title: Loss of Rho GDIα and resistance to tamoxifen via effects on estrogen receptor α.
abstract: BACKGROUND
  Estrogen receptor (ER) α is a successful therapeutic target in breast cancer, but patients eventually develop resistance to antiestrogens such as tamoxifen.
  
  
  METHODS
  To identify genes whose expression was associated with the development of tamoxifen resistance and metastasis, we used microarrays to compare gene expression in four primary tumors from tamoxifen-treated patients whose breast cancers did not recur vs five metastatic tumors from patients whose cancers progressed during adjuvant tamoxifen treatment. Because Rho guanine dissociation inhibitor (GDI) α was underexpressed in the tamoxifen-resistant group, we stably transfected ERα-positive MCF-7 breast cancer cells with a plasmid encoding a short hairpin (sh) RNA to silence Rho GDIα expression. We used immunoblots and transcription assays to examine the role of Rho GDIα in ER-related signaling and growth of cells in vitro and as xenografts in treated nude mice (n = 8-9 per group) to examine the effects of Rho GDIα blockade on hormone responsiveness and metastatic behavior. The time to tumor tripling as the time in weeks from randomization to a threefold increase in total tumor volume over baseline was examined in treated mice. The associations of Rho GDIα and MTA2 levels with tamoxifen resistance were examined in microarray data from patients. All statistical tests were two-sided.
  
  
  RESULTS
  Rho GDIα was expressed at lower levels in ERα-positive tumors that recurred during tamoxifen treatment than in ERα-positive tamoxifen-sensitive primary tumors. MCF-7 breast cancer cells in which Rho GDIα expression had been silenced were tamoxifen-resistant, had increased Rho GTPase and p21-activated kinase 1 activity, increased phosphorylation of ERα at serine 305, and enhanced tamoxifen-induced ERα transcriptional activity compared with control cells. MCF-7 cells in which Rho GDIα expression was silenced metastasized with high frequency when grown as tumor xenografts. When mice were treated with estrogen or estrogen withdrawal, tripling times for xenografts from cells with Rho GDIα silencing were similar to those from vector-containing control cells; however, tripling times were statistically significantly faster than control when mice were treated with tamoxifen (median tripling time for tumors with Rho GDIα small interfering RNA = 2.34 weeks; for control tumors = not reached, hazard ratio = 4.13, 95% confidence interval = 1.07 to 15.96, P = .040 [adjusted for multiple comparisons, P = .119]). Levels of the metastasis-associated protein MTA2 were also increased upon Rho GDIα silencing, and combined Rho GDIα and MTA2 levels were associated with recurrence in 250 tamoxifen-treated patients.
  
  
  CONCLUSIONS
  Loss of Rho GDIα enhances metastasis and resistance to tamoxifen via effects on both ERα and MTA2 in models of ERα-positive breast cancer and in tumors of tamoxifen-treated patients.
authors: Lester and Sue Smith Breast Center, Breast Center, Baylor College of Medicine, Houston, TX 77479, USA.
authors: Ines
authors: I
authors: Barone
authors: Lauren
authors: L
authors: Brusco
authors: Guowei
authors: G
authors: Gu
authors: Jennifer
authors: J
authors: Selever
authors: Amanda
authors: A
authors: Beyer
authors: Kyle R
authors: KR
authors: Covington
authors: Anna
authors: A
authors: Tsimelzon
authors: Tao
authors: T
authors: Wang
authors: Susan G
authors: SG
authors: Hilsenbeck
authors: Gary C
authors: GC
authors: Chamness
authors: Sebastiano
authors: S
authors: Andò
authors: Suzanne A W
authors: SA
authors: Fuqua
mesh_terms: D000818:Animals; D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology* / Q000627:therapeutic use; D001943:Breast Neoplasms / Q000378:metabolism* / Q000517:prevention & control*; D045744:Cell Line, Tumor; D015536:Down-Regulation; D019008:Drug Resistance, Neoplasm / Q000187:drug effects / Q000235:genetics; D004789:Enzyme Activation; D004965:Estrogen Antagonists / Q000494:pharmacology* / Q000627:therapeutic use; D047628:Estrogen Receptor alpha / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D020868:Gene Silencing; D055106:Genome-Wide Association Study; D020730:Guanine Nucleotide Dissociation Inhibitors / Q000235:genetics / Q000378:metabolism*; D006655:Histone Deacetylases / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D015151:Immunoblotting; D007150:Immunohistochemistry; D047468:Immunoprecipitation; D051379:Mice; D008819:Mice, Nude; D009364:Neoplasm Recurrence, Local / Q000378:metabolism / Q000517:prevention & control*; D016017:Odds Ratio; D010641:Phenotype; D010957:Plasmids; D040081:Protein Array Analysis; D034741:RNA, Small Interfering / Q000378:metabolism; D011897:Random Allocation; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism*; D012189:Retrospective Studies; D055502:Secondary Prevention / Q000379:methods; D020845:Selective Estrogen Receptor Modulators / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics; D013629:Tamoxifen / Q000494:pharmacology* / Q000627:therapeutic use; D013997:Time Factors; D015533:Transcriptional Activation; D014183:Transplantation, Heterologous; D014410:Tumor Stem Cell Assay; D020741:rho GTP-Binding Proteins / Q000378:metabolism; D062948:rho Guanine Nucleotide Dissociation Inhibitor alpha; D062947:rho-Specific Guanine Nucleotide Dissociation Inhibitors
chemical_list: D018931:Antineoplastic Agents, Hormonal; C569639:Arhgdia protein, mouse; D004965:Estrogen Antagonists; D047628:Estrogen Receptor alpha; D020730:Guanine Nucleotide Dissociation Inhibitors; D034741:RNA, Small Interfering; D012097:Repressor Proteins; D020845:Selective Estrogen Receptor Modulators; D062948:rho Guanine Nucleotide Dissociation Inhibitor alpha; D062947:rho-Specific Guanine Nucleotide Dissociation Inhibitors; D013629:Tamoxifen; C120917:MTA2 protein, human; D006655:Histone Deacetylases; D020741:rho GTP-Binding Proteins