Retinoids increase cell-cell adhesion strength, β-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 expresses no E-cadherin and very low levels of β-catenin protein and exhibits a poorly adhesive phenotype in Matrigel responds to retinoic acid (RA) by a marked increase in epithelial differentiation. Specifically, treatment of cells with all-trans-RA, 9-cis-RA, or a RA receptor α-specific ligand resulted in a large increase in cell-cell adhesive strength and stimulated the formation of fused cell aggregates in Matrigel. A retinoid X receptor-specific ligand was ineffective. Exposure 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 required protein and RNA synthesis, but were not mediated by factors secreted by stimulated cells or by direct cell contact and did not require serum. These 9-cis-RA-induced morphological effects were completely reversed by growing cells in 50 μM Ca²⁺, suggesting a mechanism involving a 9-cis-RA-induced increase in Ca²⁺- dependent adhesion. Consistent with this, β-catenin protein levels were markedly elevated in the 9-cis-RA-treated cells, and β-catenin became localized to a Triton-insoluble pool at regions of cell-cell contact. No change could be detected in β-catenin steady state messenger RNA levels, but 9-cis-RA did increase β-catenin protein stability. Treatment of cells with low calcium medium did not prevent the 9-cis-RA-induced increase in total β- 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 changes induced by 9-cis-RA. Like treatment with low calcium medium, these inhibitors did not prevent the 9-cis-RA-induced increase in total β-catenin protein levels, but completely prevented the movement of β-catenin to the cell membrane. These results point to a role for β-catenin and serine kinase activity in mediating the action of 9-cis-RA in epithelial differentiation.