title: Rac1 is required for reorientation of polarity and lumen formation through a PI 3-kinase-dependent pathway.
abstract: Epithelial cells are characterized by the ability to form sheets of cells that surround fluid-filled lumens. Cells in these sheets exhibit a characteristic subcellular polarity, with an apical pole that faces the lumen and a basolateral pole that is in contact with other cells and the extracellular matrix (ECM). To investigate the signaling events required for polarization and lumen formation, we have taken advantage of the ability of Madin-Darby canine kidney (MDCK) cells to dynamically remodel their polarity in response to changes in ECM cues. When MDCK cells are grown in suspension culture, they form multicellular "inside-out" cysts with apical proteins found on the peripheral surface and basolateral markers on the interior surface. When these inside-out cysts are embedded in ECM, they rapidly reorient their polarity: apical proteins become localized to the inside surface, and basolateral proteins are found on the surface that contacts ECM. Here we have characterized the signaling requirements for these early molecular reorientation events. Specifically, expression of a dominant-negative form of Rac1 (DN-Rac1) blocks the reorientation of polarity. Phosphoinositide 3'-kinase is required for apical membrane protein remodeling from the initial apical membrane surface. Cells expressing DN-Rac1 fail to detectably activate the PI 3-kinase/protein kinase B pathway. Last, we found that atypical protein kinase C (aPKC) is also required for reorientation of polarity, since an inhibitor of atypical PKC blocks reorientation. This effect cannot be overcome by constitutively active Rac1, demonstrating that both Rac1 and atypical PKC are required for reorientation of cellular polarity.
authors.affiliation: Division of Nephrology, University of California, San Francisco, Box 2140, San Francisco, CA 94143-2140, USA.
authors.forename: Kathleen D
authors.initials: KD
authors.lastname: Liu
authors.forename: Anirban
authors.initials: A
authors.lastname: Datta
authors.forename: Wei
authors.initials: W
authors.lastname: Yu
authors.forename: Paul R
authors.initials: PR
authors.lastname: Brakeman
authors.forename: Tzuu-Shuh
authors.initials: TS
authors.lastname: Jou
authors.forename: Michael A
authors.initials: MA
authors.lastname: Matthay
authors.forename: Keith E
authors.initials: KE
authors.lastname: Mostov
mesh_terms: D000818:Animals; D015415:Biomarkers / Q000378:metabolism; D002460:Cell Line; D016764:Cell Polarity / Q000187:drug effects / Q000502:physiology*; D003560:Cysts / Q000209:etiology / Q000378:metabolism; D003584:Cytological Techniques; D004285:Dogs; D004789:Enzyme Activation; D005109:Extracellular Matrix / Q000378:metabolism; D005799:Genes, Dominant; D007425:Intracellular Membranes / Q000378:metabolism; D007668:Kidney / Q000166:cytology* / Q000378:metabolism / Q000502:physiology*; D008565:Membrane Proteins / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology*; D011493:Protein Kinase C / Q000502:physiology*; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology; D020830:rac1 GTP-Binding Protein / Q000235:genetics / Q000502:physiology*