Loss of Rho GDI? and resistance to tamoxifen via effects on estrogen receptor ?.

ABSTRACT: Estrogen receptor (ER) ? 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) ? was underexpressed in the tamoxifen-resistant group, we stably transfected ER?-positive MCF-7 breast cancer cells with a plasmid encoding a short hairpin (sh) RNA to silence Rho GDI? expression. We used immunoblots and transcription assays to examine the role of Rho GDI? 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 GDI? 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 GDI? and MTA2 levels with tamoxifen resistance were examined in microarray data from patients. All statistical tests were two-sided.Rho GDI? was expressed at lower levels in ER?-positive tumors that recurred during tamoxifen treatment than in ER?-positive tamoxifen-sensitive primary tumors. MCF-7 breast cancer cells in which Rho GDI? expression had been silenced were tamoxifen-resistant, had increased Rho GTPase and p21-activated kinase 1 activity, increased phosphorylation of ER? at serine 305, and enhanced tamoxifen-induced ER? transcriptional activity compared with control cells. MCF-7 cells in which Rho GDI? 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 GDI? 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 GDI? 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 GDI? silencing, and combined Rho GDI? and MTA2 levels were associated with recurrence in 250 tamoxifen-treated patients.Loss of Rho GDI? enhances metastasis and resistance to tamoxifen via effects on both ER? and MTA2 in models of ER?-positive breast cancer and in tumors of tamoxifen-treated patients.

SUBMITTER: Barone I 

PROVIDER: S-EPMC3071355 | biostudies-literature | 2011 Apr

REPOSITORIES: biostudies-literature