title: Synergistic growth inhibition by 9-cis-retinoic acid plus trastuzumab in human hepatocellular carcinoma cells.
abstract: OBJECTIVE
  A malfunction of retinoid X receptor-alpha (RXRalpha) due to phosphorylation by the Ras/mitogen-activated protein kinase signaling pathway is associated with the development of hepatocellular carcinoma (HCC). The humanized anti-HER2 monoclonal antibody trastuzumab inhibits the activation of HER2 and its multiple downstream signaling pathways, including the Ras/mitogen-activated protein kinase pathway. In this study, the effects of phosphorylation of RXRalpha on the ability of RXRalpha ligand 9-cis-retinoic acid (9cRA) and trastuzumab to inhibit growth of HCC cells was examined.
  
  
  METHODS
  The effects of a combination of 9cRA plus trastuzumab on the inhibition of cell growth in HLF human HCC cells which express constitutive activation of HER2 protein were examined.
  
  
  RESULTS
  The combination of 9cRA plus trastuzumab synergistically inhibited the growth of HLF cells without affecting the growth of Hc normal human hepatocytes. Combined treatment with these agents acted synergistically to induce apoptosis in HLF cells. The treatment of HLF cells with trastuzumab alone inhibited the phosphorylation of HER2, RXRalpha, ERK, Akt, and Stat3 proteins and these effects were enhanced when the cells were cotreated with 9cRA. Reporter assays indicated that the combination of 9cRA plus trastuzumab markedly increased both the retinoic acid responsive element and retinoid X responsive element promoter activities in HLF cells.
  
  
  CONCLUSIONS
  9cRA and trastuzumab cooperatively inhibit the activation of HER2 and its downstream signaling pathways, subsequently inhibiting the phosphorylation of RXRalpha and the growth of HCC cells. This combination might therefore be effective for the chemoprevention and chemotherapy of HCC.
authors.affiliation: Department of Medicine, Gifu University Graduate School of Medicine, Gifu, Japan.
authors.forename: Hideharu
authors.initials: H
authors.lastname: Tatebe
authors.forename: Masahito
authors.initials: M
authors.lastname: Shimizu
authors.forename: Yohei
authors.initials: Y
authors.lastname: Shirakami
authors.forename: Hisashi
authors.initials: H
authors.lastname: Tsurumi
authors.forename: Hisataka
authors.initials: H
authors.lastname: Moriwaki
mesh_terms: D000911:Antibodies, Monoclonal / Q000494:pharmacology*; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects; D006528:Carcinoma, Hepatocellular / Q000378:metabolism / Q000473:pathology*; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D004357:Drug Synergism; D066246:ErbB Receptors / Q000378:metabolism; D022781:Hepatocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D010766:Phosphorylation; D011401:Promoter Regions, Genetic; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000378:metabolism; D020218:Response Elements; D047490:Retinoid X Receptor alpha / Q000378:metabolism; D000068878:Trastuzumab; D014212:Tretinoin / Q000494:pharmacology*
chemical_list: D000911:Antibodies, Monoclonal; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents; D066246:ErbB Receptors; D018719:Erb-b2 Receptor Tyrosine Kinases; D047490:Retinoid X Receptor alpha; D000068878:Trastuzumab; D014212:Tretinoin; C512478:EGFR protein, human