2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) induces degradation of adherens junction proteins and inhibits beta-catenin-dependent transcription in liver epithelial cells

Abstract

The toxic modes of action of non-dioxin-like polychlorinated biphenyls (PCBs) in liver cells are still only partially understood. Several recent studies have indicated that PCBs may interfere with cell membrane protein functions. Therefore, we analyzed in the present study the effects of di-ortho-substituted 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) on proteins involved in the formation of adherens junctions in a model of rat liver progenitor cells - WB-F344 cell line. PCB 153, at micromolar concentrations, induced a gradual degradation of E-cadherin, beta-catenin or plakoglobin (gamma-catenin) proteins. This effect was not due to changes in gene expression, as PCB 153 had no effect on mRNA levels of the above mentioned proteins. Moreover, apart from the reduction of total beta-catenin pool, PCB 153 also decreased levels of the active beta-catenin form, dephosphorylated at residues Ser37 and Thr41, which is the key co-activator of Wnt-induced TCF/LEF-dependent gene expression. Therefore, we also evaluated the impact of PCB 153 on expression of Axin2, a known transcriptional target of canonical Wnt signaling. PCB 153 reduced basal Axin2 mRNA levels and it inhibited induction of Axin2 expression by recombinant mouse Wnt3a. Nevertheless, PCB 153 had no effect on phosphorylation of glycogen synthase kinase-3beta (GSK-3beta), which is supposed to target beta-catenin for its proteasomal degradation. This suggested that GSK-3beta activity is not modulated by PCB 153 and, consequently, not involved in the observed PCB 153-induced decrease of both total and active beta-catenin levels. Protein levels of E-cadherin and beta-catenin were partially restored with lysosomal inhibitor leupeptin, thus suggesting a possible role of lysosomes in the observed degradation of adherens junction proteins. Taken together, the present data suggest that PCB 153 may interfere with functions of adherens junction proteins involved in both cell-to-cell communication and intracellular signaling. Such mechanisms might be involved in the effects of non-dioxin-like PCBs contributing to liver tumor promotion.