Phosphorylation of axin, a Wnt signal negative regulator, by glycogen synthase kinase-3beta regulates its stability.
Axin forms a complex with glycogen synthase kinase-3beta (GSK-3beta) and beta-catenin and promotes GSK-3beta-dependent phosphorylation of beta-catenin, thereby stimulating the degradation of beta-catenin. Because GSK-3beta also phosphorylates Axin in the complex, the physiological significance of the phosphorylation of Axin was examined. Treatment of COS cells with LiCl, a GSK-3beta inhibitor, and okadaic acid, a protein phosphatase inhibitor, decreased and increased, respectively, the cellular protein level of Axin. Pulse-chase analyses showed that the phosphorylated form of Axin was more stable than the unphosphorylated form and that an Axin mutant, in which the possible phosphorylation sites for GSK-3beta were mutated, exhibited a shorter half-life than wild type Axin. Dvl-1, which was genetically shown to function upstream of GSK-3beta, inhibited the phosphorylation of Axin by GSK-3beta in vitro. Furthermore, Wnt-3a-containing conditioned medium down-regulated Axin and accumulated beta-catenin in L cells and expression of Dvl-1(DeltaPDZ), in which the PDZ domain was deleted, suppressed this action of Wnt-3a. These results suggest that the phosphorylation of Axin is important for the regulation of its stability and that Wnt down-regulates Axin through Dvl.
Yamamoto
H
H
Department of Biochemistry, Hiroshima University School of Medicine, 1-2-3, Kasumi, Minami-ku, Hiroshima 734-8551, Japan.
Kishida
S
S
Kishida
M
M
Ikeda
S
S
Takada
S
S
Kikuchi
A
A
D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D060466:Axin Protein; D019556:COS Cells; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism*; D000072261:Dishevelled Proteins; D015536:Down-Regulation; D038362:Glycogen Synthase Kinase 3; D038341:Glycogen Synthase Kinases; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D011506:Proteins / Q000378:metabolism*; D012097:Repressor Proteins; D051153:Wnt Proteins; D060508:Wnt3 Protein
D048868:Adaptor Proteins, Signal Transducing; D060466:Axin Protein; D000072261:Dishevelled Proteins; D010750:Phosphoproteins; D011506:Proteins; D012097:Repressor Proteins; D051153:Wnt Proteins; D060508:Wnt3 Protein; D038341:Glycogen Synthase Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D038362:Glycogen Synthase Kinase 3