CHARACTERIZATION OF HUMAN GADD45, A P53-REGULATED PROTEIN

GADD45 (growth arrest and DNA damage) is a DNA-damage-inducible gene r egulated in part by the tumor suppressor p53. A role in negative growt h control has recently been suggested based on significant (more than 75%) reduction of colony formation following over expression of Gadd45 . To better understand the role of Gadd45, we have developed specific rabbit and murine antibodies raised against the human recombinant prot ein. Using these antibodies, we have found that in ML-1 cells Gadd45 i s predominantly a nuclear protein. MyD118, a protein induced by termin al differentiation sharing 57% homology with Gadd45, does not cross-re act with any of the antibodies produced. As expected, the induction of Gadd45 protein by ionizing radiation (IR) was also found to be depend ent on a mild type p53 phenotype. Interestingly, WI-L2-NS, a human lym phoid cell line, showed very high basal levels of Gadd45 mRNA and prot ein in addition to a high constitutive level of a mutated p53 protein. In this cell Line, the high levels of GADD45 did not inhibit cellular growth in spite of the fact that no mutations were found in GADD45 se quence. These results indicate that some cell line(s) can tolerate hig h levels of Gadd45 and abrogate its growth suppression function.