BIOCHEMICAL-CHANGES ASSOCIATED WITH THE ADAPTIVE RESPONSE OF HUMAN KERATINOCYTES TO N-METHYL-N'-NITRO-N-NITROSOGUANIDINE

Exposure of cells to low doses of radiation or chemicals renders them more resistant to higher doses of these agents. This phenomenon, terme d adaptive response, was studied in quiescent human keratinocytes expo sed to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG ). The cells were adapted with 2.5 nM MNNG for 60 min and challenged i mmediately thereafter with 2.5 mu M MNNG for 30, 45 or 60 min. Clonoge nic survival studies revealed that adapted cells were more resistant t o the subsequent challenge treatment (up to 30% higher survival) than unadapted cells. In addition, formation of DNA strand breaks was lower in adapted cells. We monitored poly-ADP-ribosylation activity during expression of the adaptive response both at the substrate as well as t he product level. NAD(+) utilization in adapted and non-adapted cells exposed to the high dose of MNNG was similar, but recovery from NAD(+) depletion was faster in low-dose pretreated cells. Induction of poly( ADP-ribose) formation was more than 2 times higher in low-dose adapted cells and this was associated with the formation of a distinct class of ADP-ribose polymers, i.e., branched polymers. These polymers exhibi t a very high binding affinity for histones and can displace them from DNA. Elevated levels of poly(ADP-ribose) and, particularly, synthesis of branched polymers may play a critical role in low-dose adaptation.