Resistance to UV-induced cell-killing in nucleophosmin/B23 over-expressed NIH 3T3 fibroblasts: enhancement of DNA repair and up-regulation of PCNA in association with nucleophosmin/B23 over-expression.

Nucleophosmin/B23 was rapidly up-regulated after UV irradiation as p53, PCNA and c-Jun. UV induction of nucleophosmin/B23 was evidently increased at 3 h post-irradiation, and reached a maximum at 12 h, and remained high for at least 24 h. Over-expression of nucleophosmin/B23 made cells more resistant to UV-induced cell growth inhibition and death as compared with control vector-transfected cells through three main observations: cell growth/death percentage determination; clonogenic survival assay; and flow cytometric analysis. Moreover, nucleophosmin/B23 over-expressed cells had a greater capacity to repair UV-damaged reporter plasmid, indicating a higher nucleotide excision repair (NER) activity. Furthermore, PCNA, an essential component for DNA repair machinery, was correlated with nucleophosmin/B23 expression. Both protein level and promoter activity of PCNA were higher in nucleophosmin/B23 over-expressed cells than in control vector-transfected cells. On the other hand, treatment of cells with nucleophosmin/B23 antisense oligonucleotides decreased nucleophosmin/B23 and PCNA proteins, and potentiated the UV-induced cell killing. The effect of PCNA up-regulation may be one of the reasons that nucleophosmin/B23 over-expression made cells resistant to UV-induced growth inhibition and cell-killing.