The 3-OH Derivative of the Polychlorinated Biphenyl (PCB)-28 Inhibits Telomerase Expression and Accelerates Telomere Shortening in Vitro: A Rationale for the Significantly Shortened Telomere Length Found in Peripheral Blood Lymphocytes of Workers Exposed to High Doses of Lower Chlorinated PCBs

Polychlorinated biphenyls (PCBs) are technical mixtures with a varying three-dimensional structure, depending on the degree and the position of the chlorine atom on two benzene rings. The International Agency for Research on Cancer (IARC) classified PCBs as possible carcinogens. In Germany, the employees of a transformer recycling company were contaminated with high blood levels of PCBs. Within the HELPcB-program (Health Effects in High-level Exposure to PCB), the telomere lengths (TL) of peripheral blood granulocytes and lymphocytes of 208 exposed workers were measured using flow-cytometry-based in situ hybridization (flow-FISH). Whereas the mean age-adjusted TL within granulocytes was not affected, the TL within lymphocytes of PCB exposed individuals was significantly shortened, compared to normal individuals (µ = -0,77kb; p<0.0001). Significantly shortened TL in lymphocytes was associated with a high body burden of lower chlorinated PCBs but not of dioxin-like PCBs or higher chlorinated PCBs (PCB 28 (KK – 0.140, p=0.045), PCB 52 (KK – 0.184, p=0.008), PCB 101 (KK – 0.150, p=0.003)). As PCB28 represented the most sensitive indicator of additional PCB contamination within the group of PCB exposed workers, we chose PCB28 to directly test the effects of lower chlorinated PCBs on telomere shortening in vitro. We first applied a structure-based design approach to develop 3OH-PCB28, a potential bioactive derivative of PCB28. Biotransformation of PCB28 by HePG2 liver cells confirmed the formation of 3OH-PCB28, and 3OH-PCB28 showed dose-dependent, significant antiproliferative and proapoptotic effects on K562 cells as well as on stimulated T-cells from healthy blood donors. In addition, when T-cell proliferation was stimulated by phytohemagglutinin (PHA), tetanus toxoid (TT), or cytomegalovirus (CMV) antigen, the upregulation of telomerase expression was inhibited by 3OH-PCB28 at subtoxic concentrations. As telomerase expression levels determine the lifespan of T-cells, these results suggest that 3OH-PCB28 might affect T- cell capacity for cell division and clonal expansion. Furthermore, 3OH-PCB28 inhibited telomerase expression in K562 cells and led to significantly shorter telomeres, compared to controls, in long-term incubation experiments. We therefore provide mechanistic evidence for the shortened TL found in the lymphocytes of workers contaminated with lower chlorinated PCBs. As short TL in peripheral blood cells have been shown to be associated with increased risk for cancer, long-time monitoring of PCB exposed workers might provide further insights into the abovementioned functional correlations.