Effect of 2,2',4,4',5,5'-hexachlorobiphenyl (PCB-153) on hepatocyte proliferation and apoptosis in mice deficient in the p50 subunit of the transcription factor NF-kappaB.

Polychlorinated biphenyls (PCBs) are a group of synthetic chemicals that induce and promote liver tumors in rodents. We previously showed hepatic nuclear factor kappaB (NF-kappaB) activation and increased hepatocyte proliferation in PCB-treated rats. In this study, the role of NF-kappaB in hepatocyte proliferation and apoptosis after PCB administration was analyzed in wild-type mice and in mice deficient in the NF-kappaB p50 subunit (p50-/-). In a 2-day study, mice received a single intraperitoneal (ip) injection of corn oil or PCB-153. Hepatic NF-kappaB DNA binding activity and cell proliferation were increased by PCB-153 in wild-type mice but not in p50-/- mice. In a 21-day study, mice received six ip injections of corn oil or PCB-153 (twice weekly for 3 weeks) and were euthanized 4 days after the last injection. In this study, NF-kappaB DNA binding activity was not increased after PCB-153 treatment in wild-type or p50-/- mice. Cell proliferation was significantly increased in the wild-type mice treated with PCB-153; in the p50-/- mice treated with PCB-153, cell proliferation was greater than in untreated mice but less than in wild-type mice treated with PCB-153. The livers of p50-/- mice showed greater apoptosis than those of wild-type mice; PCB-153 decreased apoptosis in p50-/- mice, with higher inhibition in the 21-day study than in the 2-day study. RNase protection assays indicated that PCB-153 decreased the mRNA level of cyclin A2, B1, B2, and C in the 2-day study, but not in the 21-day study; however, it did not affect cyclin D1 and D2 mRNA levels at either time point. Cyclin D1 protein levels were not affected by PCB-153. Taken together, these data indicate that the absence of the NF-kappaB p50 subunit alters the proliferative and apoptotic changes in mouse liver in the response to PCB-153.