Decrease in Class Pi Glutathione Transferase mRNA Levels by Ultraviolet Irradiation of Cultured Rat Keratinocytes

The effect of ultraviolet (UV) B irradiation on pi class glutathione transferase (GST‐P) gene expression was examined in cultured rat keratinocytes. Immunoblotting demonstrated GST‐P to be the major GST form in the cells, and it was significantly decreased following irradiation. Northern blot analysis revealed that the mRNA decreased to 10–25% of the initial value 24 h after irradiation at a dose of 40 mJ/cm(2). No remarkable changes were observed at earlier time points. Hydrogen peroxide treatment enhanced GST‐P mRNA expression, with a 70% increase at 250 μM concentration. Alterations in possible trans‐acting factors were examined to clarify the mechanism of repression by UV irradiation. c‐Jun mRNA was induced 3.5‐fold at 4 h after irradiation, but by 24 h fell to a lower level than that observed initially. c‐Fos mRNA was increased 10‐fold at 1 h but was completely suppressed at 12 and 24 h. Thus, the changes of c‐Jun and c‐Fos mRNA differed from that of GST‐P mRNA. The level of mRNA for silencer factor‐B was decreased to less than 10% at 12 h. UV irradiation of cells transfected with the chloramphenicol acetyltransferase (CAT) reporter gene containing enhancer (GPE I) or silencer regions of the GST‐P gene did not suppress CAT activity. Although basal expression of the GST‐P gene was mainly dependent on GPE I, altered expression of c‐jun, c‐fos and other genes coding for factors possibly trans‐acting on GPE I did not appear to be responsible for the decreased GST‐P mRNA levels.