Action of caffeine on DNA replication after ultraviolet irradiation in Indian muntjac cells: no connection with action on cell cycle delay.

Indian muntjac fibroblasts of the SV40-transformed line SVM are hypersensitivity to UV, and after UV irradiation have defective post-replication recovery and a high level of sister chromatid exchanges and chromosome aberrations. The lethal and clastogenic effects of UV on SVM have elsewhere been shown to be aggravated by caffeine, which overcomes the block to cycle traverse imposed by DNA damage; however, in DM cells, an Indian muntjac line of normal UV sensitivity, caffeine has no effect on cycle traverse, but nevertheless enhances UV killing and sister chromatid exchanges. In this paper, the effects of caffeine on irradiated DM cells are shown to be due to its inhibition of post-replication recovery, with subsequent formation of DNA double-strand breaks at the strand gaps thus produced. By contrast, in SVM cells the limited capacity for post-replication recovery is relatively insensitive to caffeine after UV fluences which permit significant cell survival; however, caffeine still strongly induces DNA double-strand breaks and chromosome aberrations, apparently by an alternative mechanism. The SVM and DM cell lines therefore exemplify separate actions of caffeine on mammalian cells, deficient in the caffeine effects on post-replication recovery and cell cycle progression, respectively.

Excessive chromosome fragility and abundance of sister-chromatid exchanges induced by UV in an Indian muntjac cell line defective in postreplication (daughter strand) repair.

Two UV-hypersensitive animal cell mutants defective in postreplication recovery (daughter strand synthesis) display quite different patterns of induced sister-chromatid exchange (SCE). One, an SV40-transformed Indian muntjac cell (SVM), shows extremely high frequencies of SCE after UV; induced exchanges can be measured after UV doses as low as 0.01 J/m2. This cell also displays exaggerated levels of induced and spontaneous chromosome aberrations. By contrast SCE rates in the Chinese hamster cell mutant, UV-1, are essentially normal. In both SVM and UV-1, however, there is a clear correlation between the cell density and spontaneous frequencies of SCE, a feature which could be related to the observed density-dependent rate of DNA maturation.

Defective post-replication recovery and u.v. sensitivity in a simian virus 40-transformed Indian muntjac cell line.

The responses to u.v. of two cell lines derived from the Indian muntjac are described. The u.v. sensitivity of the diploid cell falls within the range of most normal mammalian cells while the other, a heteroploid cell, transformed by SV40, is much more sensitive to killing. This hypersensitivity cannot be explained by defective excision repair: the two cell types are indistinguishable in this activity as judged by inhibitor-associated DNA break accumulation and unscheduled DNA synthesis. Rather, the SV40 transformed cells have a pronounced inability to recover normal DNA replication after u.v. These cells are, therefore, defective in a post-replication recovery mechanism and in this respect resemble the behaviour of the variant form of xeroderma pigmentosum. Their limited ability to recover normal levels of RNA synthesis after u.v. hints at the complexity of the phenotype.