Vinculin localization and actin stress fibers differ in thyroid cells organized as monolayers or follicles

In epithelial cells interactions between the actin cytoskeleton and cell‐cell junctions regulate paracellular permeability and partcipate in morphogenesis. We have studied the relationship between supracellular morphology and actin‐junction interactions using primary cultures of porcine thyroid cells grown either as three‐dimensional follicles or as open monolayers. Regardless of morphology, thyroid cells assembled occluding and adhesive junctions containing ZO‐1 and E‐cadherin, respectively, and showed F‐actin staining in apical microvilli and a perijunctional ring. In monolayers, actin stress fibers were also observed in the apical and basal poles of cells, where they terminated in the vinculin‐rich zonula adherens and in cell‐substrate focal adhesions, respectively. Surprisingly, we were unable to detect vinculin localization in follicular cells, which also did not form stress fibers. Immunoblotting confirmed significantly greater vinculin in triton‐insoluble fractions from monolayer cells compared with follicular cells. Incubation of monolayers with 8 chloro(phenylthio)‐cyclic AMP decreased the level of immunodetectable vinculin in the zonula adherens, indicating that junctional incorporation of vinculin was regulated by cyclic AMP. In monolayer cultures, cytochalasin D (1 μM) caused actin filaments to aggregate associated with retraction of cells from one another and the disruption of cell junctions. Despite morphologically similar perturbations of actin organization in follicular cultures treated with cytochalasin D, junctional staining of ZO‐1 and E‐cadherin was preserved and cells remained adherent to one another. We conclude that in cultured thyroid cells structural and functional associations between actin filaments and cellular junctions differ depending upon the supracellular morphology in which cells are grown. One important underlying mechanism appears to be regulation of vinculin incorporation into adhesive junctions by cyclic AMP. © 1995 Wiley‐Liss, Inc.