title: Regulation of karyopherin α1 and nuclear import by mammalian target of rapamycin.
abstract: Under conditions of reduced mitogen or nutritional substrate levels, the serine/threonine kinase target of rapamycin can augment the nuclear content of distinct transcription factors and promote the induction of stress response genes. In its latent (i.e., unphosphorylated) form, the transcription factor STAT1 regulates a subset of genes involved in immune modulation and apoptosis. Based on previous work indicating a functional relationship between mammalian target of rapamycin (mTOR) and the nuclear content of latent STAT1, we investigated the mechanism by which mTOR controls STAT1 nuclear import. By fluorescence confocal microscopy, inactivation of mTOR with rapamycin promoted the nuclear translocation of unphosphorylated STAT1, but not that of a STAT1 mutant incapable of binding its nuclear import adaptor karyopherin-α1 (KPNA1). By immunoprecipitation, KPNA1 was physically associated with mTOR and STAT1 in a complex that translocated to the nucleus in response to rapamycin. Although mTOR is not a kinase for KPNA1, the mTOR-associated phosphatase protein phosphatase 2A catalytic interacted directly with KPNA1 and regulated nuclear import of the mTOR-KPNA1 complex. KPNA1, or its interaction with STAT1, was required for the nuclear import of latent STAT1, transcriptional induction of the STAT1 gene, and caspase-3 activation under conditions of reduced mTOR activity (i.e. rapamycin, glucose starvation, serum withdrawal). Therefore, at low mitogen or nutrient levels, mTOR and protein phosphatase 2A catalytically control the constitutive nuclear import of latent STAT1 by KPNA1, which are key modulators of STAT1 expression and apoptosis.
authors.affiliation: Critical Care Division and Meakins-Christie Laboratories, Faculty of Medicine, McGill University, Montreal, Quebec H3A 1A1, Canada.
authors.forename: Jill A
authors.initials: JA
authors.lastname: Fielhaber
authors.forename: Jason
authors.initials: J
authors.lastname: Tan
authors.forename: Kwang-Bo
authors.initials: KB
authors.lastname: Joung
authors.forename: Ortal
authors.initials: O
authors.lastname: Attias
authors.forename: Stefanie
authors.initials: S
authors.lastname: Huegel
authors.forename: Michael
authors.initials: M
authors.lastname: Bader
authors.forename: Philippe P
authors.initials: PP
authors.lastname: Roux
authors.forename: Arnold S
authors.initials: AS
authors.lastname: Kristof
mesh_terms: D021581:Active Transport, Cell Nucleus / Q000502:physiology; D000818:Animals; D017209:Apoptosis / Q000502:physiology; D053148:Caspase 3 / Q000502:physiology; D002467:Cell Nucleus / Q000235:genetics / Q000378:metabolism*; D004789:Enzyme Activation / Q000502:physiology; D005786:Gene Expression Regulation / Q000502:physiology; D057809:HEK293 Cells; D006801:Humans; D051379:Mice; D046912:Multiprotein Complexes / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D010766:Phosphorylation / Q000502:physiology; D054648:Protein Phosphatase 2 / Q000235:genetics / Q000378:metabolism; D050794:STAT1 Transcription Factor / Q000235:genetics / Q000378:metabolism; D058570:TOR Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D028901:alpha Karyopherins / Q000235:genetics / Q000378:metabolism*
chemical_list: C423142:KPNA1 protein, human; C527077:KPNA1 protein, mouse; D046912:Multiprotein Complexes; D050794:STAT1 Transcription Factor; C494079:STAT1 protein, human; C494080:Stat1 protein, mouse; D028901:alpha Karyopherins; C546842:MTOR protein, human; C546843:mTOR protein, mouse; D058570:TOR Serine-Threonine Kinases; C517189:PPP2CA protein, human; D054648:Protein Phosphatase 2; C505407:CASP3 protein, human; C505409:Casp3 protein, mouse; D053148:Caspase 3