A constitutively active Rac impairs the acquisition of epithelial cell polarity.

We demonstrated that treatment of FRT thyroid epithelial cells with a Rac-specific inhibitor causes an impairment in the acquisition of polarity. FRT-β8i cells, in which the signal transduction from the β1A integrin is impaired, manifest a similar defect. We tested here the hypothesis that an active Rac might correct the FRT-β8i polarity defect. FRT-β8i cells that stably expressed an inducible, constitutively active Rac, ER-Rac(QL), were obtained. Upon tamoxifen treatment the ER-Rac(QL) protein became active, localized at the plasma membrane and in confluent cells it was mostly found on the lateral plasmamembrane, at sites of cell-cell contact. In these cultured cells cytokinesis was progressively impaired. Furthermore, activation of ER-Rac(QL) interfered with the acquisition of transepithelial resistance by confluent monolayers on filters, impaired cyst formation by cells in suspension culture and reduced the wound healing efficiency in a scratch test. Similar results were obtained with wild-type FRT cells expressing the same ER-Rac(QL). We conclude that a constitutively active Rac does not promote, but rather hampers, the acquisition of cell polarity in epithelial cells.