CHARACTERIZATION OF THE GADD45 RESPONSE TO IONIZING-RADIATION IN WI-L2-NS CELLS, A P53 MUTANT-CELL LINE

We have previously reported that WI-L2-NS, a human lymphoblastoid cell line, has very high basal levels of GADD45 mRNA and protein in spite of a p53 mutation at amino acid 237. Regardless of the amount of Gadd4 5 in this cell line, no growth suppression activity was detected. We r eport here that in WI-L2-NS, the mutated p53 protein adopts predominan tly a wild type (wt) conformation and binds to the p53 binding site in the GADD45 third intron. In this cell line, the already high levels o f mutated p53 protein can be induced further by ionizing radiation (IR ) but the response of the p53 downstream effector genes is altered. In duction of GADD45 and CIP1/WAF1 is reduced compared to p53 wt cell lin es but is still substantially higher than the average fold induction o btained from 39 p53 mutant cell lines. Induction of the MDM2 gene was not detected in WI-L2-NS following IR. The induction pattern of the th ree p53 effector genes by the alkylating agent methylmethane sulfonate (MMS) was also attenuated in WI-L2-NS cells. In TK6 cells, a WI-L2-NS sister cell line having a p53 wt genotype, the induction of the p53 d ownstream effecters is normal, i.e. induced, both at the protein and t he mRNA levels. These results indicate that the DNA binding activity o f the mutated p53 protein in WI-L2-NS might be responsible, at least i n part, for the high basal levels of GADD45 but can not mediate the fu ll induction of the p53 downstream effector genes. The reason(s) for-t he inability of Gadd45 to suppress growth in this cell line remains ho wever unknown.