title: Smad7 knockdown activates protein kinase RNA-associated eIF2α pathway leading to colon cancer cell death.
abstract: Upregulation of Smad7, an inhibitor of transforming growth factor-β1 (TGF-β1), occurs in sporadic colorectal cancer (CRC) and knockdown of Smad7 inhibits CRC cell growth, a phenomenon that associates with decreased expression of cell division cycle 25 homolog A and arrest of cells in the S phase of the cell cycle. These findings occur in CRC cells unresponsive to TGF-β1, thus suggesting the existence of a Smad7-mediated TGF-β1-independent mechanism that controls CRC cell behavior. Here we show that Smad7 inhibition with a specific Smad7 antisense oligonucleotide upregulates eukaryotic translation initiation factor 2α (eIF2α) phosphorylation, a transcription factor involved in the regulation of cell cycle arrest and induction of cell death, and induces activating transcription factor 4 (ATF4) and CCAAT/enhancer binding protein homology protein (CHOP), two downstream targets of eIF2α. Among the upstream kinases that control eIF2α phosphorylation, the serine-threonine protein kinase RNA (PKR), but not general control non-derepressible 2 (GCN2) and protein kinase RNA-like endoplasmic reticulum kinase (PERK), is activated by Smad7 knockdown. PKR silencing abolishes Smad7 antisense-induced eIF2α phosphorylation and ATF4/CHOP induction, thereby preventing Smad7 antisense-driven cell death. Smad7 inhibition diminishes interaction of PKR with protein kinase inhibitor p58 (p58IPK), a cellular inhibitor of PKR, but does not change the expression and/or activity of other factors involved in the control of PKR activation. These findings delineate a novel mechanism by which Smad7 knockdown promotes CRC cell death.
authors.affiliation: Department of Systems Medicine, University of 'Tor Vergata', Rome, Italy.
authors.forename: Veronica
authors.initials: V
authors.lastname: De Simone
authors.affiliation: Department of Systems Medicine, University of 'Tor Vergata', Rome, Italy.
authors.forename: Gerolamo
authors.initials: G
authors.lastname: Bevivino
authors.affiliation: Department of Systems Medicine, University of 'Tor Vergata', Rome, Italy.
authors.forename: Silvia
authors.initials: S
authors.lastname: Sedda
authors.affiliation: Department of Systems Medicine, University of 'Tor Vergata', Rome, Italy.
authors.forename: Roberta
authors.initials: R
authors.lastname: Izzo
authors.affiliation: Department of Systems Medicine, University of 'Tor Vergata', Rome, Italy.
authors.forename: Federica
authors.initials: F
authors.lastname: Laudisi
authors.affiliation: Department of Systems Medicine, University of 'Tor Vergata', Rome, Italy.
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Dinallo
authors.affiliation: Department of Systems Medicine, University of 'Tor Vergata', Rome, Italy.
authors.forename: Eleonora
authors.initials: E
authors.lastname: Franzè
authors.affiliation: Department of Systems Medicine, University of 'Tor Vergata', Rome, Italy.
authors.forename: Alfredo
authors.initials: A
authors.lastname: Colantoni
authors.affiliation: Department of Systems Medicine, University of 'Tor Vergata', Rome, Italy.
authors.forename: Angela
authors.initials: A
authors.lastname: Ortenzi
authors.affiliation: Department of Systems Medicine, University of 'Tor Vergata', Rome, Italy.
authors.forename: Silvia
authors.initials: S
authors.lastname: Salvatori
authors.affiliation: Department of Surgery, University of 'Tor Vergata', Rome, Italy.
authors.forename: Piero
authors.initials: P
authors.lastname: Rossi
authors.affiliation: Department of Surgery, University of 'Tor Vergata', Rome, Italy.
authors.forename: Giuseppe S
authors.initials: GS
authors.lastname: Sica
authors.affiliation: Department of Systems Medicine, University of 'Tor Vergata', Rome, Italy.
authors.forename: Massimo C
authors.initials: MC
authors.lastname: Fantini
authors.affiliation: Department of Systems Medicine, University of 'Tor Vergata', Rome, Italy.
authors.forename: Carmine
authors.initials: C
authors.lastname: Stolfi
authors.affiliation: Department of Systems Medicine, University of 'Tor Vergata', Rome, Italy.
authors.forename: Giovanni
authors.initials: G
authors.lastname: Monteleone
mesh_terms: D051701:Activating Transcription Factor 4 / Q000378:metabolism; D059447:Cell Cycle Checkpoints / Q000502:physiology; D016923:Cell Death / Q000502:physiology*; D003110:Colonic Neoplasms / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism*; D045325:HCT116 Cells; D050956:HSP40 Heat-Shock Proteins / Q000378:metabolism; D006801:Humans; D010766:Phosphorylation / Q000502:physiology; D014176:Protein Biosynthesis / Q000502:physiology; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D051906:Smad7 Protein / Q000378:metabolism*; D051742:Transcription Factor CHOP / Q000378:metabolism; D053773:Transforming Growth Factor beta1 / Q000378:metabolism; D015854:Up-Regulation / Q000502:physiology; D019892:eIF-2 Kinase / Q000378:metabolism*
chemical_list: D051701:Activating Transcription Factor 4; D015852:Eukaryotic Initiation Factor-2; D050956:HSP40 Heat-Shock Proteins; D017346:Protein Serine-Threonine Kinases; D051906:Smad7 Protein; D051742:Transcription Factor CHOP; D053773:Transforming Growth Factor beta1; D019892:eIF-2 Kinase; C497806:SMAD7 protein, human