title: Glucocorticoid-induced degradation of glycogen synthase kinase-3 protein is triggered by serum- and glucocorticoid-induced protein kinase and Akt signaling and controls beta-catenin dynamics and tight junction formation in mammary epithelial tumor cells.
abstract: Glucocorticoid hormones stimulate adherens junction and tight junction formation in Con8 mammary epithelial tumor cells and induce the production of a stable nonphosphorylated beta-catenin protein localized exclusively to the cell periphery. Glycogen synthase kinase-3 (GSK3) phosphorylation of beta-catenin is known to trigger the degradation of this adherens junction protein, suggesting that steroid-activated cascades may be targeting this protein kinase. We now demonstrate that treatment with the synthetic glucocorticoid dexamethasone induces the ubiquitin-26S proteasome-mediated degradation of GSK3 protein with no change in GSK3 transcript levels. In transfected cells, deletion of the N-terminal nine amino acids or mutation of the serine-9 phosphorylation site on GSK3-beta prevented its glucocorticoid-induced degradation. Expression of stabilized GSK3 mutant proteins ablated the glucocorticoid-induced tight junction sealing and resulted in production of a nonphosphorylated beta-catenin that localizes to both the nucleus and the cell periphery in steroid-treated cells. Serine-9 on GSK3 can be phosphorylated by Sgk (serum- and glucocorticoid-induced protein kinase) and by Akt. Expression of dominant-negative forms of either Sgk- or Akt-inhibited glucocorticoid induced GSK3 ubiquitination and degradation and disrupted the dexamethasone-induced effects on beta-catenin dynamics. Furthermore, the steroid-induced tight junction sealing is attenuated in cells expressing dominant-negative forms of either Sgk or Akt, although the effect of blunting Sgk signaling was significantly greater. Taken together, we have uncovered a new cellular cascade in which Sgk and Akt trigger the glucocorticoid-regulated phosphorylation, ubiquitination, and degradation of GSK3, which alters beta-catenin dynamics, leading to the formation of adherens junctions and tight junction sealing.
authors.affiliation: Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, California 94720-3200, USA.
authors.forename: Kim L
authors.initials: KL
authors.lastname: Failor
authors.forename: Yelena
authors.initials: Y
authors.lastname: Desyatnikov
authors.forename: Lindsay A
authors.initials: LA
authors.lastname: Finger
authors.forename: Gary L
authors.initials: GL
authors.lastname: Firestone
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D003907:Dexamethasone / Q000494:pharmacology*; D004847:Epithelial Cells / Q000187:drug effects / Q000201:enzymology / Q000648:ultrastructure; D038362:Glycogen Synthase Kinase 3 / Q000235:genetics / Q000378:metabolism*; D017874:Immediate-Early Proteins / Q000378:metabolism; D008321:Mammary Glands, Animal / Q000187:drug effects* / Q000201:enzymology / Q000648:ultrastructure; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D051381:Rats; D012694:Serine / Q000235:genetics / Q000378:metabolism; D019108:Tight Junctions / Q000187:drug effects* / Q000378:metabolism; D025801:Ubiquitin / Q000378:metabolism; D051176:beta Catenin / Q000378:metabolism*; D000099184:Serum-Glucocorticoid Regulated Kinases
chemical_list: D003907:Dexamethasone; D038362:Glycogen Synthase Kinase 3; D017874:Immediate-Early Proteins; D046988:Proteasome Endopeptidase Complex; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D012694:Serine; D025801:Ubiquitin; D051176:beta Catenin; D000099184:Serum-Glucocorticoid Regulated Kinases; C061553:ATP dependent 26S protease