title: Specific chemopreventive agents trigger proteasomal degradation of G1 cyclins: implications for combination therapy.
abstract: OBJECTIVE
  There is a need to identify cancer chemoprevention mechanisms. We reported previously that all-trans-retinoic acid (RA) prevented carcinogenic transformation of BEAS-2B immortalized human bronchial epithelial cells by causing G(1) arrest, permitting repair of genomic DNA damage. G(1) arrest was triggered by cyclin D1 proteolysis via ubiquitin-dependent degradation. This study investigated which chemopreventive agents activated this degradation program and whether cyclin E was also degraded.
  
  
  METHODS
  This study examined whether: (a) cyclin E protein was affected by RA treatment; (b) cyclin degradation occurred in derived BEAS-2B-R1 cells that were partially resistant to RA; and (c) other candidate chemopreventive agents caused cyclin degradation.
  
  
  RESULTS
  RA treatment triggered degradation of cyclin E protein, and ALLN, a proteasomal inhibitor, inhibited this degradation. Induction of the retinoic acid receptor beta, growth suppression, and cyclin degradation were each inhibited in BEAS-2B-R1 cells. Transfection experiments in BEAS-2B cells indicated that RA treatment repressed expression of wild-type cyclin D1 and cyclin E, but ALLN inhibited this degradation. Mutation of threonine 286 stabilized transfected cyclin D1, and mutations of threonines 62 and 380 stabilized transfected cyclin E, despite RA treatment. Specific chemopreventive agents triggered cyclin degradation. Nonclassical retinoids (fenretinide and retinoid X receptor agonists) and a synthetic triterpenoid (2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid) each suppressed BEAS-2B growth and activated this degradation program. However, a vitamin D3 analog (RO-24-5531), a cyclooxygenase inhibitor (indomethacin), and a peroxisome proliferator-activated receptor gamma agonist (rosiglitazone) each suppressed BEAS-2B growth, but did not cause cyclin degradation. BEAS-2B-R1 cells remained responsive to nonclassical retinoids and to 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid.
  
  
  CONCLUSIONS
  Specific chemopreventive agents activate cyclin proteolysis. Yet, broad resistance did not occur after acquired resistance to a single agent. This provides a therapeutic rationale for combination chemoprevention with agents activating non-cross-resistant pathways.
authors.affiliation: Norris Cotton Cancer Center, and Department of Medicine, Dartmouth Medical School, Hanover, New Hampshire 03755, USA.
authors.forename: Konstantin H
authors.initials: KH
authors.lastname: Dragnev
authors.forename: Ian
authors.initials: I
authors.lastname: Pitha-Rowe
authors.forename: Yan
authors.initials: Y
authors.lastname: Ma
authors.forename: W Jeffrey
authors.initials: WJ
authors.lastname: Petty
authors.forename: David
authors.initials: D
authors.lastname: Sekula
authors.forename: Bryan
authors.initials: B
authors.lastname: Murphy
authors.forename: Mara
authors.initials: M
authors.lastname: Rendi
authors.forename: Nanjoo
authors.initials: N
authors.lastname: Suh
authors.forename: Neil B
authors.initials: NB
authors.lastname: Desai
authors.forename: Michael B
authors.initials: MB
authors.lastname: Sporn
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: Freemantle
authors.forename: Ethan
authors.initials: E
authors.lastname: Dmitrovsky
mesh_terms: D016588:Anticarcinogenic Agents / Q000627:therapeutic use*; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000627:therapeutic use*; D001980:Bronchi / Q000166:cytology; D018929:Cell Culture Techniques; D002455:Cell Division; D002460:Cell Line; D019938:Cyclin D1 / Q000096:biosynthesis / Q000378:metabolism; D019927:Cyclin E / Q000378:metabolism; D056746:Cyclin G; D056766:Cyclin G1; D016213:Cyclins / Q000378:metabolism*; D004249:DNA Damage; D004305:Dose-Response Relationship, Drug; D004847:Epithelial Cells / Q000378:metabolism; D016193:G1 Phase; D006801:Humans; D015151:Immunoblotting; D007976:Leupeptins / Q000494:pharmacology; D009154:Mutation; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D061988:Proteasome Inhibitors; D012176:Retinoids / Q000737:chemistry; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013912:Threonine / Q000737:chemistry; D014158:Transcription, Genetic; D014162:Transfection; D014212:Tretinoin / Q000378:metabolism
chemical_list: D016588:Anticarcinogenic Agents; C534702:CCNG1 protein, human; D019927:Cyclin E; D056746:Cyclin G; D056766:Cyclin G1; D016213:Cyclins; D007976:Leupeptins; D061988:Proteasome Inhibitors; D012176:Retinoids; C063261:acetylleucyl-leucyl-norleucinal; D019938:Cyclin D1; D013912:Threonine; D014212:Tretinoin; D046988:Proteasome Endopeptidase Complex