RETINOIDS INCREASE CELL-CELL ADHESION STRENGTH, BETA-CATENIN PROTEIN STABILITY, AND LOCALIZATION TO THE CELL-MEMBRANE IN A BREAST-CANCER CELL-LINE - A ROLE FOR SERINE KINASE-ACTIVITY

In this study we show that a breast cancer cell line (SKBR3) that expr esses no E-cadherin and very low levels of beta-catenin protein and ex hibits a poorly adhesive phenotype in Matrigel responds to retinoic ac id (RA) by a marked increase in epithelial differentiation. Specifical ly, treatment of cells with all-trans-RA, 9-cis-RA, or a RA receptor a lpha-specific ligand resulted in a large increase in cell-cell adhesiv e strength and stimulated the formation of fused cell aggregates in Ma trigel. A retinoid X receptor-specific ligand was ineffective. Exposur e of cells to 9-cis-RA for as little as 4 h was sufficient to maintain the adhesive phenotype for at least 4 days. The effects of 9-cis-RA r equired protein and RNA synthesis, but were not mediated by factors se creted by stimulated cells or by direct cell contact and did not requi re serum. These 9-cis-RA-induced morphological effects were completely reversed by growing cells in 50 mu M Ca2+, suggesting a mechanism inv olving a 9-cis-RA-induced increase in Ca2+-dependent adhesion. Consist ent with this, beta-catenin protein levels were markedly elevated in t he 9-cis-RA-treated cells, and beta-catenin became localized to a Trit on-insoluble pool at regions of cell-cell contact. No change could be detected in beta-catenin steady state messenger RNA levels, but 9-cis- RA did increase beta-catenin protein stability. Treat ment of cells wi th low calcium medium did not prevent the 9-cis-RA-induced increase in total beta-catenin protein, but did prevent its movement to a Triton- insoluble pool at the cell membrane. Among several kinase inhibitors, only the broad spectrum kinase inhibitor staurosporine and the protein kinase C inhibitor bisindoylmaleimide reversed the morphological chan ges induced by 9-cis-RA. Like treatment with low calcium medium, these inhibitors did not prevent the 9-cis-RA-induced increase in total bet a-catenin protein levels, but completely prevented the movement of bet a-catenin to the cell membrane. These results point to a role for beta -catenin and serine kinase activity in mediating the action of 9-cis-R A in epithelial differentiation.