Cytotoxicity of benzo[a]pyrene diol epoxide is associated with premature arrest of nascent strand elongation in serum-stimulated monkey kidney cells

Effects of 4-nitroquinoline-1-oxide on population growth, cell-cycle compartmentalization and viability in human lymphoblastoid cells

The mechanism by which the carcinogen 4-nitroquinoline-1-oxide (4-NQO) kills mammalian cells is unclear; however, damage to DNA is presumed to be involved. We examined the relationship between the kinetics of cell death and alteration of cell-cycle compartmentalization after exposure of T5-1 human lymphoblastoid cells to 4-NQO (50 to 500 ng/ml) to establish whether cytotoxicity was related to the perturbation of DNA replication. Dose-dependent reductions in cell proliferation and cell viability were present from 1 day after treatment. Maximal reductions in viability were observed 2 days after exposure. Concentrations of 4-NQO of up to 175 ng/ml did not affect cell-cycle compartmentalization, but 250 ng 4-NQO/ml caused a transient accumulation of cells in S phase after 1 day. Only after 500 ng 4-NQO/ml was a marked and prolonged S-phase block observed from 1 day onwards. The lack of a strong correlation between S-phase block and cell death after exposure to 4-NQO suggests that responses to DNA damage in addition to perturbation of DNA replication entrain 4-NQO-induced cell lethality.

Inhibitory effect of theophylline on repair of potentially lethal MMS-induced damages to DNA in V79 cells

In a study of Chinese hamster V79 cells growing in the presence of sublethal concentrations of theophylline, we followed both the nature of DNA replication and the cells' response to toxic and DNA-damaging effects of methyl methanesulfonate (MMS). We found that cells cultured at low concentrations of theophylline (less than or equal to 0.3 mg/ml medium) showed deviations in the rate of DNA replication which, however, did not depress either the growth activity of the cells or their colony-forming ability. Considerable differences as against the controls appear in theophylline-cultured cells after treatment with MMS. Not only are they more sensitive to the toxic effects of this alkylating agent, but also their DNA synthesis is strikingly inhibited. More unrepaired lesions remain in parental DNA, and short fragments of daughter DNA, synthesized following cell treatment with MMS, are not elongated during a 2-h post-MMS treatment. Theophylline obviously belongs among agents inhibiting repair of potentially lethal MMS-induced DNA damages in Chinese hamster V79 cells.

Benzo[a]pyrene-induced immunotoxicity: comparison to DNA adduct formation in vivo, in cultured splenocytes, and in microsomal systems

Benzo[a]pyrene (BaP)/DNA adduct formation appears to be involved in carcinogenesis, but the relationship between adduct formation and BaP-induced immunotoxicity is unknown. We compared DNA adduct formation (32P-postlabeling analysis) to suppression of polyclonal immune responses (3H-TdR incorporation and IgM secretion) and decreases in cell viability in B6C3F1 female mouse splenic leukocytes (SPL). BaP administration (200 mg/kg, ip) resulted in suppression of polyclonal responses and substantial DNA adduct formation in mouse SPL. SPL adduct levels were similar to those in liver, lung, kidney, and stomach. In vitro exposure of SPL to BaP without rat liver activation enzymes (S9) caused decreases in SPL viability and immune responses that were dependent on dose and exposure period. However, DNA adduct formation in SPL was very low between 1 and 200 microM BaP. S9 enhanced the toxicity of BaP for SPL cultures. Adduct formation was rapid and dose related in +S9 incubates. The low level of BaP activation by SPL was confirmed in microsomal incubations in which splenic microsomes exhibited much lower aryl hydrocarbon hydroxylase (AAH) activity and ability to form DNA-adducting metabolites than did microsomes from liver or lung. Results indicate that immunosuppression produced by BaP in these systems was due to cytotoxic effects. It appears that these effects were caused by two separate mechanisms, one dependent on and one independent of DNA adduct formation. Since SPL had high levels of DNA adducts after ip injection of BaP, reactive metabolites of BaP may be involved in the immunotoxicity seen in vivo.

S-phase block and cell death in human lymphoblasts exposed to benzo[a]pyrene diol epoxide or N-acetoxy-2-acetylaminofluorene

The relationship between perturbation of the cell cycle and induction of cell death by benzo[a]pyrene diol epoxide (BPDE) or N-acetoxy-2-acetylaminofluorene (AcAAF) in exponentially proliferating T5-1 human lymphoblastoid cells was studied. Both BPDE and AcAAF caused cells to accumulate in the S phase of the cell cycle. Perturbation of the cell cycle preceded reduction of cell viability and was associated with inhibition of population growth. Effects on each of the three parameters were noted during the first population doubling, suggesting that they occurred during the first cell cycle after exposure. BPDE-exposed cells accumulated initially in early to mid-S phase and then moved parasynchronously through the remainder of this phase. In contrast, AcAAF-exposed cells accumulated uniformly at all points of the S phase. High doses of either compound froze cell cycle progression, completely inhibited population growth, and killed nearly all cells in the population. Our results suggest that perturbation of DNA replication mediates cell death after exposure to doses of either chemical that cause less than complete inhibition of cell proliferation. However, additional processes, such as perturbation of transcription, may be involved in lethality after exposure to doses that immediately and completely inhibit population growth.