Studies on the mechanisms of cytotoxicity induced by lightly chlorinated PCBs.

摘要

Recently accumulating evidence shows that certain PCB congeners can be cytotoxic, especially to neurons. Several groups have reported that the mechanisms involving non-planar congeners that produce PCB-induced cytotoxicity differ substantially from than those of coplanar congeners, which exert their adverse effects via activation of Ah-receptors. However, the mechanisms of PCB-induced cytotoxicity remain elusive. A series of experiments have been performed to study the mechanisms of PCB cytotoxicity, mainly using flow cytometry. Cerebellar granule cells, thymocytes and artificial lipid vesicles were employed to test the effects of various PCB congeners. A number of biochemical endpoints were monitored in these two distinct types of cells in order to delineate the mechanism(s) of PCB cytotoxicity. Endpoints studied included measurements of intracellular calcium ([Ca2+]_i), free radicals, mitochondrial membrane potential (Δψ_m), cell viability, and plasma membrane integrity and fluidity. Non-planar, but not coplanar PCBs (0.1–20 μM), caused a cell loss in both cell types. Loss of viability was accompanied by increased [Ca2+]_i and decreased Δψ _m. No significant changes of free radical level were observed during exposure. The coplanar congeners had no significant effect on viability, [Ca 2+]_i or Δψ_m, in the concentration range studied. Our data also showed that neither the increase in [Ca 2+]_i nor the decrease in Δψ_m was the cause of non-planar PCB-induced cell death. Plasma membrane integrity was compromised by addition of non-planar congeners before cell death, as indicated by an increase in fluorescence of DNA-binding dyes in presumably viable cells. As cell membrane integrity is lost, there is an increase in [Ca2+] _i, and a progressive loss of fluidity. Non-planar PCBs cause an increase in membrane fluidity in both cell types and artificial lipid vesicles, as indicated by 1,6-dipheny-1,3,5-hexatriene (DPH) fluorescence polarization.; These studies show that non-planar PCBs are toxic to cerebellar granule cells and immune cells; however their toxic nature could not be correlated with [Ca2+]_i, a change in Δψ_m, or free radical generation. The data suggests that a change in plasma membrane integrity including protein-lipid interaction may be associated with its toxicity.