Rac activity regulates polarity of thyroid epithelial cells

Thyroid derived FRT cells express in culture the polarized epithelial phenotype. FRT-β8i cells, in which the signal transduction from the β1A integrin is impaired by the expression of the β1B variant, have a partial polarization defect. We tested the hypothesis that this is due to a lack of activity of the Rac GTPase. By confocal microscopy the endogenous Rac, as well as Rac-GFP, is present in the cytosol of FRT cells but also on the plasma membrane. In confluent monolayers on filters Rac forms a ring that is associated to the tight junction. The Rac-specific inhibitor NSC23766 inhibits FRT cell migration, reduces the amount of Rac on the plasma membrane and prevents the full acquisition of transepithelial resistance (TER). This effect is reversible and is not observed if the drug is added when the monolayer has acquired its full TER. The Rac inhibitor also, in part, impairs the formation of polarized cysts in suspension culture. The phenotype of the NSC23766-treated FRT cells is fully reminiscent of that of FRT-β8i suggesting that Rac signaling regulates the polarized phenotype. We are now trying to correct the FRT-β8i phenotype by using an inducible ER-Rac(QL).