The ras-related gene rhoB is an immediate-early gene inducible by v-Fps, epidermal growth factor, and platelet-derived growth factor in rat fibroblasts

Tumor necrosis factor-induced ArhGEF10 selectively activates RhoB contributing to human microvascular endothelial cell tight junction disruption

Capillary endothelial cells (ECs) maintain a semi-permeable barrier between the blood and tissue by forming inter-EC tight junctions (TJs), regulating selective transport of fluid and solutes. Overwhelming inflammation, as occurs in sepsis, disrupts these TJs, leading to leakage of fluid, proteins, and small molecules into the tissues. Mechanistically, disruption of capillary barrier function is mediated by small Rho-GTPases, such as RhoA, -B, and -C, which are activated by guanine nucleotide exchange factors (GEFs) and disrupted by GTPase-activating factors (GAPs). We previously reported that a mutation in a specific RhoB GAP (p190BRhoGAP) underlays a hereditary capillary leak syndrome. Tumor necrosis factor (TNF) treatment disrupts TJs in cultured human microvascular ECs, a model of capillary leak. This response requires new gene transcription and involves increased RhoB activation. However, the specific GEF that activates RhoB in capillary ECs remains unknown. Transcriptional profiling of cultured tight junction-forming human dermal microvascular endothelial cells (HDMECs) revealed that 17 GEFs were significantly induced by TNF. The function of each candidate GEF was assessed by short interfering RNA depletion and trans-endothelial electrical resistance screening. Knockown of ArhGEF10 reduced the TNF-induced loss of barrier which was phenocopied by RhoB or dual ArhGEF10/RhoB knockdown. ArhGEF10 knockdown also reduced the extent of TNF-induced RhoB activation and disruption at tight junctions. In a cell-free assay, immunoisolated ArhGEF10 selectively catalyzed nucleotide exchange to activate RhoB, but not RhoA or RhoC. We conclude ArhGEF10 is a TNF-induced RhoB-selective GEF that mediates TJ disruption and barrier loss in human capillary endothelial cells.

High accuracy gene expression profiling of sorted cell subpopulations from breast cancer PDX model tissue

Intratumor Heterogeneity (ITH) is a functionally important property of tumor tissue and may be involved in drug resistance mechanisms. Although descriptions of ITH can be traced back to very early reports about cancer tissue, mechanistic investigations are still limited by the precision of analysis methods and access to relevant tissue sources. PDX models have provided a reproducible source of tissue with at least a partial representation of naturally occurring ITH. We investigated the properties of phenotypically distinct cell populations by Fluorescence activated cell sorting (FACS) tissue derived cells from multiple tumors from a triple negative breast cancer patient derived xenograft (PDX) model. We subsequently subjected each population to in depth gene expression analysis. Our findings suggest that process related gene expression changes (caused by tissue dissociation and FACS sorting) are restricted to Immediate Early Genes (IEGs). This allowed us to discover highly reproducible gene expression profiles of distinct cellular compartments identifiable by cell surface markers in this particular tumor model. Within the context of data from a previously published model our work suggests that gene expression profiles associated with hypoxia, stemness and drug resistance may reside in tumor subpopulations predictably growing in PDX models. This approach provides a novel opportunity for prospective mechanistic studies of ITH.

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.

Building Blood Vessels-One Rho GTPase at a Time

Blood vessels are required for the survival of any organism larger than the oxygen diffusion limit. Blood vessel formation is a tightly regulated event and vessel growth or changes in permeability are linked to a number of diseases. Elucidating the cell biology of endothelial cells (ECs), which are the building blocks of blood vessels, is thus critical to our understanding of vascular biology and to the development of vascular-targeted disease treatments. Small GTPases of the Rho GTPase family are known to regulate several processes critical for EC growth and maintenance. In fact, many of the 21 Rho GTPases in mammals are known to regulate EC junctional remodeling, cell shape changes, and other processes. Rho GTPases are thus an attractive target for disease treatments, as they often have unique functions in specific vascular cell types. In fact, some Rho GTPases are even expressed with relative specificity in diseased vessels. Interestingly, many Rho GTPases are understudied in ECs, despite their known expression in either developing or mature vessels, suggesting an even greater wealth of knowledge yet to be gleaned from these complex signaling pathways. This review aims to provide an overview of Rho GTPase signaling contributions to EC vasculogenesis, angiogenesis, and mature vessel barrier function. A particular emphasis is placed on so-called "alternative" Rho GTPases, as they are largely understudied despite their likely important contributions to EC biology.

Transcriptional and post-transcriptional regulation of the genes encoding the small GTPases RhoA, RhoB, and RhoC: implications for the pathogenesis of human diseases

Rho GTPases are highly conserved proteins that play critical roles in many cellular processes including actin dynamics, vesicular trafficking, gene transcription, cell-cycle progression, and cell adhesion. The main mode of regulation of Rho GTPases is through guanine nucleotide binding (cycling between an active GTP-bound form and an inactive GDP-bound form), but transcriptional, post-transcriptional, and post-translational modes of Rho regulation have also been described. In the present review, we summarize recent progress on the mechanisms that control the expression of the three members of the Rho-like subfamily (RhoA, RhoB, and RhoC) at the level of gene transcription as well as their post-transcriptional regulation by microRNAs. We also discuss the progress made in deciphering the mechanisms of cross-talk between Rho proteins and the transforming growth factor β signaling pathway and their implications for the pathogenesis of human diseases such as cancer metastasis and fibrosis.

Rho GTPases, their post-translational modifications, disease-associated mutations and pharmacological inhibitors

The 20 members of the Rho GTPase family are key regulators of a wide-variety of biological activities. In response to activation, they signal via downstream effector proteins to induce dynamic alterations in the organization of the actomyosin cytoskeleton. In this review, post-translational modifications, mechanisms of dysregulation identified in human pathological conditions, and the ways that Rho GTPases might be targeted for chemotherapy will be discussed.

RhoB: Team Oncogene or Team Tumor Suppressor?

Although Rho GTPases RhoA, RhoB, and RhoC share more than 85% amino acid sequence identity, they play very distinct roles in tumor progression. RhoA and RhoC have been suggested in many studies to contribute positively to tumor development, but the role of RhoB in cancer remains elusive. RhoB contains a unique C-terminal region that undergoes specific post-translational modifications affecting its localization and function. In contrast to RhoA and RhoC, RhoB not only localizes at the plasma membrane, but also on endosomes, multivesicular bodies and has even been identified in the nucleus. These unique features are what contribute to the diversity and potentially opposing functions of RhoB in the tumor microenvironment. Here, we discuss the dualistic role that RhoB plays as both an oncogene and tumor suppressor in the context of cancer development and progression.

RhoB induces the production of proinflammatory cytokines in TLR-triggered macrophages

Toll-like receptors (TLRs) are the primary sensors detecting conserved molecular patterns on microorganisms, thus acting as important components of innate immunity against invading pathogens. Many positive and negative regulators of TLR-triggered signaling have been identified. The Rho GTPase RhoB plays a key role in cell migration, division and polarity; however, the function and regulatory mechanisms of RhoB in TLR ligand-triggered innate immune responses remain to be investigated. Here, we report that the expression of RhoB is induced by TLR agonists (lipopolysaccharide (LPS), CpG, poly(I:C)) in macrophages. Knockdown of RhoB expression markedly decreased TLR ligand-induced activation of mitogen activated protein kinases and nuclear factor-κB (NF-κB), and the production of tumor necrosis factor α (TNFα), interleukin (IL)-6 and IL-1β in macrophages stimulated with TLR ligands. Furthermore, we demonstrated that RhoB interacts with major histocompatibility complex class II (MHCII) α chain, but not β chain, in endosomes of macrophages. Knockdown of MHCII expression greatly reduced the interaction of RhoB with Btk, and attenuated the induction of NF-κB and interferon β activity by RhoB upon LPS stimulation. These findings suggest that RhoB is a positive physiological regulator of TLRs signaling via binding to MHCII in macrophages, and therefore RhoB may be a potential therapeutic target in inflammatory diseases.

Involvement of small G protein RhoB in the regulation of proliferation, adhesion and migration by dexamethasone in osteoblastic cells

Long-term exposure to therapeutic doses of glucocorticoids (GCs) results in bone remodeling, which frequently causes osteoporosis and fracture healing retardation because of the abnormality of osteoblastic proliferation and differentiation. The mechanisms of GCs’ effect on osteoblasts are largely unknown. In this present study, we found that dexamethasone (Dex) could induce the expression of the small G protein, RhoB, in mRNA and protein levels in the osteoblast-derived osteosarcoma cell lines MG-63. The up-regulation of RhoB mRNA by Dex mainly occurs at posttranscriptional level by increasing its mRNA stability through PI-3K/Akt and p38 mitogen-activated protein kinase signaling pathways. Over-expression of RhoB in MG-63 cells magnified while down-regulation of RhoB level by RNA interference impaired Dex-induced growth inhibition but not differentiation. What’s more, over-expression of RhoB mimicked the effect of Dex on cell adhesion and migration. And interfering RhoB expression partially suppressed Dex-induced pro-adhesion and anti-migration in MG-63 cells. In conclusion, these results indicate that RhoB plays an important role in the pathological effect of Dex on osteoblastic growth and migration, which is a part of the mechanisms of GCs’ adverse effect on bone remodeling.

Immunological and Functional Characterization of RhoGDI3 and Its Molecular Targets RhoG and RhoB in Human Pancreatic Cancerous and Normal Cells

RhoGDI proteins have been implicated in several human cancers; changes in their expression levels have shown pro- or anti-tumorigenic effects. Pancreatic Ductal Adenocarcinoma (PDAC) is a complex pathology, with poor prognosis, and most patients die shortly after diagnosis. Efforts have been focused on understanding the role of RhoGDI's in PDAC, specially, RhoGDI1 and RhoGDI2. However, the role of RhoGDI3 has not been studied in relation to cancer or to PDAC. Here, we characterized the expression and functionality of RhoGDI3 and its target GTPases, RhoG and RhoB in pancreatic cell lines from both normal pancreatic tissue and tissue in late stages of PDAC, and compared them to human biopsies. Through immunofluorescences, pulldown assays and subcellular fractionation, we found a reduction in RhoGDI3 expression in the late stages of PDAC, and this reduction correlates with tumor progression and aggressiveness. Despite the reduction in the expression of RhoGDI3 in PDAC, we found that RhoB was underexpressed while RhoG was overexpressed, suggesting that cancerous cells preserve their capacity to activate this pathway, thus these cells may be more eager to response to the stimuli needed to proliferate and become invasive unlike normal cells. Surprisingly, we found nuclear localization of RhoGDI3 in non-cancerous pancreatic cell line and normal pancreatic tissue biopsies, which could open the possibility of novel nuclear functions for this protein, impacting gene expression regulation and cellular homeostasis.

RhoB Acts as a Tumor Suppressor That Inhibits Malignancy of Clear Cell Renal Cell Carcinoma

This study aims to investigate the biological role of RhoB in clear cell renal cell carcinoma (ccRCC). The expression of RhoB was examined in specimens of patients and cell lines by Western blot and Immunohistochemistry. The correlation between RhoB expression and clinicopathologic variables was also analyzed. The effects of RhoB on cell proliferation, cell cycle, cell apoptosis, and invasion/migration were detected by over-expression and knockdown of RhoB level in ccRCC cells via plasmids and RNAi. The results showed that RhoB was low-expressed in ccRCC surgical specimens and cell lines compared with adjacent normal renal tissues and normal human renal proximal tubular epithelial cell lines (HKC), and its protein expression level was significantly associated with the tumor pathologic parameter embracing tumor size(P = 0.0157), pT stage(P = 0.0035), TNM stage(P = 0.0024) and Fuhrman tumor grade(P = 0.0008). Further, over-expression of RhoB remarkably inhibited the cancer cell proliferation, colony formation and promoted cancer cell apoptosis, and aslo reduced the invasion and migration ability of ccRCC cells. Interestingly, up-regulation of RhoB could induce cell cycle arrest in G2/M phase and led to cell cycle regulators(CyclineB1,CDK1) and pro-apoptotic protein(casp3,casp9) aberrant expression. Moreover, knockdown of RhoB in HKC cells promoted cell proliferation and migration. Taken together, our study indicates that RhoB expression is decreased in ccRCC carcinogenesis and progression. Up-regulation of RhoB significantly inhibits ccRCC cell malignant phenotype. These findings show that RhoB may play a tumor suppressive role in ccRCC cells, raising its potential value in futural therapeutic target for the patients of ccRCC.

The mechanisms and significance of up-regulation of RhoB expression by hypoxia and glucocorticoid in rat lung and A549 cells

Small guanosine triphosphate (GTP)‐binding protein RhoB is an important stress sensor and contributes to the regulation of cytoskeletal organization, cell proliferation and survival. However, whether RhoB is involved in the hypoxic response and action of glucocorticoid (GC) is largely unknown. In this study, we investigated the effects of hypoxia or/and GC on the expression and activition of RhoB in the lung of rats and human A549 lung carcinoma cells, and further studied its mechanism and significance. We found that hypoxia and dexamethasone (Dex), a synethic GC, not only significantly increased the expression and activation of RhoB independently but also coregulated the expresion of RhoB in vitro and in vivo. Up‐regulation of RhoB by hypoxia was in part through stabilizing the RhoB mRNA and protein. Inhibiting hypoxia‐activated hypoxia‐inducible transcription factor‐1α (HIF‐1α), c‐Jun N‐terminal kinase (JNK) or extracellular signal‐regulated kinase (ERK) with their specific inhibitors significantly decreased hypoxia‐induced RhoB expression, indicating that HIF‐1α, JNK and ERK are involved in the up‐regulation of RhoB in hypoxia. Furthermore, we found that knockdown of RhoB expression by RhoB siRNA not only significantly reduced hypoxia‐enhanced cell migration and cell survival in hypoxia but also increased the sensitivity of cell to paclitaxel (PTX), a chemotherapeutic agent, and reduced Dex‐enhanced resistance to PTX‐chemotherapy in A549 cells. Taken together, the novel data revealed that hypoxia and Dex increased the expression and activation of RhoB, which is important for hypoxic adaptation and hypoxia‐accelerated progression of lung cancer cells. RhoB also enhanced the resistance of cell to PTX‐chemotherapy and mediated the pro‐survival effect of Dex.

RhoB regulates the function of macrophages in the hypoxia-induced inflammatory response

Immune cells, particularly macrophages, play critical roles in the hypoxia-induced inflammatory response. The small GTPase RhoB is usually rapidly induced by a variety of stimuli and has been described as an important regulator of cytoskeletal organization and vesicle and membrane receptor trafficking. However, it is unknown whether RhoB is involved in the hypoxia-induced inflammatory response. Here, we investigated the effect of hypoxia on the expression of RhoB and the mechanism and significance of RhoB expression in macrophages. We found that hypoxia significantly upregulated the expression of RhoB in RAW264.7 cells, mouse peritoneal macrophages, and the spleen of rats. Hypoxia-induced expression of RhoB was significantly blocked by a specific inhibitor of hypoxia-inducible factor-1α (HIF-1α), c-Jun N-terminal kinase (JNK), or extracellular-signal regulated protein kinase (ERK), indicating that hypoxia-activated HIF-1α, JNK, and ERK are involved in the upregulation of RhoB by hypoxia. Knockdown of RhoB expression not only significantly suppressed basal production of interleukin-1 beta (IL-1β), interleukin 6 (IL-6), and tumor necrosis factor alpha (TNF-α) in normoxia but also more markedly decreased the hypoxia-stimulated production of these cytokines. Furthermore, we showed that RhoB increased nuclear factor-kappa B (NF-κB) activity, and the inhibition of NF-κB transcriptional activity significantly decreased the RhoB-increased mRNA levels of IL-1β, IL-6, and TNF-α. Finally, we demonstrated that RhoB enhanced cell adhesion and inhibited cell migration in normoxia and hypoxia. Taken together, these results suggest that RhoB plays an important role in the hypoxia-induced activation of macrophages and the inflammatory response.Cellular & Molecular Immunology advance online publication, 21 September 2015; doi:10.1038/cmi.2015.78.

Liposomal trichostatin A: therapeutic potential in hormone-dependent and -independent breast cancer xenograft models

BACKGROUND

Trichostatin A (TSA) is one of the most potent histone deacetylase inhibitors (HDACi) in vitro but it lacks biological activity in vivo when injected intravenously owing to its fast metabolism.

MATERIALS AND METHODS

TSA was incorporated into Stealth® liposomes (TSA-lipo) at a high loading and its anticancer activity was evaluated in several types of breast cancer cells and xenografts.

RESULTS

In estrogen receptor α (ERα)-positive MCF-7 and T47-D cells, TSA induced a long-term degradation of cyclin A and a proteasome-dependent loss of ERα and cyclin D1, allowed derepression of p21WAF1/CIP1, HDAC1 and RhoB GTPase, concomitantly with blockade in G2/M of the cell cycle and apoptosis induction. In MDA-MB-231 (MDA) and SKBr-3 cells, TSA increased ERα mRNA and p21WAF1/CIP1 protein expression, but decreased cyclin A with a G2/M blockade and cleavage of polyADP-ribose polymerase (PARP). No significant restoration of any ER protein was noticed in any cells. TSA-lipo markedly inhibited tumor growth in MCF-7 and MDA cells xenografts following intravenous injection. Their anticancer effects were characterized by inhibition of Ki-67 labeling, the inhibition of tumor vasculature and an increase of p21WAF1/CIP1 in both tumors. In MCF-7 cell tumors, enhanced RhoB accumulation in the cytoplasm of epithelial cells was noticed, inversely to ERα that was strongly decreased.

CONCLUSION

Such anticancer activity of TSA-lipo is exp-lained by the protection provided by HDACi encapsulation and by the strong tumor accumulation of the nanocarriers as revealed by fluorescence confocal microscopy experi-ments. Together with its lack of toxicity, the enhanced stability of TSA-lipo in vivo justifies its development for therapeutic use in the treatment estradiol-dependent and -independent breast cancers.

ATR/Chk1/Smurf1 pathway determines cell fate after DNA damage by controlling RhoB abundance

ATM- and RAD3-related (ATR)/Chk1 and ataxia-telangiectasia mutated (ATM)/Chk2 signalling pathways play critical roles in the DNA damage response. Here we report that the E3 ubiquitin ligase Smurf1 determines cell apoptosis rates downstream of DNA damage-induced ATR/Chk1 signalling by promoting degradation of RhoB, a small GTPase recognized as tumour suppressor by promoting death of transformed cells. We show that Smurf1 targets RhoB for degradation to control its abundance in the basal state. DNA damage caused by ultraviolet light or the alkylating agent methyl methanesulphonate strongly activates Chk1, leading to phosphorylation of Smurf1 that enhances its self-degradation, hence resulting in a RhoB accumulation to promote apoptosis. Suppressing RhoB levels by overexpressing Smurf1 or blocking Chk1-dependent Smurf1 self-degradation significantly inhibits apoptosis. Hence, our study unravels a novel ATR/Chk1/Smurf1/RhoB pathway that determines cell fate after DNA damage, and raises the possibility that aberrant upregulation of Smurf1 promotes tumorigenesis by excessively targeting RhoB for degradation.

RhoB controls coordination of adult angiogenesis and lymphangiogenesis following injury by regulating VEZF1-mediated transcription

Mechanisms governing the distinct temporal dynamics that characterize post-natal angiogenesis and lymphangiogenesis elicited by cutaneous wounds and inflammation remain unclear. RhoB, a stress-induced small GTPase, modulates cellular responses to growth factors, genotoxic stress and neoplastic transformation. Here we show, using RhoB null mice, that loss of RhoB decreases pathological angiogenesis in the ischaemic retina and reduces angiogenesis in response to cutaneous wounding, but enhances lymphangiogenesis following both dermal wounding and inflammatory challenge. We link these unique and opposing roles of RhoB in blood versus lymphatic vasculatures to the RhoB-mediated differential regulation of sprouting and proliferation in primary human blood versus lymphatic endothelial cells. We demonstrate that nuclear RhoB-GTP controls expression of distinct gene sets in each endothelial lineage by regulating VEZF1-mediated transcription. Finally, we identify a small-molecule inhibitor of VEZF1-DNA interaction that recapitulates RhoB loss in ischaemic retinopathy. Our findings establish the first intra-endothelial molecular pathway governing the phased response of angiogenesis and lymphangiogenesis following injury.

RhoA and RhoC differentially modulate estrogen receptor α recruitment, transcriptional activities, and expression in breast cancer cells (MCF-7)

PURPOSE

RhoA and RhoC are closely related, small GTPases that are clearly involved in breast cancer tumorigenesis. Nonetheless, their specific roles in the control of estrogen receptor alpha (ERα) activities have not been elucidated.

METHODS

We used siRNA sequences to specifically down-regulate RhoA and RhoC expression in ERα-positive breast adenocarcinoma MCF-7 cells. We then analyzed the consequences of down-regulation on ERα expression, ERα recruitment to the promoters of four target genes, and the mRNA levels of those genes.

RESULTS

We demonstrated that RhoA and RhoC clearly and similarly modulated ERα recruitment to the vitellogenin estrogen responsive element (ERE) present in a luciferase reporter gene and to the promoters of progesterone receptor (PR), cathepsin D, and pS2 genes. Besides, RhoA up-regulated the ERE-luciferase reporter gene activity and PR mRNA expression and tended to down-regulate cathepsin D and pS2 mRNA expression. Conversely, RhoC inhibition had no significant effect at the mRNA level. Furthermore, RhoA inhibition, and to a lesser extent RhoC inhibition, increased ERα expression. No alteration in ERα mRNA levels was observed, suggesting potential post-translational control.

CONCLUSIONS

Taken together, our results strongly suggest that RhoA and RhoC play different, but clear, roles in ERα signaling. These GTPases are definitely involved, along with RhoB, in ERα recruitment and, to some extent, ERα cofactor balance. We hypothesize a differential role of RhoA in breast cancer tumors that depend on hormone status.

The small GTPase RhoB regulates TNFα signaling in endothelial cells

The inflammatory response of endothelial cells triggered by cytokines such as TNFα and IL1β plays a pivotal role in innate immunity. Upon pro-inflammatory cytokine stimulation, endothelial cells produce chemokines and cytokines that attract and activate leukocytes, and express high levels of leukocyte adhesion molecules. This process is mediated by intracellular signaling cascades triggered by activation of e.g. the TNFα receptor (TNFR) that lead to the activation of the NFκB transcription factor and of MAP kinases, which in turn activate inflammatory gene transcription. We found that the small GTPase RhoB was strongly and rapidly upregulated in primary human endothelial cells by TNFα, IL1β and LPS. We subsequently investigated the role of RhoB in the regulation of TNFR signaling in endothelial cells by silencing RhoB expression with siRNA. We provide evidence that the TNFα-induced activation of p38 MAP kinase is strongly dependent on RhoB, but not on RhoA, while JNK activation is regulated by both RhoB and RhoA. Consistent with the important role of p38 MAP kinase in inflammation, we demonstrate that loss of RhoB impairs TNFα-induced ICAM-1 expression and reduces cell production of IL6 and IL8. In addition, we show that RhoB silencing alters the intracellular traffic of TNFα after endocytosis. Since RhoB is a known regulator of the intracellular traffic of membrane receptors, our data suggest that RhoB controls TNFα signaling through the regulation of the TNFR traffic.

RhoB modifies estrogen responses in breast cancer cells by influencing expression of the estrogen receptor

INTRODUCTION

RhoB has been reported to exert positive and negative effects on cancer pathophysiology but an understanding of its role in breast cancer remains incomplete. Analysis of data from the Oncomine database showed a positive correlation between RhoB expression and positivity for both estrogen receptor alpha (ERα) and progesterone receptor (PR).

METHODS

This finding was validated by our analysis of a tissue microarray constructed from a cohort of 113 patients and then investigated in human cell models.

RESULTS

We found that RhoB expression in tissue was strongly correlated with ERα and PR expression and inversely correlated with tumor grade, tumor size and count of mitosis. In human breast cancer cell lines, RhoB attenuation was associated with reduced expression of both ERα and PR, whereas elevation of RhoB was found to be associated with ERα overexpression. Mechanistic investigations suggested that RhoB modulates ERα expression, controlling both its protein and mRNA levels, and that RhoB modulates PR expression by accentuating the recruitment of ERα and other major co-regulators to the promoter of PR gene. A major consequence of RhoB modulation was that RhoB differentially regulated the proliferation of breast cancer cell lines. Interestingly, we documented crosstalk between RhoB and ERα, with estrogen treatment leading to RhoB activation.

CONCLUSION

Taken together, our findings offer evidence that in human breast cancer RhoB acts as a positive function to promote expression of ERα and PR in a manner correlated with cell proliferation.

RhoB is involved in lipopolysaccharide-induced inflammation in mouse in vivo and in vitro

Small GTPase RhoB has been well documented in regulating cell adhesion, motility, proliferation, and survival, but to date, there is little information about the relationship between RhoB and inflammation. In this study, the mRNA and protein levels of RhoB were induced by lipopolysaccharide (LPS) in RAW264.7 cells determined by real-time PCR and Western blot. The upregulation of RhoB by LPS was also observed in mouse peritoneal macrophages and in mouse lung, liver, and kidney. RhoB overexpression by transfecting with wild RhoB plasmid increased the secretion of tumor necrosis factor alpha (TNF-α) and nitric oxide (NO) in RAW264.7 cells, while RhoB knockdown by RNA interference decreased the secretion of TNF-α and NO in RAW264.7 cells. TNF-α and NO synthase are the target genes of nuclear factor-kappaB (NF-κB), and overexpression of RhoB increased, whereas inhibition of RhoB decreased the basal and LPS-activated transcriptional activity of NF-κB in the cells. These results demonstrated that LPS induced RhoB expression in mouse in vivo and in vitro and in RAW264.7 cells, and the role of RhoB on LPS-induced secretion of TNF-α and NO was at least partly mediated via NF-κB. These results indicated that RhoB was involved in LPS-induced inflammation in mouse in vivo and in vitro.

Cell cycle regulation of Rho signaling pathways

The dynamics of the actin cytoskeleton and its regulation by Rho GTPases are essential to maintain cell shape, to allow cell motility and are also critical during cell cycle progression and mitosis. Rho GTPases and their effectors are involved in cell rounding at mitosis onset, in chromosomes alignment and are required for contraction of the actomyosin ring that separates daughter cells at the end of mitosis. Recent studies have revealed how a number of nucleotide exchange factors and GTPase-activating proteins regulate the activity of Rho GTPases during these processes. This review will focus on how the cell cycle machinery, in turn, regulates expression of proteins in the Rho signaling pathways through transcriptional activation, ubiquitylation and proteasomal degradation and modulates their activity through phosphorylation by mitotic kinases.

Role of RhoB in the regulation of pulmonary endothelial and smooth muscle cell responses to hypoxia

RATIONALE

RhoA and Rho kinase contribute to pulmonary vasoconstriction and vascular remodeling in pulmonary hypertension. RhoB, a protein homologous to RhoA and activated by hypoxia, regulates neoplastic growth and vasoconstriction but its role in the regulation of pulmonary vascular function is not known.

OBJECTIVE

To determine the role of RhoB in pulmonary endothelial and smooth muscle cell responses to hypoxia and in pulmonary vascular remodeling in chronic hypoxia-induced pulmonary hypertension.

METHODS AND RESULTS

Hypoxia increased expression and activity of RhoB in human pulmonary artery endothelial and smooth muscle cells, coincidental with activation of RhoA. Hypoxia or adenoviral overexpression of constitutively activated RhoB increased actomyosin contractility, induced endothelial permeability, and promoted cell growth; dominant negative RhoB or manumycin, a farnesyltransferase inhibitor that targets the vascular function of RhoB, inhibited the effects of hypoxia. Coordinated activation of RhoA and RhoB maximized the hypoxia-induced stress fiber formation caused by RhoB/mammalian homolog of Drosophila diaphanous-induced actin polymerization and RhoA/Rho kinase-induced phosphorylation of myosin light chain on Ser19. Notably, RhoB was specifically required for hypoxia-induced factor-1α stabilization and for hypoxia- and platelet-derived growth factor-induced cell proliferation and migration. RhoB deficiency in mice markedly attenuated development of chronic hypoxia-induced pulmonary hypertension, despite compensatory expression of RhoA in the lung.

CONCLUSIONS

RhoB mediates adaptational changes to acute hypoxia in the vasculature, but its continual activation by chronic hypoxia can accentuate vascular remodeling to promote development of pulmonary hypertension. RhoB is a potential target for novel approaches (eg, farnesyltransferase inhibitors) aimed at regulating pulmonary vascular tone and structure.

RhoB regulates uPAR signalling

Urokinase-type plasminogen activator (uPA) and its receptor, uPAR, play important roles in promoting cancer cell adhesion, migration and invasion. Rho GTPases are key coordinators of these processes; the Rho GTPase Rac1 has previously been implicated in uPA- and/or uPAR-induced migratory or morphological cell responses. We used RNAi to deplete 12 different Rho GTPases to screen for effects on uPA-stimulated migration, and found that depletion of RhoB significantly reduces uPA-induced migration and invasion of prostate carcinoma cells. RhoB depletion did not affect the expression or surface levels of uPAR but reduced the uPAR-induced increase in levels of several integrins and inhibited uPAR signalling to the actin regulator cofilin, the cell-adhesion signal-transduction adaptor molecule paxillin and the serine/threonine kinase Akt. uPAR rapidly activated RhoB and increased RhoB expression. RhoB depletion also reduced cell adhesion to and spreading on vitronectin, which is a uPAR ligand. This correlated with decreased association between integrins and uPAR and reduced integrin β1 activity. Our results indicate that RhoB is a key regulator of uPAR signalling in cell adhesion, migration and invasion.

miR-21 targets the tumor suppressor RhoB and regulates proliferation, invasion and apoptosis in colorectal cancer cells

It has become increasingly clear that microRNAs play an important role in many human diseases including cancer. Here, we show that expression of miR-21 in HEK293 and several colorectal cancer cells was found inversely correlated with ras homolog gene family, member B (RhoB) expression. miR-21 expression significantly suppressed RhoB 3' UTR luciferase-reporter activity, but the inhibitory effect was lost when the putative target sites were mutated. Exogenous miR-21 over-expression mimicked the effect of RhoB knockdown in promoting proliferation and invasion and inhibiting apoptosis, whereas anti-miR-21 or RhoB expression yielded opposite effects, in colorectal cancer cells. These results suggest that miR-21 is a regulator of RhoB expression and RhoB could be a useful target in exploring the potential therapeutic benefits of miR-21 mediated tumor cell behaviors in colorectal cancer.

The activation of p38 MAPK primarily contributes to UV-induced RhoB expression by recruiting the c-Jun and p300 to the distal CCAAT box of the RhoB promoter

The Ras-related small GTP-binding protein RhoB is rapidly induced in response to genotoxic stresses caused by ionizing radiation. It is known that UV-induced RhoB expression results from the binding of activating transcription factor 2 (ATF2) via NF-Y to the inverted CCAAT box (-23) of the RhoB promoter. Here, we show that the association of c-Jun with the distal CCAAT box (-72) is primarily involved in UV-induced RhoB expression and p38 MAPK regulated RhoB induction through the distal CCAAT box. UV-induced RhoB expression and apoptosis were markedly attenuated by pretreatment with the p38 MAPK inhibitor. siRNA knockdown of RhoB, ATF2 and c-Jun resulted in decreased RhoB expression and eventually restored the growth of UV-irradiated Jurkat cells. In the reporter assay using luciferase under the RhoB promoter, inhibition of RhoB promoter activity by the p38 inhibitor and knockdown of c-Jun using siRNA occurred through the distal CCAAT box. Immunoprecipitation and DNA affinity protein binding assays revealed the association of c-Jun and p300 via NF-YA and the dissociation of histone deacetylase 1 (HDAC1) via c-Jun recruitment to the CCAAT boxes of the RhoB promoter. These results suggest that the activation of p38 MAPK primarily contributes to UV-induced RhoB expression by recruiting the c-Jun and p300 proteins to the distal CCAAT box of the RhoB promoter in Jurkat cells.

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.

A distinct role of RhoB in gastric cancer suppression

Although Rho family GTPases RhoA, RhoB and RhoC share more than 85% amino acid sequence identity, they may play distinct roles in tumor progression. RhoA and RhoC have been suggested to have positive effects on tumor progression, but the role of RhoB in cancer, particularly in gastric cancer, remains unclear. In our study, we have examined the expression levels of these three Rho GTPases in a large panel of specimens from gastric cancer patients by immunohistochemistry. We found that RhoA and RhoC expression were significantly elevated, while RhoB was reduced or absent, in surgically removed gastric cancer tissues when compared to normal gastric tissues. The significant reduction of RhoB expression was confirmed in another group of gastric cancer samples in comparison to the adjacent non-neoplastic tissues. Then we transfected the plasmids containing RhoA, RhoB or RhoC cDNA into two gastric cancer cell lines, SGC7901 and AGS cells, respectively. By overexpression experiments, we found that RhoA promoted the gastric cancer cell proliferation and RhoC stimulated migration and invasion of the cancer cell. RhoB expression, however, significantly inhibited the proliferation, migration and invasion of the gastric cancer cells and also enhanced the chemosensitivity of these cells to anticancer drugs. It appears that RhoB plays an opposing role from that of RhoA and/or RhoC in gastric cancer cells. Our work suggests that RhoB may play a tumor suppressor role and subsequently may have potential implications in future targeted therapy.

Induction of small G protein RhoB by non-genotoxic stress inhibits apoptosis and activates NF-κB

It has been reported by us and other groups that the expression of small GTP binding protein RhoB can be induced by genotoxic stressors and glucocorticoid (GC), a stress hormone that plays a key role in stress response. Until now stress-induced genes that confer cytoprotection under stressed conditions are largely unknown. In this study, we investigated the effects and mechanism of non-genotoxic stressors, including scalding in vivo and heat stress in vitro on the expression of RhoB. We found for the first time that both scalding, which could induce typical neuroendocrine responses of acute stress and cellular heat stress significantly increased the expression of RhoB at mRNA and protein levels. Moreover, in vitro experiments in human lung epithelial cells (A549) showed that induction of RhoB by heat stress was in a glucocorticoid receptor (GR)-independent manner and through multiple pathways including stabilization of RhoB mRNA and activation of p38 MAPK. Further experiments demonstrated that up-regulation of RhoB significantly inhibited heat stress-induced apoptosis and elevated transcriptional activity of NF-κB, but did not affect the expression of Hsp70 in A549 cells. In conclusion, we showed for the first time that RhoB was up-regulated by scalding in vivo and heat stress in vitro and played an important cytoprotective role during heat stress-induced apoptotic cell death.

Simvastatin inhibits glucose-stimulated vascular smooth muscle cell migration involving increased expression of RhoB and a block of Ras/Akt signal

BACKGROUND

Diabetic patients are at high risk to develop atherosclerotic cardiovascular disease and have a higher restenotic rate after percutaneous coronary intervention (PCI). Statins improve cardiovascular outcome and reduce restenosis after PCI by inhibiting proliferation and migration of vascular smooth muscle cells (VSMCs). But the effect of statins on diabetes without dyslipidemia was still not fully understood. Our previous study has demonstrated that simvastatin inhibits VSMC proliferation in high glucose status without dyslipidemia, inducing a G0/G1 phase cell cycle growth arrest by acting on multiple steps upstream of pRb, including inhibition of CDK2/4 expression and upregulation of p53, p21, p16, and p27.

METHOD

Following our previous study, we investigated the mechanism of simvastatin inhibition of VSMC migration in a diabetes-like model (A7r5 cells under high glucose conditions without dyslipidemia).

RESULTS

Under high glucose conditions, simvastatin dose-dependently inhibited VSMC migration, decreased PI3K/Akt pathway activity, reduced c-Raf and Ras expression, increased RhoB but not RhoA, Rac1, and Cdc2 expression, dose-dependently inhibited MMP-2, but not MMP-9, activity, and dose-dependently inhibited NF-κB activity.

CONCLUSION

The inhibition of VSMC migration under high glucose conditions was via two different pathways. The first pathway is mevalonate-related but not RhoA protein-related and involves suppression of Ras and PI3K/Akt signals. The second pathway is not mevalonate-related and involves increasing RhoB expression directly.

Possible role of the RhoC/ROCK pathway in progression of clear cell renal cell carcinoma

To clarify the role of the Rho small GTP-binding protein (Rho) and its major downstream target, ROCK (Rho-associated serine-threonine protein kinase), in progression of renal cell carcinoma (RCC), we examined mRNA expression for Rho and ROCK genes in surgical specimen of RCC tissues from 78 Japanese patients and in the corresponding non-tumor tissues originating from the same patient using a real-time reverse transcription polymerase chain reaction (RT-PCR). Expression of mRNA for RhoA did not differ between tumor and non-tumor tissues. RhoB mRNA expression was higher in the tumor (P < 0.05), but expression was not associated with tumor grade, stage, or prognosis. However, degree of RhoC and ROCK mRNA expression was related to tumor grade (P < 0.05) and stage (P < 0.0001). A positive relationship was seen between expression of mRNA for RhoC and that for ROCK in tumor tissues (P < 0.0001). Kaplan-Meier plots showed high RhoC and ROCK mRNA expression to be negatively associated with overall survival (P < 0.0001). Multivariate analysis showed mRNA expression of RhoC and ROCK to be independent poor prognostic factors concerning overall survival. Our findings implicate the RhoC/ROCK pathway in carcinogenesis and progression of RCC, indicating that RhoC/ROCK may be a useful prognostic marker and a possible molecular target for treatment of the disease.

Phosphorylation of RhoB by CK1 impedes actin stress fiber organization and epidermal growth factor receptor stabilization

RhoB is a small GTPase implicated in cytoskeletal organization, EGF receptor trafficking and cell transformation. It is an immediate-early gene, regulated at many levels of its biosynthetic pathway. Herein we show that the serine/threonine protein kinase CK1 phosphorylates RhoB in vitro but not RhoA or RhoC. With the use of specific CK1 inhibitors, IC261 and D4476, we show that the kinase phosphorylates also RhoB in HeLa cells. Mass spectrometry analysis demonstrates that RhoB is monophosphorylated by CK1, in its C-terminal end, on serine 185. The substitution of Ser185 by Ala dramatically inhibited the phosphorylation of RhoB in cultured cells. Lastly we show that the inhibition of CK1 activates RhoB and promotes RhoB dependent actin fiber formation and EGF-R level. Our data provide the first demonstration of RhoB phosphorylation and indicate that this post-translational maturation would be a novel critical mechanism to control the RhoB functions.

MAP1A light chain-2 interacts with GTP-RhoB to control epidermal growth factor (EGF)-dependent EGF receptor signaling

Rho GTPases have been implicated in the control of several cellular functions, including regulation of the actin cytoskeleton, cell proliferation, and oncogenesis. Unlike RhoA and RhoC, RhoB localizes in part to endosomes and controls endocytic trafficking. Using a yeast two-hybrid screen and a glutathione S-transferase pulldown assay, we identified LC2, the light chain of the microtubule-associated protein MAP1A, as a novel binding partner for RhoB. GTP binding and the 18-amino acid C-terminal hypervariable domain of RhoB are critical for its binding to MAP1A/LC2. Coimmunoprecipitation and immunofluorescence experiments showed that this interaction occurs in U87 cells. Down-regulation of MAP1A/LC2 expression decreased epidermal growth factor (EGF) receptor expression and modified the signaling response to EGF treatment. We concluded that MAP1A/LC2 is critical for RhoB function in EGF-induced EGF receptor regulation. Because MAP1A/LC2 is thought to function as an adaptor between microtubules and other molecules, we postulate that the RhoB and MAP1A/LC2 interactions facilitate endocytic vesicle trafficking and regulate the trafficking of signaling molecules.

ROCK I-mediated activation of NF-kappaB by RhoB

RhoB is a short-lived protein whose expression is increased by a variety of extra-cellular stimuli including UV irradiation, epidermal growth factor (EGF) and transforming growth factor beta (TGF-beta). Whereas most Rho proteins are modified by the covalent attachment of a geranylgeranyl group, RhoB is unique in that it can exist in either a geranylgeranylated (RhoB-GG) or a farnesylated (RhoB-F) form. Although each form is proposed to have different cellular functions, the signaling events that underlie these differences are poorly understood. Here we show that RhoB can activate NF-kappaB signaling in multiple cell types. Whereas RhoB-F is a potent activator of NF-kappaB, much weaker activation is observed for RhoB-GG, RhoA, and RhoC. NF-kappaB activation by RhoB is not associated with increased nuclear translocation of RelA/p65, but rather, by modification of the RelA/p65 transactivation domain. Activation of NF-kappaB by RhoB is dependent upon ROCK I but not PRK I. Thus, ROCK I cooperates with RhoB to activate NF-kappaB, and suppression of ROCK I activity by genetic or pharmacological inhibitors blocks NF-kappaB activation. Suppression of RhoB activity by dominant-inhibitory mutants, or siRNA, blocks NF-kappaB activation by Bcr, and TSG101, but not by TNFalpha or oncogenic Ras. Collectively, these observations suggest the existence of an endosome-associated pathway for NF-kappaB activation that is preferentially regulated by the farnesylated form of RhoB.

GTP-binding proteins of the Rho/Rac family: regulation, effectors and functions in vivo

Rho/Rac proteins constitute a subgroup of the Ras superfamily of GTP hydrolases. Although originally implicated in the control of cytoskeletal events, it is currently known that these GTPases coordinate diverse cellular functions, including cell polarity, vesicular trafficking, the cell cycle and transcriptomal dynamics. In this review, we will provide an overview on the recent advances in this field regarding the mechanism of regulation and signaling, and the roles in vivo of this important GTPase family.

Up-regulation of RhoB by glucocorticoids and its effects on the cell proliferation and NF-kappaB transcriptional activity

Although there is ample evidence that glucocorticoids (GCs) have an antiproliferative effect on many cell types, the molecular mechanism remains elusive. We reported in our previous study that Dex treatment led to cell growth arrest in a human ovarian cancer cell HO-8910. RhoB, as a member of Rho GTPases, have been implicated to be a negative regulator of cell proliferation. In this study, we provided novel evidence that Dex induced the expressions of small GTPase RhoB mRNA and protein, but not RhoA and RhoC mRNA in a dose- and time-dependent fashion via glucocorticoid receptor (GR). Over-expression of RhoB increased while inhibition of RhoB expression by RNA interference reversed Dex-induced growth arrest, indicating that RhoB signaling is involved in Dex-induced proliferation inhibition. We also presented the novel observation that over-expression or activation of RhoB signaling elevated the basal transcriptional activity of the transcription factor NF-kappaB in HO-8910 cells. Furthermore, elevating RhoB signaling enhanced the inhibitory effect of Dex on NF-kappaB activity, while attenuating RhoB signaling almost abrogated Dex suppression of NF-kappaB signaling, indicating that RhoB pathway is involved in the regulation of NF-kappaB activity and is essential for Dex transcriptional repression on NF-kappaB signaling in HO-8910 cells.

Microarray and bioinformatic detection of novel and established genes expressed in experimental anti-Thy1 nephritis

BACKGROUND

Microarray technology is a powerful tool that can probe the molecular pathogenesis of renal injury. In this present study microarray analysis was used to monitor serial changes in the renal transcriptome of a rat model of mesangial proliferative glomerulonephritis. Administration of anti-Thy1 antibody results in phases of acute mesangial injury (day 2), cell proliferation (day 5), matrix expansion (days 5 and 7), and subsequent healing (day 14).

METHODS

Using Affymetrix (RAE230A) microarrays coupled with sequential primary biologic function-focused and secondary "baited" global cluster analysis, a cohort of established and putative novel modulators of mesangial cell turnover was identified.

RESULTS

Cluster analysis of proliferative genes identified a number of gene expression profiles. The most striking pattern was increased gene expression at day 5, a cluster that included platelet-derived growth factor (PDGF), cyclins and transforming growth factor-beta (TGF-beta). The gene expression patterns identified by primary focused cluster analysis were used as bioinformatic bait and resulted in the identification of novel families of genes such as the S100 family. The expression of established and novel genes was confirmed using reverse transcription-polymerase chain reaction (RT-PCR). Next, in vivo gene expression was compared to PDGF-stimulated mesangial cells in vitro revealing similar patterns of dysregulation.

CONCLUSION

Transcriptomic analysis defined both known and novel molecules involved in mesangial cell proliferation in vitro and in vivo and defined a panel of molecules that are potential contributors to mesangial cell dysfunction in glomerular disease.

Geranylgeranylated, but not farnesylated, RhoB suppresses Ras transformation of NIH-3T3 cells

RhoB is a low molecular weight GTPase that is both farnesylated (RhoB-F) and geranylgeranylated (RhoB-GG) in cells. Based on data from rodent cell models, it has been suggested that RhoB displays differential effects on cell transformation, according to the nature of its prenylation. To test directly this hypothesis, we generated GTPase-deficient RhoB mutants that are exclusively either farnesylated or geranylgeranylated. We show that in Ras-transformed murine NIH-3T3 cells, RhoB-F enhances, whereas RhoB-GG and RhoB (F/GG) suppresses anchorage-dependent and -independent cell growth as well as tumor growth in nude mice. We then demonstrate that Ras constitutive activation of the tumor survival pathways Akt and NF-kappa B are blocked by RhoB-GG, but not by RhoB-F, providing further support for the opposing role of RhoB-F and RhoB-GG in Ras malignant transformation in NIH-3T3 cells. In addition, both RhoB (F/GG) and RhoB-GG induce apoptosis in Ras-transformed NIH-3T3 cells whereas RhoB-F has no effect. Our data demonstrate that RhoB-F and RhoB-GG which differ only by a 5-carbon isoprene behave differently in rodent cells highlighting the important role of prenyl groups in protein function and emphasize the potency of RhoB to regulate negatively the oncogenic signal.

RhoB mRNA is stabilized by HuR after UV light

RhoB is a small GTP-binding protein that is involved in apoptotic signal transduction. We have cloned the mouse RhoB mRNA including a 1377 nucleotide 3'-untranslated region (UTR) that contains six AU-rich elements (AREs) as well as several uridine-rich stretches. There is 94% homology overall between the mouse and rat RhoB genes and 92% homology between the mouse and a putative human clone. Ultraviolet light (UVL) induces RhoB production through regulated changes in gene transcription and mRNA stabilization although the latter mechanism is unknown. We observed that UVL increased the half-life of RhoB mRNA from 63 min to 3.3 h in NIH/3T3 cells and from 87 min to 2.7 h in normal human keratinocyte cells. In vitro mobility shift assays demonstrated that HuR bound the 3'-UTR of RhoB at three distinct locations (nucleotides 1342-1696, 1765-1920 and 1897-1977) suggesting a regulatory role for this RNA-binding protein. HuR immunoprecipitations were positive for RhoB mRNA indicating an in vivo association, and Western blot analysis and immunofluorescence demonstrated that HuR rapidly partitions from the nucleus to the cytoplasm after UVL. Therefore, we propose a model in which UVL induces stress-activated signal transduction leading to nuclear/cytoplasmic shuttling of HuR and subsequent stabilization of RhoB mRNA.

RhoB and actin polymerization coordinate Src activation with endosome-mediated delivery to the membrane

We have used a c-Src-GFP fusion protein to address the spatial control of Src activation and the nature of Src-associated intracellular structures during stimulus-induced transit to the membrane. Src is activated during transit, particularly in RhoB-containing cytoplasmic endosomes associated with the perinuclear recycling compartment. Knocking out RhoB or expressing a dominant-interfering Rab11 mutant suppresses both catalytic activation of Src and translocation of active kinase to peripheral membrane structures. In addition, the Src- and RhoB-containing endosomes harbor proteins involved in actin polymerization and filament assembly, for example Scar1, and newly polymerized actin can associate with these endosomes in a Src-dependent manner. This implies that Src may regulate an endosome-associated actin nucleation activity. In keeping with this, Src controls the actin dependence of RhoB endosome movement toward the plasma membrane. This work identifies RhoB as a component of "outside-in" signaling pathways that coordinate Src activation with translocation to transmembrane receptors.

Rho-family GTPases: it's not only Rac and Rho (and I like it)

The Rho-family proteins make up a major branch of the Ras superfamily of small GTPases. To date, 22 human genes encoding at least 25 proteins have been described. The best known 'classical' members are RhoA, Rac1 and Cdc42. Highly related isoforms of these three proteins have not been studied as intensively, in part because it has been assumed that they are functionally identical to their better-studied counterparts. This now appears not to be the case. Variations in C-terminal-signaled modifications and subcellular targeting cause otherwise highly biochemically related isoforms (e.g. RhoA, RhoB and RhoC) to exhibit surprisingly divergent biological activities. Whereas the classical Rho GTPases are regulated by GDP/GTP cycling, other Rho GTPases are also regulated by other mechanisms, particularly by transcriptional regulation. Newer members of the family possess additional sequence elements beyond the GTPase domain, which suggests they exhibit yet other mechanisms of regulation.

Fasudil, a Rho-kinase inhibitor, attenuates glomerulosclerosis in Dahl salt-sensitive rats

OBJECTIVE

The present study was designed to clarify whether the Rho-Rho-kinase pathway is involved in the process of hypertensive glomerulosclerosis and to assess the therapeutic effect of fasudil, a specific Rho-kinase inhibitor.

METHOD AND RESULTS

Dahl salt-sensitive rats (DS) and Dahl salt-resistant rats (DR) were fed a high-salt diet at 6 weeks of age. Fasudil (30 mg/kg per day) was administered for 7 weeks to DS starting at the age of 11 weeks. After 7 weeks, untreated DS were characterized by decreased kidney function, increased proteinuria, abnormal morphological findings, increased adrenomedullin and atrial natriuretic peptide (ANP) levels, and increased renal messenger RNA expression of RhoB, Rho-kinasealpha, Rho-kinasebeta, collagen I and collagen III, and transforming growth factor-beta (TGF-beta) in the renal cortex compared with DR. Chronic fasudil treatment significantly improved renal function (serum creatinine, -26%; blood urea nitrogen, -41%; creatinine clearance, +42%), proteinuria (-24%) and histological findings (glomerular injury score, -49%; afferent arteriolar injury score, -17%) without changing blood pressure compared with untreated DS. Interestingly, long-term fasudil treatment decreased the plasma adrenomedullin (-25%) and ANP (-49%), but did not change the plasma renin or aldosterone. Furthermore, fasudil significantly decreased the messenger RNA expression of TGF-beta (-20%), collagen I (-23%), and collagen III (-24%) in the renal cortex. However, there were still significant differences in the aforementioned parameters between DR and fasudil-treated DS.

CONCLUSION

These results suggest that the Rho-Rho-kinase pathway may be partly responsible for the pathogenesis of hypertensive glomerulosclerosis independently of blood pressure in DS, and that chronic inhibition of the Rho-Rho-kinase pathway may be a new strategy for treating hypertensive nephrosclerosis.

Farnesyltransferase inhibitors disrupt EGF receptor traffic through modulation of the RhoB GTPase

The Rho family of small GTPases play a pivotal role in the dynamic regulation of the actin cytoskeleton. Recent studies have suggested that these signalling proteins also have wide-ranging functions in membrane trafficking pathways. The Rho family member RhoB was shown to localise to vesicles of the endocytic compartment, suggesting a potential function in regulation of endocytic traffic. In keeping with this, we have previously shown that expression of active RhoB causes a delay in the intracellular trafficking of the epidermal growth factor (EGF) receptor; however, the site of action of RhoB within the endocytic pathway is still unknown. RhoB exists as two prenylated forms in cells: geranylgeranylated RhoB (RhoB-GG) and farnesylated RhoB (RhoB-F). Here we use farnesyltransferase inhibitors (FTIs) to show that prenylation specifies the cellular localisation of RhoB. RhoB-GG localises to multivesicular late endosomes and farnesylated RhoB (RhoB-F) localises to the plasma membrane. The gain of endosomal RhoB-GG elicited by FTI treatment reduces sorting of EGF receptor to the lysosome and increases recycling to the plasma membrane. Ultrastructural analysis shows that activation of RhoB through drug treatment or mutation has no effect the sorting of receptor into late endosomes, but instead inhibits the subsequent transfer of late endosomal receptor to the lysosome.

EGFR, ErbB2 and Ras but not Src suppress RhoB expression while ectopic expression of RhoB antagonizes oncogene-mediated transformation

While some low molecular weight GTPases such as Ras and RhoA contribute to malignant transformation, a closely related family member, RhoB, has tumor-suppressive activity, but little is known about its regulation by oncogenes. In this study, we show that H-Ras, N-Ras, K-Ras, EGFR and ErbB2 but not v-Src suppress RhoB promoter transcriptional activity in NIH3T3 cells and human cancer cell lines derived from lung (A-549), pancreatic (Panc-1) and cervical (C33A) tumors. The EGFR and ErbB2 suppression of RhoB promoter activity is mediated by Ras. Furthermore, Ras suppresses basal as well as 5-fluorouracil (5-FU)-induced RhoB promoter activity and RhoB protein levels. Ectopic expression of RhoB, but not the closely related family member RhoA, antagonizes the ability of EGFR, ErbB2, H-Ras, N-Ras and K-Ras but not v-Src to transform NIH3T3 cells. Furthermore, RhoB, but not RhoA, inhibits colony formation and proliferation and induces anoikis in A-549 cells and Ras-transformed NIH3T3 cells. Finally, Ras-mediated resistance to 5-FU-induced apoptosis is reversed by RhoB. These results demonstrate that RhoB expression is negatively regulated by oncogenes that are prevalent in human cancers, and that ectopic expression of RhoB antagonizes the ability of these oncogenes to induce transformation. Taken together the data suggest that certain oncogenes suppress RhoB as one of the critical steps leading to malignant transformation.