AbstractThe promoting effects of polychlorinated biphenyls (PCBs) have been studied extensively in a variety of two‐stage carcinogenesis models. However, the molecular mechanisms responsible for the promotion effects of PCBs have not been elucidated. We measured the effect of PCBs on DNA‐binding proteins involved in cell proliferation and transformation. Male Sprague‐Dawley rats were injected intraperitoneally with mono‐, di‐, tri‐, tetra‐, or hexachlorobiphenyls (300 μmol/kg/d) each day for 4 d and killed 4 h after the last injection. To detect alterations in nuclear proteins that could explain the tumor‐promoter activity of PCBs, liver nuclear extracts were analyzed by electrophoretic mobility shift assays. Electrophoretic mobility shift assay analysis of signal transducers and activators of transcription (STAT)–binding activity to a consensus γ‐interferon–activated sequence (GAS) element was compared in liver nuclear extracts from treated rats. STAT‐binding activity was eightfold to tenfold higher in nuclear extracts from animals treated with 2,4,4′‐trichloro‐ (PCB 28) and 2,2′,4,4′,5,5′‐hexachlorobiphenyl (PCB 153). Analysis of the protein complex binding to the GAS element, with antibodies specific for STAT3, STAT5, and STAT6, indicated that the protein complex was made up of STAT5 and STAT6 proteins. HepG2 cells transiently transfected with a luciferase reporter gene construct containing many STAT5 binding sites were treated with PCB 28 and PCB 153. PCB 28 stimulated a greater than 25‐fold increase in luciferase activity at the highest concentration tested, 1.0 μg/mL. However, enhanced luciferase activity did not occur with PCB 153 treatment. 4‐Chlorobiphenyl (PCB 3), PCB 28, and PCB 153 treatment of Sprague‐Dawley rats resulted in a large increase in protein binding to a consensus activated protein‐1 (AP‐1) element. However, 3,4‐dichlorobiphenyl (PCB 12) and 3,3′,4,4′‐tetrachlorobiphenyl (PCB 77) treatments did not increase AP‐1 transcription activity. Further analysis of the proteins binding to the AP‐1 consensus sequence with antibodies specific for c‐fos, junD, and junB indicated that the protein composition consists of junD proteins. These data showed functional differences between noncoplanar and coplanar PCBs with respect to STAT activation and AP‐1–DNA binding. © 2001 Wiley‐Liss, Inc.