Mitogen-activated protein kinase signaling alterations and functional consequences of arsenite and ultraviolet irradiation in human keratinocytes

Arsenic is widely distributed within the environment and its dermatotoxicity is well established. Epidemiological studies have identified an increased incidence of keratinocytic tumors associated with arsenic exposure. The mechanisms involved in arsenic-mediated skin carcinogenesis are not well defined, but activation of mitogen-activated protein (MAP) kinases and the downstream regulation of target genes may be contributing factors. In these studies we investigated the activation of MAP kinases and the subsequent gene regulation by arsenite in human keratinocytes. We find that acute exposure (≤ 4h) to high concentrations of arsenite (≥ 100 μM) stimulates rapid activation of extracellular signal regulated kinase (ERK) and the stress-regulated p38 MAP kinase in human keratinocytes. Extended exposure (24h) to low micromolar concentrations of arsenite results in delayed, but persistent ERK and p38 activation. However the mechanisms for activation for these two MAP kinases differ. ERK activation by arsenite is dependent upon epidermal growth factor (EGF) receptor activation while p38 is independent of EGF receptor activity. Arsenite exposure also induces the production of reactive oxygen species (ROS) which were determined to signal upstream of p38 activation. Arsenite-induced stimulation of these MAP kinases results in increased production of matrix metalloproteinase (MMP)-9 and Hemeoxygenase-1 (HO-1). MMP-9 is upregulated in an EGF receptor-dependent manner, while HO-1 is independent of EGF receptor activation. These results demonstrate that arsenite acts via at least two distinct mechanisms in the stimulation of MAP kinases and subsequent gene regulation in human keratinocytes. However, arsenite does not act as a prototypical tumor initiator or promoter. Recent studies have suggested that arsenic may require a carcinogenic partner to fully stimulate cellular transformation and ultraviolet radiation (UVR) has been proposed as one potential candidate. These studies show that keratinocytes exposed to UVA following arsenite pretreatment exhibit additive ROS production with a synergistic increase in p38, but not ERK activation. The combined exposure also resulted in the synergistic production of MMP-9 and HO-1. These studies provide evidence of the cooperative activity of arsenite and UVA in human keratinocytes and contribute to our understanding of arsenite's actions in a more complex and environmentally relevant manner.