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

Runhua Liu,¹ Lizhong Wang,¹ Chong Chen,¹ Yan Liu,¹ Penghui Zhou,¹ Yin Wang,¹ Xirui Wang,¹ Julie Turnbull,⁴ Berge A. Minassian,⁴ Yang Liu,^1,2, and Pan Zheng^1,3*

Division of Immunotherapy, Department of Surgery, Program of Molecular Mechanism of Diseases and Comprehensive Cancer Center,¹ Division of Molecular Medicine and Genetics, Department of Internal Medicine,² Department of Pathology, University of Michigan, Ann Arbor, Michigan 48109,³ Program in Genetics and Genome Biology, The Hospital for Sick Children, Toronto, Ontario, Canada⁴

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-3. 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.

* Corresponding author. Mailing address: Department of Surgery, 1810 BSRB, 109 Zina Pitcher Place, University of Michigan, Ann Arbor, MI 48109. Phone: (734) 615-3464. Fax: (734) 763-2162. E-mail: panz{at}umich.edu

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

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