Cellular signaling crosstalk between Wnt signaling and gap junctions inbenzo[a]pyrene toxicity

Gap junctional intercellular communication (GJIC) is considered a key biological mechanism to maintain homeostasis in cell differentiation and growth. In addition, as another major signaling pathway associated with cell proliferation and differentiation, Wnt/beta-catenin signaling appears to trigger several cellular responses against injury. The purpose of the present study was to investigate the effects of a known toxic agent, benzo[a]pyrene (BaP), on the regulation and interaction between GJIC and Wnt/beta-catenin signaling. BaP treatment resulted in GJIC inhibition and decreases the major GJIC protein connexin 43 (Cx43) in WB-F344 rat liver epithelial cells. We also found BaP-mediated downregulation of Wnt/beta-catenin signaling related to the PI3K-Akt pathway. To identify the relationship between GJIC and Wnt/beta-catenin signaling, we treated WB-F344 cells with the Wnt agonist CHIR99021 and found that it inhibited GJIC while causing a significant reduction in Cx43 expression at both the mRNA and protein levels, through the repression of promoter activity. This Wnt agonist-mediated GJIC inhibition was confirmed using a small interfering RNA directed against the Wnt antagonist Dact2, indicating that Wnt/beta-catenin signaling negatively regulates GJIC. Despite the inverse correlation between Wnt/beta-catenin signaling and Cx43 promoter activation as indicated by downregulation of beta-catenin nuclear translocation and upregulation of Cx43 promoter activation involving HNF3 beta, BaP treatment decreased the Cx43 protein expression, which was associated with protein degradation, possibly through protein kinase C activation. In conclusion, our results revealed the mechanism of BaP-induced inhibition of GJIC and Wnt/beta-catenin signaling. More importantly, linking Wnt/beta-catenin signaling to Cx protein expression will have profound implications in understanding the relationships among different major signaling pathways associated with cell proliferation and differentiation in toxicity.

Automated summary

This study investigated the effects of benzo[a]pyrene (BaP) on gap junctional intercellular communication (GJIC) and Wnt/beta-catenin signaling. BaP treatment inhibited GJIC and decreased the expression of connexin 43 (Cx43), a major GJIC protein. Treatment with the Wnt agonist CHIR99021 also inhibited GJIC and reduced Cx43 expression. It was found that Wnt/beta-catenin signaling negatively regulated GJIC.