Laforin Negatively Regulates Cell Cycle Progression through Glycogen Synthase Kinase 3β-Dependent Mechanisms

Abstract

Glycogen synthase kinase 3β (GSK-3β) represses cell cycle progression by directly phosphorylating cyclin D1 and indirectly regulating cyclin D1 transcription by inhibiting Wnt signaling. Recently, we reported that the Epm2a-encoded laforin is a GSK-3β phosphatase and a tumor suppressor. The cellular mechanism for its tumor suppression remains unknown. Using ex vivo thymocytes and primary embryonic fibroblasts from Epm2a^−/− mice, we show here a general function of laforin in the cell cycle regulation and repression of cyclin D1 expression. Moreover, targeted mutation of Epm2a increased the phosphorylation of Ser9 on GSK-3β while having no effect on the phosphorylation of Ser21 on GSK-3α. In the GSK-3β^+/+ but not the GSK-3β^−/− cells, Epm2a small interfering RNA significantly enhanced cell growth. Consistent with an increased level of cyclin D1, the phosphorylation of retinoblastoma protein (Rb) and the levels of Rb-E2F-regulated genes cyclin A, cyclin E, MCM3, and PCNA are also elevated. Inhibitors of GSK-3β selectively increased the cell growth of Epm2a^+/+ but not of Epm2a^−/− cells. Taken together, our data demonstrate that laforin is a selective phosphatase for GSK-3β and regulates cell cycle progression by GSK-3β-dependent mechanisms. These data provide a cellular basis for the tumor suppression activity of laforin.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/ .

ACKNOWLEDGMENTS

We thank Christopher J. Phiel from Columbus Children's Hospital for the MEF used in the study and Lynde Shaw and Todd Brown for secretarial assistance.

This study is supported by grants from the American Cancer Society (P.Z.), the U.S. Department of Defense (P.Z.), the National Institutes of Health (Y.L.), and the Canadian Institutes of Health Research (B.A.M.).