title: Notch prevents transforming growth factor-beta-assisted epithelial-mesenchymal transition in cultured limbal progenitor cells through the induction of Smad7.
abstract: OBJECTIVE
  Continuous culture of limbal epithelial stem cells (LSCs) slows down proliferation, which inevitably results in differentiation. Transforming growth factor-beta (TGFβ)-assisted epithelial-mesenchymal transition (EMT) is often found in the late stage of LSC culture. Thus, EMT is proposed to be part of the mechanism responsible for the loss of LSCs in culture. To explore the regulation mechanism of EMT, we investigated the early stage culture for factor(s) that may potentially prevent EMT.
  
  
  METHODS
  LSCs from the corneal limbus region of rabbits were isolated and expanded to confluence in culture (P0), and then serial passage of these LSCs (P1 to P3) was performed. EMT in LSCs was induced with TGFβ1, and the corresponding EMT signaling was confirmed with Smad2/3 phosphorylation. The expression of mesenchymal markers, including alpha-smooth muscle actin (α-SMA) and vimentin, was determined with western blot analysis. Proteins extracted from different passaged cells were also subjected to western blot analysis of TGFβ signaling components, including TGFβ1, TGFβ receptor I/II, and Smad2/3 as well as Smad7, the main negative regulator of TGFβ signaling. The mitogenic response was measured with the bromodeoxyuridine (BrdU) labeling index and real-time PCR using primers for Ki67. N-(N-[3,5-difluorophenacetyl]-l-alanyl)-S-phenylglycine t-butyl ester (DAPT), a gamma-secretase inhibitor, and Jagged-1 Notch ligand were used to block and activate Notch signaling, respectively, and their efficacy was evaluated by determining the expression of Hes1, a Notch signaling target.
  
  
  RESULTS
  Mesenchymal marker induction and growth arrest were found in the TGFβ1-treated P1 cells, and the changes were less significant in the TGFβ1-treated P0 cells. Western blot analysis confirmed that the expressed levels of TGFβ signaling components, including TGFβ1, TGFβ receptor I/II, and Smad2/3, were relatively stable with passages. In contrast, the expression of Hes1 and Smad7 markedly decreased after the first passage, and with each passage, the levels diminished even further. Hes1 and Smad7 were expressed only in the limbal epithelium and not in the corneal epithelium. DAPT effectively blocked the expression of Hes1. DAPT also dose-dependently suppressed Smad7 expression in P0 cells, which was associated with the susceptibility of P0 cells to TGFβ1-induced Smad2/3 phosphorylation, EMT formation, and growth arrest. Reciprocally, Jagged-1 upregulated Smad7 expression in LSCs against TGFβ signaling.
  
  
  CONCLUSIONS
  These findings indicate that Smad7 plays a crucial role in antagonizing EMT induced by TGFβ signaling and support our proposition that Smad7 is a Notch signaling target in LSCs, and may mediate the Notch function in preventing the occurrence of EMT.
authors.affiliation: Department of Neurosurgery, Tri-Service General Hospital, National Defense Medical Center, Taipei, Taiwan.
authors.forename: Tung-Han
authors.initials: TH
authors.lastname: Tsai
authors.affiliation: Department of Ophthalmology, Chang Gung Memorial Hospital, Chang Gung University, Taoyuan, Taiwan.
authors.forename: Ming-Hui
authors.initials: MH
authors.lastname: Sun
authors.affiliation: Department of Medical Research, Mackay Memorial Hospital, Taipei, Taiwan.
authors.forename: Tsung-Chuan
authors.initials: TC
authors.lastname: Ho
authors.affiliation: Department of Neurosurgery, Tri-Service General Hospital, National Defense Medical Center, Taipei, Taiwan.
authors.forename: Hsin-I
authors.initials: HI
authors.lastname: Ma
authors.affiliation: Department of Neurosurgery, Tri-Service General Hospital, National Defense Medical Center, Taipei, Taiwan.
authors.forename: Ming-Ying
authors.initials: MY
authors.lastname: Liu
authors.affiliation: Department of Medical Research, Mackay Memorial Hospital, Taipei, Taiwan ; Department of Ophthalmology, Mackay Memorial Hospital, Taipei, Taiwan.
authors.forename: Yeou-Ping
authors.initials: YP
authors.lastname: Tsao
mesh_terms: D000818:Animals; D015415:Biomarkers / Q000378:metabolism; D049109:Cell Proliferation / Q000187:drug effects; D002478:Cells, Cultured; D004847:Epithelial Cells / Q000187:drug effects / Q000378:metabolism; D058750:Epithelial-Mesenchymal Transition / Q000187:drug effects; D018398:Homeodomain Proteins / Q000378:metabolism; D016850:Limbus Corneae / Q000166:cytology*; D051379:Mice; D041681:NIH 3T3 Cells; D011817:Rabbits; D051381:Rats; D051880:Receptors, Notch / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D051906:Smad7 Protein / Q000378:metabolism*; D013234:Stem Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D016212:Transforming Growth Factor beta / Q000378:metabolism* / Q000494:pharmacology; D015854:Up-Regulation / Q000187:drug effects
chemical_list: D015415:Biomarkers; D018398:Homeodomain Proteins; D051880:Receptors, Notch; D051906:Smad7 Protein; D016212:Transforming Growth Factor beta