Inhibition of unscheduled DNA synthesis in human lymphocytes by chemical carcinogens

Tanning salon exposure suppression of DNA repair capacity and mitogen-induced DNA synthesis

Mitogen responsiveness and the capacity to repair genetic damage were measured in lymphocytes collected from young, healthy, adult Caucasians immediately before exposure in commercial tanning salons and again 24 h after exposure. For every individual studied, tanning exposure produced significant inhibition of phytohemagglutinin-induced mitogenesis or of the ability to repair DNA lesions by unscheduled DNA synthesis. The results imply that such exposure could: (1) pose a significant hazard for individuals who are already immunosuppressed (e.g. cancer patients, AIDS patients or carriers of latent HIV) and (2) increase the carcinogenic effects of environmental mutagens.

Use of scintillometric quantitation of unscheduled DNA synthesis in isolated rat hepatocytes for the screening of genotoxic agents

The induction of unscheduled DNA synthesis has been considered as a suitable endpoint for the screening of genotoxic agents. Experimentally, unscheduled DNA synthesis is most frequently measured by autoradiography. The purpose of this report was to examine the usefulness of the liquid scintillation counting technique in measuring unscheduled DNA synthesis response in isolated rat hepatocytes. The various liquid scintillation counting-based unscheduled DNA synthesis assay procedures were examined according to the following groupings: (1) procedures based on the acid precipitation of cellular macromolecules, (2) procedures based on isopycnic gradient centrifugation of solubilized cells, (3) procedures based on nuclei isolation in conjunction with other DNA purification methods, and (4) procedures based on the selective retention of hepatocellular DNA. Limited cases in which test chemicals gave positive unscheduled DNA synthesis response in liquid scintillation counting-based assays and negative unscheduled DNA synthesis response in autoradiography-based assays are presented. It is concluded that liquid scintillation counting-based unscheduled DNA synthesis assays represent an appropriate system for inclusion in carcinogenicity and mutagenicity testing programs.

Serum effects on DNA repair in human cells

Freshly isolated human lymphocytes were used to determine how serum supplements affect cellular capacity to repair UV damage. Repair capacity was always found to be greatest in medium supplemented with autologous plasma. Variability in repair capacity among individuals was greater in serum supplemented medium than in unsupplemented medium. Thus, in vitro cellular responses will most accurately represent in vivo responses if autologous serum factors are present in the culture medium. This is of particular importance in studies attempting to correlate DNA repair capacity with age or susceptibility to carcinogenesis.

Sensitivity of carcinogen-treated DNA to inactivation by ultraviolet radiation

The DNA of bacteriophage SPO2c12 was treated with methylmethane sulfonate (MMS), beta-propiolactone (BPL), 2-anthramine (AA) or benzo[a]pyrene (BP) and then exposed to 254-nm radiation. Competent Bacillus subtilis host cells were transfected with DNA subjected to the carcinogen-UV treatment or with DNA treated with carcinogen only. Survival curves were obtained for loss of plaque-forming ability as a function of UV dose. The UV sensitivity of DNA treated with MMS, BPL or AA was not significantly different from that of untreated DNA. The results indicate that in competent B. subtilis the pathways for repair of alkylating agent damage and for repair of UV damage are probably different.

Inhibition of DNA synthesis by an electrophilic metabolite of benzo[a]pyrene

Mitogen-stimulated scheduled DNA synthesis and DNA excision repair in human lymphocytes, as well as DNA polymerase a activity in a cell-free system, were inhibited by an electrophilic metabolite of benzo[a]pyrene. This metabolite, (+/-)-anti-(7r,8t)-dihydroxy-(9,10t)-epoxy-7,8,9,10-tetrahyd robenzo[a]pyrene (BPDE), covalently binds to cellular macromolecules and is mutagenic, carcinogenic, and cytotoxic. Human lymphocytes treated with BPDE at concentrations greater than 500-800 ng/ml showed decreases in both mitogen-stimulated DNA synthesis and excision repair of damaged DNA but did not exhibit overt cytotoxicity (excluded trypan blue and maintained an adenylate charge of greater than 0.7). Formation of, and total concentration of, BPDE-DNA adducts was not correlated with inhibition of DNA synthesis. DNA polymerase alpha studies using a cell-free system showed that enzymatic activity was not diminished when purified polymerase was treated with BPDE prior to the addition of template DNA. When the template DNA concentration was varied, BPDE inhibition of enzyme activity was uncompetitive. BPDE inhibition of enzyme activity was found to be noncompetitive when concentrations of dATP, dCTP, or dTTP were varied and competitive when the concentration of dGTP was varied. The data indicate that BPDE competitively inhibits interaction of dGTP with the template-DNA polymerase alpha complex.