Protection from radiation-induced DNA single-strand breaks by induction ofnuclear metallothionein

Purpose: To examine the extent to which nuclear metallothionein protects fr om radiation-induced DNA damage under aerobic and hypoxic conditions. Materials and methods: A semiquantitative fluorescence image analysis metho d measured the nuclear content of metallothionein (MT) in ME180 and SiHa hu man squamous cervical carcinoma cell lines under normal growth conditions, and following MT induction by zinc. The extent of initial DNA damage follow ing Co-60 irradiation under aerobic and hypoxic conditions was assessed usi ng the alkaline comet assay. Results: Provided that cells were maintained at 37 degrees C, most of the c ellular content of MT was in the nucleus. Incubation at 4 degrees C caused the rapid translocation of MT from the nucleus into the cytoplasm in both c ell lines, with no net loss of cellular MT. Baseline nuclear MT levels were about four times greater in ME180 cells, and were much more readily induce d by treatment with 100 mu M zinc acetate, compared with SiHa cells. Under aerobic conditions, MT induction by zinc resulted in no protection in eithe r of the cell lines. Under hypoxic conditions, however, the number of DNA s ingle-strand breaks in zinc-treated cells was reduced by similar to 40% in ME180, but not in SiHa cells, when compared with non-induced controls. Conclusions: Nuclear MT can exert a significant level of protection from ra diation by a mechanism that involves competition with oxygen for DNA radica l sites and/or scavenging of free radicals. Because increased MT levels hav e been reported in hypoxic micro-regions of some solid tumours, this protec tive mechanism might have clinical relevance.