Glycogen Synthase Kinase 3β Is Positively Regulated by Protein Kinase Cζ-Mediated Phosphorylation Induced by Wnt Agonists

Abstract

? 2016, American Society for Microbiology.The molecular events that drive Wnt-induced regulation of glycogen synthase kinase 3?; (GSK-3?) activity are poorly defined. In this study, we found that protein kinase C? (PKC?) and GSK-3? interact mainly in colon cancer cells. Wnt stimulation induced a rapid GSK-3? redistribution from the cytoplasm to the nuclei in malignant cells and a transient PKC-mediated phosphorylation of GSK-3? at a different site from serine 9. In addition, while Wnt treatment induced a decrease in PKC-mediated phosphorylation of GSK-3? in nonmalignant cells, in malignant cells, this phosphorylation was increased. Pharmacological inhibition and small interfering RNA (siRNA)-mediated silencing of PKC? abolished all of these effects, but unexpectedly, it also abolished the constitutive basal activity of GSK-3?. In vitro activity assays demonstrated that GSK-3? phosphorylation mediated by PKC? enhanced GSK-3? activity. We mapped Ser147 of GSK-3? as the site phosphorylated by PKC?, i.e., its mutation into alanine abolished GSK-3? activity, resulting in ?-catenin stabilization and increased transcriptional activity, whereas phosphomimetic replacement of Ser147 by glutamic acid maintained GSK-3? basal activity. Thus, we found that PKC? phosphorylates GSK-3? at Ser147 to maintain its constitutive activity in resting cells and that Wnt stimulation modifies the phosphorylation of Ser147 to regulate GSK-3? activity in opposite manners in normal and malignant colon cells.