Adaptation to chromosomal damage in an Indian muntjac fibroblast cell line conditioned with low intensity laser irradiation.

Exposure to DNA damaging agents is known to induce an adaptive response in a variety of eukaryotic cells. Cells can become radioresistant to the damaging effects of ionizing radiation when pretreated with a lower, conditioning radiation dose [l]. An adaptive response has been described for a variety of eukaryotic cells exposed to numerous chemical and physical agents, in particular ioinizing radiation [2]. In experiments with human peripheral lymphocytes, it has been demonstrated that cells exposed to a low conditioning dose of ionizing radiation (d.1Gy) exhibit a reduced chromosome aberration frequency when challenged with a subsequent, higher radiation dose (>lGy) [3,4]. The adaptive response can also be reflected by a change in mutation frequency, cell survival, and frequency of micronuclei. The triggering mechanism for the adaptive response has yet to be elucidated. One hypothesis is that DNA damage induced by a low dose of radiation stimulates a DNA repair mechanism that renders the cell less susceptible to a subsequent higher, challenging dose [5]. The present work investigates the effect of low intensity laser irradiation (LILI), as a source of monochromatic light, on the induction of a cellular adaptive response. Cells of an Indian muntjac fibroblast cell line were laser-irradiated (Gallium Aluminium Arsenide diode laser, 15mW, 5 W , 660nm or 820nm) in vitro at 11.5J/cmz and, following one hour incubation at 37’C, exposed to 2, 4, or 6Gy of 137Cs-gamma irradiation (2radkec). Cells were then incubated for 33h to allow progression through one cell cycle. Post incubation, cells were incubated with colchicine for 90min, harvested and prepared for cytogenetic analysis. Chromosome aberrations were scored (blind) in 100 first-division metaphase cells. Chromosome aberration analysis revealed that LILI at 660nm caused a significant reduction (16.4%, PcO.05, Students’ t-test) in the frequency of chromosomal aberrations induced by the higher gamma irradiation dose of 6Gy. When cells were preexposed to LILI at 820nm instead of 660nm, a significant reduction in aberration frequency was not evident. This finding indicates that the protective effect of LILI was wavelengthdependent. Experiments in which cells were conditioned with varying doses of LILI at 660nm and subsequently challenged with 6Gy of gamma irradiation showed that the radioadaptive effect was laser dose-deuendent CTable 1).