Polychlorinated biphenyls activate caspase-3-like death protease in vitro but not in vivo

Divergent Effects of Dioxin- or Non-Dioxin-Like Polychlorinated Biphenyls on the Apoptosis of Primary Cell Culture from the Mouse Pituitary Gland

Polychlorinated biphenyls (PCBs) can disrupt the endocrine function, promote neoplasms and regulate apoptosis in some tissues; however, it is unknown whether PCBs can affect the apoptosis of pituitary cells. The study evaluated the effect of PCBs on the apoptosis of normal pituitary cells and the underlying mechanisms. Primary cell cultures obtained from mouse pituitary glands were exposed to Aroclor 1254 or selected dioxin-like (PCB 77, PCB 126) or non-dioxin-like (PCB 153, PCB 180) congeners. Apoptosis was evaluated by Annexin V staining, DNA fragmentation, and TUNEL assay. Both the expression and activity of caspases were analyzed. Selective thyroid hormone receptor (TR) or aryl-hydrocarbon receptor (AhR) or CYP1A1 antagonist were used to explore the mechanisms underlying PCBs action. Our results showed that Aroclor 1254 induced the apoptosis of pituitary cells as well as the final caspase-3 level and activity through the extrinsic pathway, as shown by the increased caspase-8 level and activity. On the other hand, the intrinsic pathway evaluated by measuring caspase-9 expression was silent. The selected non-dioxin-like congeners either increased (PCB 180) or reduced (PCB 153) pituitary cell apoptosis, affecting the extrinsic pathway (PCB 180), or both the extrinsic and intrinsic pathways (PCB 153), respectively. In contrast, the dioxin-like congeners (PCB 77 and PCB 126) did not affect apoptosis. The anti-apoptotic phenotype of PCB 153 was counteracted by a TR or a CYP1A1 antagonist, whereas the pro-apoptotic effect of PCB 180 was counteracted by an AhR antagonist. The induced apoptosis of Aroclor 1254 or PCB 180 was associated with a reduction of cell proliferation, whereas the decreased apoptosis due to PCB 153 increased cell proliferation by 30%. In conclusion, our data suggest that non-dioxin-like PCBs may modulate apoptosis and the proliferation rate of pituitary cells that have either pro- or anti-apoptotic effects depending on the specific congeners. However, the impact of PCBs on the process of pituitary tumorigenesis remains to be elucidated.

Polychlorinated biphenyls (PCB-153) and (PCB-77) absorption in human liver (HepG2) and kidney (HK2) cells in vitro: PCB levels and cell death

An understanding of congener specific cellular absorption of PCBs is important to the study of the organ specific body burden of an individual and to their toxic effects. We have previously demonstrated that single PCB congeners induce cytotoxicity, as evidenced by decreased cellular viability and accelerated apoptotic death. There is very little, if any, information available on the differences in toxicity due to the nature of absorption of PCBs in different cells. To obtain such information human liver (HepG2) cells (in medium with 10% FBS) were exposed to 70 μM of both PCB-153 (non-coplanar hexachlorobiphenyl) and PCB-77 (coplanar tetrachlorobiphenyl), and human kidney (HK2) cells in serum free medium were exposed to 80 and 40 μM of PCB-153 and PCB-77 respectively, according to their LC(50) values in these cells. Medium and cells were collected separately at each time interval from 30 min to 48 h, and PCB concentrations were analyzed in both by GC-MS using biphenyl as an internal standard following hexane:acetone (50:50) extraction. We also performed trypan blue exclusion, DNA fragmentation and fluorescence microscopic studies in assessing cell viability and apoptotic cell death. About 40% of PCB-153 (35 μM, 50% of the maximum value) was detected in HepG2 cells within 30 min, and it reached its highest concentration at 6h (60 μM), concomitant with the PCB depletion in the medium (5 μM). For PCB-77, the highest concentrations within the cells were reached at 3h. However, the absorption levels of PCB-153 and PCB-77 in HK2 cells reached their peaks at 3 and 6h respectively. Exposure of human liver and kidney cells to PCB-153 and PCB-77 caused accelerated apoptotic cell death in a time-dependent manner. The studies demonstrated that (1) liver cells initiate the absorption of PCBs much faster than kidney cells; however, the concentration reaches its maximum level much earlier in kidney cells; (2) both PCB-153 and PCB-77 induced enhanced apoptotic death in liver and kidney cells; and (3) kidney cells are more vulnerable to PCBs based on the results of apoptosis and cellular viability, even with almost similar absorption or tissue burden of PCBs.

Polychlorinated biphenyl mixtures (Aroclors) induce apoptosis via Bcl-2, Bax and caspase-3 proteins in neuronal cell cultures

Polychlorinated biphenyls (PCBs) are a group of persistent and widely dispersed environmental pollutants, some of which may be neurotoxic. In the present study, we have investigated the effect of PCB commercial mixtures (Aroclors) on neuronal cell cultures by assessing cell viability and apoptotic cell death. We have combined morphological and biochemical techniques to establish the relevance of apoptosis in neuronal cell death induced by Aroclors. Treatment with both Aroclor 1248 and Aroclor 1260 caused the loss of cell viability and accelerated apoptosis both in a concentration- and time-dependent manner. However, the extent of apoptosis resulted greater for Aroclor 1248 than for Aroclor 1260. This is correlated with the loss of cell viability since Aroclor 1248 is more cytotoxic. The apoptosis induced by Aroclors involves the increase of caspase-3 activity. To correlate the caspase-3 activity with respect to changes in protein processing, caspase-3 precursor protein (procaspase-3) was evaluated by Western blot analysis. Also, Bcl-2 and Bax protein were assessed in order to elucidate the cell death machinery induced in cortical neuronal cell cultures by Aroclor 1248. The results indicate that the increase in Aroclor-induced apoptosis correlates with a reduction in the expression of antiapoptotic Bcl-2 and an increase in the expression of proapoptotic Bax. These results suggest that, with our experimental conditions, Aroclors induce apoptosis in primary cultures of cortical neurons via proteins of the Bcl-2 and caspase families.