| The skin is a major target organ for both arsenic and ultraviolet
irradiation. UV irradiation (UVR) and arsenic are known to activate the epidermal growth factor
(EGF) receptor and stimulate intracellular signaling cascades. These agents can also induce
signaling via the stress-activated kinases without involvement of EGF receptor activity. In
order to investigate the potential synergistic effects of arsenite and UV irradiation, dose
response and time course experiments were performed on cultured human keratinocytes; HaCat
(spontaneously immortalized human keratinocytes) cells. Western analyses for proteins involved
in mitogen-activated protein (MAP) kinase signaling cascades and downstream transcriptional
targets were performed and pharmacological inhibitors used to determine the relative contribution
of EGF receptor-MAP kinase activation, stress-activated signaling and/or reactive oxygen species
(ROS) generation in the induction of these proteins. Both UVR and arsenite induced ROS as
observed via dihydroethidium (DHE) staining and fluorescence microscopy in a dose and time
dependent manner, but with differing kinetics. Western analyses and zymography show activation
of the p38 MAP kinase and induction of the downstream gene products, HO-1 and MMP-9 in response
to 24h arsenite treatment. UV irradiation also induced p38 activation and slight upregulation
of HO-1. Irradiation of cells pretreated with arsenite for 24h resulted in activation or
upregulation of the proteins of interest to a greater degree than seen with either treatment
alone. These results indicate that arsenite and UVR act synergistically within these human
keratinocytes potentially greatly magnifying the tumorigenic potential of these environmentally
relevant agents. |
