Indolo[3,2-b]carbazole inhibits gap junctional intercellular communication in rat primary hepatocytes and acts as a potential tumor promoter

Indole-3-carbinol (I3C) is a naturally occurring substance that shows anti-carcinogenic properties in animal models. Besides its clear anti-carcinogenic effects, some studies indicate that I3C may sometimes act as a tumor promoter. Indolo[3,2-b]carbazole (ICZ), which is formed in the acidic environment of the stomach after intake of I3C, has a similar structure to, and shares biological effects with, the well-known tumor promoter 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Therefore, we hypothesized that ICZ could be responsible for the potential tumor-promoting activity of I3C. The aim of the present study was to investigate the effect of ICZ on gap junctional intercellular communication (GJIC) in primary cultured rat hepatocytes co-cultured with the rat liver epithelial cell line WB-F344. Indolo[3,2b]carbazole inhibited GJIC in the rat hepatocytes in a dose- and time-dependent manner. Significant inhibition was observed after 8 and 12 h of treatment with I and 0.1 muM ICZ, respectively. Maximum GJIC inhibition (cell-cell communication only 5% of control values) was observed after 24-48 h of ICZ treatment. Continued exposure to 1 muM ICZ suppressed GJIC until similar to120 h. Both ICZ and TCDD treatment reduced the Cx32 mRNA level as well as the plasma membrane Cx32 staining. Indolo[3,2-b]carbazole increased the Cyp1a1, Cyp1a2 and Cyp1b1 mRNA levels concurrently with an increase in 7-ethoxyresorufin O-deethylase (EROD) activities. Maximum EROD activity and Cyp1a1 mRNA levels were observed after similar to12 h, whereas Cyp1a2 and Cyp1b1 mRNA levels peaked after 48 h. This study shows that ICZ may possess tumor promoter activity down-regulating GJIC by mechanisms, which seem to include activation of the Ah receptor and/or Cyp1 activity. Further studies are needed in order to clarify the anticarcinogenic/carcinogenic effects of I3C and ICZ before high doses of I3C may be recommended as a dietary supplement.