Lead induced DNA strand breaks in lymphocytes of exposed workers: role of reactive oxygen species and protein kinase C

Lead and lead compounds play a significant role in modern industry; a wide variety of population is at risk of occupational exposure and lead is suspected to be a human carcinogen. The biochemical and molecular mechanisms of lead toxicity are poorly understood, but emerging data suggest that some of the effects of lead may be due to its interference with calcium in the activation of protein kinase C (PKC) and/or through production of reactive oxygen species (ROS). Many of these results are conducted in vitro on cell lines or ex vivo on human lymphocytes treated in vitro. We, therefore, performed a study on the induction of DNA damage, using the alkaline comet assay, in lymphocytes of battery plant workers. To elucidate in vivo the mechanism(s) responsible for this effect, we determined ROS production, and glutathione (GSH) levels in living cells using the fluorescent probe (2',7'-dichlorofluorescein and monochlorobimane, respectively). Subcellular fractions were obtained from sonicated lymphocytes; cytosolic and membrane expression of PKC isoforms (alpha, and zeta) was evaluated after electrophoresis by immunoblot analysis. The results indicate that lead-exposed workers have significantly elevated levels of DNA breaks compared to the unexposed group. A multivariate analysis of variance (ANOVA) shows that the most common confounding factors (smoking, drinking and age) have no synergistic effects with lead-exposure on the comet parameters or on GSH levels and ROS production. The logistic regression analysis distinguishing the exposed and non-exposed indicates that only GSH with tail moment are selected as significant risk factors. There is a significant positive correlation with ROS production and negative correlation with GSH levels. The content of PKC alpha in cytosol and membranes is decreased 40% (indicating a down-regulation of protein), whereas PKC zeta isoform is not modified in an evident manner. Our results suggest that lead-exposure induces an increase of DNA breakage with an alternate cellular redox state and a significant down-regulation of PKC alpha, suggesting that this metal may act as a tumor promoter.