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/cm2. No remarkable changes were observed at earlier time points. Hydrogen peroxide treatment enhanced GST-P mRNA expression, with a 70% increase at 250 microM 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.