Increased gap junctional intercellular communication is directly related to the anti‐tumor effect of all‐trans‐retinoic acid plus tamoxifen in a human mammary cancer cell line

Additive effects against tumor cells might be achieved by combining anti‐neoplastic agents directed against one or more altered mechanisms in cancer. We investigated the participation of gap junctional intercellular communication (GJIC), which is commonly dysfunctional in tumor cells as a possible mediating mechanism of the effect of all‐trans‐retinoic acid (RA) and tamoxifen (Tx) in MCF‐7 human breast cancer cell lines. The combination of RA + Tx stimulated GJIC in approximately 53 ± 3% of MCF‐7 cells as early as after 6 h of treatment remaining communicated through 144 h of culture. The GJIC enhancement occurred along with immunolocalization of Cx26 and 43 at the membrane of contacting cells and correlated with higher protein levels. Cx40 immunoreactive plaques were detected at cell‐to‐cell contacts during 48 h of RA + Tx treatment that did not involve higher protein expression, to the contrary, a downregulation occurred after 72 h of treatment. Cell proliferation inhibition upon RA + Tx exposure was observed with optimal effects at 96–120 h of culture with an accumulation of cells primarily in G2/M and G0/G1 cell cycle boundaries. An enhancement of the pre‐existing E‐cadherin levels was observed after drug exposure along with a downregulation of Bcl‐2 and C‐myc protein levels and a reduction of telomerase activity, suggesting partial tumor phenotype reversion. Blockage of the RA + Tx‐induced GJIC with 18‐β‐glycyrrhetinic acid (β‐Gly) prevented in 34% the inhibition of MCF‐7 proliferation and the E‐cadherin increment in 30% at 96 h of culture. GJIC blockage did not alter the downregulation of Bcl‐2, c‐Myc, or telomerase activity induced by RA + Tx. Our results showed the participation of GJIC as a mediator mechanism of the combined action of RA and Tx in MCF‐7 cells. The chemopreventive modulation of GJIC might represent an approachable alternative for the improvement of cancer therapy. J. Cell. Biochem. 89: 450–461, 2003. © 2003 Wiley‐Liss, Inc.