RhoB is regulated by hypoxia and modulates metastasis in breast cancer

BACKGROUND

RhoB is a Rho family GTPase that is highly homologous to RhoA and RhoC. RhoA and RhoC have been shown to promote tumor progression in many cancer types; however, a distinct role for RhoB in cancer has not been delineated. Additionally, several well-characterized studies have shown that small GTPases such as RhoA, Rac1, and Cdc42 are induced in vitro under hypoxia, but whether and how hypoxia regulates RhoB in breast cancer remains elusive.

AIMS

To determine whether and how hypoxia regulates RhoB expression and to understand the role of RhoB in breast cancer metastasis.

METHODS

We investigated the effects of hypoxia on the expression and activation of RhoB using real-time quantitative polymerase chain reaction and western blotting. We also examined the significance of both decreased and increased RhoB expression in breast cancer using CRISPR depletion of RhoB or a vector overexpressing RhoB in 3D in vitro migration models and in an in vivo mouse model.

RESULTS

We found that hypoxia significantly upregulated RhoB mRNA and protein expression resulting in increased levels of activated RhoB. Both loss of RhoB and gain of RhoB expression led to reduced migration in a 3D collagen matrix and invasion within a multicellular 3D spheroid. We showed that neither the reduction nor overexpression of RhoB affected tumor growth in vivo. While the loss of RhoB had no effect on metastasis, RhoB overexpression led to decreased metastasis to the lungs, liver, and lymph nodes of mice.

CONCLUSION

Our results suggest that RhoB may have an important role in suppressing breast cancer metastasis.

RhoB controls endothelial barrier recovery by inhibiting Rac1 trafficking to the cell border

Endothelial barrier dysfunction underlies chronic inflammatory diseases. In searching for new proteins essential to the human endothelial inflammatory response, we have found that the endosomal GTPase RhoB is up-regulated in response to inflammatory cytokines and expressed in the endothelium of some chronically inflamed tissues. We show that although RhoB and the related RhoA and RhoC play additive and redundant roles in various aspects of endothelial barrier function, RhoB specifically inhibits barrier restoration after acute cell contraction by preventing plasma membrane extension. During barrier restoration, RhoB trafficking is induced between vesicles containing RhoB nanoclusters and plasma membrane protrusions. The Rho GTPase Rac1 controls membrane spreading and stabilizes endothelial barriers. We show that RhoB colocalizes with Rac1 in endosomes and inhibits Rac1 activity and trafficking to the cell border during barrier recovery. Inhibition of endosomal trafficking impairs barrier reformation, whereas induction of Rac1 translocation to the plasma membrane accelerates it. Therefore, RhoB-specific regulation of Rac1 trafficking controls endothelial barrier integrity during inflammation.

Genetic response to DNA damage: Proapoptotic targets of RhoB include modules for p53 response and susceptibility to alzheimer’s disease

Knockout mouse studies indicate that the small GTPase RhoB is critical for apoptosis triggered by genotoxic stress in transformed mouse cells. However, the mechanisms used by RhoB to sensitize cells to cell death are obscure. To gain insight into this question, we compared the genetic response of cells with different rhoB genotypes to the DNA damaging anticancer drug doxorubicin (DOX). The microarray hybridization strategy focused on events occurring by 6 hr of DOX treatment, preceding the execution phase of RhoB-dependent apoptosis by 12-16 hr. Genes controlling cytoskeletal organization, adhesion, transcription, trafficking, apoptosis, and protein turnover were represented prominently. Gene clustering revealed a module of p53 target genes, suggesting that RhoB may modify the p53 response, and a module for susceptibility to Alzheimer's disease, suggesting a link between RhoB and age-associated dementia. The findings of this study suggest mechanisms by which RhoB may act to elevate the sensitivity of cells to apoptosis following genotoxic stress.

Rho-kinase: regulation, (dys)function, and inhibition

In a variety of normal and pathological cell types, Rho-kinases I and II (ROCKI/II) play a pivotal role in the organization of the nonmuscle and smooth muscle cytoskeleton and adhesion plaques as well as in the regulation of transcription factors. Thus, ROCKI/II activity regulates cellular contraction, motility, morphology, polarity, cell division, and gene expression. Emerging evidence suggests that dysregulation of the Rho-ROCK pathways at different stages is linked to cardiovascular, metabolic, and neurodegenerative diseases as well as cancer. This review focuses on the current status of understanding the multiple functions of Rho-ROCK signaling pathways and various modes of regulation of Rho-ROCK activity, thereby orchestrating a concerted functional response.

GTPases Rho distribution in intraepithelial and invasive neoplasias of the uterine cervix

PURPOSE OF INVESTIGATION

To evaluate the distribution of GTPases RhoA, RhoB, and Cdc42 in cervical intraepithelial neoplasias (CIN) and invasive neoplasias of the uterine cervix.

MATERIALS AND METHODS

samples of neoplastic lesions of the uterine cervix of 44 patients were classified in: CIN I (n = 10), CIN II (n = 10), CIN III (n = 09), and invasive carcinoma (n = 15). Antibodies anti-RhoA, anti-RhoB, and anti-Cdc42 were used and staining was classified as: negative, mild, moderate, and intense positive.

RESULTS

When compared with dysplastic cells, superficial cells showed: higher expression of RhoB in CIN I (p = 0.0018), and lower expression of Cdc42 in CIN I (p = 0.0225). The authors observed higher expression of RhoA (p = 0.0002) and RhoB (p = 0.0046) in CIN dysplastic cells when compared with invasive carcinoma cells.

CONCLUSIONS

GTPases Rho may be involved with the regulation of biological processes, important to the progression of cervical neoplasias. Probably, RhoA is important for maintenance of cell differentiation and RhoB protects cells from malignant cervical neoplasia.

The nuclear guanine nucleotide exchange factors Ect2 and Net1 regulate RhoB-mediated cell death after DNA damage

Commonly used antitumor treatments, including radiation and chemotherapy, function by damaging the DNA of rapidly proliferating cells. However, resistance to these agents is a predominant clinical problem. A member of the Rho family of small GTPases, RhoB has been shown to be integral in mediating cell death after ionizing radiation (IR) or other DNA damaging agents in Ras-transformed cell lines. In addition, RhoB protein expression increases after genotoxic stress, and loss of RhoB expression causes radio- and chemotherapeutic resistance. However, the signaling pathways that govern RhoB-induced cell death after DNA damage remain enigmatic. Here, we show that RhoB activity increases in human breast and cervical cancer cell lines after treatment with DNA damaging agents. Furthermore, RhoB activity is necessary for DNA damage-induced cell death, as the stable loss of RhoB protein expression using shRNA partially protects cells and prevents the phosphorylation of c-Jun N-terminal kinases (JNKs) and the induction of the pro-apoptotic protein Bim after IR. The increase in RhoB activity after genotoxic stress is associated with increased activity of the nuclear guanine nucleotide exchange factors (GEFs), Ect2 and Net1, but not the cytoplasmic GEFs p115 RhoGEF or Vav2. Importantly, loss of Ect2 and Net1 via siRNA-mediated protein knock-down inhibited IR-induced increases in RhoB activity, reduced apoptotic signaling events, and protected cells from IR-induced cell death. Collectively, these data suggest a mechanism involving the nuclear GEFs Ect2 and Net1 for activating RhoB after genotoxic stress, thereby facilitating cell death after treatment with DNA damaging agents.

Loss of RhoB expression enhances the myelodysplastic phenotype of mammalian diaphanous-related Formin mDia1 knockout mice

Myelodysplastic syndrome (MDS) is characterized by ineffective hematopoiesis and hyperplastic bone marrow. Complete loss or interstitial deletions of the long arm of chromosome 5 occur frequently in MDS. One candidate tumor suppressor on 5q is the mammalian Diaphanous (mDia)-related formin mDia1, encoded by DIAPH1 (5q31.3). mDia-family formins act as effectors for Rho-family small GTP-binding proteins including RhoB, which has also been shown to possess tumor suppressor activity. Mice lacking the Drf1 gene that encodes mDia1 develop age-dependent myelodysplastic features. We crossed mDia1 and RhoB knockout mice to test whether the additional loss of RhoB expression would compound the myelodysplastic phenotype. Drf1^−/−RhoB^−/− mice are fertile and develop normally. Relative to age-matched Drf1^−/−RhoB^+/− mice, the age of myelodysplasia onset was earlier in Drf1^−/−RhoB^−/− animals—including abnormally shaped erythrocytes, splenomegaly, and extramedullary hematopoiesis. In addition, we observed a statistically significant increase in the number of activated monocytes/macrophages in both the spleen and bone marrow of Drf1^−/−RhoB^−/− mice relative to Drf1^−/−RhoB^+/− mice. These data suggest a role for RhoB-regulated mDia1 in the regulation of hematopoietic progenitor cells.

RhoB-dependent modulation of postendocytic traffic in polarized Madin-Darby canine kidney cells

The Rho family of GTPases is implicated in the control of endocytic and biosynthetic traffic of many cell types; however, the cellular distribution of RhoB remains controversial and its function is not well understood. Using confocal microscopy, we found that endogenous RhoB and green fluorescent protein-tagged wild-type RhoB were localized to early endosomes, and to a much lesser extent to recycling endosomes, late endosomes or Golgi complex of fixed or live polarized Madin-Darby canine kidney cells. Consistent with RhoB localization to early endosomes, we observed that expression of dominant-negative RhoBN19 or dominant-active RhoBV14 altered postendocytic traffic of ligand-receptor complexes that undergo recycling, degradation or transcytosis. In vitro assays established that RhoB modulated the basolateral-to-apical transcytotic pathway by regulating cargo exit from basolateral early endosomes. Our results indicate that RhoB is localized, in part, to early endosomes where it regulates receptor egress through the early endocytic system.

RhoB plays an essential role in CXCR2 sorting decisions

The CXCR2 chemokine receptor is a G-protein-coupled receptor that undergoes clathrin-mediated endocytosis upon ligand binding. The trafficking of CXCR2 is crucial for cells to maintain a proper chemotactic response. The mechanisms that regulate the recycling/degradation sorting decision are unknown. In this study, we used dominant-negative (T19N) and GTPase-deficient activated (Q63L) RhoB mutants, as well as RhoB small interfering RNA (siRNA) to investigate the role of RhoB in CXCR2 trafficking. Expression of either of the RhoB mutants or transfection of RhoB siRNA impaired CXCR2-mediated chemotaxis. Expression of RhoB T19N and transfection of RhoB siRNA impaired sorting of CXCR2 to the lysosome after 3 hours of CXCL8 stimulation and impaired CXCL8-induced CXCR2 degradation. In cells expressing the RhoB Q63L mutant, CXCR2 recycling through the Rab11a recycling compartment was impaired after 30 minutes of CXCL8 stimulation as was CXCL8-induced CXCR2 degradation. For cells expressing activated RhoB, CXCR2 colocalized with Rab4, a marker for the rapid recycling pathway, and with the mannose-6-phosphate receptor, which traffics between the trans-Golgi network and endosomes. These data suggest that CXCR2 recycles through alternative pathways. We conclude that oscillation of RhoB GTPase activity is essential for appropriate sorting decisions, and for directing CXCR2 degradation and recycling--events that are required for optimal chemotaxis.

Modulation pathways of NCX mRNA stability: involvement of RhoB

Cardiac Na+/Ca2+ exchanger 1 (NCX1) expression levels change under various pathophysiological conditions. However, its mechanism is unknown. We found that fluvastatin, an HMG-CoA reductase inhibitor, decreased NCX1 mRNA and protein by inhibiting a small G protein, RhoB, in H9c2 cardiomyoblasts. Conversely, lysophosphatidylcholine (LPC) increased NCX1 mRNA and protein by activating RhoB. The effect of LPC was mediated by geranylgeranylation but not farnesylation of RhoB. Furthermore, we also detected that activation of RhoB increased NCX1 mRNA stability. Our results suggest that RhoB is involved in modulation of cardiac NCX1 mRNA expression.

Transcriptomal profiling of the cellular transformation induced by Rho subfamily GTPases

We have used microarray technology to identify the transcriptional targets of Rho subfamily guanosine 5'-triphosphate (GTP)ases in NIH3T3 cells. This analysis indicated that murine fibroblasts transformed by these proteins show similar transcriptomal profiles. Functional annotation of the regulated genes indicate that Rho subfamily GTPases target a wide spectrum of functions, although loci encoding proteins linked to proliferation and DNA synthesis/transcription are upregulated preferentially. Rho proteins promote four main networks of interacting proteins nucleated around E2F, c-Jun, c-Myc and p53. Of those, E2F, c-Jun and c-Myc are essential for the maintenance of cell transformation. Inhibition of Rock, one of the main Rho GTPase targets, leads to small changes in the transcriptome of Rho-transformed cells. Rock inhibition decreases c-myc gene expression without affecting the E2F and c-Jun pathways. Loss-of-function studies demonstrate that c-Myc is important for the blockage of cell-contact inhibition rather than for promoting the proliferation of Rho-transformed cells. However, c-Myc overexpression does not bypass the inhibition of cell transformation induced by Rock blockage, indicating that c-Myc is essential, but not sufficient, for Rock-dependent transformation. These results reveal the complexity of the genetic program orchestrated by the Rho subfamily and pinpoint protein networks that mediate different aspects of the malignant phenotype of Rho-transformed cells.

TRIF-GEFH1-RhoB pathway is involved in MHCII expression on dendritic cells that is critical for CD4 T-cell activation

Dendritic cells (DC) play a central role in immune responses by presenting antigenic peptides to CD4+ T cells through MHCII molecules. Here, we demonstrate a TRIF-GEFH1-RhoB pathway is involved in MHCII surface expression on DC. We show the TRIF (TIR domain-containing adapter inducing IFNbeta)- but not the myeloid differentiation factor 88 (MyD88)-dependent pathway of lipopolysaccharide (LPS)-signaling in DC is crucial for the MHCII surface expression, followed by CD4+ T-cell activation. LPS increased the activity of RhoB, but not of RhoA, Cdc42, or Rac1/2 in a manor dependent on LPS-TRIF- but not LPS-Myd88-signaling. RhoB colocalized with MHCII+ lysosomes in DC. A dominant-negative (DN) form of RhoB (DN-RhoB) or RhoB's RNAi in DC inhibited the LPS-induced MHCII surface expression. Moreover, we found GEFH1 associated with RhoB, and DN-GEFH1 or GEFH1's RNAi suppressed the LPS-mediated RhoB activation and MHCII surface expression. DN-RhoB attenuated the DC's CD4+ T-cell stimulatory activity. Thus, our results provide a molecular mechanism relating how the MHCII surface expression is regulated during the maturation stage of DC. The activation of GEFH1-RhoB through the TRIF-dependent pathway of LPS in DC might be a critical target for controlling the activation of CD4+ T cells.

RhoB in cancer suppression

RhoB is a mainly endosomal small GTPase that regulates actin organization and vesicle trafficking. Expression of RhoB is elevated rapidly by many stimuli, including growth factors, cytokines, and genotoxic stress. In cancer, RhoB can limit cell proliferation, survival, invasion, and metastasis, and during malignant progression its levels are attenuated commonly. In support of its role as a negative modifier of cancer progression, targeted deletion of RhoB in mice can increase tumor formation initiated by Ras mutation. How RhoB acts to suppress different aspects of cancer pathophysiology has emerged as a question of significant interest.

RhoB protects human keratinocytes from UVB-induced apoptosis through epidermal growth factor receptor signaling

Exposure of the skin to UVB light results in the formation of DNA photolesions that can give rise to cell death, mutations, and the onset of carcinogenic events. Specific proteins are activated by UVB and then trigger signal transduction pathways that lead to cellular responses. An alteration of these signaling molecules is thought to be a fundamental event in tumor promotion by UVB irradiation. RhoB, encoding a small GTPase has been identified as a DNA damage-inducible gene. RhoB is involved in epidermal growth factor (EGF) receptor trafficking, cytoskeletal organization, cell transformation, and survival. We have analyzed the regulation of RhoB and elucidated its role in the cellular response of HaCaT keratinocytes to relevant environmental UVB irradiation. We report here that the activated GTP-bound form of RhoB is increased rapidly within 5 min of exposure to UVB, and then RhoB protein levels increased concomitantly with EGF receptor (EGFR) activation. Inhibition of UVB-induced EGFR activation prevents RhoB protein expression and AKT phosphorylation but not the early activation of RhoB. Blocking UVB-induced RhoB expression with specific small interfering RNAs inhibits AKT and glycogen synthase kinase-3beta phosphorylation through inhibition of EGFR expression. Moreover, down-regulation of RhoB potentiates UVB-induced cell apoptosis. In contrast, RhoB overexpression protects keratinocytes against UVB-induced apoptosis. These results indicated that RhoB is regulated upon UVB exposure by a two-step process consisting of an early EGFR-independent RhoB activation followed by an EGFR-dependent induction of RhoB expression. Moreover, we have demonstrated that RhoB is essential in regulating keratinocyte cell survival after UVB exposure, suggesting its potential role in photocarcinogenesis.

RhoB facilitates c-Myc turnover by supporting efficient nuclear accumulation of GSK-3

The small GTPase RhoB suppresses cancer in part by limiting cell proliferation. However, the mechanisms it uses to achieve this are poorly understood. Recent studies link RhoB to trafficking of Akt, which through its regulation of glycogen synthase kinase-3 (GSK-3) has an important role in controlling the stability of the c-Myc oncoprotein. c-Myc stabilization may be a root feature of human tumorigenesis as it phenocopies an essential contribution of SV40 small T antigen in human cell transformation. In this study we show that RhoB directs efficient turnover of c-Myc in established or transformed mouse fibroblasts and that the attenuation of RhoB which occurs commonly in human cancer is a sufficient cause to elevate c-Myc levels. Increased levels of c-Myc elicited by RhoB deletion increased the proliferation of nullizygous cells, whereas restoring RhoB in null cells decreased the stability of c-Myc and restrained cell proliferation. Mechanistic analyses indicated that RhoB facilitated nuclear accumulation of GSK-3 and GSK-3-mediated phosphorylation of c-Myc T58, the critical site for ubiquitination and degradation of c-Myc. RhoB deletion restricted nuclear localization of GSK-3, reduced T58 phosphorylation, and stabilized c-Myc. These effects were not associated with changes in phosphorylation or localization of Akt, however, differences were observed in phosphorylation and localization of the GSK-3 regulatory Akt-related kinase, serum- and glucocorticoid-inducible protein kinase (SGK). The ability of RhoB to support GSK-3-dependent turnover of c-Myc offers a mechanism by which RhoB acts to limit the proliferation of neoplastically transformed cells.

The association between RhoB and caspase-2: changes with lovastatin-induced apoptosis

Because cytoskeletal actin is regulated, in part, by Rho, and because Rho and caspases are involved in apoptosis, we sought to determine whether there was an association between RhoB and caspase-2. A RhoB–caspase-2 association was consistently demonstrated in neonatal mouse cardiomyocytes with Western Blotting, either after im mun o precipitation with RhoB followed by immunoblotting with caspase-2, or in reciprocal experiments after immuno precipitation with caspase-2 and immunoblotting with RhoB (n = 14). Although the RhoB–caspase-2 complex was constitutively present, the link between RhoB and caspase-2 may be operative in apoptosis because the HMG-CoA reductase inhibitor lovastatin increased the RhoB–caspase complex, especially in the nuclear fraction of the cell, with a peak occurrence 2 h after treatment. This association was unaffected by the caspase-2 inhibitor zVDVAD. Lovastatin produced apoptosis that was accompanied by an activation of caspase-2, as demonstrated by its immunohistochemistry and by the fact that the caspase-2 inhibitor zVDVAD reduced lovastatin-induced apoptosis. Lovastatin induced dramatic changes in cell morphology and a reduction in F-actin. Immunoblotting for actin suggests that lovastatin does not induce a degradation of the actin molecule, but rather affects filamentous F-actin. Caspase-2 inhibition with zVDVAD reduced lovastatin-induced alteration in cytoskeletal F-actin. The Rho inhibitor, Clostridium difficile toxin B, blunted the ability of lovastatin to induce apoptosis. In summary, these data show a previously unrecognized association between RhoB and caspase-2 in the cytosolic and nuclear fractions, which has ramifications for processes regulated by RhoB and caspase-2, including apoptosis.Key words: actin, apoptosis, caspase-2, cardiomyocyte, heart, lovastatin.

Actin polymerization in the equatorial and postacrosomal regions of guinea pig spermatozoa during the acrosome reaction is regulated by G proteins

The acrosome reaction (AR) is an exocytotic process of spermatozoa, and an absolute requirement for fertilization. During AR, actin polymerization is necessary in the equatorial and postacrosomal regions of guinea pig sperm for spermatozoa incorporation deep into the egg cytoplasm, but not for plasma membrane (PM) fusion nor the early steps of egg activation. To identify the mechanisms involved in this sperm actin polymerization, we searched for the protein members, known to be involved in a highly conserved model, that may apply to any cellular process in which de novo actin polymerization occurs from G protein activation. WASP, Arp 2/3, profilins I and II, and Cdc42, RhoA and RhoB GTPases were localized by indirect immunofluorescence (IIF) in guinea pig spermatozoa and their presence corroborated by Western blotting. WASP and profilin II were translocated to the postacrosomal region (Arp2/3 already were there) in long-term capacitated and acrosome-reacted spermatozoa, at the same time as actin polymerization occurred. These events were inhibited by GDP-beta-S and promoted by lysophosphatidic acid (LPA) and GTP-gamma-S, a small GTPase inhibitor and two activators, respectively. By immunoprecipitation, Cdc42-WASp association was identified in capacitated but not in noncapacitated gametes. Polymerized actin in the postacrosomal region is apparently anchored both to the postacrosomal perinuclear theca region and the overlying PM. Results suggest that GTPases are involved in sperm actin polymerization, in the postacrosomal region and the mechanism for polymerization might fit a previously proposed model (Mullins, 2000: Curr Opin Cell Biol 12:91-96).

The transcriptional control of trunk neural crest induction, survival, and delamination

Trunk neural crest cells are generated at the border between the neural plate and nonneural ectoderm, where they initiate a distinct program of gene expression, undergo an epithelial-mesenchymal transition (EMT), and delaminate from the neuroepithelium. Here, we provide evidence that members of three families of transcription induce these properties in premigratory neural crest cells. Sox9 acts to provide the competence for neural crest cells to undergo an EMT and is required for trunk neural crest survival. In the absence of Sox9, cells apoptose prior to or shortly after delamination. Slug/Snail, in the presence of Sox9, is sufficient to induce an EMT in neural epithelial cells, while FoxD3 regulates the expression of cell-cell adhesion molecules required for neural crest migration. Together, the data suggest a model in which a combination of transcription factors regulates the acquisition of the diverse properties of neural crest cells.

Palmitoylated cysteine 192 is required for RhoB tumor-suppressive and apoptotic activities

RhoA and RhoB share 86% amino acid sequence identity, yet RhoA promotes whereas RhoB suppresses malignant transformation. Amino acids 29, 100, 116, 123, 129, 140-143, 141, 146, 152, 154, 155, 173, 181, 183-187, 189, 190, 191, 192, and 193 in RhoB were mutated to the corresponding RhoA residues to determine those critical for RhoB tumor-suppressive activity. Of all the mutants made, only the cysteine 192 (one of two palmitoylation sites) and cysteine 193 (the prenylation site) point mutations abolish RhoB functions. In contrast, mutation of the other palmitoylation site, cysteine 189, did not affect RhoB functions. Moving cysteine 192 to position 190 did not affect RhoB function either. Mutation of cysteine 192 to glycine, alanine, or serine blocks the ability of RhoB to suppress transforming growth factor beta type II receptor, p2lwaf, and AP-1 promoter transcriptional activities. Furthermore, mutations of cysteines 192 and 193, but not 189, mislocalize RhoB and prevent RhoB from inhibiting anchorage-dependent and anchorage-independent tumor growth and colony formation as well as prevent it from inducing apoptosis. The cysteine 192 RhoB mutant is farnesylated and geranylgeranylated as efficiently as wild type RhoB. A RhoA-(1-180)/RhoB-(181-196) chimera inhibited tumor cell proliferation and induced apoptosis as efficiently as RhoB. These results demonstrate that the presence of neither cysteine 193 nor cysteine 192 alone is sufficient and that both palmitoylated cysteine 192 and prenylated cysteine 193, but not palmitoylated cysteine 189, are required for RhoB tumor-suppressive and proapoptotic activities.

Prenylation inhibitors stimulate both estrogen receptor alpha transcriptional activity through AF-1 and AF-2 and estrogen receptor beta transcriptional activity

Introduction

We showed in a previous study that prenylated proteins play a role in estradiol stimulation of proliferation. However, these proteins antagonize the ability of estrogen receptor (ER) α to stimulate estrogen response element (ERE)-dependent transcriptional activity, potentially through the formation of a co-regulator complex. The present study investigates, in further detail, how prenylated proteins modulate the transcriptional activities mediated by ERα and by ERβ.

Methods

The ERE-β-globin-Luc-SV-Neo plasmid was either stably transfected into MCF-7 cells or HeLa cells (MELN cells and HELN cells, respectively) or transiently transfected into MCF-7 cells using polyethylenimine. Cells deprived of estradiol were analyzed for ERE-dependent luciferase activity 16 hours after estradiol stimulation and treatment with FTI-277 (a farnesyltransferase inhibitor) or with GGTI-298 (a geranylgeranyltransferase I inhibitor). In HELN cells, the effect of prenyltransferase inhibitors on luciferase activity was compared after transient transfection of plasmids coding either the full-length ERα, the full-length ERβ, the AF-1-deleted ERα or the AF-2-deleted ERα. The presence of ERα was then detected by immunocytochemistry in either the nuclei or the cytoplasms of MCF-7 cells. Finally, Clostridium botulinum C3 exoenzyme treatment was used to determine the involvement of Rho proteins in ERE-dependent luciferase activity.

Results

FTI-277 and GGTI-298 only stimulate ERE-dependent luciferase activity in stably transfected MCF-7 cells. They stimulate both ERα-mediated and ERβ-mediated ERE-dependent luciferase activity in HELN cells, in the presence of and in the absence of estradiol. The roles of both AF-1 and AF-2 are significant in this effect. Nuclear ERα is decreased in the presence of prenyltransferase inhibitors in MCF-7 cells, again in the presence of and in the absence of estradiol. By contrast, cytoplasmic ERα is mainly decreased after treatment with FTI-277, in the presence of and in the absence of estradiol. The involvement of Rho proteins in ERE-dependent luciferase activity in MELN cells is clearly established.

Conclusions

Together, these results demonstrate that prenylated proteins (at least RhoA, RhoB and/or RhoC) antagonize the ability of ERα and ERβ to stimulate ERE-dependent transcriptional activity, potentially acting through both AF-1 and AF-2 transcriptional activities.

A novel strategy for specifically down-regulating individual Rho GTPase activity in tumor cells

The Rho family GTPases RhoA, RhoB, and RhoC regulate the actin cytoskeleton, cell movement, and cell growth. Unlike Ras, up-regulation or overexpression of these GDP/GTP binding molecular switches, but not activating point mutations, has been associated with human cancer. Although they share over 85% sequence identity, RhoA, RhoB, and RhoC appear to play distinct roles in cell transformation and metastasis. In NIH 3T3 cells, RhoA or RhoB overexpression causes transformation whereas RhoC increases the cell migration rate. To specifically target RhoA, RhoB, or RhoC function, we have generated a set of chimeric molecules by fusing the RhoGAP domain of p190, a GTPase-activating protein that accelerates the intrinsic GTPase activity of all three Rho GTPases, with the C-terminal hypervariable sequences of RhoA, RhoB, or RhoC. The p190-Rho chimeras were active as GTPase-activating proteins toward RhoA in vitro, co-localized with the respective active Rho proteins, and specifically down-regulated Rho protein activities in cells depending on which Rho GTPase sequences were included in the chimeras. In particular, the p190-RhoA-C chimera specifically inhibited RhoA-induced transformation whereas p190-RhoC-C specifically reversed the migration phenotype induced by the active RhoC. In human mammary epithelial-RhoC breast cancer cells, p190-RhoC-C, but not p190-RhoA-C or p190-RhoB-C, reversed the anchorage-independent growth and invasion phenotypes caused by RhoC overexpression. In the highly metastatic A375-M human melanoma cells, p190-RhoC-C specifically reversed migration, and invasion phenotypes attributed to RhoC up-regulation. Thus, we have developed a novel strategy utilizing RhoGAP-Rho chimeras to specifically down-regulate individual Rho activity and demonstrate that this approach may be applied to multiple human tumor cells to reverse the growth and/or invasion phenotypes associated with disregulation of a distinct subtype of Rho GTPase.

Effect of Rho family GTP-binding proteins on Amoeba proteus

While there is a number of studies on the effects of Rho GTPases on the actin-based cytoskeleton in higher eukaryotes, studies in protozoans are rather limited. The problem seems to be intriguing since the structure of protozoan cytoskeletons is distinct from most vertebrate cells. By blocking endogenous Rho family proteins of highly motile Amoeba proteus with C3 transferase and antibodies against human RhoA and Rac1, we tried to assess the in vivo role of these proteins in amoebae. In migrating amoebae, both proteins are concentrated in the cortical layer and seem to colocalize with filamentous actin. Endogenous Rac1, but not RhoA, is accumulated in the perinuclear cytoskeleton. Blocking Rac- or Rho-like proteins caused distinct and irreversible changes in the locomotive shape of the examined amoebae and significant inhibition of their migration. Amoebae microinjected with anti-Rac1 antibodies were contracted, shortened, and developed only few wide pseudopodia. More pronounced changes were observed in cells treated with anti-RhoA antibodies. They exhibited an atypical locomotion not leading to their effective displacement. After treatment with 50 microg of C3 transferase per ml, cells rapidly contracted and almost completely rounded up, became refractile with the granules beaten into a dense mass, detached from the surface and died. Ten times lower concentration of the enzyme caused similar changes as the inhibition of endogenous RhoA-like protein. These results indicate that Rho family-based regulation plays a key role in amoebic migration.

RhoB controls the 24 kDa FGF-2-induced radioresistance in HeLa cells by preventing post-mitotic cell death

Farnesylated Ras oncoprotein induces a cellular resistance to ionizing radiation that can be reversed by farnesyltransferase inhibitors (FTI). We previously demonstrated that, expression of the 24 kDa FGF2 isoform in wild type ras bearing HeLa cells, induced radioresistance which was also reversed by FTI. We tested the hypothesis that wild type Ras or RhoB, which has been proposed as a potential FTI target, could control the FGF-2-induced radioresistance mechanisms. For this, we expressed inducible dominant negative forms of Ras (RasN17) and Rho (RhoBN19) in 24 kDa FGF2 transfected HeLa cells and analysed their survival after irradiation. While no cell survival modification was observed after RasN17 induction, the expression of RhoBN19 induced a radiosensitization of FGF2 radioresistant HeLa cells in the same range as the one observed after a 48 h treatment with the specific FTI, R115777. Moreover, we showed that activated RhoB but not RhoA induced radioresistance in NIH3T3 cells. The radiosensitizer effect of RhoBN19 expression was due to the induction of the radiation induced post-mitotic cell death. Taken together, these data demonstrate that 24 kDa FGF-2-induced radioresistance is controlled by Rho pathways and suggest that RhoB should be a major determinant in cellular resistance to ionizing radiation.

Farnesyltransferase inhibitor, R115777, reverses the resistance of human glioma cell lines to ionizing radiation

We investigated for the first time the ability of farnesyltransferase inhibitors (FTI) to radiosensitize human glioma. For this, human glioma cell lines were treated with the specific FTI, R115777, 48 hr prior to a 2Gy irradiation. The treatment with R115777 decreased by 45% the SF2 value of the more radioresistant glioma cell lines (SF763 and U87) without any significant effect on the radioresistance of the radiosensitive ones (SF767 and U251-MG). This radiosensitizer effect was due to the induction of post-mitotic necrotic cell death. We then tested the hypothesis that wild-type Ras or RhoB, which has been proposed as potential FTI target, could control the glioma radioresistance. For this, we expressed inducible dominant negative forms of Ras (RasN17) and RhoB (RhoBN19) in radioresistant U87 glioma cell line and analyzed the survival after irradiation of the obtained clones. While blocking Ras pathways by expression of RasN17 did not affect the SF2 value of the U87 glioma cell line, the expression of RhoBN19 dramatically reduced the cell survival after irradiation of these cells. Taken together, these data demonstrated that RhoB, but not Ras, is implicated in glioma radioresistance. Furthermore, the R115777 differential radiosensitizer effect underlines the potential therapeutic interest of using this drug as a radiosensitizer of human glioma.

Farnesyltransferase inhibitors reverse Ras-mediated inhibition of Fas gene expression

Factors that govern host-tumor interaction play a critical role in tumor progression. In previous studies we have shown that oncogenic Ras inhibits the expression of Fas (CD95) and renders Ras-transformed cells resistant to Fas-induced death. We now demonstrate that culture of Ras-transformed cells in the presence of the farnesyltransferase inhibitor (FTI) LB42722 leads to up-regulation of Fas expression, both under basal growth conditions and in the presence of the inflammatory cytokines IFN-gamma and tumor necrosis factor alpha. This is manifested by an increase in fas mRNA, Fas cell surface expression, and Fas-induced apoptosis. Whereas FTI up-regulates expression of FAS in Ras-transformed cells, it inhibits the expression of vascular endothelial growth factor. Culture of Ras-transformed cells in the presence of the histone deacetylase inhibitor trichostatin A resulted in morphological reversion and G(1) arrest (as observed with FTI); however, no induction of Fas was observed. Furthermore, the effects of FTI on Fas-induced death were shown to be independent of RhoB. Therefore, inhibition of oncogenic Ras by FTI can result in two events that alter host-tumor interactions: up-regulation of Fas, rendering tumors more sensitive to immune cytotoxic effector cells, and down-reglation of VEGF, which may inhibit tumor angiogenesis.

Actin' up: RhoB in cancer and apoptosis

RhoB is a small GTPase that regulates actin organization and vesicle transport. It is required for signalling apoptosis in transformed cells that are exposed to farnesyltransferase inhibitors, DNA-damaging agents or taxol. Genetic analysis in mice indicates that RhoB is dispensable for normal cell physiology, but that it has a suppressor or negative modifier function in stress-associated processes, including cancer.

Ras and RhoA suppress whereas RhoB enhances cytokine-induced transcription of nitric oxide synthase-2 in human normal liver AKN-1 cells and lung cancer A-549 cells

While both nitric oxide synthase-2 (NOS-2) and low molecular weight GTPases, such as Ras and Rho, have been implicated in malignant transformation, the cross talk between these important proteins is ill understood. In this study we examined the ability of H-Ras, RhoA, RhoB and Rac1 to modulate cytokine-induced NOS2. In the normal human liver AKN-1 cell line and in the human non-small cell lung carcinoma cell line, A-549, the ability of the cytokines (INF-gamma, IL-1beta and TNF-alpha) to activate NOS-2 was blocked by activated L61-H-Ras whereas dominant negative N17-H-Ras enhanced NOS-2 activation. Consistent with this dominant negative Erk2 as well as a MEK inhibitor also enhanced cytokine activation of NOS-2. Furthermore, activated L63-RhoA blocked whereas activated V14-RhoB enhanced cytokine NOS-2 activation. Activated I115-Racl did not affect NOS-2 activation. These results demonstrate that the Ras/Erk and the Ras/RhoA pathways negatively regulate whereas RhoB enhances cytokine-induced NOS-2. This is the first demonstration that genes that promote malignant transformation such as Ras and RhoA inhibit, whereas genes with tumor suppressor activity such as RhoB enhance NOS2 induction.

RhoB is dispensable for mouse development, but it modifies susceptibility to tumor formation as well as cell adhesion and growth factor signaling in transformed cells

RhoB is an endosomal small GTPase that is implicated in the response to growth factors, genotoxic stress, and farnesyltransferase inhibitors. To gain insight into its physiological functions we examined the consequences of homozygous gene deletion in the mouse. Loss of RhoB did not adversely affect mouse development, fertility, or wound healing. However, embryo fibroblasts cultured in vitro exhibited a defect in motility, suggesting that RhoB has a role in this process that is conditional on cell stress. Neoplastic transformation by adenovirus E1A and mutant Ras yielded differences in cell attachment and spreading that were not apparent in primary cells. In addition, transformed -/- cells displayed altered actin and proliferative responses to transforming growth factor beta. A negative modifier role in transformation was suggested by the increased susceptibility of -/- mice to 7,12-dimethylbenz[a]anthracene-induced skin carcinogenesis and by the increased efficiency of intraperitoneal tumor formation by -/- cells. Our findings suggest that RhoB is a negative regulator of integrin and growth factor signals that are involved in neoplastic transformation and possibly other stress or disease states.

RhoB is required to mediate apoptosis in neoplastically transformed cells after DNA damage

The effect of neoplastic transformation on the response to genotoxic stress is of significant clinical interest. In this study, we offer genetic evidence that the apoptotic response of neoplastically transformed cells to DNA damage requires RhoB, a member of the Rho family of actin cytoskeletal regulators. Targeted deletion of the rhoB gene did not affect cell cycle arrest in either normal or transformed cells after exposure to doxorubicin or gamma irradiation, but rendered transformed cells resistant to apoptosis. This effect was specific insofar as rhoB deletion did not affect apoptotic susceptibility to agents that do not damage DNA. However, rhoB deletion also affected apoptotic susceptibility to Taxol, an agent that disrupts microtubule dynamics. We have demonstrated that RhoB alteration mediates the proapoptotic and antineoplastic effects of farnesyltransferase inhibitors, and we show here that RhoB alteration is also crucial for farnesyltransferase inhibitors to sensitize neoplastic cells to DNA damage-induced cell death. We found RhoB to be an important determinant of long-term survival in vitro and tumor response in vivo after gamma irradiation. Our findings identify a pivotal role for RhoB in the apoptotic response of neoplastic cells to DNA damage at a novel regulatory point that may involve the actin cytoskeleton.

Farnesyltransferase inhibitors define a role for RhoB in controlling neoplastic pathophysiology

A long-standing goal in cancer research is to identify cellular functions that have selective roles in regulating neoplastic pathophysiology. Farnesyl-transferase inhibitors (FTIs) are a novel class of cancer chemotherapeutics which have little effect on normal cell physiology but which inhibit or reverse malignant cell phenotypes. FTIs were originally developed as a strategy to inhibit oncogenic Ras, the activity of which depends upon posttranslational farnesylation. However, recent work indicates the antineoplastic effects of FTIs are not linked to Ras inhibition but instead to alteration of RhoB, a small GTPase of the Rho family of cytoskeletal regulators that controls trafficking of cell surface receptors. Rho proteins integrate signals from integrins and cytokine receptors with cell shape via the actin cytoskeleton. A connection between FTIs and Rho alteration is interesting given that histological differences have long been used to define clinical cancer. RhoB is dispensable for normal cell growth and differentiation in mice. Thus, research into the antineoplastic effects of FTIs has led to the identification of a function(s) that is unnecessary for normal cell physiology but crucial for controlling malignant phenotypes.

Regulation of constitutive cyclooxygenase-2 expression in colon carcinoma cells

Cyclooxygenase-2 (COX-2) is not normally expressed in the human large intestine, but its levels are increased in the majority of human colorectal carcinomas. Here we investigate the regulation of constitutive COX-2 expression and prostaglandin production in human colorectal carcinoma cells. Both COX-2 mRNA and protein were expressed in well differentiated HCA-7, Moser, LS-174, and HT-29 cells, albeit at different levels. COX-2 expression was not detected in several poorly differentiated colon cancer cell lines including DLD-1. Transcriptional regulation played a key role for the expression of COX-2 in human colon carcinoma cells, and both the nuclear factor for interleukin-6 regulatory element and the cAMP-response element were responsible for regulation of COX-2 transcription. COX-2 mRNA was more stable in HCA-7 cells than in the other cell lines tested. Both transcriptional and post-transcriptional regulation of COX-2 involved the MAP kinase pathway. Modulation of the Akt/protein kinase B or Rho B signaling pathways altered the levels of COX-2 expression. Furthermore, COX-2 protein is degraded through ubiquitin proteolysis, and its half-life was approximately 3.5-8 h. HCA-7 cells produced significant quantities of prostaglandin E(2) and other prostaglandins. Moser and LS-174 cells also generated prostaglandins, but levels were significantly lower than that observed in HCA-7 cells.

Geranylgeranylated RhoB is sufficient to mediate tissue-specific suppression of Akt kinase activity by farnesyltransferase inhibitors

Farnesyltransferase inhibitors (FTIs) induce apoptosis by elevating the levels of geranylgeranylated RhoB (RhoB-GG) in cells. However, the mechanism by which RhoB-GG acts is unclear. Here we report that RhoB-GG is sufficient to mediate the suppressive effects of FTIs on the activity of the survival kinase Akt-1 in epithelial cells. This mechanism is tissue-specific insofar as it does not operate in fibroblasts. We discuss how the cell survival functions of RhoB and Akt may be linked biochemically in certain cell types.

RhoB is expressed in migrating neural crest and endocardial cushions of the developing mouse embryo

RhoB mRNA expression was examined in the developing mouse embryo between E8.5 and E11.5. Specific expression was found in migrating neural crest (NC) cells, from the first stages of their migration at E9.5, throughout the migration period. Expression is maintained in NC derivatives for at least one embryonic day after they reach their final destinations, but is then down-regulated. RhoB is also expressed in non NC-derived neural tissues, including motor neurones and the floor plate of the neural tube. RhoB mRNA expression is also found in the developing endocardial cushions of the atrioventricular and outflow regions of the developing heart.

Ras-related GTPase RhoB forces alkylation-induced apoptotic cell death

rhoB encoding a Ras-related GTPase is immediate-early inducible by genotoxic treatments. To address the question of the physiological role of RhoB in cellular defense, cells stably overexpressing wild-type RhoB protein were generated. Overexpression of RhoB renders cells hypersensitive to the killing effect of alkylating agents including antineoplastic drugs but not to UV-light and doxorubicin. As compared to control cells, RhoB overexpressing cells revealed an increase in the frequency of alkylation-induced apoptotic cell death. This indicates that RhoB is involved in modulating apoptotic signaling. Furthermore, overexpression of RhoB resulted in a prolonged transient block to DNA replication upon MMS treatment. UV-induced replication blockage was not affected by RhoB. Based on the data we suggest RhoB to be a novel regulatory factor which takes influence on the level of cytotoxicity of DNA damaging drugs and forces cells to alkylation-induced apoptosis. The data indicate that this might be due to RhoB mediated delay in cell cycle progression upon alkylation treatment.

Regulation of epidermal growth factor receptor traffic by the small GTPase rhoB

Members of the Rho family of small GTPases control cell adhesion and motility through dynamic regulation of the actin cytoskeleton. Although twelve family members have been identified, only three of these - RhoA, Rac and Cdc42 - have been studied in detail. RhoA regulates the formation of focal adhesions and the bundling of actin filaments into stress fibres. It is also involved in other cell signalling pathways including the regulation of gene expression and the generation of lipid second messengers [1] [2]. RhoA is very closely related to two other small GTPases about which much less is known: RhoB and RhoC (which are approximately 83% identical). Perhaps the most intriguing of these is RhoB. RhoA is largely cytosolic but translocates to the plasma membrane on activation. RhoB, however, is entirely localised to the cytosolic face of endocytic vesicles [3] [4]. This suggests a potential role for RhoB in regulating endocytic traffic; however, no evidence has been presented to support this. RhoA has been shown to act at the plasma membrane to regulate the clathrin-mediated internalisation of transferrin receptor [5] and of the muscarinic acetylcholine receptor [6]. We have recently demonstrated that RhoB binds the RhoA effector, PRK1 and targets it to the endosomal compartment [7]. We show here that RhoB acts through PRK1 to regulate the kinetics of epidermal growth factor receptor traffic.