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AMP-Activated Protein Kinase Signaling Results in Cytoplasmic Sequestration of p27

Departments of ¹ Carcinogenesis, ² Molecular Pathology, and ³ Systems Biology, The University of Texas M. D. Anderson Cancer Center, Smithville, Texas; ⁴ Department of Microbiology, Immunology, and Molecular Genetics, University of California, Los Angeles, Los Angeles, California; and ⁵ Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts

Requests for reprints: Cheryl Lyn Walker, Department of Carcinogenesis, The University of Texas M. D. Anderson Cancer Center, 1808 Park Road 1C, P.O. Box 389, Smithville, TX 78957. Phone: 512-237-9550; Fax: 512-237-2475; E-mail: cwalker{at}wotan.mdacc.tmc.edu.

Key Words: AMPK • TSC2 • p27

Tuberin, the Tsc2 gene product, integrates the phosphatidylinositol 3-kinase/mitogen-activated protein kinase (mitogenic) and LKB1/AMP-activated protein kinase (AMPK; energy) signaling pathways, and previous independent studies have shown that loss of tuberin is associated with elevated AMPK signaling and altered p27 function. In Tsc2-null tumors and tumor-derived cells from Eker rats, we observed elevated AMPK signaling and concordant cytoplasmic mislocalization of p27. Cytoplasmic localization of p27 in Tsc2-null cells was reversible pharmacologically using inhibitors of the LKB1/AMPK pathway, and localization of p27 to the cytoplasm could be induced directly by activating AMPK physiologically (glucose deprivation) or genetically (constitutively active AMPK) in Tsc2-proficient cells. Furthermore, AMPK phosphorylated p27 in vitro on at least three sites including T170 near the nuclear localization signal, and T170 was shown to determine p27 localization in response to AMPK signaling. p27 functions in the nucleus to suppress cyclin-dependent kinase-2 (Cdk2) activity and has been reported to mediate an antiapoptotic function when localized to the cytoplasm. We found that cells with elevated AMPK signaling and cytoplasmic p27 localization exhibited elevated Cdk2 activity, which could be suppressed by inhibiting AMPK signaling. In addition, cells with elevated AMPK signaling and cytoplasmic p27 localization were resistant to apoptosis, which could be overcome by inhibition of AMPK signaling and relocalization of p27 to the nucleus. These data show that AMPK signaling determines the subcellular localization of p27, and identifies loss of integration of pathways controlling energy balance, the cell cycle, and apoptosis due to aberrant AMPK and p27 function as a feature of cells that have lost the Tsc2 tumor suppressor gene. [Cancer Res 2008;68(16):6496–506]

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