Recent studies have demonstrated an induction of expression of inducible nitric oxide synthase that is associated with several inflammatory diseases of the skin. To define the mechanisms of action of nitric oxide (NO) in the skin, we attempted to identify genes that are regulated by NO in keratinocytes. Using the human keratinocyte cell line HaCaT as a model system, we identified a Cu/Zn superoxide dismutase (SOD) that was strongly induced by high concentrations (500 microM) of NO-donating agents ¿S-nitrosoglutathione, sodium nitroprusside and (Z)-1-[2-(2-aminoethyl)-N-(2-ammonioethyl) amino] diazen-1-ium-1,2 -diolate (DETA-NO)¿, but not by serum or by single recombinant growth factors and inflammatory cytokines or by treatment with superoxide anions. Furthermore, endogenously produced NO increased the expression of Cu/Zn SOD mRNA in keratinocytes. Moreover, treatment of HaCaT cells with NO was associated with a biphasic effect on cell proliferation, because low doses (100 microM) of different NO donors (S-nitrosoglutathione and DETA-NO) mediated a proliferative signal to the cells, whereas high concentrations (500 microM) were cytostatic. To determine a possible correlation between the close regulation of Cu/Zn SOD expression and proliferation by NO in keratinocytes, we established a cell line (psp1CZ1N) carrying a human Cu/Zn SOD cDNA under the control of a ponasterone-inducible promoter construct. Ponasterone-induced overexpression of Cu/Zn SOD caused a cytostatic effect in proliferating psp1CZ1N cells. We therefore suggest that the up-regulation of Cu/Zn SOD expression by NO establishes an inhibitory mechanism on keratinocyte proliferation.