RhoB is stabilized by transforming growth factor beta and antagonizes transcriptional activation

Urine Measurements of the Renin-Angiotensin System-Regulated Proteins Predict Death and Graft Loss in Kidney Transplant Recipients Enrolled in a Ramipril versus Placebo Randomized Controlled Trial

The renin-angiotensin system (RAS) is involved in kidney fibrosis. We previously identified six RAS-regulated proteins (RHOB, BST1, LYPA1, GLNA, TSP1, and LAMB2) that were increased in the urine of patients with kidney allograft fibrosis, compared to patients without fibrosis. We hypothesized that these urinary RAS-regulated proteins predicted primary outcomes in kidney transplant recipients enrolled in the largest RAS inhibitor randomized controlled trial. Urine excretion of 10 peptides corresponding to the six RAS-regulated proteins was quantified using parallel reaction monitoring mass spectrometry assays (normalized by urine creatinine) in a subset of patients in the trial. Machine learning models predicting outcomes based on urine peptide excretion rates were developed and evaluated. Urine samples (n = 111) from 56 patients were collected at 0, 6, 12, and 24 months. Twenty-four primary outcomes (doubling of serum creatinine, graft loss, or death) occurred in 17 patients. Logistic regression utilizing eight peptides of TSP1, BST1, LAMB2, LYPA1, and RHOB, from the last urine sample prior to outcomes, predicted a graft loss with an AUC of 0.78 (p = 0.00001). A random forest classifier utilizing BST1 and LYPA1 peptides predicted death with an AUC of 0.80 (p = 0.0016). Urine measurements of RAS-regulated proteins may predict outcomes in kidney transplant recipients, although further prospective studies are required.

Golgi screen identifies the RhoGEF Solo as a novel regulator of RhoB and endocytic transport

The control of intracellular membrane trafficking by Rho GTPases is central to cellular homeostasis. How specific guanine nucleotide exchange factors and GTPase-activating proteins locally balance GTPase activation in this process is nevertheless largely unclear. By performing a microscopy-based RNAi screen, we here identify the RhoGEF protein Solo as a functional counterplayer of DLC3, a RhoGAP protein with established roles in membrane trafficking. Biochemical, imaging and optogenetics assays further uncover Solo as a novel regulator of endosomal RhoB. Remarkably, we find that Solo and DLC3 control not only the activity, but also total protein levels of RhoB in an antagonistic manner. Together, the results of our study uncover the first functionally connected RhoGAP-RhoGEF pair at endomembranes, placing Solo and DLC3 at the core of endocytic trafficking.

Association between renin-angiotensin system and chronic lung allograft dysfunction

Chronic lung allograft dysfunction (CLAD) is the major cause of death after lung transplantation. Angiotensin II (AngII), the main effector of the renin-angiotensin (RA) system, elicits fibrosis in both kidney and lung. We identified 6 AngII-regulated proteins (RHOB, BST1, LYPA1, GLNA, TSP1, LAMB1) increased in urine of patients with kidney allograft fibrosis. We hypothesized that RA system is active in CLAD and that AngII-regulated proteins are increased in bronchoalveolar lavage fluid (BAL) of CLAD patients.We performed immunostaining of AngII receptors (AGTR1 and AGTR2) and TSP1/GLNA in 10 CLAD lungs and 5 controls. Using mass spectrometry, we quantified peptides corresponding to AngII-regulated proteins in BAL of 40 lung transplant recipients (CLAD, stable and acute lung allograft dysfunction (ALAD)). Machine learning algorithms were developed to predict CLAD based on BAL peptide concentrations.Immunostaining demonstrated significantly more AGTR1+ cells in CLAD versus control lungs (p=0.02). TSP1 and GLNA immunostaining positively correlated with the degree of lung fibrosis (R²=0.42 and 0.57, respectively). In BAL, we noted a trend toward higher concentrations of AngII-regulated peptides in patients with CLAD at the time of bronchoscopy, and significantly higher concentrations of BST1, GLNA and RHOB peptides in patients that developed CLAD at follow-up (p<0.05). Support vector machine classifier discriminated CLAD from stable and ALAD patients at the time of bronchoscopy with AUC 0.86, and accurately predicted subsequent CLAD development (AUC 0.97).Proteins involved in the RA system are increased in CLAD lung and BAL. AngII-regulated peptides measured in BAL may accurately identify patients with CLAD and predict subsequent CLAD development.

Actin Cytoskeleton and Regulation of TGFβ Signaling: Exploring Their Links

Human tissues, to maintain their architecture and function, respond to injuries by activating intricate biochemical and physical mechanisms that regulates intercellular communication crucial in maintaining tissue homeostasis. Coordination of the communication occurs through the activity of different actin cytoskeletal regulators, physically connected to extracellular matrix through integrins, generating a platform of biochemical and biomechanical signaling that is deregulated in cancer. Among the major pathways, a controller of cellular functions is the cytokine transforming growth factor β (TGFβ), which remains a complex and central signaling network still to be interpreted and explained in cancer progression. Here, we discuss the link between actin dynamics and TGFβ signaling with the aim of exploring their aberrant interaction in cancer.

Arf6 regulates RhoB subcellular localization to control cancer cell invasion

The ADP-ribosylation factor 6 (Arf6) is a small GTPase that regulates endocytic recycling processes in concert with various effectors. Arf6 controls cytoskeletal organization and membrane trafficking; however, the detailed mechanisms of regulation remain poorly understood. Here, we report that Arf6 forms a complex with RhoB. The interaction between RhoB and Arf6 is mediated by the GCI (glycine, cysteine, and isoleucine) residues (188-190) of RhoB. Specific targeting of Arf6 to plasma membrane or mitochondrial membranes promotes recruitment and colocalization of RhoB to these membrane microdomains. Arf6 depletion promotes the loss of RhoB from endosomal membranes and leads to RhoB degradation through an endolysosomal pathway. This results in defective actin and focal adhesion dynamics and increased 3D cell migration upon activation of the Met receptor tyrosine kinase. Our findings identify a novel regulatory mechanism for RhoB localization and stability by Arf6 and establish the strict requirement of Arf6 for RhoB-specific subcellular targeting to endosomes and biological functions.

Urine Angiotensin II Signature Proteins as Markers of Fibrosis in Kidney Transplant Recipients

BACKGROUND

Interstitial fibrosis/tubular atrophy (IFTA) is an important cause of kidney allograft loss; however, noninvasive markers to identify IFTA or guide antifibrotic therapy are lacking. Using angiotensin II (AngII) as the prototypical inducer of IFTA, we previously identified 83 AngII-regulated proteins in vitro. We developed mass spectrometry-based assays for quantification of 6 AngII signature proteins (bone marrow stromal cell antigen 1, glutamine synthetase [GLNA], laminin subunit beta-2, lysophospholipase I, ras homolog family member B, and thrombospondin-I [TSP1]) and hypothesized that their urine excretion will correlate with IFTA in kidney transplant patients.

METHODS

Urine excretion of 6 AngII-regulated proteins was quantified using selected reaction monitoring and normalized by urine creatinine. Immunohistochemistry was used to assess protein expression of TSP1 and GLNA in kidney biopsies.

RESULTS

The urine excretion rates of AngII-regulated proteins were found to be increased in 15 kidney transplant recipients with IFTA compared with 20 matched controls with no IFTA (mean log2[fmol/µmol of creatinine], bone marrow stromal cell antigen 1: 3.8 versus 3.0, P = 0.03; GLNA: 1.2 versus -0.4, P = 0.03; laminin subunit beta-2: 6.1 versus 5.4, P = 0.06; lysophospholipase I: 2.1 versus 0.6, P = 0.002; ras homolog family member B: 1.2 versus -0.1, P = 0.006; TSP1_GGV: 2.5 versus 1.9; P = 0.15; and TSP1_TIV: 2.0 versus 0.6, P = 0.0006). Receiver operating characteristic curve analysis demonstrated an area under the curve = 0.86 for the ability of urine AngII signature proteins to discriminate IFTA from controls. Urine excretion of AngII signature proteins correlated strongly with chronic IFTA and total inflammation. In a separate cohort of 19 kidney transplant recipients, the urine excretion of these 6 proteins was significantly lower following therapy with AngII inhibitors (P < 0.05).

CONCLUSIONS

AngII-regulated proteins may represent markers of IFTA and guide antifibrotic therapies.

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.

The Cullin-3-Rbx1-KCTD10 complex controls endothelial barrier function via K63 ubiquitination of RhoB

The RhoA GTPase controls endothelial cell migration, adhesion, and barrier formation but the role of RhoB is unclear. Kovačević et al. now discover that RhoB is ubiquitinated by the CUL3–Rbx1–KCTD10 complex and that this is a prerequisite for lysosomal degradation of RhoB and the maintenance of endothelial barrier integrity. 

RhoGTPases control endothelial cell (EC) migration, adhesion, and barrier formation. Whereas the relevance of RhoA for endothelial barrier function is widely accepted, the role of the RhoA homologue RhoB is poorly defined. RhoB and RhoA are 85% identical, but RhoB’s subcellular localization and half-life are uniquely different. Here, we studied the role of ubiquitination for the function and stability of RhoB in primary human ECs. We show that the K63 polyubiquitination at lysine 162 and 181 of RhoB targets the protein to lysosomes. Moreover, we identified the RING E3 ligase complex Cullin-3–Rbx1–KCTD10 as key modulator of endothelial barrier integrity via its regulation of the ubiquitination, localization, and activity of RhoB. In conclusion, our data show that ubiquitination controls the subcellular localization and lysosomal degradation of RhoB and thereby regulates the stability of the endothelial barrier through control of RhoB-mediated EC contraction.

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.

Smad-independent TGF-β2 signaling pathways in human trabecular meshwork cells

Aberrant expression and signaling of Transforming Growth Factor (TGF)-β is strongly associated with development of elevated intraocular pressure (IOP) and primary open-angle glaucoma (POAG). In cells of the trabecular meshwork, a key component of the conventional outflow pathway, TGF-β is well-known to promote expression of multiple ocular hypertensive mediators, including genes associated with fibrosis as well as cellular contractility. These effects are mediated by induction of canonical (Smad) as well as non-canonical (MAPK, Rho GTPase) signaling cascades. In the present review, we will highlight the non-canonical, Smad-independent signaling pathways activated by TGF-β2 in human TM cells, as well as the genes known to be induced by non-canonical TGF-β2 signaling.

RhoGTPases - A novel link between cytoskeleton organization and cisplatin resistance

For more than three decades, platinum compounds have been the first line treatment for a wide spectrum of solid tumors. Yet, cisplatin resistance is a major impediment in cancer therapy, and deciphering the mechanisms underlying chemoresistance is crucial for the development of novel therapies with enhanced efficacy. The Rho subfamily of small GTPases plays a significant role in cancer progression, and a growing body of evidence points toward the involvement of these proteins in anticancer drug resistance, including cisplatin resistance. The cycling between active and inactive states, governed by the balance between their GEFs, GAPs and GDIs, RhoGTPases, acts as molecular switches with a pivotal role in actin cytoskeleton organization. The Rho subfamily of proteins is involved in many key cellular processes including adhesion, vesicular trafficking, proliferation, survival, cell morphology and cell-matrix interactions. Although RhoA, RhoB and RhoC are highly homologous and share some upstream regulators and downstream effectors, they each have different roles in cancer progression and chemoresistance. While RhoA and RhoC are upregulated in many tumors and can stimulate transformation, RhoB appears to exhibit tumor suppressor characteristics with proapoptotic effects. In the current review, we discuss the role of Rho subfamily of proteins in cancer, and focus on their involvement in intrinsic and acquired drug resistance.

The neddylation-cullin 2-RBX1 E3 ligase axis targets tumor suppressor RhoB for degradation in liver cancer

The neddylation-cullin-RING E3 ligase (CRL) pathway has recently been identified as a potential oncogenic event and attractive anticancer target; however, its underlying mechanisms have not been well elucidated. In this study, RhoB, a well known tumor suppressor, was identified and validated with an iTRAQ-based quantitative proteomic approach as a new target of this pathway in liver cancer cells. Specifically, cullin 2-RBX1 E3 ligase, which requires NEDD8 conjugation for its activation, interacted with RhoB and promoted its ubiquitination and degradation. In human liver cancer tissues, the neddylation-CRL pathway was overactivated and reversely correlated with RhoB levels. Moreover, RhoB accumulation upon inhibition of the neddylation-CRL pathway for anticancer therapy contributed to the induction of tumor suppressors p21 and p27, apoptosis, and growth suppression. Our findings highlight the degradation of RhoB via the neddylation-CRL pathway as an important molecular event that drives liver carcinogenesis and RhoB itself as a pivotal effector for anticancer therapy targeting this oncogenic pathway.

Threonine 32 (Thr32) of FoxO3 is critical for TGF-β-induced apoptosis via Bim in hepatocarcinoma cells

Transforming growth factor-β (TGF-β) exerts apoptotic effects on various types of malignant cells, including liver cancer cells. However, the precise mechanisms by which TGF-β induces apoptosis remain poorly known. In the present study, we have showed that threonine 32 (Thr32) residue of FoxO3 is critical for TGF-β to induce apoptosis via Bim in hepatocarcinoma Hep3B cells. Our data demonstrated that TGF-β induced FoxO3 activation through specific de-phosphorylation at Thr32. TGF-β-activated FoxO3 cooperated with Smad2/3 to mediate Bim up-regulation and apoptosis. FoxO3 (de)phosphorylation at Thr32 was regulated by casein kinase I-ε (CKI-ε). CKI inhibition by small molecule D4476 could abrogate TGF-β-induced FoxO/Smad activation, reverse Bim up-regulation, and block the sequential apoptosis. More importantly, the deregulated levels of CKI-ε and p32FoxO3 were found in human malignant liver tissues. Taken together, our findings suggest that there might be a CKI-FoxO/Smad-Bim engine in which Thr32 of FoxO3 is pivotal for TGF-β-induced apoptosis, making it a potential therapeutic target for liver cancer treatment.

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.

MicroRNA-21 in glomerular injury

TGF-β(1) is a pleotropic growth factor that mediates glomerulosclerosis and podocyte apoptosis, hallmarks of glomerular diseases. The expression of microRNA-21 (miR-21) is regulated by TGF-β(1), and miR-21 inhibits apoptosis in cancer cells. TGF-β(1)-transgenic mice exhibit accelerated podocyte loss and glomerulosclerosis. We determined that miR-21 expression increases rapidly in cultured murine podocytes after exposure to TGF-β(1) and is higher in kidneys of TGF-β(1)-transgenic mice than wild-type mice. miR-21-deficient TGF-β(1)-transgenic mice showed increased proteinuria and glomerular extracellular matrix deposition and fewer podocytes per glomerular tuft compared with miR-21 wild-type TGF-β(1)-transgenic littermates. Similarly, miR-21 expression was increased in streptozotocin-induced diabetic mice, and loss of miR-21 in these mice was associated with increased albuminuria, podocyte depletion, and mesangial expansion. In cultured podocytes, inhibition of miR-21 was accompanied by increases in the rate of cell death, TGF-β/Smad3-signaling activity, and expression of known proapoptotic miR-21 target genes p53, Pdcd4, Smad7, Tgfbr2, and Timp3. In American-Indian patients with diabetic nephropathy (n=48), albumin-to-creatinine ratio was positively associated with miR-21 expression in glomerular fractions (r=0.6; P<0.001) but not tubulointerstitial fractions (P=0.80). These findings suggest that miR-21 ameliorates TGF-β(1) and hyperglycemia-induced glomerular injury through repression of proapoptotic signals, thereby inhibiting podocyte loss. This finding is in contrast to observations in murine models of tubulointerstitial kidney injury but consistent with findings in cancer models. The aggravation of glomerular disease in miR-21-deficient mice and the positive association with albumin-to-creatinine ratio in patients with diabetic nephropathy support miR-21 as a feedback inhibitor of TGF-β signaling and functions.

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.

RHOB influences lung adenocarcinoma metastasis and resistance in a host-sensitive manner

Lung adenocarcinoma (ADC) is the most common lung cancer subtype and presents a high mortality rate. Clinical recurrence is often associated with the emergence of metastasis and treatment resistance. The purpose of this study was to identify genes with high prometastatic activity which could potentially account for treatment resistance. Global transcriptomic profiling was performed by robust microarray analysis in highly metastatic subpopulations. Extensive in vitro and in vivo functional studies were achieved by overexpression and by silencing gene expression. We identified the small GTPase RHOB as a gene that promotes early and late stages of metastasis in ADC. Gene silencing of RHOB prevented metastatic activity in a systemic murine model of bone metastasis. These effects were highly dependent on tumor-host interactions. Clinical analysis revealed a marked association between high RHOB levels and poor survival. Consistently, high RHOB levels promote metastasis progression, taxane-chemoresistance, and contribute to the survival advantage to γ-irradiation. We postulate that RHOB belongs to a novel class of "genes of recurrence" that have a dual role in metastasis and treatment resistance.

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.

Cytoprotective effect of the small GTPase RhoB expressed upon treatment of fibroblasts with the Ras-glucosylating Clostridium sordellii lethal toxin

Mono-glucosylation of (H/K/N)Ras by Clostridium sordellii lethal toxin (TcsL) blocks critical survival signaling pathways, resulting in apoptosis. In this study, TcsL and K-Ras knock-down by siRNA are presented to result in expression of the cell death-regulating small GTPase RhoB. TcsL-induced RhoB expression is based on transcriptional activation involving p38(alpha) MAP kinase. Newly synthesized RhoB protein is rapidly degraded in a proteasome- and a caspase-dependent manner, providing first evidence for caspase-dependent degradation of a Rho family protein. Although often characterised as a pro-apoptotic protein, RhoB suppresses caspase-3 activation in TcsL-treated fibroblasts. The finding on the cytoprotective activity of RhoB in TcsL-treated cells re-enforces the concept that RhoB exhibits cytoprotective rather than pro-apoptotic activity in a cellular background of inactive Ras.

Acetylsalicylic acid regulates overexpressed small GTPase RhoA in vascular smooth muscle cells through prevention of new synthesis and enhancement of protein degradation

RhoA has been shown to play a major role in vascular processes and acetylsalicylic acid (aspirin) is known to exert a cytoprotective effect via multiple mechanisms. In the present study, we aimed at investigating the effect of aspirin on RhoA expression under a stress state in rat VSMCs (vascular smooth muscle cells) and the underlying mechanisms. The expression of iNOS (inducible nitric oxide synthase) and iNOS activity as well as NO concentration was significantly promoted by LPS (lipopolysaccharide) accompanying the elevation of RhoA expression, which was blocked by the addition of the iNOS inhibitor L-NIL [L-N6-(1-iminoethyl)lysine dihydrochloride]. Aspirin (30 μM) significantly attenuated the elevation of RhoA, while indomethacin and salicylate had no similar effect. The sGC (soluble guanylate cyclase) inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) showed the same effect as aspirin in down-regulating RhoA but was reversed by the addition of the cGMP analogue 8-Br-PET-cGMP (β-phenyl-1,N2-ethano-8-bromoguanosine 3',5'-cyclic monophosphorothioate). 8-Br-PET-cGMP solely enhanced the RhoA expression that was abrogated by preincubation with aspirin. Degradation analysis indicated that aspirin enhanced the protein degradation rate of RhoA and GDP-bound RhoA seemed to be more susceptible to aspirin-enhanced degradation compared with the GTP-bound form. Our results indicate that aspirin attenuates the LPS-induced overexpression of RhoA both by inhibiting new synthesis and accelerating protein degradation, which may help elucidate the multiple beneficial effects of aspirin.

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.

RhoB deficiency in thymic medullary epithelium leads to early thymic atrophy

RhoB, a member of the Rho subfamily of small GTPases, mediates diverse cellular functions, including cytoskeletal organization, cell transformation and vesicle trafficking. The thymus undergoes progressive decline in its structure and function after puberty. We found that RhoB was expressed in thymic medullary epithelium. To investigate a role of RhoB in the regulation of thymic epithelial organization or thymocyte development, we analyzed the thymi of RhoB-deficient mice. RhoB-deficient mice were found to display earlier thymic atrophy. RhoB deficiency showed significant reductions in thymus weight and cellularity, beginning as early as 5 weeks of age. The enhanced expression of TGF-β receptor type II (TGFβRII) in thymic medullary epithelium was observed in RhoB-null mice. In addition, the expression of fibronectin, which is shown to be regulated by TGF-β signaling, was accordingly increased in the mutant thymic medulla. Since there is no age-related change of RhoB expression in the thymus, it is unlikely that RhoB in thymic epithelium directly contributes to age-related thymic involution. Nevertheless, our findings strongly support a physiological role of RhoB in regulation of thymus development and maintenance through the inhibition of TGF-β signaling in thymic medullary epithelium.

JNK-mediated transcriptional upregulation of RhoB is critical for apoptosis of HCT-116 colon cancer cells by a novel diarylsulfonylurea derivative

Diarylsulfonylureas are potent antitumor agents that have been tested in clinical trials. However, detailed mechanisms of their apoptotic activity remain unclear. Here, we report a new diarylsulfonylurea derivative, LB2A, that upregulates RhoB, thereby inducing potent apoptosis in HCT-116 human colon cancer cells independently of p53 status. LB2A decreased procaspase-3, increased phospho-JNK, and cleaved PARP, leading to apoptosis of HCT-116 cells. Prior treatment of HCT-116 cells with the JNK inhibitor SP600125 and the RNA synthesis inhibitor DRB blocked apoptosis, implying that JNK activation and mRNA production are important for apoptosis by LB2A. Western blotting, RT-PCR, and RhoB-promoter luciferase reporter assays revealed that LB2A increased RhoB via JNK-mediated transcriptional activation. LB2A decreased HDAC1 and increased acetyl-H3, both of which activate the RhoB promoter and were blocked by SP600125. Ectopic expression of RhoB induced apoptosis of HCT-116 cells, suggesting that RhoB is critical for the anti-cancer activity of LB2A in human colon cancer cells. LB2A also exhibited potent tumor growth inhibition of HCT-116 cells in vivo using a mouse xenograft assay. Taken together, these results show that LB2A induces apoptosis of HCT-116 cells via JNK-mediated transcriptional upregulation of RhoB and may therefore provide a potential therapy for human colon cancer.

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.

Direct modifications of Rho proteins: deconstructing GTPase regulation

Small GTPases of the Rho protein family are master regulators of the actin cytoskeleton and are targeted by potent virulence factors of several pathogenic bacteria. Their dysfunctional regulation can lead to severe human pathologies. Both host and bacterial factors can activate or inactivate Rho proteins by direct post-translational modifications: such as deamidation and transglutamination for activation, or ADP-ribosylation, glucosylation, adenylylation and phosphorylation for inactivation. We review and compare these unconventional ways in which both host cells and bacterial pathogens regulate Rho proteins.

The RhoA activator GEF-H1/Lfc is a transforming growth factor-beta target gene and effector that regulates alpha-smooth muscle actin expression and cell migration

TGFβ induces various responses, including Rho signaling. How TGFβ stimulates Rho is poorly understood. Our data indicate that GEF-H1 is a target and effector of TGFβ to regulate Rho signaling, gene expression, and cell migration, suggesting that it represents a new marker and possible therapeutic target for degenerative and fibrotic diseases.

Maintenance of the epithelial phenotype is crucial for tissue homeostasis. In the retina, dedifferentiation and loss of integrity of the retinal pigment epithelium (RPE) leads to retinal dysfunction and fibrosis. Transforming growth factor (TGF)-β critically contributes to RPE dedifferentiation and induces various responses, including increased Rho signaling, up-regulation of α-smooth muscle actin (SMA), and cell migration and dedifferentiation. Cellular TGF-β responses are stimulated by different signal transduction pathways: some are Smad dependent and others Smad independent. Alterations in Rho signaling are crucial to both types of TGF-β signaling, but how TGF-β-stimulates Rho signaling is poorly understood. Here, we show that primary RPE cells up-regulated GEF-H1 in response to TGF-β. GEF-H1 was the only detectable Rho exchange factor increased by TGF-β1 in a genome-wide expression analysis. GEF-H1 induction was Smad4-dependant and led to Rho activation. GEF-H1 inhibition counteracted α-SMA up-regulation and cell migration. In patients with retinal detachments and fibrosis, migratory RPE cells exhibited increased GEF-H1 expression, indicating that induction occurs in diseased RPE in vivo. Our data indicate that GEF-H1 is a target and functional effector of TGF-β by orchestrating Rho signaling to regulate gene expression and cell migration, suggesting that it represents a new marker and possible therapeutic target for degenerative and fibrotic diseases.

The C-terminal sequence of RhoB directs protein degradation through an endo-lysosomal pathway

BACKGROUND

Protein degradation is essential for cell homeostasis. Targeting of proteins for degradation is often achieved by specific protein sequences or posttranslational modifications such as ubiquitination.

METHODOLOGY/PRINCIPAL FINDINGS

By using biochemical and genetic tools we have monitored the localization and degradation of endogenous and chimeric proteins in live primary cells by confocal microscopy and ultra-structural analysis. Here we identify an eight amino acid sequence from the C-terminus of the short-lived GTPase RhoB that directs the rapid degradation of both RhoB and chimeric proteins bearing this sequence through a lysosomal pathway. Elucidation of the RhoB degradation pathway unveils a mechanism dependent on protein isoprenylation and palmitoylation that involves sorting of the protein into multivesicular bodies, mediated by the ESCRT machinery. Moreover, RhoB sorting is regulated by late endosome specific lipid dynamics and is altered in human genetic lipid traffic disease.

CONCLUSIONS/SIGNIFICANCE

Our findings characterize a short-lived cytosolic protein that is degraded through a lysosomal pathway. In addition, we define a novel motif for protein sorting and rapid degradation, which allows controlling protein levels by means of clinically used drugs.

A negative modulatory role for rho and rho-associated kinase signaling in delamination of neural crest cells

Background

Neural crest progenitors arise as epithelial cells and then undergo a process of epithelial to mesenchymal transition that precedes the generation of cellular motility and subsequent migration. We aim at understanding the underlying molecular network. Along this line, possible roles of Rho GTPases that act as molecular switches to control a variety of signal transduction pathways remain virtually unexplored, as are putative interactions between Rho proteins and additional known components of this cascade.

Results

We investigated the role of Rho/Rock signaling in neural crest delamination. Active RhoA and RhoB are expressed in the membrane of epithelial progenitors and are downregulated upon delamination. In vivo loss-of-function of RhoA or RhoB or of overall Rho signaling by C3 transferase enhanced and/or triggered premature crest delamination yet had no effect on cell specification. Consistently, treatment of explanted neural primordia with membrane-permeable C3 or with the Rock inhibitor Y27632 both accelerated and enhanced crest emigration without affecting cell proliferation. These treatments altered neural crest morphology by reducing stress fibers, focal adhesions and downregulating membrane-bound N-cadherin. Reciprocally, activation of endogenous Rho by lysophosphatidic acid inhibited emigration while enhancing the above. Since delamination is triggered by BMP and requires G1/S transition, we examined their relationship with Rho. Blocking Rho/Rock function rescued crest emigration upon treatment with noggin or with the G1/S inhibitor mimosine. In the latter condition, cells emigrated while arrested at G1. Conversely, BMP4 was unable to rescue cell emigration when endogenous Rho activity was enhanced by lysophosphatidic acid.

Conclusion

Rho-GTPases, through Rock, act downstream of BMP and of G1/S transition to negatively regulate crest delamination by modifying cytoskeleton assembly and intercellular adhesion.

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.

TGF-beta in neural stem cells and in tumors of the central nervous system

Mechanisms that regulate neural stem cell activity in the adult brain are tightly coordinated. They provide new neurons and glia in regions associated with high cellular and functional plasticity, after injury, or during neurodegeneration. Because of the proliferative and plastic potential of neural stem cells, they are currently thought to escape their physiological control mechanisms and transform to cancer stem cells. Signals provided by proteins of the transforming growth factor (TGF)-beta family might represent a system by which neural stem cells are controlled under physiological conditions but released from this control after transformation to cancer stem cells. TGF-beta is a multifunctional cytokine involved in various physiological and patho-physiological processes of the brain. It is induced in the adult brain after injury or hypoxia and during neurodegeneration when it modulates and dampens inflammatory responses. After injury, although TGF-beta is neuroprotective, it may limit the self-repair of the brain by inhibiting neural stem cell proliferation. Similar to its effect on neural stem cells, TGF-beta reveals anti-proliferative control on most cell types; however, paradoxically, many brain tumors escape from TGF-beta control. Moreover, brain tumors develop mechanisms that change the anti-proliferative influence of TGF-beta into oncogenic cues, mainly by orchestrating a multitude of TGF-beta-mediated effects upon matrix, migration and invasion, angiogenesis, and, most importantly, immune escape mechanisms. Thus, TGF-beta is involved in tumor progression. This review focuses on TGF-beta and its role in the regulation and control of neural and of brain-cancer stem cells.

Rho GTPase-activating bacterial toxins: from bacterial virulence regulation to eukaryotic cell biology

Studies on the interactions of bacterial pathogens with their host have provided an invaluable source of information on the major functions of eukaryotic and prokaryotic cell biology. In addition, this expanding field of research, known as cellular microbiology, has revealed fascinating examples of trans-kingdom functional interplay. Bacterial factors actually exploit eukaryotic cell machineries using refined molecular strategies to promote invasion and proliferation within their host. Here, we review a family of bacterial toxins that modulate their activity in eukaryotic cells by activating Rho GTPases and exploiting the ubiquitin/proteasome machineries. This family, found in human and animal pathogenic Gram-negative bacteria, encompasses the cytotoxic necrotizing factors (CNFs) from Escherichia coli and Yersinia species as well as dermonecrotic toxins from Bordetella species. We survey the genetics, biochemistry, molecular and cellular biology of these bacterial factors from the standpoint of the CNF1 toxin, the paradigm of Rho GTPase-activating toxins produced by urinary tract infections causing pathogenic Escherichia coli. Because it reveals important connections between bacterial invasion and the host inflammatory response, the mode of action of CNF1 and its related Rho GTPase-targetting toxins addresses major issues of basic and medical research and constitutes a privileged experimental model for host-pathogen interaction.

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.

CNF1-induced ubiquitylation and proteasome destruction of activated RhoA is impaired in Smurf1-/- cells

Ubiquitylation of RhoA has emerged as an important aspect of both the virulence of Escherichia coli producing cytotoxic necrotizing factor (CNF) 1 toxin and the establishment of the polarity of eukaryotic cells. Owing to the molecular activity of CNF1, we have investigated the relationship between permanent activation of RhoA catalyzed by CNF1 and subsequent ubiquitylation of RhoA by Smurf1. Using Smurf1-deficient cells and by RNA interference (RNAi)-mediated Smurf1 knockdown, we demonstrate that Smurf1 is a rate-limiting and specific factor of the ubiquitin-mediated proteasomal degradation of activated RhoA. We further show that the cancer cell lines HEp-2, human embryonic kidney 293 and Vero are specifically deficient in ubiquitylation of either activated Rac, Cdc42, or Rho, respectively. In contrast, CNF1 produced the cellular depletion of all three isoforms of Rho proteins in the primary human cell types we have tested. We demonstrate that ectopic expression of Smurf1 in Vero cells, deficient for RhoA ubiquitylation, restores ubiquitylation of the activated forms of RhoA. We conclude here that Smurf1 ubiquitylates activated RhoA and that, in contrast to human primary cell types, some cancer cell lines have a lower ubiquitylation capacity of specific Rho proteins. Thus, both CNF1 and transforming growth factor-beta trigger activated RhoA ubiquitylation through Smurf1 ubiquitin-ligase.

Bacterial toxins activating Rho GTPases

The CNF1 toxin is produced by some uropathogenic (UPECs) andmeningitis-causing Escherichia coli strains. It belongs to a large family of bacterial virulence factors and toxins modifying cellular regulators of the actin cytoskeleton, namely the Rho GTPases. CNF1 autonomously enters the host cell cytosol, where it catalyzes the constitutive activation of Rho GTPases by deamidation. This activation is, however, attenuated because of activated Rho protein ubiquitin-mediated proteasomal degradation. Both Rho protein activation and deactivation confer phagocytic properties on epithelial and endothelial cells, as well as epithelial cell motility and cell-cell junction dynamics. Transcriptome analysis using DNA microarray revealed that endothelial cells respond to high doses of CNF1 by launching a genetic program of host alarm. This host cell reaction to CNF1 intoxication also indicates that degradation of activated Rho proteins by the proteasome may lead to a lowering of the threshold of the intoxicated cell inflammatory response. These results are consistent with growing evidence that Rho proteins control the cell inflammatory responses. It is tempting to assume that Rho deregulation may participate in various immunological disorders also involved in cancer.

Mechanisms of statin-mediated inhibition of small G-protein function

3-Hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) have been reported to reduce the risk of Alzheimer disease. We have shown previously that statins inhibit a beta-amyloid (Abeta)-mediated inflammatory response through mechanisms independent of cholesterol reduction. Specifically, statins exert anti-inflammatory actions through their ability to prevent the isoprenylation of members of the Rho family of small G-proteins, resulting in the functional inactivation of these G-proteins. We report that statin treatment of microglia results in perturbation of the cytoskeleton and morphological changes due to alteration in Rho family function. Statins also block Abeta-stimulated phagocytosis through inhibition of Rac action. Paradoxically, the statin-mediated inactivation of G-protein function was associated with increased GTP loading of Rac and RhoA, and this effect was observed in myeloid lineage cells and other cell types. Statin treatment disrupted the interaction of Rac with its negative regulator the Rho guanine nucleotide dissociation inhibitor (RhoGDI), an interaction that is dependent on protein isoprenylation. We propose that lack of negative regulation accounts for the increased GTP loading. Isoprenylation of Rac is also required for efficient interaction with the plasma membrane, and we report that statin treatment dramatically reduces the capacity of Rac to interact with membranes. These results suggest a mechanism by which statins inhibit the actions of Rho GTPases and attenuate Abeta-stimulated inflammation.

Phosphorylation of serine 188 protects RhoA from ubiquitin/proteasome-mediated degradation in vascular smooth muscle cells

cAMP and cyclic GMP-dependent kinases (PKA and PKG) phosphorylate the small G protein RhoA on Ser188. We have previously demonstrated that phosphorylation of Ser188 inhibits RhoA-dependent functions and positively regulates RhoA expression, and that the nitric oxide (NO)/cGMP-dependent protein kinase pathway plays an essential role, both in vitro and in vivo, in the regulation of RhoA protein expression and functions in vascular smooth muscle cells. Here we analyze the consequences of Ser188 phosphorylation on RhoA protein degradation. By expressing Ser188 phosphomimetic wild-type (WT-RhoA-S188E) and active RhoA proteins (Q63L-RhoA-S188E), we show that phosphorylation of Ser188 of RhoA protects RhoA, particularly its active form, from ubiquitin-mediated proteasomal degradation. Coimmunoprecipitation experiments indicate that the resistance of the phosphorylated active form of RhoA to proteasome-mediated degradation is because of its cytoplasmic sequestration through enhanced RhoGDI interaction. In rat aortic smooth muscle cells, stimulation of PKG and inhibition of proteasome by lactacystin, induce nonadditive increases in RhoA protein expression. In addition, stimulation of PKG leads to the accumulation of GTP-bound RhoA in the cytoplasm. In vivo stimulation of the NO/PKG signaling by treating rats with sildenafil increased RhoA level and RhoA phosphorylation, and enhanced its association to RhoGDI in the pulmonary artery, whereas opposite effects are induced by chronic inhibition of NO synthesis in N-omega-nitro-L-arginine-treated rats. Our results thus suggest that Ser188 phosphorylation-mediated protection against degradation is a physiological process regulating the level of endogenous RhoA and define a novel function for RhoGDI, as an inhibitor of Rho protein degradation.

Elucidation of IL-1/TGF-beta interactions in mouse chondrocyte cell line by genome-wide gene expression

OBJECTIVE

To elucidate the antagonism between interleukin-1 (IL-1) and transforming growth factor-beta (TGF-beta) at the gene expression level, as IL-1 and TGF-beta are postulated to be critical mediators of cartilage degeneration/protection in rheumatic diseases.

METHODS

The H4 chondrocyte cell line was validated by comparing metalloproteinase expression profile with intact murine cartilage by reverse transcription polymerase chain reaction. Genome-wide gene expression in the H4 cells in response to IL-1 and TGF-beta, alone and in combination, was analyzed by using oligonucleotide arrays negotiating approximately 12,000 genes.

RESULTS

The response of cartilage and the H4 cell line to IL-1 and TGF-beta was comparable. Oligonucleotide array analysis demonstrated a mutual but asymmetrical antagonism as the dominant mode of interaction between IL-1 and TGF-beta. Cluster analysis revealed a remarkable selectivity in the mode of action exerted by TGF-beta on IL-1 regulated genes: antagonistic on pro-inflammatory genes whereas additive on growth regulators such as vascular endothelial growth factor (VEGF) and connective tissue growth factor (CTGF). While the former cluster underlined the protective effect of TGF-beta, the latter underscored the adverse effect of TGF-beta. We further identified potentially novel classes of target genes under control of TGF-beta such as ras family, histones, proteasome components, and ubiquitin family, highlighting the importance of such genes in TGF signaling besides the well-characterized SMAD pathway.

CONCLUSIONS

We identified a cluster of genes as potential targets mediating the adverse effect of TGF-beta such as fibrosis. Transcriptional regulation of ras GTPase and ubiquitin/proteasome pathways is likely to be a novel mechanism mediating the effect of TGF-beta and its interaction with IL-1. These down-stream genes and pathways can be targets in future therapy.

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.