Nucleophosmin/B23 (NPM/B23), a nucleolar protein, was rapidly up-regulated after UV irradiation (at 254 nm; 30 J/m(2)) in NIH 3T3 cells and HeLa/S3 cells. Levels of NPM/B23 mRNA peaked 45-60 min after UV treatment and returned to baseline by 12 h. Transcription inhibitor actinomycin D (5 microg/ml) prevented the UV-induced increase of NPM/B23 mRNA, suggesting that UV induction of NPM/B23 was mediated at the transcriptional level. Moreover, UV-induced NPM/B23 expression was super-induced by cycloheximide (20 microg/ml), which was characteristic of immediate-early gene response. The transcriptional activation of NPM/B23 by UV was also confirmed by NPM/B23 promoter activity assay. Thymine dinucleotide, mimicking the effects of UV-induced DNA damage, was able to trigger NPM/B23 expression in the absence of genomic DNA damage. UV-induced activation of NPM/B23 promoter could not be blocked by UV-inducible pathway inhibitors, such as those of growth factor tyrosine kinase, mitogen-activated protein kinase, AP-1, NF-kappaB, and DNA-dependent kinase. Our results indicate that UV stimulation of NPM/B23 expression may be mediated through a novel UV-inducible pathway and is an immediate-early gene response induced by damaged DNA. Induction of immediate-early gene is an initial step in the regulation of cellular and genomic responses to external stimuli. Our results thus provide important evidence for an involvement of NPM/B23 in the acute response of mammalian cells to environmental stress.