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

P. aeruginosa type III and type VI secretion systems modulate early response gene expression in type II pneumocytes in vitro

BACKGROUND

Lung airway epithelial cells are part of innate immunity and the frontline of defense against bacterial infections. During infection, airway epithelial cells secrete proinflammatory mediators that participate in the recruitment of immune cells. Virulence factors expressed by bacterial pathogens can alter epithelial cell gene expression and modulate this response. Pseudomonas aeruginosa, a Gram-negative opportunistic pathogen, expresses numerous virulence factors to facilitate establishment of infection and evade the host immune response. This study focused on identifying the role of two major P. aeruginosa virulence factors, type III (T3SS) and type VI (T6SS) secretion systems, on the early transcriptome response of airway epithelial cells in vitro.

RESULTS

We performed RNA-seq analysis of the transcriptome response of type II pneumocytes during infection with P. aeruginosa in vitro. We observed that P. aeruginosa differentially upregulates immediate-early response genes and transcription factors that induce proinflammatory responses in type II pneumocytes. P. aeruginosa infection of type II pneumocytes was characterized by up-regulation of proinflammatory networks, including MAPK, TNF, and IL-17 signaling pathways. We also identified early response genes and proinflammatory signaling pathways whose expression change in response to infection with P. aeruginosa T3SS and T6SS mutants in type II pneumocytes. We determined that T3SS and T6SS modulate the expression of EGR1, FOS, and numerous genes that are involved in proinflammatory responses in epithelial cells during infection. T3SS and T6SS were associated with two distinct transcriptomic signatures related to the activation of transcription factors such as AP1, STAT1, and SP1, and the secretion of pro-inflammatory cytokines such as IL-6 and IL-8.

CONCLUSIONS

Taken together, transcriptomic analysis of epithelial cells indicates that the expression of immediate-early response genes quickly changes upon infection with P. aeruginosa and this response varies depending on bacterial viability and injectosomes. These data shed light on how P. aeruginosa modulates host epithelial transcriptome response during infection using T3SS and T6SS.

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.

MiR-19a Promotes Migration And Invasion By Targeting RHOB In Osteosarcoma

Introduction

Osteosarcoma is the most common bone tumor with high metastasis and recurrence rate. MicroRNA-19a (miR-19a) has been reported to act as tumor oncogene in multiple cancers. The objective of the study was to explore the molecular mechanisms of miR-19a in osteosarcoma cell migration and invasion.

Materials and methods

Real-time quantitative polymerase chain reaction (RT-qPCR) and Western blotting were employed to measure the levels of miR-19a and RhoB in osteosarcoma tissues and cell lines. Transwell assay was employed to analyze the tissues and cell lines’ migratory and invasive abilities. Dual luciferase reporter assay was utilized to analyze the association between miR-19a and RhoB.

Results

MiR-19a was overexpressed in osteosarcoma tissues and cell lines. MiR-19a promoted osteosarcoma cell migration and invasion in vitro. RhoB was thus confirmed as a direct and functional target of miR-19a, and it could partially reverse the function of miR-19a. Knockdown miR-19a inhibited osteosarcoma cell epithelial-mesenchymal transition (EMT) and suppressed osteosarcoma xenograft growth.

Conclusion

MiR-19a enhanced cell migration, invasion and EMT through RhoB in osteosarcoma. The newly identified miR-19a/RhoB axis provides novel insight into the progression of osteosarcoma and offers a promising target for osteosarcoma therapy.

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.

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.

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.

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.

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.

Rho GTPase expression in human myeloid cells

Myeloid cells are critical for innate immunity and the initiation of adaptive immunity. Strict regulation of the adhesive and migratory behavior is essential for proper functioning of these cells. Rho GTPases are important regulators of adhesion and migration; however, it is unknown which Rho GTPases are expressed in different myeloid cells. Here, we use a qPCR-based approach to investigate Rho GTPase expression in myeloid cells.

We found that the mRNAs encoding Cdc42, RhoQ, Rac1, Rac2, RhoA and RhoC are the most abundant. In addition, RhoG, RhoB, RhoF and RhoV are expressed at low levels or only in specific cell types. More differentiated cells along the monocyte-lineage display lower levels of Cdc42 and RhoV, while RhoC mRNA is more abundant. In addition, the Rho GTPase expression profile changes during dendritic cell maturation with Rac1 being upregulated and Rac2 downregulated. Finally, GM-CSF stimulation, during macrophage and osteoclast differentiation, leads to high expression of Rac2, while M-CSF induces high levels of RhoA, showing that these cytokines induce a distinct pattern. Our data uncover cell type specific modulation of the Rho GTPase expression profile in hematopoietic stem cells and in more differentiated cells of the myeloid lineage.

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.

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.

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.

HDJ-2 as a Target for Radiosensitization of Glioblastoma Multiforme Cells by the Farnesyltransferase Inhibitor R115777 and the Role of the p53/p21 Pathway

Resistance of glioblastoma multiforme to radiotherapy poses a major clinical challenge. Farnesyltransferase inhibitors (FTI), such as R115777, have potential to increase radiotherapeutic benefit in this disease, although their mechanism of action is unclear. In our study with eight glioblastoma multiforme cell lines, the most sensitive ones underwent cell cycle arrest in response to FTI treatment. Radiosensitization by FTIs, however, seemed to involve other pathways. If R115777 treatment was initiated < 6 hours before irradiation, all eight glioblastoma multiforme lines were radiosensitized. However, if the time between drug and radiation was extended to 24 hours, cells harboring wild type but not mutated p53 were able to counteract drug-induced radiosensitization. The involvement of the p53/p21 pathway in the development of resistance was confirmed by showing that U87 cells transfected with human papillomavirus E6 to block p53 or interfering RNA to inhibit p21 stayed radiosensitive for 24 hours after drug treatment. The time dependency of R115777-induced radiosensitization suggested that the initial FTI target for early radiosensitization was short-lived, and that a p21-directed pathway restored resistance. Consideration of prenylated molecules that could potentially be involved led us to consider HDJ-2, a co-chaperone of heat shock protein 70. This hypothesis was strengthened by finding that cellular radiosensitivity was increased by genetic inhibition of HDJ-2, whereas overexpression conferred radioresistance. Importantly, irradiation of cells caused HDJ-2 to migrate from the cytoplasm to the nucleus, and this migration was inhibited by prior FTI treatment. These results have clinical relevance in that they help explain the variability in responses to FTIs that occurs following radiotherapy and elucidate some of the reasons for the complexity underlying FTI-induced radiosensitization.

Akt mediates Ras downregulation of RhoB, a suppressor of transformation, invasion, and metastasis

Although recent evidence supports a tumor-suppressive role for the GTPase RhoB, little is known about its regulation by signal transduction pathways. Here we demonstrate that Ras downregulates RhoB expression by a phosphatidylinositol 3-kinase (PI3K)- and Akt- but not Mek-dependent mechanism. Furthermore, genetic and pharmacological blockade of PI3K/Akt results in upregulation of RhoB expression. We also provide evidence for the importance of the downregulation of RhoB in oncogenesis by demonstrating that RhoB antagonizes Ras/PI3K/Akt malignancy. Ectopic expression of RhoB, but not the close relative RhoA, inhibits Ras, PI3K, and Akt induction of transformation, migration, and invasion and induces apoptosis and anoikis. Finally, RhoB inhibits melanoma metastasis to the lung in a mouse model. These studies identify suppression of RhoB as a mechanism by which the Ras/PI3K/Akt pathway induces tumor survival, transformation, invasion, and metastasis.

Histone deacetylase 1 represses the small GTPase RhoB expression in human nonsmall lung carcinoma cell line

The dynamic balance between histone acetylation and deacetylation plays a significant role in the regulation of gene transcription. Much of our current understanding of this transcriptional control comes from the use of HDAC inhibitors such as trapoxin A (TPX), which leads to hyperacetylated histone, alters local chromatin architecture and transcription and results in tumor cell death. In this study, we treated tumor cells with TPX and HDAC1 antisense oligonucleotides, and analysed the transcriptional consequences of HDAC inhibition. Among other genes, the small GTPase RhoB was found to be significantly upregulated by TPX and repressed by HDAC1. The induction of RhoB by HDAC inhibition was mediated by an inverted CCAAT box in the RhoB promoter. Interestingly, measurement of RhoB transcription in approximately 130 tumor-derived cell lines revealed low expression in almost all of these samples, in contrast to RhoA and RhoC. Accumulating evidence indicates that the small GTPase Rho proteins are involved in a variety of important processes in cancer, including cell transformation, survival, invasion, metastasis and angiogenesis. This study for the first time demonstrates a link between HDAC inhibition and RhoB expression and provides an important insight into the mechanisms of HDAC-mediated transcriptional control and the potential therapeutic benefit of HDAC inhibition.

Mitogen-responsive expression of RhoB is regulated by RNA stability

The small GTPase-encoding gene RhoB is strongly induced as part of the immediate early response of serum-stimulated fibroblasts. In this report, we have characterized the mechanism for growth factor responsiveness of RhoB in Rat-2 fibroblasts. By Northern blotting and ribonuclease protection, we observed low or barely detectable levels of RhoB mRNA in quiescent cells, but expression was transiently induced in response to serum stimulation, such that the mRNA peaked within 30 min and then declined over the next hour. Analysis of the rat promoter revealed cis-elements conserved with the mouse and human genes, including a pair of CEBP sites near the transcriptional start site. However, in contrast to the analysis of RNA, RhoB promoter fusions were constitutively expressed in quiescent cells in transient transfections, and were unaffected by serum. Similarly, stable RhoB promoter integrants were highly expressed in quiescent cells, and growth factor caused a slight decrease in activity. This indicates that growth factor-inducible RhoB expression cannot be mediated by transcriptional activation. We then examined decay of the RhoB mRNA and found that serum caused significant stabilization. Additionally, fusion of the 3' RhoB untranslated region (UTR) to a constitutively expressed reporter gene caused serum and growth factor as well as DNA damage-inducible expression. These observations are consistent with the view that RhoB mRNA is produced constitutively but its abundance is controlled in response to growth factors, and other signals including DNA damage, by stabilization through elements within the 3' UTR.

Lovastatin inhibits brain endothelial cell Rho-mediated lymphocyte migration and attenuates experimental autoimmune encephalomyelitis

Neuroinflammatory diseases, such as multiple sclerosis (MS), result from aberrant leukocyte traffic into the central nervous system (CNS). To breach the specialized blood-brain barrier, activated leukocytes interact with CNS endothelial cells (EC) and activate a CD54-mediated signaling pathway controlling the Rho GTPase. To function correctly Rho requires posttranslational prenylation, and this can be inhibited by depleting the supply of isoprenoids through inhibition of the cholesterol synthesis pathway with 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoA reductase) inhibitors (statins). Here we show that treatment of brain EC in vitro with lovastatin inhibits Rho-mediated transendothelial T cell migration. This effect can be reversed by supplementation with mevalonolactone, the downstream product of HMG-CoA reductase, or by ectopic expression of myristoylated Rho, which remains active in the absence of prenylation. In a relapsing-remitting mouse model of MS, lovastatin treatment inhibited leukocyte migration into the CNS and significantly attenuated the development of both acute and relapsing clinical disease. These studies demonstrate that the indirect pharmacological inhibition of Rho proteins in brain EC by statins can inhibit a key stage in the pathogenesis of neuroinflammation, namely leukocyte migration across the blood-brain barrier. These studies demonstrate a novel effect of statins in modulating the immune response in neuroinflammtory diseases and may provide additional rationale for their use in the treatment of MS.

Inhibition of Rho GTPases with protein prenyltransferase inhibitors prevents leukocyte recruitment to the central nervous system and attenuates clinical signs of disease in an animal model of multiple sclerosis

The ICAM-1-mediated brain endothelial cell (EC)-signaling pathway induced by adherent lymphocytes is a central element in facilitating lymphocyte migration through the tight endothelial barrier of the brain. Rho proteins, which must undergo posttranslational prenylation to be functionally active, have been shown to be an essential component of this signaling cascade. In this study, we have evaluated the effect of inhibiting protein prenylation in brain ECs on their ability to support T lymphocyte migration. ECs treated in vitro with protein prenylation inhibitors resulted in a significant reduction in transendothelial T lymphocyte migration. To determine the therapeutic potential of this approach, an animal model of multiple sclerosis, experimental autoimmune encephalomyelitis, was induced in Biozzi ABH mice. Animals treated before disease onset with protein prenylation inhibitors exhibited a dramatic and significant reduction in both leukocyte infiltration into the CNS and clinical presentation of disease compared with untreated animals. These studies demonstrate, for the first time, the potential for pharmacologically targeting CNS EC signaling responses, and particularly endothelial Rho proteins, as a means of attenuating leukocyte recruitment to the CNS.

Analysis of DNA microarrays using algorithms that employ rule-based expert knowledge

The ability to investigate the transcription of thousands of genes concurrently by using DNA microarrays offers both major scientific opportunities and significant analytical challenges. Here we describe GABRIEL, a rule-based system of computer programs designed to apply domain-specific and procedural knowledge systematically and uniformly for the analysis and interpretation of data from DNA microarrays. GABRIEL'S problem-solving rules direct stereotypical tasks, whereas domain-specific knowledge pertains to gene functions and relationships or to experimental conditions. Additionally, GABRIEL can learn novel rules through genetic algorithms, which define patterns that best match the data being analyzed and can identify groupings in gene expression profiles preordered by chromosomal position or by a nonsupervised algorithm such as hierarchical clustering. GABRIEL subsystems explain the logic that underlies conclusions and provide a graphical interface and interactive platform for the acquisition of new knowledge. The present report compares GABRIEL'S output with published findings in which expert knowledge has been applied post hoc to microarray groupings generated by hierarchical clustering.

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.

Wrch-1, a novel member of the Rho gene family that is regulated by Wnt-1

We report the isolation and cloning of the Wrch-1 (Wnt-1 responsive Cdc42 homolog) cDNA. Wrch-1 is a novel gene whose mRNA level increases in response to Wnt-1 signaling in Wnt-1 transformed cells, Wnt-1 transgene induced mouse mammary tumors, and Wnt-1 retrovirus infected cells. Wrch-1 encodes a homolog of the Rho family of GTPases. It shares 57% amino acid sequence identity with Cdc42, but possesses a unique N-terminal domain that contains several putative PXXP SH3-binding motifs. Like Cdc42, Wrch-1 can activate PAK-1 and JNK-1, and induce filopodium formation and stress fiber dissolution. Active Wrch-1 stimulates quiescent cells to reenter the cell cycle. Moreover, overexpression of Wrch-1 phenocopies Wnt-1 in morphological transformation of mouse mammary epithelial cells. Taken together, Wrch-1 could mediate the effects of Wnt-1 signaling in the regulation of cell morphology, cytoskeletal organization, and cell proliferation.

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.

Transcriptional activation of the small GTPase gene rhoB by genotoxic stress is regulated via a CCAAT element

The gene encoding the Ras-related GTPase RhoB-specific is immediate-early inducible by genotoxic treatments. Regulation of transcriptional activation of rhoB is still unclear. Here we show that cells lacking either p53 or c-Fos are not different from wild-type cells with respect to the level of rhoB induction upon UV irradiation, indicating that these transcription factors are not crucial for stimulation of rhoB mRNA expression. Extracts from UV-irradiated and non-irradiated cells revealed similar DNA-binding activities to a 0.17 kb rhoB promoter fragment harboring the functional element(s) necessary for stimulation of rhoB by UV light. By means of immunoprecipitation we found that an ATF-2-specific antibody co-precipitates the (32)P-labeled 0.17 kb rhoB fragment, whereas an anti-AP1 antibody did not. Since no consensus sequence for binding of ATF-2 is present within the rhoB promoter, ATF-2 is likely to be associated with another factor that binds to the minimal promoter. Deletion analysis and site-directed mutagenesis of the 0.17 kb rhoB fragment revealed a CCAAT box to be an essential requirement for stimulation of rhoB by UV light and methyl methanesulfonate. Moreover, immunoprecipitation experiments showed that the CCAAT-binding factor NF-YA is complexed with ATF-2. Overall, the data strongly indicate that transcriptional activation of the rhoB gene by genotoxic stress is regulated via a CCAAT box and that interaction of CCAAT-binding factor and ATF-2 triggers the stress-inducible expression of rhoB.

Inhibition of farnesyltransferase increases TGFbeta type II receptor expression and enhances the responsiveness of human cancer cells to TGFbeta

Several small GTPases of the Ras superfamily have been shown to antagonize TGFbeta signaling in human tumor cell lines. Some of these GTPases are post-translationally modified by farnesylation, a lipid modification catalyzed by farnesyltransferase and required for the proteins to attach to membranes and to function. In this study, we investigated the effect of the farnesyltransferase inhibitor FTI-277 on TGFbeta-regulated cell growth and transcription. Treatment of the human pancreatic tumor cell line, Panc-1, with FTI-277 enhanced the ability of TGFbeta to inhibit both anchorage-dependent and -independent tumor cell growth. FTI-277 also enhanced the ability of TGFbeta to induce transcription, as measured by p3TP-lux reporter activity and collagen synthesis. The enhancement of TGFbeta responses by FTI-277 correlated with the stimulation of transcription and protein expression of type II TGFbeta receptor (TbetaRII). Consequently, FTI-277-treated cells exhibited a higher level of TGFbeta binding to its receptor. Thus, inhibition of protein farnesylation stimulates TbetaRII expression, which leads to increased TGFbeta receptor binding and signaling as well as inhibition of tumor cell growth and transformation.

RhoB alteration is necessary for apoptotic and antineoplastic responses to farnesyltransferase inhibitors

Farnesyltransferase inhibitors (FTIs) are in clinical trials, but how they selectively inhibit malignant cell growth remains uncertain. One important player in this process appears to be RhoB, an endosomal Rho protein that regulates receptor trafficking. FTI treatment elicits a gain of the geranylgeranylated RhoB isoform (RhoB-GG) that occurs due to modification of RhoB by geranylgeranyltransferase I in drug-treated cells. Notably, this event is sufficient to mediate antineoplastic effects in murine models and human carcinoma cells. To further assess this gain-of-function mechanism and determine whether RhoB-GG has a necessary role in drug action, we examined the FTI response of murine fibroblasts that cannot express RhoB-GG due to homozygous deletion of the rhoB gene. Nullizygous (-/-) cells were susceptible to cotransformation by adenovirus E1A plus activated H-Ras but defective in their FTI response, despite complete inhibition of H-Ras prenylation. Actin cytoskeletal and phenotypic events were disrupted in -/- cells, implicating RhoB-GG in these effects. Interestingly, -/- cells were resistant to FTI-induced growth inhibition under anchorage-dependent but not anchorage-independent conditions, indicating that, while RhoB-GG is sufficient, it is not necessary for growth inhibition under all conditions. In contrast, -/- cells were resistant to FTI-induced apoptosis in vitro and in vivo. Significantly, the apoptotic defect of -/- cells compromised the antitumor efficacy of FTI in xenograft assays. This study offers genetic proof of the hypothesis that RhoB-GG is a crucial mediator of the antineoplastic effects of FTIs.

The p21 Rho-activating toxin cytotoxic necrotizing factor 1 is endocytosed by a clathrin-independent mechanism and enters the cytosol by an acidic-dependent membrane translocation step

Cytotoxic necrotizing factor 1 (CNF1), a protein produced by pathogenic strains of Escherichia coli, activates the p21 Rho-GTP-binding protein, inducing a profound reorganization of the actin cytoskeleton. CNF1 binds to its cell surface receptor on HEp-2 cells with high affinity (K(d) = 20 pM). In HEp-2 cells the action of CNF1 is not blocked in the presence of filipin, a drug described to reduce cholera toxin internalization by the caveolae-like mechanism. Moreover, HEp-2 cells, which express a dominant negative form of proteins that impair the formation of clathrin coated-vesicles and internalization of transferrin (Eps15, dynamin or intersectin-Src homology 3), are still sensitive to CNF1. In this respect, the endocytosis of CNF1 is similar to the plant toxin ricin. However, unlike ricin toxin, CNF1 does not cross the Golgi apparatus and requires an acidic cell compartment to transfer its enzymatic activity into the cytosol in a manner similar to that required by diphtheria toxin. As shown for diphtheria toxin, the pH-dependent membrane translocation step of CNF1 could be mimicked at the level of the plasma membrane by a brief exposure to a pH of </=5.2. CNF1 is the first bacterial toxin described that uses both a clathrin-independent endocytic mechanism and an acidic-dependent membrane translocation step in its delivery of the catalytic domain to the cell cytosol.

Lymphocyte Migration Through Brain Endothelial Cell Monolayers Involves Signaling Through Endothelial ICAM-1 Via a Rho-Dependent Pathway

Lymphocyte extravasation into the brain is mediated largely by the Ig superfamily molecule ICAM-1. Several lines of evidence indicate that at the tight vascular barriers of the central nervous system (CNS), endothelial cell (EC) ICAM-1 not only acts as a docking molecule for circulating lymphocytes, but is also involved in transducing signals to the EC. In this paper, we examine the signaling pathways in brain EC following Ab ligation of endothelial ICAM-1, which mimics adhesion of lymphocytes to CNS endothelia. ICAM-1 cross-linking results in a reorganization of the endothelial actin cytoskeleton to form stress fibers and activation of the small guanosine triphosphate (GTP)-binding protein Rho. ICAM-1-stimulated tyrosine phosphorylation of the actin-associated molecule cortactin and ICAM-1-mediated, Ag/IL-2-stimulated T lymphocyte migration through EC monolayers were inhibited following pretreatment of EC with cytochalasin D. Pretreatment of EC with C3 transferase, a specific inhibitor of Rho proteins, significantly inhibited the transmonolayer migration of T lymphocytes, endothelial Rho-GTP loading, and endothelial actin reorganization, without affecting either lymphocyte adhesion to EC or cortactin phosphorylation. These data show that brain vascular EC are actively involved in facilitating T lymphocyte migration through the tight blood-brain barrier of the CNS and that this process involves ICAM-1-stimulated rearrangement of the endothelial actin cytoskeleton and functional EC Rho proteins.

Cell growth inhibition by farnesyltransferase inhibitors is mediated by gain of geranylgeranylated RhoB

Recent results have shown that the ability of farnesyltransferase inhibitors (FTIs) to inhibit malignant cell transformation and Ras prenylation can be separated. We proposed previously that farnesylated Rho proteins are important targets for alternation by FTIs, based on studies of RhoB (the FTI-Rho hypothesis). Cells treated with FTIs exhibit a loss of farnesylated RhoB but a gain of geranylgeranylated RhoB (RhoB-GG), which is associated with loss of growth-promoting activity. In this study, we tested whether the gain of RhoB-GG elicited by FTI treatment was sufficient to mediate FTI-induced cell growth inhibition. In support of this hypothesis, when expressed in Ras-transformed cells RhoB-GG induced phenotypic reversion, cell growth inhibition, and activation of the cell cycle kinase inhibitor p21WAF1. RhoB-GG did not affect the phenotype or growth of normal cells. These effects were similar to FTI treatment insofar as they were all induced in transformed cells but not in normal cells. RhoB-GG did not promote anoikis of Ras-transformed cells, implying that this response to FTIs involves loss-of-function effects. Our findings corroborate the FTI-Rho hypothesis and demonstrate that gain-of-function effects on Rho are part of the drug mechanism. Gain of RhoB-GG may explain how FTIs inhibit the growth of human tumor cells that lack Ras mutations.

Overexpression of the rhoC gene correlates with progression of ductal adenocarcinoma of the pancreas

It has been reported that the rho genes, which consist of a ras-related small GTPase protein family, regulate cytoskeletal structures and have the potential to transform cultured cells. To investigate the biological relevance of the rho genes in pancreatic carcinogenesis, we examined expressions of the rhoA, B and C genes by polymerase chain reaction after reverse transcription (RT-PCR) in 33 cases of ductal adenocarcinoma of the pancreas. In addition, mutations of the K-ras, rhoA, B and C genes were studied in the same series of tumour tissues to correlate with rho gene expressions. The expression levels of the rhoC gene were significantly higher in tumours than in non-malignant portions (P < 0.001). Metastatic lesions overexpressed the rhoC gene compared with primary tumours (P < 0.05). Carcinoma tissues with perineural invasion and lymph node metastasis exhibited significantly higher expressions of the rhoC gene than tumours without these manifestations (P < 0.001 and P < 0.05 respectively). Overexpression of the rhoC gene significantly correlated with poorer prognosis of patients with pancreatic adenocarcinoma (P < 0.05). In contrast, the expression levels of the rhoA and B genes showed no significant relationship with clinicopathological findings. Mutation was not found either in the rhoA, B or C gene sequences examined. K-ras gene mutation, detected in 27 out of 33 (81.8%) cases, did not affect the expression levels in any of the rho genes. These suggest that elevated expression of the rhoC gene may be involved in the progression of pancreatic carcinoma independent of K-ras gene activation.

Cell type-specific roles for Cdc42, Rac, and RhoL in Drosophila oogenesis

The Rho subfamily of GTPases has been shown to regulate cellular morphology. We report the discovery of a new member of the Rho family, named RhoL, which is equally similar to Rac, Rho, and Cdc42. Expression of a dominant-negative RhoL transgene in the Drosophila ovary caused nurse cells to collapse and fuse together. Mutant forms of Cdc42 mimicked this effect. Expression of constitutively active RhoL led to nurse cell subcortical actin breakdown and disruption of nurse cell-follicle cell contacts, followed by germ cell apoptosis. In contrast, Rac activity was specifically required for migration of a subset of follicle cells called border cells. All three activities were necessary for normal transfer of nurse cell cytoplasm to the oocyte. These results suggest that Rho protein activities have cell type-specific effects on morphogenesis.

Evidence that farnesyltransferase inhibitors suppress Ras transformation by interfering with Rho activity

Small-molecule inhibitors of the housekeeping enzyme farnesyltransferase (FT) suppress the malignant growth of Ras-transformed cells. Previous work suggested that the activity of these compounds reflected effects on actin stress fiber regulation rather than Ras inhibition. Rho proteins regulate stress fiber formation, and one member of this family, RhoB, is farnesylated in vivo. Therefore, we tested the hypothesis that interference with RhoB was the principal basis by which the peptidomimetic FT inhibitor L-739,749 suppressed Ras transformation. The half-life of RhoB was found to be approximately 2 h, supporting the possibility that it could be functionally depleted within the 18-h period required by L-739,749 to induce reversion. Cell treatment with L-739,749 disrupted the vesicular localization of RhoB but did not effect the localization of the closely related RhoA protein. Ras-transformed Rat1 cells ectopically expressing N-myristylated forms of RhoB (Myr-rhoB), whose vesicular localization was unaffected by L-739,749, were resistant to drug treatment. The protective effect of Myr-rhoB required the integrity of the RhoB effector domain and was not due to a gain-of-function effect of myristylation on cell growth. In contrast, Rat1 cells transformed by a myristylated Ras construct remained susceptible to growth inhibition by L-739,749. We concluded that Rho is necessary for Ras transformation and that FT inhibitors suppress the transformed phenotype at least in part by direct or indirect interference with Rho, possibly with RhoB itself.

ADP-ribosylation of the GTP-binding protein RhoA blocks cytoplasmic division in human myelomonocytic cells

To test the role of Rho GTP-binding proteins in growth regulation of human myelomonocytic tumour cells we used recombinant C3 exoenzyme of Clostridium botulinum to specifically ADP-ribosylate and inactivate Rho proteins in situ. In homogenates of HL60 cells, the C3 exoenzyme [32P]ADP-ribosylated one protein that was identified as RhoA by immunoblot and two-dimensional gel electrophoresis. [32P]ADP ribosylation of RhoA in HL60 homogenates in vitro was reduced to 10-20% when cells in culture were pretreated with C3 exoenzyme (10 micrograms, 24 h), indicating that 80-90% of RhoA could be ADP-ribosylated in situ. The C3 exoenzyme inhibited HL60 cell proliferation by up to 80% and the degree of growth inhibition correlated with the amount of in situ ADP-ribosylated RhoA in a time- and dose-dependent manner. Cell cycle analysis demonstrated that the C3 exoenzyme-treated HL60 cells accumulated in mitosis, and nuclear staining revealed binucleated cells. These findings suggest that RhoA has a key role in human myelomonocytic tumour cell growth by regulating cytoplasmic division.

The Ras signal transduction pathway

Considerable progress has been made over the past year in elucidating the mechanisms by which extracellular signals are transduced via cell surface receptors to trigger changes in gene expression which determine the growth and differentiated state of a cell. In particular, Ras proteins have been implicated as key intermediates that mediate the signal from upstream tyrosine kinases to a downstream cascade of serine/threonine kinases, which then activate nuclear factors that control gene expression and protein synthesis. How Ras proteins function is regulated in this role as a molecular switch, and how the signal is transmitted between the various components of the pathway, are now being determined. Finally, the Rho family of Ras-related proteins, which regulate the actin cytoskeleton, have also been implicated as mediators of oncogenic Ras transformation. The brisk pace at which the key components of Ras-mediated signal transduction pathways are being identified hold great promise that new targets for therapeutic intervention in cancer may now be identified.

Transcriptional downregulation of the retina-specific QR1 gene by pp60v-src and identification of a novel v-src-responsive unit

The embryonic avian neuroretina (NR) is part of the central nervous system and is composed of various cell types: photoreceptors and neuronal and Müller (glial) cells. These cells are derived from proliferating neuroectodermal precursors which differentiate after terminal mitosis and become organized in cell strata. Proliferation of differentiating NR cells can be induced by infection with Rous sarcoma virus (RSV) and requires the expression of a functional v-src gene. To understand the mechanisms involved in the regulation of neural cell growth and differentiation, we studied the transcriptional regulation of QR1, a gene specifically expressed in postmitotic NR cells. Transcription of this gene is detected primarily in Müller cells and is strongly downregulated by the v-src gene product. Moreover, QR1 expression takes place only during the late phase of retinal development and is shut off abruptly at hatching. We have isolated a promoter region(s) of the QR1 gene that confers v-src responsiveness. By transfection of QR1-CAT constructs into quail NR cells infected with the temperature-sensitive mutant of RSV, PA101, we have identified a v-src-responsive region located between -1208 and -1161 upstream of the transcription initiation site. This sequence is able to form two DNA-protein complexes, C1 and C2. Formation of complex C2 is specifically induced in cells expressing an active v-src product, while formation of C1 is detected mainly in nonproliferating quail NR cells upon pp60v-src inactivation. C1 is also a target for regulation during development. We have identified the DNA binding site for the C1 complex, a repeated GCTGAC sequence, and shown that mutations in this element abolish binding of this factor as well as transcription of the gene at the nonpermissive temperature. Neither formation of C1 nor that of C2 seems to involve factors known to be targeted in the pp60v-src cascade. Our data suggest that C1 could be a novel target for both developmental control and oncogene-induced cell growth regulation.

Transcriptional downregulation of the retina-specific QR1 gene by pp60v-src and identification of a novel v-src-responsive unit

The embryonic avian neuroretina (NR) is part of the central nervous system and is composed of various cell types: photoreceptors and neuronal and Müller (glial) cells. These cells are derived from proliferating neuroectodermal precursors which differentiate after terminal mitosis and become organized in cell strata. Proliferation of differentiating NR cells can be induced by infection with Rous sarcoma virus (RSV) and requires the expression of a functional v-src gene. To understand the mechanisms involved in the regulation of neural cell growth and differentiation, we studied the transcriptional regulation of QR1, a gene specifically expressed in postmitotic NR cells. Transcription of this gene is detected primarily in Müller cells and is strongly downregulated by the v-src gene product. Moreover, QR1 expression takes place only during the late phase of retinal development and is shut off abruptly at hatching. We have isolated a promoter region(s) of the QR1 gene that confers v-src responsiveness. By transfection of QR1-CAT constructs into quail NR cells infected with the temperature-sensitive mutant of RSV, PA101, we have identified a v-src-responsive region located between -1208 and -1161 upstream of the transcription initiation site. This sequence is able to form two DNA-protein complexes, C1 and C2. Formation of complex C2 is specifically induced in cells expressing an active v-src product, while formation of C1 is detected mainly in nonproliferating quail NR cells upon pp60v-src inactivation. C1 is also a target for regulation during development. We have identified the DNA binding site for the C1 complex, a repeated GCTGAC sequence, and shown that mutations in this element abolish binding of this factor as well as transcription of the gene at the nonpermissive temperature. Neither formation of C1 nor that of C2 seems to involve factors known to be targeted in the pp60v-src cascade. Our data suggest that C1 could be a novel target for both developmental control and oncogene-induced cell growth regulation.

Inhibition of PMA-induced, LFA-1-dependent lymphocyte aggregation by ADP ribosylation of the small molecular weight GTP binding protein, rho

Botulinum C3 exoenzyme specifically ADP-ribosylates a group of ras-related small molecular weight GTP-binding proteins, rho, and inhibits their biological activity. Using this enzyme, we examined the function of rho in PMA-induced activation of lymphocyte function-associated antigen-1 (LFA-1) in a B lymphoblastoid cell line, JY. Northern blot analysis revealed that among the three rho genes, rhoA mRNA was predominantly expressed in JY cells. Consistently, only one [32P]ADP-ribosylated band was found when the lysate of the cells was subjected to ADP ribosylation by C3 exoenzyme. When the cells were cultured with C3 exoenzyme, this substrate was ADP-ribosylated in situ in a time- and concentration-dependent manner. Concomitant with this ADP ribosylation, PMA-induced LFA-1/intercellular adhesion molecule (ICAM)-1-dependent aggregation of JY cells was inhibited. This inhibition was blocked by prior treatment of the enzyme with an anti-C3 monoclonal antibody, and overcome by stimulation with higher concentrations of PMA. The C3 exoenzyme-induced inhibition was not affected by shaking of the cell suspension, while inhibition of aggregation by cytochalasin B was abolished by this procedure, suggesting that the inhibitory effect of the C3 exoenzyme treatment was not due to decrease in cell motility. The C3 exoenzyme treatment affected neither protein phosphorylation in JY cells before and after PMA stimulation, nor affected surface expression of LFA-1 and ICAM-1. These results suggest that rhoA protein works downstream of protein kinase C activation linking PMA stimulation to LFA-1 activation and aggregation in JY cells.

Growth-regulated expression of rhoG, a new member of the ras homolog gene family

Cellular transition from the resting state to DNA synthesis involves master switches genes encoding transcriptional factors (e.g., fos, jun, and egr genes), whose targets remain to be fully characterized. To isolate coding sequences specifically accumulated in late G1, a differential screening was performed on a cDNA library prepared from hamster lung fibroblasts stimulated for 5 h with serum. One of the positive clones which displayed a sevenfold induction, turned out to code for a protein sharing homology to Ras-like products. Cloning and sequence analysis of the human homolog revealed that this putative new small GTPase, referred to as rhoG, is more closely related to the rac, CDC42, and TC10 members of the rho (ras homolog) gene family and might have diverged very early during evolution. rhoG mRNA accumulates in proportion to the mitogenic strength of various purified growth factors used for the stimulation, as a consequence of transcriptional activation. G1-specific RNA accumulation is impaired upon addition of antimitogenic cyclic AMP and is enhanced when protein synthesis is inhibited, mainly as a result of RNA stabilization. rhoG mRNA expression is observed in a wide variety of human organs but reaches a particularly high level in lung and placental tissues.

Growth-regulated expression of rhoG, a new member of the ras homolog gene family

Cellular transition from the resting state to DNA synthesis involves master switches genes encoding transcriptional factors (e.g., fos, jun, and egr genes), whose targets remain to be fully characterized. To isolate coding sequences specifically accumulated in late G1, a differential screening was performed on a cDNA library prepared from hamster lung fibroblasts stimulated for 5 h with serum. One of the positive clones which displayed a sevenfold induction, turned out to code for a protein sharing homology to Ras-like products. Cloning and sequence analysis of the human homolog revealed that this putative new small GTPase, referred to as rhoG, is more closely related to the rac, CDC42, and TC10 members of the rho (ras homolog) gene family and might have diverged very early during evolution. rhoG mRNA accumulates in proportion to the mitogenic strength of various purified growth factors used for the stimulation, as a consequence of transcriptional activation. G1-specific RNA accumulation is impaired upon addition of antimitogenic cyclic AMP and is enhanced when protein synthesis is inhibited, mainly as a result of RNA stabilization. rhoG mRNA expression is observed in a wide variety of human organs but reaches a particularly high level in lung and placental tissues.

Transcriptional activation of the CEF-4/9E3 cytokine gene by pp60v-src

The CEF-4/9E3 gene is expressed constitutively in Rous sarcoma virus (RSV)-transformed cells. This expression is largely determined by an increase in transcription of the gene. In this report, we characterize the regulatory elements responsible for the transformation-dependent activation of CEF-4/9E3. Three sequences corresponding to AP-1, PRD II/kappa B, and TAACGCAATT are involved in the process and therefore define the src-responsive unit (SRU) of the CEF-4 promoter. In constructs containing a deletion of the SRU, multiple copies of AP-1 or PRD II/kappa B, but not TAACGCAATT, led to activation of the promoter. Thus, factors interacting with these elements are constitutively activated in RSV-transformed chicken embryo fibroblasts. In agreement with the results of transient expression assays, protein binding to AP-1, PRD II/kappa B, and TAACGCAATT were more abundant in the nuclei of transformed cells. The expression of the CEF-4 promoter was investigated in cells infected by a temperature-sensitive mutant of RSV. No significant increase in CEF-4 promoter activity was detected early after activation of pp60v-src. In contrast, a substantial activation of the CEF-4 promoter was detected late after a temperature shift. Factors interacting with the TAACGCAATT, PRD II/kappa B, and AP-1 elements accumulated gradually over a period of several hours. Therefore, transcriptional activation plays an important role in the late, constitutive expression of the CEF-4 gene in stably transformed cells.

Transcriptional activation of the CEF-4/9E3 cytokine gene by pp60v-src

The CEF-4/9E3 gene is expressed constitutively in Rous sarcoma virus (RSV)-transformed cells. This expression is largely determined by an increase in transcription of the gene. In this report, we characterize the regulatory elements responsible for the transformation-dependent activation of CEF-4/9E3. Three sequences corresponding to AP-1, PRD II/kappa B, and TAACGCAATT are involved in the process and therefore define the src-responsive unit (SRU) of the CEF-4 promoter. In constructs containing a deletion of the SRU, multiple copies of AP-1 or PRD II/kappa B, but not TAACGCAATT, led to activation of the promoter. Thus, factors interacting with these elements are constitutively activated in RSV-transformed chicken embryo fibroblasts. In agreement with the results of transient expression assays, protein binding to AP-1, PRD II/kappa B, and TAACGCAATT were more abundant in the nuclei of transformed cells. The expression of the CEF-4 promoter was investigated in cells infected by a temperature-sensitive mutant of RSV. No significant increase in CEF-4 promoter activity was detected early after activation of pp60v-src. In contrast, a substantial activation of the CEF-4 promoter was detected late after a temperature shift. Factors interacting with the TAACGCAATT, PRD II/kappa B, and AP-1 elements accumulated gradually over a period of several hours. Therefore, transcriptional activation plays an important role in the late, constitutive expression of the CEF-4 gene in stably transformed cells.