Abstract
Cadmium has been associated with a number of tumors but its role in tumor promotion has not been elucidated clearly or the results obtained from various studies have been conflicting. This study was designed to investigate the effects of cadmium on the gap junctional intercellular communication (GJIC), number of gap junctions per cell, and cell proliferation in WB-F344 rat liver epithelial cells from the viewpoint of tumor promotion. GJIC was monitored by counting the cells stained with Lucifer yellow CH dye, using the scrape-loading and dye-transfer method. The numbers of gap junctions per cell were visually quantitated after an indirect immunostaining for gap junction protein using an antibody to connexin 43. Cell proliferation was assayed by direct counting of the living cells using the trypan blue dye exclusion method. In the time course study, cells treated with 200 μM CdCl2 showed rapid and nearly complete inhibition of GJIC (approximately 14% of the control) and a decrease in the number of gap junctions per cell (approximately 21% of the control) at 30 min, and the decrease continued up to 4 h without any changes in the cell viability. Treatment with CdCl2 7.4-200 μM) for 4 h resulted in the decrease of GJIC and gap junction numbers per cell in a dose-response pattern without changes in the cell viability. In the long-term (14 days) exposure studies at doses of 0.01-7.4 μM CdCl2, an increase in cell proliferation was observed at low doses of 0.03-2.5 μM CdCl2, with GJIC also decreasing. These data demonstrate that cadmium inhibits GJIC, reduces the number of gap junctions per cell, and induces cell proliferation while decreasing the function of the gap junction.