Loss of Rho GDIα and Resistance to Tamoxifen via Effects on Estrogen Receptor α

• Published in: JNCI : Journal of the National Cancer Institute 2011-04, Vol.103 (7), p.538-552
• Main Authors: BARONE, Ines, BRUSCO, Lauren, ANDO, Sebastiano, FUQUA, Suzanne A. W, GUOWEI GU, SELEVER, Jennifer, BEYER, Amanda, COVINGTON, Kyle R, TSIMELZON, Anna, TAO WANG, HILSENBECK, Susan G, CHAMNESS, Gary C
• Format: Article
• Language: English
• Subjects: Animals; Antineoplastic agents; Antineoplastic Agents, Hormonal - pharmacology; Antineoplastic Agents, Hormonal - therapeutic use; Biological and medical sciences; Breast Neoplasms - metabolism; Breast Neoplasms - prevention & control; Cell Line, Tumor; Chemotherapy; Down-Regulation; Drug Resistance, Neoplasm - drug effects; Drug Resistance, Neoplasm - genetics; Enzyme Activation; Estrogen Antagonists - pharmacology; Estrogen Antagonists - therapeutic use; Estrogen Receptor alpha - drug effects; Estrogen Receptor alpha - metabolism; Female; Gene Expression Regulation, Neoplastic; Gene Silencing; Genome-Wide Association Study; Guanine Nucleotide Dissociation Inhibitors - genetics; Guanine Nucleotide Dissociation Inhibitors - metabolism; Histone Deacetylases - genetics; Histone Deacetylases - metabolism; Humans; Immunoblotting; Immunohistochemistry; Immunoprecipitation; Medical sciences; Mice; Mice, Nude; Neoplasm Recurrence, Local - metabolism; Neoplasm Recurrence, Local - prevention & control; Odds Ratio; Pharmacology. Drug treatments; Phenotype; Plasmids; Protein Array Analysis; Random Allocation; Repressor Proteins - genetics; Repressor Proteins - metabolism; Retrospective Studies; rho GTP-Binding Proteins - metabolism; rho Guanine Nucleotide Dissociation Inhibitor alpha; rho-Specific Guanine Nucleotide Dissociation Inhibitors; RNA, Small Interfering - metabolism; Secondary Prevention - methods; Selective Estrogen Receptor Modulators - pharmacology; Signal Transduction - drug effects; Signal Transduction - genetics; Tamoxifen - pharmacology; Tamoxifen - therapeutic use; Time Factors; Transcriptional Activation; Transplantation, Heterologous; Tumor Stem Cell Assay; Tumors
• ISSN: 0027-8874; 1460-2105
• DOI: 10.1093/jnci/djr058

Estrogen receptor (ER) α is a successful therapeutic target in breast cancer, but patients eventually develop resistance to antiestrogens such as tamoxifen. 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. 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