Casein kinase 2 prevents mesenchymal transformation by maintaining Foxc2 in the cytoplasm

Nuclear Foxc2 is a transcriptional regulator of mesenchymal transformation during developmental epithelial-mesenchymal transition (EMT) and has been associated with EMT in malignant epithelia. Our laboratory has shown that in normal epithelial cells Foxc2 is maintained in the cytoplasm where it promotes an epithelial phenotype. The Foxc2 amino terminus has a consensus casein kinase 2 (CK2) phosphorylation site at serine 124, and we now show that CK2 associates with Foxc2 and phosphorylates this site in vitro. Knockdown or inhibition of the CK2 alpha/alpha a' kinase subunit in epithelial cells causes de novo accumulation of Foxc2 in the nucleus. Mutation of serine 124 to leucine promotes constitutive nuclear localization of Foxc2 and expression of mesenchymal genes, whereas an S124D phosphomimetic leads to constitutive cytoplasmic localization and epithelial maintenance. In malignant breast cancer cells, the CK2 beta regulatory subunit is downregulated and FOXC2 is found in the nucleus, correlating with an increase in a-smooth muscle actin (SMA) expression. Restoration of CK2 beta expression in these cells results in cytoplasmic localization of Foxc2, decreased alpha-SMA expression and reduced cell migration and invasion. In contrast, knockdown of CK2 beta in normal breast epithelial cells leads to FOXC2 nuclear localization, decreased E-cadherin expression, increased alpha-SMA and vimentin expression, and enhanced cell migration and invasion. Based on these findings, we propose that Foxc2 is functionally maintained in the cytoplasm of normal epithelial cells by CK2 alpha/alpha'-mediated phosphorylation at serine 124, which is dependent on proper targeting of the holoenzyme via the CK2 beta regulatory subunit.