Absence of DNA repair replication after chemical mutagen damage in Vicia faba

Exposure of human (Hela) cells to the mutagens 4-nitroquinoline-1-oxide (4NQO) and N-methyl-N′-nitro-nitrosoguanidine (MNNG) produces damage in DNA that is repaired by a mechanism involving the insertion of new bases into DNA (repair replication). Vicia faba root tips, either from soaked seeds containing non-proliferating cells or from growing roots, do not perform detectable amounts of repair replication even though the mutagens inhibit DNA synthesis and cause chromosome aberrations. In view of similar failures to resolve excision in Chlamydomonas, Haplopappus, and Nicotiana after irradiation with UV light and in Vicia faba after X-irradiation it appears that plants in general might lack this repair process.     

Reassociation of human lymphoblastoid cell DNA repair replicated following methyl methanesulfonate treatment

The reassociation rates of repair replicated DNA of two human lymphoblastoid cell lines, the WIL₂-A3 ‘normal’ line and the RAJI line of Burkitt's lymphoma, were examined using the DNA/DNA ‘C₀t’ hybridization technique. The cells were treated with methyl methanesulfonate (MMS), an alkylating agent and mutagen, to induce the repair.The incorporated repair replication radioactivity in highly repetitive sequences of WIL₂-A3 cell DNA reassociates as expected for a randomly distributed incorporation. The reassociation of repair radioactivity in sequences of fewer numbers of copies, however, is less than expected for a random distribution. It is less than that occurring for semiconservatively synthesized DNA of WIL₂-A3 cells co-incubated with the repair labeled DNA as an internal control.The observed difference could be due to an over-representation of repair replication radioactivity in DNA sequences with fewer copies. It is unlikely to be due to residual alkali labile damage resulting from MMS treatment, since a similar difference was not observed when semiconservatively labeled DNA from cells which had been treated with MMS for the same time and at the same concentration as in the repair experiments was substituted for repair replicated DNA in the reassociation reactions. Other possible causes of the apparent difference in the reassociation rates observed are discussed.     

Retinyl acetate modulation of cell growth kinetics and carcinogen-cellular interaction in mouse epidermal cell cultures

Exposure of mouse epidermal cell cultures to β-retinyl acetate (RA) affects a number of parameters presumed to be important in chemical carcinogenesis. (1) RA alters the course of differentiation of the epidermal cells in culture resulting in a reduced rate of cell death which normally follows cellular maturation during the first two weeks in culture. The extended life span of the cultures appeared due to prolonged survival of cells and not to increased growth rate since RA inhibited the rate of cellular proliferation. This inhibition took place only after completion of a full cell cycle in the presence of RA. (2) DNA repair in response to physical and chemical agents was quantitatively unaffected in the presence of RA. (3) The activity of constitutive aryl hydrocarbon hydroxylase (AHH) was slightly decreased after exposure to RA but the level of enzyme induced by benz[a]anthracene was strongly reduced to 20% of the controls. (4) In the presence of RA, binding of 7,12-dimethylbenz[a]anthracene to epidermal cell DNA was markedly decreased. In contrast, binding to cellular protein was significantly increased by the retinoid.