title: Proteasome inhibition as a novel mechanism of the proapoptotic activity of γ-secretase inhibitor I in cutaneous T-cell lymphoma.
abstract: BACKGROUND
  We have previously discovered that Notch1 is expressed on malignant T cells in cutaneous T-cell lymphoma (CTCL), and is required for survival of CTCL cell lines. Notch can be inhibited by γ-secretase inhibitors (GSIs), which differ widely in their ability to induce apoptosis in CTCL.
  
  
  OBJECTIVE
  To investigate whether GSI-I, in addition to inhibiting Notch, induces apoptosis in CTCL by proteasome inhibition, as GSI-I is very potent and has structural similarity to the proteasome inhibitor MG-132.
  
  
  METHODS
  Cell lines derived from CTCL (MyLa, SeAx, JK, Mac1 and Mac2a) were treated with GSI-I and two other proteasome inhibitors (MG-132 and bortezomib). The effects on cell viability, apoptosis and proteasome activity were measured, as was the impact on the prosurvival, nuclear factor κB (NF-κB) pathway.
  
  
  RESULTS
  In CTCL, GSI-I had proteasome-blocking activity with a potency comparable to the proteasome inhibitors MG-132 and bortezomib. Proteasome inhibition was the main mechanism responsible for GSI-I-induced cell death, as tiron, a compound known to reverse the effect of MG-132, restored proteasome activity and largely abrogated the cytotoxic effect of GSI-I. Although inactivation of NF-κB is an important mechanism of action for proteasome inhibitors, we demonstrated an apparent activation of NF-κB. Furthermore, we showed that while the tumour suppressor protein p53 was induced during proteasome inhibition, it was dispensable for CTCL apoptosis, as both SeAx cells, which harbour p53 mutations that attenuate the apoptotic capacity, and HuT-78 cells, which have a deleted p53 gene, demonstrated potent apoptotic response.
  
  
  CONCLUSIONS
  GSI-I represents an interesting drug with a dual mechanism of action comprising inhibition of both Notch and the proteasome.
authors: Department of Dermatology, Bispebjerg Hospital, University of Copenhagen, Bispebjerg Bakke 23, Copenhagen-2400, Denmark.
authors: E
authors: E
authors: Biskup
authors: M R
authors: MR
authors: Kamstrup
authors: V
authors: V
authors: Manfé
authors: R
authors: R
authors: Gniadecki
mesh_terms: D053829:Amyloid Precursor Protein Secretases / Q000037:antagonists & inhibitors*; D000970:Antineoplastic Agents / Q000494:pharmacology; D017209:Apoptosis / Q000187:drug effects*; D001897:Boronic Acids / Q000494:pharmacology; D000069286:Bortezomib; D045744:Cell Line, Tumor; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology; D016410:Lymphoma, T-Cell, Cutaneous / Q000188:drug therapy* / Q000201:enzymology; D016328:NF-kappa B / Q000378:metabolism; D009842:Oligopeptides / Q000494:pharmacology; D046988:Proteasome Endopeptidase Complex / Q000187:drug effects*; D061988:Proteasome Inhibitors / Q000494:pharmacology; D011719:Pyrazines / Q000494:pharmacology; D051881:Receptor, Notch1 / Q000037:antagonists & inhibitors*; D012878:Skin Neoplasms / Q000188:drug therapy* / Q000201:enzymology; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism
chemical_list: D000970:Antineoplastic Agents; D001897:Boronic Acids; D004791:Enzyme Inhibitors; D007976:Leupeptins; D016328:NF-kappa B; C497761:NOTCH1 protein, human; D009842:Oligopeptides; D061988:Proteasome Inhibitors; D011719:Pyrazines; D051881:Receptor, Notch1; D016159:Tumor Suppressor Protein p53; C523462:benzyloxycarbonyl-leucyl-leucyl-norleucinal; D000069286:Bortezomib; D053829:Amyloid Precursor Protein Secretases; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde