IEX-1, an immediate early gene, increases the rate of apoptosis in keratinocytes

Abstract

IEX-1, an immediate early gene, is widely expressed in epithelial and endothelial tissues, and is altered by a variety of growth regulatory factors. We have shown that expression of IEX-1 in keratinocytes increases the growth rate of these cells. The effects of IEX-1 on apoptosis, however, are unclear. To clarify the effects of IEX-1 on apoptosis, we investigated the effects of IEX-1 expression in keratinocytes (HaCaT cells) in the basal state and after the induction of cellular stress. Under normal, non-stressed conditions, both control (HaCaT) and IEX-1-transfected (IEX–HaCaT) cell lines showed no significant differences in the activity of a key apoptotic enzyme, caspase 3 despite significantly higher levels of IEX-1 expression. IEX–HaCaT cells grew faster than HaCaT cells. When both cell lines were irradiated with ultraviolet B radiation, caspase 3 activity increased to a greater extent in the IEX–HaCaT cells than in HaCaT cells. Camptothecin increased caspase 3 activity twice as much in the IEX–HaCaT cells when compared to HaCaT cells. When histone-complex DNA fragments were measured in IEX–HaCaT or HaCaT cells following UVB irradiation or treatment with camptothecin, significantly higher amounts of nucleosomes were seen in the IEX–HaCaT transfected cells. Likewise, serum deprivation induced higher degrees of apoptosis in IEX–HaCaT cells than in HaCaT cells. We conclude that overexpression of IEX-1 in HaCaT keratinocytes increases the growth rate of cells under basal conditions; in the basal state the rate of apoptosis is unchanged. However, the rate of apoptosis increases in IEX-1 overexpressing HaCaT keratinocytes after cells are subjected to stress.