Adaptive Change of AP DNA Endonuclease Against Genotoxic Agents in Normal and Transformed Cells

Lee, Young Hee; Jeong, In Cheol; Oh, Sang Hwan; Cho, Moo Youn

Abstract

PURPOSE: AP DNA endonuclease (APE), an enzyme responsible for the repair of damaged DNAs, is essential for the maintenance of genetic information of cells. Deficiency of APE in certain hereditary skin tumor and senescent cells has been implicated but the regulation of APE activity as well as the expression of APE gene in response to DNA damage has not been well documented. Genotoxic agents including ultimate carcinogens that can damage DNA were treated to cultured normal and transformed human cells and adaptive response of APE gene expression to these treatments was measured in order to evaluate the role of APE in chemical carcinogenesis.
MATERIALS AND METHODS
Hydroxyl radical ('OH) generated from H2O2 (60 uM) through Fenton reaction, each 100 uM of N-nitrosomethylurea (NMU), 3-methyl-4-monomethyl- aminoazobenzene (3'-MeMAB) and N-acetoxy-2-acetaminofluorene (AAAF) were treated to umbilical cord blood cells (UCBC), HepG2 cells and HL-60 cells. APEX mRNA and APEX protein contents expressed in these cells exposed to each of these agents were measured by Northern blot hybridization and Western blot immunodetection analysis. The changes of APE activity in cells exposed to these genetoxic agents were measured.
RESULTS
Treatment of H2O2 (60 uM) to UCBC, HepG2, and HL-60 cells increased APE activity significantly and pretreatment of a catalytic agent for OH, FeSO4 (60 pM) to the cells prior to H2O2 exposure did not further increase the APE activity in cells. Adaptive response to H2O2 in HL-60 cells increased in proportion to the concentration of H2O2 up to 60 pM. However, further increase in H2O2 concentration had no effect on the enzyme activity. Treatment of NMU (100 pM), 3-MeMAB (100 pM) and AAAF (100 pM) to these cells brought about a slight increase in the APE activity. APEX mRNA expression in UCBC and HepG2 cells exposed to H2O2, NMU, 3-MeMAB was markedly increased in APEX mRNA expression. APEX mRNA expression was also increased in HL-60 cells exposed to H2O2 (60 pM) and 3-MeMAB (100 uM) but NMU (100 pM) exposure to the cells resulted in a slight increase of it (Fig. 2). APEX protein expression was increased in all UCBC, HepG2 and HL-60 cells exposed to these genotoxic agents (Fig. 3).
CONCLUSION
These results implicate that exposure of genotoxic agents to the cultured cells may cause DNA damage and lead to adaptive increase in APE activity as well as APE gene expression. It is probable that APE gene is transcriptionally regulated in response to the exposure of H2O2 or 3-MeMAB in cultured human cells as a consequence of activation of DNA repair system for the adaptation to the crisis.