (Dot1L−/−) MEF cells. Our RNA-seq and Western blot data demonstrated that, in both cell lines, acute As exposure abolished histone acetyltransferase p300 at the RNA level and subsequent protein level. Consequently, p300-specific main target histone H3K27ac, a marker separating active from poised enhancers, decreased dramatically as validated by both Western blot and ChIP-qPCR/seq analyses. Concomitantly, H3K4me1 as another well-known marker for enhancers also showed significant decreases, suggesting an underappreciated crosstalk between H3K4me1 and H3K27ac involved in As exposure. Significantly, As exposure-reduced H3K27ac and H3K4me1 inhibited the expression of genes including EP300 itself and Kruppel Like Factor 4(Klf4) that both are tumor suppressor genes. Collectively, our investigations identified p300 as an internal bridging factor within cells to sense external environmental As exposure to alter chromatin, thereby changing gene transcription for disease pathogenesis.