INDUCTION OF PBP74 MORTALIN/GRP75, A MEMBER OF THE HSP70 FAMILY, BY LOW-DOSES OF IONIZING-RADIATION - A POSSIBLE ROLE IN INDUCED RADIORESISTANCE/

The identification of genes whose expression is altered following expo sure to a low dose of ionizing radiation (IR) is an important step in understanding the phenomenon of the adaptive response. Using the diffe rential mRNA display method we have identified a gene whose expression is up-regulated following exposure to 0.25 Gy IR. Partial DNA sequenc e and restriction endonuclease analysis of this gene showed that it is identical to the gene encoding for the human peptide-binding protein 74 (PBP74/mortalin/Grp75), a member of the heal shock 70 protein famil y. Time-course measurement of the PBP74/mortalin/Grp75 mRNA showed tha t its level was elevated after a lag of at least 15 min. The maximum i nduction appears to be at 30 min following gamma-irradiation and there is then a steady decline to control levels within 5 h in the HT29 cel l line. On the other hand, the level of the PBP74/mortalin/Grp75 mRNA in the human breast adenocarcinoma cell line MCF-7 is consistently ele vated after gamma-irradiation for up to 6 h post-irradiation. Furtherm ore, a cell line that does not demonstrate the induced radioresistance phenomenon (SW48) shows no induction of the PBP74/mortalin/Grp75 mRNA in contrast with HT29 or MCF-7. Treatment of the HT29 cells with anti sense oligonucleotide directed towards the initiation codon of PBP74 s ensitized cells to ionizing radiation.