Untersuchungen zur metabolischen Aktivierung und Toxizität niedrigchlorierter PCBs

Polychlorinated biphenyls (PCBs) are pollutants that can be found everywhere in the environment due to their persistence. Depending on the number and position of chlorine atoms, their chemical and toxicological properties vary. In this thesis, an inhibitory influence of PCB-contaminated plasma samples on telomerase (hTERT) gene expression could be determined ex vivo using two methods. A dose-response relationship could be established between the concentration of lower chlorinated (lc) PCBs in plasma and the inhibition of hTERT expression in hTERT BAC B6B5.1 fibroblasts, which is 1 μg/L. In vitro, qRT-PCR revealed a concentration-dependent inhibitory effect of single chemically synthesized hydroxy (OH)-metabolites of PCB 28 and PCB 101 on hTERT expression in Jurkat T lymphocytes. Lower concentrations of the OH-metabolites led to stronger effects than the parental congeners, which suggests a metabolic activation of lc PCBs as a prerequisite for their mediated toxicity. A combination of the four most reactive metabolites 3-OH-CB28, 3'-OH-CB28, 3-OH-CB101 and 4-OH-CB101 (OH-PCB mix) led to strong inhibitory effects on hTERT expression in concentration ranges which were found to be subtoxic in the MTT assay and approached the content of lc PCBs in the plasma samples. In vitro these concentrations of the OH-PCB mix led to a reduction in ATP production and an increased production of reactive oxygen species (ROS), which could be responsible for inhibition of hTERT expression and PCB-induced senescence. Incubation with the subtoxic OH-PCB mix also led to increased apoptosis of primary and Jurkat T lymphocytes, which could occur via caspase-dependent as well as caspase-independent signaling pathways. In addition, a higher concentration of prostaglandin E2 (PGE2) was detected in PCB-contaminated plasma samples, which inhibited hTERT expression in vitro depending on the concentration. Thus, in addition to mitochondrial dysfunction and energetic deficiency conditions, another inhibition mechanism for hTERT expression of the lc PCBs contained in plasma could exist via PGE2.In the second part of this work, Drosophila melanogaster was established as a model organism for PCB metabolism and PCB-induced neurotoxicity. Wild type Drosophila metabolized PCB 28 into the same OH-metabolites as were detected in human plasma. Contact with PCB 28 in Drosophila led to neurotoxic signs and concentration-dependent lethality. This could be attributed by flow cytometry to a PCB-dependent induction of apoptosis in differentiated neurons. A neuronal knock-down of the cytochrome P450 monooxygenase Cyp18a1 involved in PCB 28 metabolism caused a complete vitality of the flies at high PCB 28 concentration and a strong decrease of apoptosis in differentiated neurons due to the strongly reduced conversion of PCB 28