The gap-junction-mediated intercellular communication (GJIC) is required for embryonic development, cell growth regulation and tissue homeostasis. It has been repeatedly emphasized that GJIC is defective in most human malignancies. However, the potential role of GJIC in the carcinogenesis and tumor progression of several endocrine-related cancers, including prostate and liver, remains unclear. In a previous work, we inspected GJIC activity in nontumorigenic (RWPE1) and malignant (RWPE2, LNCaP, DU145) human prostate epithelial cells, as model systems of prostate multistep carcinogenesis; the rat liver WB1 cell line was used as positive control since it is known to be GJIC-proficient. Scrutiny of GJIC activity was achieved by either the Scrape-Loading/Dye Transfer (SL/DT) or the Fluorescence Recovery After Photobleaching (FRAP) method. Both SL/DT and FRAP revealed that all cell lines studied have defective GJIC, while, as expected, WB1 cells show functional GJIC. However, both forskolin (FK) and estrone (E1), which elevate intracellular cAMP levels, significantly increased GJIC activity in RWPE1 cells only. Western blot analysis revealed that E1 induced a 3.5-fold increase of Cx43 expression, while both E1 and FK caused a marked reduction of Cx32 expression in RWPE-1 cells, suggesting that these agents may restore GJIC activity by increasing the Cx43:Cx32 ratio. Estrogen also induced a marked reduction of growth in both RWPE1 and RWPE2 cells. More recently, we have investigated GJIC in both nontumoral (Chang Liver, CL) and malignant (HepG2, Huh7) human liver cell lines. Using the SL/DT assay, only Huh7 cells exhibited a moderate degree of junctional activity in basic conditions, while neither CL nor HepG2 cells showed functional GJIC. Under exactly the same experimental approach used for prostate studies, we observed that, once again, both estrogen (either estradiol or estrone) and FK induce a significant increase of GJIC in Huh7 cells, while exposure of HepG2 cells to FK produces only a limited rise of junctional activity in this cell line. However, estrogen induced a significant increase and reduction of the proliferative activity of CL and Huh7 cells, respectively, while growth of HepG2 cells was not affected. While the above evidence suggests that estrogens are primarily implicated in growth regulation and communication of both prostate and liver epithelial cells, it also implies that compounds able to restore GJIC in junctionally-deficient cells or prevent its disruption in junctionally-proficient cells may be used for development of new strategies in the prevention and/or treatment of several human malignancies. Studies partly supported by Italian AIRC and CNR.
[Proc Amer Assoc Cancer Res, Volume 45, 2004]