Isoprenylation of RhoB is necessary for its degradation. A novel determinant in the complex regulation of RhoB expression by the mevalonate pathway

Simvastatin activates the spindle assembly checkpoint and causes abnormal cell division by modifying small GTPases

Simvastatin is an inhibitor of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, which is a rate-limiting enzyme of the cholesterol synthesis pathway. It has been used clinically as a lipid-lowering agent to reduce low-density lipoprotein (LDL) cholesterol levels. In addition, antitumor activity has been demonstrated. Although simvastatin attenuates the prenylation of small GTPases, its effects on cell division in which small GTPases play an important role, have not been examined as a mechanism underlying its cytostatic effects. In this study, we determined its effect on cell division. Cell cycle synchronization experiments revealed a delay in mitotic progression in simvastatin-treated cells at concentrations lower than the IC50. Time-lapse imaging analysis indicated that the duration of mitosis, especially from mitotic entry to anaphase onset, was prolonged. In addition, simvastatin increased the number of cells exhibiting misoriented anaphase/telophase and bleb formation. Inhibition of the spindle assembly checkpoint (SAC) kinase Mps1 canceled the mitotic delay. Additionally, the number of cells exhibiting kinetochore localization of BubR1, an essential component of SAC, was increased, suggesting an involvement of SAC in the mitotic delay. Enhancement of F-actin formation and cell rounding at mitotic entry indicates that cortical actin dynamics were affected by simvastatin. The cholesterol removal agent methyl-β-cyclodextrin (MβCD) accelerated mitotic progression differently from simvastatin, suggesting that cholesterol loss from the plasma membrane is not involved in the mitotic delay. Of note, the small GTPase RhoA, which is a critical factor for cortical actin dynamics, exhibited upregulated expression. In addition, Rap1 was likely not geranylgeranylated. Our results demonstrate that simvastatin affects actin dynamics by modifying small GTPases, thereby activating the spindle assembly checkpoint and causing abnormal cell division.

Geranylgeraniol prevents zoledronic acid-mediated reduction of viable mesenchymal stem cells via induction of Rho-dependent YAP activation

Long-term use of zoledronic acid (ZA) increases the risk of medication-related osteonecrosis of the jaw (MRONJ). This may be attributed to ZA-mediated reduction of viable mesenchymal stem cells (MSCs). ZA inhibits protein geranylgeranylation, thus suppressing cell viability and proliferation. Geranylgeraniol (GGOH), which is a naturally found intermediate compound in the mevalonate pathway, has positive effects against ZA. However, precise mechanisms by which GGOH may help preserve stem cell viability against ZA are not fully understood. The objective of this study was to investigate the cytoprotective mechanisms of GGOH against ZA. The results showed that while ZA dramatically decreased the number of viable MSCs, GGOH prevented this negative effect. GGOH-rescued ZA-exposed MSCs formed mineralization comparable to that produced by normal MSCs. Mechanistically, GGOH preserved the number of viable MSCs by its reversal of ZA-mediated Ki67+ MSC number reduction, cell cycle arrest and apoptosis. Moreover, GGOH prevented ZA-suppressed RhoA activity and YAP activation. The results also established the involvement of Rho-dependent YAP and YAP-mediated CDK6 in the cytoprotective ability of GGOH against ZA. In conclusion, GGOH preserves a pool of viable MSCs with osteogenic potency against ZA by rescuing the activity of Rho-dependent YAP activation, suggesting GGOH as a promising agent and YAP as a potential therapeutic target for MRONJ.

FBXW7 regulates endothelial barrier function by suppression of the cholesterol synthesis pathway and prenylation of RhoB

Rho GTPases control both the actin cytoskeleton and adherens junction stability and are recognized as essential regulators of endothelial barrier function. They act as molecular switches and are primarily regulated by the exchange of GDP and GTP. However, posttranslational modifications such as phosphorylation, prenylation, and ubiquitination can additionally alter their localization, stability, and activity. F-box proteins are involved in the recognition of substrate proteins predestined for ubiquitination and subsequent degradation. Given the importance of ubiquitination, we studied the effect of the loss of 62 members of the F-box protein family on endothelial barrier function in human umbilical vein endothelial cells. Endothelial barrier function was quantified by electrical cell impedance sensing and macromolecule passage assay. Our RNA interference–based screen identified FBXW7 as a key regulator of endothelial barrier function. Mechanistically, loss of FBXW7 induced the accumulation of the RhoB GTPase in endothelial cells, resulting in their increased contractility and permeability. FBXW7 knockdown induced activation of the cholesterol biosynthesis pathway and changed the prenylation of RhoB. This effect was reversed by farnesyl transferase inhibitors and by the addition of geranylgeranyl pyrophosphate. In summary, this study identifies FBXW7 as a novel regulator of endothelial barrier function in vitro. Loss of FBXW7 indirectly modulates RhoB activity via alteration of the cholesterol biosynthesis pathway and, consequently, of the prenylation status and activity of RhoB, resulting in increased contractility and disruption of the endothelial barrier.

Effect of survivin downregulation by simvastatin on the growth and invasion of salivary adenoid cystic carcinoma

Simvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, is been used in the clinic due to its pleiotropic effects, such as breast cancer, prostate cancer, pancreatic cancer. Simvastatin has recently been demonstrated to serve a potential role in the prophylaxis and therapeutics of a number of human cancers. The majority of reports concerning simvastatin treatment in the majority of human cancers have demonstrated that survivin is significantly decreased as a result and has been implicated in tumorigenesis. However, only a limited number of studies have investigated the use of simvastatin for the treatment of salivary gland adenoid cystic carcinoma (SACC). Therefore, this agent is a candidate for further investigation. The aim of the present study was to investigate the effects of simvastatin on the proliferation, invasion and apoptosis of the human salivary adenoid cystic carcinoma cell line, SACC-83, as well as survivin expression in the cells. The Cell Counting kit-8 assay results revealed that simvastatin inhibited the proliferation of SACC-83 cells in a dose-dependent (10 to 50 µM) and time-dependent (24 to 48 h) manner when compared with the untreated cells. Flow cytometry analysis indicated that simvastatin increased the percentage of cells in early and late apoptosis. Invasion assays revealed that simvastatin treatment inhibited the invasiveness of SACC-83 cells in a dose-dependent manner. In addition, simvastatin downregulated survivin expression in SACC-83 cells. In conclusion, simvastatin significantly inhibited the proliferation and invasion of SACC-83 cells, induced apoptosis, and reduced the expression of survivin, which suggests that simvastatin may be a novel target for SACC therapy.

Rasd1, a small G protein with a big role in the hypothalamic response to neuronal activation

Background

Rasd1 is a member of the Ras family of monomeric G proteins that was first identified as a dexamethasone inducible gene in the pituitary corticotroph cell line AtT20. Using microarrays we previously identified increased Rasd1 mRNA expression in the rat supraoptic nucleus (SON) and paraventricular nucleus (PVN) of the hypothalamus in response to increased plasma osmolality provoked by fluid deprivation and salt loading. RASD1 has been shown to inhibit adenylyl cyclase activity in vitro resulting in the inhibition of the cAMP-PKA-CREB signaling pathway. Therefore, we tested the hypothesis that RASD1 may inhibit cAMP stimulated gene expression in the brain.

Results

We show that Rasd1 is expressed in vasopressin neurons of the PVN and SON, within which mRNA levels are induced by hyperosmotic cues. Dexamethasone treatment of AtT20 cells decreased forskolin stimulation of c-Fos, Nr4a1 and phosphorylated CREB expression, effects that were mimicked by overexpression of Rasd1, and inhibited by knockdown of Rasd1. These effects were dependent upon isoprenylation, as both farnesyltransferase inhibitor FTI-277 and CAAX box deletion prevented Rasd1 inhibition of cAMP-induced gene expression. Injection of lentiviral vector into rat SON expressing Rasd1 diminished, whereas CAAX mutant increased, cAMP inducible genes in response to osmotic stress.

Conclusions

We have identified two mechanisms of Rasd1 induction in the hypothalamus, one by elevated glucocorticoids in response to stress, and one in response to increased plasma osmolality resulting from osmotic stress. We propose that the abundance of RASD1 in vasopressin expressing neurons, based on its inhibitory actions on CREB phosphorylation, is an important mechanism for controlling the transcriptional responses to stressors in both the PVN and SON. These effects likely occur through modulation of cAMP-PKA-CREB signaling pathway in the brain.

The bisphosphonate zoledronic acid regulates key angiogenesis-related genes in primary human gingival fibroblasts

BACKGROUND

Osteonecrosis of the jaws is recognised as a serious complication for patients receiving bisphosphonates. The anti-angiogenic effects of bisphosphonates have been implicated in the pathogenesis of bisphosphonate-related osteonecrosis of the jaw (BRONJ). The purpose of this study was to determine the effects of zoledronic acid on cultured human gingival fibroblasts in relation to the modulation of genes associated with angiogenic regulation.

METHODS

Primary cultures of fibroblasts were developed from gingival tissues excised during crown-lengthening surgery from three patients. Cells were cultured with and without 30μM zoledronic acid for 6, 12 and 24h and cellular proliferation and migration investigated using CellTiter-Blue and scratch wound assays, respectively. Gene expression was determined using semi-quantitative PCR array technology that allowed the analysis of 84 pathway-focused genes known to be important in the regulation of angiogenesis.

RESULTS

Zoledronic acid increased the proliferation of the gingival fibroblasts in a dose dependent manner with 12 and 24h of exposure. Scratch wounding of the human gingival fibroblasts and treatment with increasing doses and time exposure to zoledronic acid (ZA) inhibited their migration. Statistically significant increases in gene expression were found for RHOB, VEGFA, CD55 and BMP2 (p≤0.05) in response to 30μM zoledronic acid. CCL2 and IL6 genes were significantly downregulated (p≤0.05).

CONCLUSIONS

The regulation of the prenylated protein RHOB in this study was consistent with the known effects of zoledronic acid on the mevalonate pathway. The down regulation of CCL2 and IL6 and the upregulation of CD55 may be associated with suppression of inflammation. An increase in VEGFA and BMP2 gene expression suggests that fibroblasts respond to zoledronic acid by producing a proangiogenic environment.

An isoprenylation and palmitoylation motif promotes intraluminal vesicle delivery of proteins in cells from distant species

The C-terminal ends of small GTPases contain hypervariable sequences which may be posttranslationally modified by defined lipid moieties. The diverse structural motifs generated direct proteins towards specific cellular membranes or organelles. However, knowledge on the factors that determine these selective associations is limited. Here we show, using advanced microscopy, that the isoprenylation and palmitoylation motif of human RhoB (–CINCCKVL) targets chimeric proteins to intraluminal vesicles of endolysosomes in human cells, displaying preferential co-localization with components of the late endocytic pathway. Moreover, this distribution is conserved in distant species, including cells from amphibians, insects and fungi. Blocking lipidic modifications results in accumulation of CINCCKVL chimeras in the cytosol, from where they can reach endolysosomes upon release of this block. Remarkably, CINCCKVL constructs are sorted to intraluminal vesicles in a cholesterol-dependent process. In the lower species, neither the C-terminal sequence of RhoB, nor the endosomal distribution of its homologs are conserved; in spite of this, CINCCKVL constructs also reach endolysosomes in Xenopus laevis and insect cells. Strikingly, this behavior is prominent in the filamentous ascomycete fungus Aspergillus nidulans, in which GFP-CINCCKVL is sorted into endosomes and vacuoles in a lipidation-dependent manner and allows monitoring endosomal movement in live fungi. In summary, the isoprenylated and palmitoylated CINCCKVL sequence constitutes a specific structure which delineates an endolysosomal sorting strategy operative in phylogenetically diverse organisms.

Zoledronic acid and geranylgeraniol regulate cellular behaviour and angiogenic gene expression in human gingival fibroblasts

The mevalonate pathway (MVP) and the anti-angiogenic effect of bisphosphonates have been shown to play a role in the pathogenesis of bisphosphonate-related osteonecrosis of the jaw (BRONJ). This study determined the effect of the bisphosphonate, zoledronic acid and the replenishment of the MVP by geranylgeraniol on human gingival fibroblasts. Cell viability, apoptosis, morphological analysis using transmission electron microscopy, and gene expression for vascular endothelial growth factor A, bone morphogenic protein 2, ras homologue gene family member B, epiregulin and interferon-alpha were conducted. Results showed cellular viability was decreased in the presence of zoledronic acid and the co-addition of zoledronic acid with geranylgeraniol restored cell viability to control levels. Caspase 3/7 was detected in zoledronic-acid-treated cells indicating apoptosis. Transmission electron microscopy revealed dilation of the rough endoplasmic reticulum with zoledronic acid and the appearance of multiple lipid-like vesicles following the addition of geranylgeraniol. Zoledronic acid significantly (P < 0.05, FR > ± 2) up-regulated vascular endothelial growth factor A, bone morphogenic protein 2, ras homologue gene family member B and epiregulin at one or more time points but not interferon-alpha. Addition of geranylgeraniol resulted in a reduction in the expression of all five genes compared with zoledronic-acid-treated human gingival fibroblasts. The study concluded geranylgeraniol partially reversed the effects of zoledronic acid in human gingival fibroblasts both at the cellular and genetic levels, suggesting the regulation of these genes is mediated via the mevalonate pathway.

Innate and adaptive inflammation as a therapeutic target in vascular disease: the emerging role of statins

Atherosclerosis, the main pathophysiological condition leading to cardiovascular disease (CVD), is now considered to be a chronic inflammatory condition. Statins are the most widely used and promising agents in treating CVD and are renowned for their pleiotropic lipid-lowering independent effects. Statins exert their anti-inflammatory effects on the vascular wall through a variety of molecular pathways of the innate and adaptive immune systems, their impact on the circulating levels of pro-inflammatory cytokines, and their effect on adhesion molecules. By inhibiting the mevalonate pathway and isoprenoid formation, statins account for the increase of nitric oxide bioavailability and the improvement of vascular and myocardial redox state by multiple different mechanisms (directly or indirectly through low-density lipoprotein [LDL] lowering). A large number of randomized control trials have shown that statins help in the primary and secondary prevention of cardiovascular events, not only via their lipid-lowering effect, but also due to their anti-inflammatory potential as well. In this paper, we examine the molecular pathways in which statins are implicated and exert their anti-inflammatory effects, and we focus specifically on their impact on innate and adaptive immunity systems. Finally, we review the most important clinical data for the role of statins in primary and secondary prevention of cardiovascular events.

Expression loss and revivification of RhoB gene in ovary carcinoma carcinogenesis and development

RhoB, a member of small GTPases belonging to the Ras protein superfamily, might have a suppressive activity in cancer progression. Here, expression of RhoB gene was evaluated in human benign, borderline and malignant ovary tumors by immunostaining, with normal ovary tissue as control. Malignant tumors were assessed according to Federation Internationale de Gynecologie Obstetrique (FIGO) guidelines and classified in stage I-IV. Revivification of RhoB gene was investigated by analyzing the effect of histone deacetylase (HDAC) inhibitor trichostatin (TSA) and methyltransferase inhibitor 5-azacytidine (5-Aza) on ovarian cancer cells via RT-PCR and western blot. Apoptosis of ovary cancer cells was detected using flowcytometry and fluorescence microscopy. Subsequently, RhoB expression is detected in normal ovary epithelium, borderline tumors, and decreases significantly or lost in the majority of ovarian cancer specimen (P<0.05). RhoB expression decreases significantly from stage II (71.4%) to stage III (43.5%) to stage IV (18.2%, P<0.05). TSA can both significantly revive the RhoB gene and mediate apoptosis of ovarian cancer cells, but 5-Aza couldn't. Interference into Revivification of RhoB gene results in reduction of ovary carcinoma cell apoptosis. It is proposed that loss of RhoB expression occurs frequently in ovary carcinogenesis and progression and its expression could be regulated by histone deacetylation but not by promoter hypermethylation, which may serve as a prospective gene treatment target for the patients with ovarian malignancy not responding to standard therapies.

Deciphering the signaling networks underlying simvastatin-induced apoptosis in human cancer cells: evidence for non-canonical activation of RhoA and Rac1 GTPases

Although statins are known to inhibit proliferation and induce death in a number of cancer cell types, the mechanisms through which downregulation of the mevalonate (MVA) pathway activates death signaling remain poorly understood. Here we set out to unravel the signaling networks downstream of the MVA pathway that mediate the death-inducing activity of simvastatin. Consistent with previous reports, exogenously added geranylgeranylpyrophosphate, but not farnesylpyrophosphate, prevented simvastatin's growth-inhibitory effect, thereby suggesting the involvement of geranylgeranylated proteins such as Rho GTPases in the anticancer activity of simvastatin. Indeed, simvastatin treatment led to increased levels of unprenylated Ras homolog gene family, member A (RhoA), Ras-related C3 botulinum toxin substrate 1 (Rac1) and cell division cycle 42 (Cdc42). Intriguingly, instead of inhibiting the functions of Rho GTPases as was expected with loss of prenylation, simvastatin caused a paradoxical increase in the GTP-bound forms of RhoA, Rac1 and Cdc42. Furthermore, simvastatin disrupted the binding of Rho GTPases with the cytosolic inhibitor Rho GDIα, which provides a potential mechanism for GTP loading of the cytosolic Rho GTPases. We also show that the unprenylated RhoA- and Rac1-GTP retained at least part of their functional activities, as evidenced by the increase in intracellular superoxide production and JNK activation in response to simvastatin. Notably, blocking superoxide production attenuated JNK activation as well as cell death induced by simvastatin. Finally, we provide evidence for the involvement of the B-cell lymphoma protein 2 family, Bcl-2-interacting mediator (Bim), in a JNK-dependent manner, in the apoptosis-inducing activity of simvastatin. Taken together, our data highlight the critical role of non-canonical regulation of Rho GTPases and involvement of downstream superoxide-mediated activation of JNK pathway in the anticancer activity of simvastatin, which would have potential clinical implications.

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.

The role of small GTPases in neuronal morphogenesis and polarity

The highly dynamic remodeling and cross talk of the microtubule and actin cytoskeleton support neuronal morphogenesis. Small RhoGTPases family members have emerged as crucial regulators of cytoskeletal dynamics. In this review we will comprehensively analyze findings that support the participation of RhoA, Rac, Cdc42, and TC10 in different neuronal morphogenetic events ranging from migration to synaptic plasticity. We will specifically address the contribution of these GTPases to support neuronal polarity and axonal elongation.

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.

Prenylation of Rho G-proteins: a novel mechanism regulating gene expression and protein stability in human trabecular meshwork cells

Endogenous prenylation with sesquiterpene or diterpene isoprenoids facilitates membrane localization and functional activation of small monomeric GTP-binding proteins. A direct effect of isoprenoids on regulation of gene expression and protein stability has also been proposed. In this study, we determined the role of sesquiterpene or diterpene isoprenoids on the regulation of Rho G-protein expression, activation, and stability in human trabecular meshwork (TM) cells. In both primary and transformed human TM cells, limiting endogenous isoprenoid synthesis with lovastatin, a potent HMG-CoA reductase inhibitor, elicited marked increases in RhoA and RhoB mRNA and protein content. The effect of lovastatin was dose-dependent with newly synthesized inactive protein accumulating in the cytosol. Supplementation with geranylgeranyl pyrophosphate (GGPP) prevented, while inhibition of geranylgeranyl transferase-I mimicked, the effects of lovastatin on RhoA and RhoB protein content. Similarly, lovastatin-dependent increases in RhoA and RhoB mRNA expression were mimicked by geranylgeranyl transferase-I inhibition. Interestingly, GGPP supplementation selectively promoted the degradation of newly synthesized Rho proteins which was mediated, in part, through the 20S proteasome. Functionally, GGPP supplementation prevented lovastatin-dependent decreases in actin stress fiber organization while selectively facilitating the subcellular redistribution of accumulated Rho proteins from the cytosol to the membrane and increasing RhoA activation. Post-translational prenylation with geranylgeranyl diterpenes selectively facilitates the expression, membrane translocation, functional activation, and turnover of newly synthesized Rho proteins. Geranylgeranyl prenylation represents a novel mechanism by which active Rho proteins are targeted to the 20S proteasome for degradation in human TM cells.

RhoB is associated with the anti-angiogenic effects of celiac patient transglutaminase 2-targeted autoantibodies

Celiac patient-derived anti-transglutaminase 2 (TG2) antibodies disturb several steps in angiogenesis, but the detailed molecular basis is not known. Therefore, we here analyzed by microarray technology the expression of a set of genes related to angiogenesis and endothelial cell biology in order to identify factors that could explain our previous data related to vascular biology in the context of celiac disease. To this end, in vitro models using human umbilical vein endothelial cells (HUVECs) or in vivo models of angiogenesis were used. A total of 116 genes were analyzed after treatment with celiac patient autoantibodies against TG2. Compared to treatment with control IgA celiac patient, total IgA induced a consistent expression change of 10 genes, the up-regulation of four and down-regulation of six. Of these genes the up-regulated RhoB was selected for further studies. RhoB expression was found to be up-regulated at both messenger RNA and protein level in response to celiac patient total IgA as well as anti-TG2-specific antibody derived from a celiac patient. Interestingly, down-regulation of RhoB by specific small interfering RNA treatment in endothelial cells could rescue the deranged endothelial length and tubule formation caused by celiac disease autoantibodies. RhoB function is controlled by its post-translational modification by farnesylation. This modification of RhoB required for its correct function can be prevented by the cholesterol lowering drug simvastatin, which was also able to abolish the anti-angiogenic effects of celiac anti-TG2 autoantibodies. Taken together, our results would suggest that RhoB plays a key role in the response of endothelial cells to celiac disease-specific anti-TG2 autoantibodies.

Off the beaten paths: alternative and crosstalk regulation of Rho GTPases

Rho proteins are small GTPases of the Ras superfamily that regulate a wide variety of biological processes, ranging from gene expression to cell migration. Mechanistically, the major Rho GTPases function as molecular switches cycling between an inactive GDP-bound and an active GTP-bound conformation, although several Rho proteins spontaneously exchange nucleotides or are simply devoid of GTPase activity. For over a decade, RhoGEFs and RhoGAPs have been established as the mainstream regulators of Rho proteins, respectively flipping the switch on or off. However, regulation by GEFs and GAPs leaves several fundamental questions on the operation of the Rho switch unanswered, indicating that the regulation of Rho proteins does not rely exclusively on RhoGEFs and RhoGAPs. Recent evidence indeed suggests that Rho GTPases are finely tuned by multiple alternative regulatory mechanisms, including post-translational modifications and protein degradation, as well as crosstalk mechanisms between Rho proteins. Here we review these alternative mechanisms and discuss how they alter Rho protein function and signaling. We also envision how the classic binary Rho switch may indeed function more like a switchboard with multiple switches and dials that can all contribute to the regulation of Rho protein function.

The role of ubiquitylation and degradation in RhoGTPase signalling

Rho-like guanosine triphosphatases (RhoGTPases) control many aspects of cellular physiology through their effects on the actin cytoskeleton and on gene transcription. Signalling by RhoGTPases is tightly coordinated and requires a series of regulatory proteins, including guanine-nucleotide exchange factors (GEFs), GTPase-activating proteins (GAPs) and guanine-nucleotide dissociation inhibitors (GDIs). GEFs and GAPs regulate GTPase cycling between the active (GTP-bound) and inactive (GDP-bound) states, whereas GDI is a cytosolic chaperone that binds inactive RhoGTPases. Like many other proteins, RhoGTPases are subject to degradation following the covalent conjugation of ubiquitin. There have been increasing indications that ubiquitylation of small GTPases occurs in a regulated fashion, primarily upon activation, and is an important means to control signalling output. Recent work has identified cellular proteins that control RasGTPase and RhoGTPase ubiquitylation and degradation, allowing us to amend the canonical model for GTPase (in)activation. Moreover, accumulating evidence for indirect regulation of GTPase function through the ubiquitylation of GTPase regulators makes this post-translational modification a key feature of GTPase-dependent signalling pathways. Here, we will discuss these recent insights into the regulation of RhoGTPase ubiquitylation and their relevance for cell signalling.

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

BACKGROUND

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

METHOD

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

RESULTS

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

CONCLUSION

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

Structural determinants allowing endolysosomal sorting and degradation of endosomal GTPases

Rapid control of protein degradation is usually achieved through the ubiquitin-proteasome pathway. We recently found that the short-lived GTPase RhoB is degraded in lysosomes. Moreover, the fusion of the RhoB C-terminal sequence CINCCKVL, containing the isoprenylation and palmitoylation sites, to other proteins directs their sorting into multivesicular bodies (MVBs) and rapid lysosomal degradation. Here, we show that this process is highly specific for RhoB. Alteration of late endosome lipid dynamics produced the accumulation of RhoB, but not of other endosomal GTPases, including Rab5, Rab7, Rab9 or Rab11, into enlarged MVB. Other isoprenylated and bipalmitoylated GTPases, such as H-Ras, Rap2A, Rap2B and TC10, were not accumulated into MVB and were stable. Remarkably, although TC10, which is highly homologous to RhoB, was stable, a sequence derived from its C-terminus (CINCCLIT) elicited MVB sorting and degradation of a green fluorescent protein (GFP)-chimeric protein. This led us to identify a cluster of basic amino acids (KKH) in the TC10 hypervariable region, constituting a secondary signal potentially involved in electrostatic interactions with membrane lipids. Mutation of this cluster allowed TC10 MVB sorting and degradation, whereas inserting it into RhoB hypervariable region rescued this protein from its lysosomal degradation pathway. These findings define a highly specific structural module for entering the MVB pathway and rapid lysosomal degradation.

A tagging-via-substrate approach to detect the farnesylated proteome using two-dimensional electrophoresis coupled with Western blotting

Prenylation is a post-translational modification critical for the proper function of multiple physiologically important proteins, including small G-proteins, such as Ras. Methods allowing rapid and selective detection of protein farnesylation and geranylgeranylation are fundamental for the understanding of prenylated protein function and for monitoring efficacy of drugs such as farnesyltransferase inhibitors (FTIs). Although the natural substrates for prenyltransferases are farnesyl pyrophosphate and geranylgeranyl pyrophosphate, farnesyltransferase has been shown to incorporate isoprenoid analogues into protein substrates. In this study, protein prenyltransferase targets were labeled using anilinogeraniol, the alcohol precursor to the unnatural farnesyl pyrophosphate analogue 8-anilinogeranyl diphosphate in a tagging-via-substrate approach. Antibodies specific for the anilinogeranyl moiety were used to detect the anilinogeranyl-modified proteins. Coupling this highly effective labeling/detection method with two-dimensional electrophoresis and subsequent Western blotting allowed simple, rapid analysis of the complex farnesylated proteome. For example, this method elucidated the differential effects induced by two chemically distinct FTIs, BMS-214,662 and L-778,123. Although both FTIs strongly inhibited farnesylation of many proteins such as Lamins, NAP1L1, N-Ras, and H-Ras, only the dual prenylation inhibitor L-778,123 blocked prenylation of Pex19, RhoB, K-Ras, Cdc42, and Rap1. This snapshot approach has significant advantages over traditional techniques, including radiolabeling, anti-farnesyl antibodies, or mass spectroscopy, and enables dynamic analysis of the farnesylated proteome.

Cholesterol metabolism and transport in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder, affecting millions of people worldwide. Apart from age, the major risk factor identified so far for the sporadic form of AD is possession of the epsilon4 allele of apolipoprotein E (APOE), which is also a risk factor for coronary artery disease (CAD). Other apolipoproteins known to play an important role in CAD such as apolipoprotein B are now gaining attention for their role in AD as well. AD and CAD share other risk factors, such as altered cholesterol levels, particularly high levels of low density lipoproteins together with low levels of high density lipoproteins. Statins--drugs that have been used to lower cholesterol levels in CAD, have been shown to protect against AD, although the protective mechanism(s) involved are still under debate. Enzymatic production of the beta amyloid peptide, the peptide thought to play a major role in AD pathogenesis, is affected by membrane cholesterol levels. In addition, polymorphisms in several proteins and enzymes involved in cholesterol and lipoprotein transport and metabolism have been linked to risk of AD. Taken together, these findings provide strong evidence that changes in cholesterol metabolism are intimately involved in AD pathogenic processes. This paper reviews cholesterol metabolism and transport, as well as those aspects of cholesterol metabolism that have been linked with AD.

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.

Role of RHOB in the antiproliferative effect of glucocorticoid receptor on macrophage RAW264.7 cells

Although glucocorticoid (GC) has been reported to inhibit macrophage killing activity and cytokine production in response to proinflammatory stimuli, the effect of GC on macrophage proliferation is controversial. In our previous study, we found that inhibition of glucocorticoid receptor (GR) expression in murine macrophage cell line RAW264.7 cells (RAW-GR(-) cells) by RNAi significantly promoted cell proliferation. In the present study, we provide the evidence that the expression of Rhob, a member of Rho GTPases with anti-cancer character, remarkably decreased in RAW-GR(-) and RAW264.7 cells transiently transfected with GR-RNAi vector. Overexpression or constitutive activation of Rhob in RAW-GR(-) and RAW264.7 cells by transfection with wild-type Rhob expression vector (Rhob-wt) or constitutively activated Rhob plasmid (Rhob-V14) resulted in decreased proliferation of the two cell lines. Oppositely, the proliferation of RAW264.7 cells was significantly increased when the expression of Rhob by RNA interference technique or the activity of Rhob by transfection with dominant negative Rhob mutant that is defective in nucleotide binding (Rhob-N19) was inhibited. In addition, enhanced activity of Akt, but not MAPK3/1 or MAPK14, was found in RAW-GR(-) cells. Blocking the pathway of phosphatidylinositol 3-kinase (PI3K)/Akt with the specific inhibitor LY294002 decreased the proliferation and elevated RHOB protein level, indicating that PI3K/Akt signal plays its role of proliferation modulation upstream of RHOB protein. In conclusion, these results demonstrate that Rhob plays an important role in the antiproliferative effect of GR on RAW264.7 cells by GR-->Akt-->Rhob signaling and Rhob negatively regulates the proliferation of RAW264.7 cells.

RhoA-GDP regulates RhoB protein stability. Potential involvement of RhoGDIalpha

RhoA plays a significant role in actin stress fibers formation. However, silencing RhoA alone or RhoA and RhoC did not completely suppress the stress fibers suggesting a residual "Rho-like" activity. RhoB, the third member of the Rho subclass, is a shortlived protein barely detectable in basal conditions. In various cell types, the silencing of RhoA induced a strong up-regulation of both total and active RhoB protein levels that were rescued by re-expressing RhoA and related to an enhanced half-life of the protein. The RhoA-dependent regulation of RhoB does not depend on the activity of RhoA but is mediated by its GDP-bound form. The stabilization of RhoB was not dependent on isoprenoid biosynthesis, Rho kinase, extracellular signal-regulated kinase, p38 mitogen-activated kinase, or phosphatidylinositol 3'-OH kinase pathways but required RhoGDIalpha. The forced expression of RhoGDIalpha increased RhoB half-life, whereas its knock-down antagonized the induction of RhoB following RhoA silencing. Moreover, a RhoA mutant (RhoAR68E) unable to bind RhoGDIalpha was significantly less efficient as compared with wild-type RhoA in reversing RhoB up-regulation upon RhoA silencing. These results suggest that, in basal conditions, RhoGDIalpha is rate-limiting and the suppression of RhoA makes it available to stabilize RhoB. Our results highlight RhoGDIalpha-dependent cross-talks that regulate the stability of RhoGTPases.

Role of small GTPase Rho in regulating corneal epithelial wound healing

PURPOSE

To determine the role of small GTPase Rho and its relation with epidermal growth factor receptor (EGFR) in mediating corneal epithelial wound healing.

METHODS

Rho activity in THCE cells, an SV40-immortalized human corneal epithelial cell (HCEC) line, and primary HCECs was assessed by pull-down assay followed by Western blotting. Rho functions were inhibited with specific inhibitor exoenzyme C3 (C3) and confirmed by knockdown with small interference RNA (siRNA) transfection. Effects of Rho inhibition on wound healing were determined in porcine corneal organ culture and HCEC scratch wound models. Effects of C3 on cell proliferation and focal adhesion formation were determined by BrdU incorporation assay and immunocytochemistry, respectively.

RESULTS

Wounding, lysophosphatidic acid, and heparin-binding EGF-like growth factor (HB-EGF) induced rapid and strong RhoA activation. HB-EGF-, but not wounding-, enhanced RhoA activity was sensitive to EGFR inhibition. In corneal organ and cell culture models, C3 attenuated spontaneous and HB-EGF-induced wound closures, confirmed by delayed wound healing in cells transfected with RhoA siRNA. C3 also retarded spontaneous wound healing in the presence of hydroxyurea, a cell cycle blocker. C3 significantly reduced the number of BrdU-positive cells near the leading edge. Treatment with C3 resulted in the disruption of the cortical actin cytoskeleton and in the disappearance of paxillin-containing focal adhesion and lamellipodia.

CONCLUSIONS

Wounding induces RhoA activation through an EGFR-independent pathway. Rho activity is required for modulating cell migration and proliferation through cytoskeleton reorganization and focal adhesion formation in response to wounding.

Simvastatin inhibits Staphylococcus aureus host cell invasion through modulation of isoprenoid intermediates

Patients on a statin regimen have a decreased risk of death due to bacterial sepsis. We have found that protection by simvastatin includes the inhibition of host cell invasion by Staphylococcus aureus, the most common etiologic agent of sepsis. Inhibition was due in part to depletion of isoprenoid intermediates within the cholesterol biosynthesis pathway and led to the cytosolic accumulation of the small GTPases CDC42, Rac, and RhoB. Actin stress fiber disassembly required for host invasion was attenuated by simvastatin and by the inhibition of phosphoinositide 3-kinase (PI3K) activity. PI3K relies on coupling to prenylated proteins, such as this subset of small GTPases, for access to membrane-bound phosphoinositide to mediate stress fiber disassembly. Therefore, we examined whether simvastatin restricts PI3K cellular localization. In response to simvastatin, the PI3K isoform p85, coupled to these small-GTPases, was sequestered within the cytosol. From these findings, we propose a mechanism whereby simvastatin restricts p85 localization, inhibiting the actin dynamics required for bacterial endocytosis. This approach may provide the basis for protection at the level of the host in invasive infections by S. aureus.

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.

Inhibition of serotonin-induced mitogenesis, migration, and ERK MAPK nuclear translocation in vascular smooth muscle cells by atorvastatin

The HMG-CoA reductase inhibitors, statins, have pleiotropic effects which may include interference with the isoprenylation of Ras and Rho small GTPases. Statins have beneficial effects in animal models of pulmonary hypertension, although their mechanisms of action remain to be determined. Serotonin [5-hydroxytryptamine (5-HT)] is implicated in the process of pulmonary artery smooth muscle (PASM) remodeling as part of the pathophysiology of pulmonary hypertension. We examined the effect of atorvastatin on 5-HT-induced PASM cell responses. Atorvastatin dose dependently inhibits 5-HT-induced mitogenesis and migration of cultured bovine PASM cells. Inhibition by atorvastatin was reversed by mevalonate and geranylgeranylpyrophosphate (GGPP) supplement, suggesting that the statin targets a geranylgeranylated protein such as Rho. Concordantly, atorvastatin inhibits 5-HT-induced cellular RhoA activation, membrane localization, and Rho kinase-mediated phosphorylation of myosin phosphatase-1 subunit. Atorvastatin reduced activated RhoA-induced serum response factor-mediated reporter activity in HEK293 cells, indicating that atorvastatin inhibits Rho signaling, and this was reversed by GGPP. While 5-HT-induced ERK MAP and Akt kinase activation were unaffected by atorvastatin, 5-HT-induced ERK nuclear translocation was attenuated in a GGPP-dependent fashion. These studies suggest that atorvastatin inhibits 5-HT-induced PASM cell mitogenesis and migration through targeting isoprenylation which may, in part, attenuate the Rho pathway, a mechanism that may apply to statin effects on in vivo models of pulmonary hypertension.

X-ray crystal structures reveal two activated states for RhoC

RhoC is a member of the Rho family of Ras-related (small) GTPases and shares significant sequence similarity with the founding member of the family, RhoA. However, despite their similarity, RhoA and RhoC exhibit different binding preferences for effector proteins and give rise to distinct cellular outcomes, with RhoC being directly implicated in the invasiveness of cancer cells and the development of metastasis. While the structural analyses of the signaling-active and -inactive states of RhoA have been performed, thus far, the work on RhoC has been limited to an X-ray structure for its complex with the effector protein, mDia1 (for mammalian Diaphanous 1). Therefore, in order to gain insights into the molecular basis for RhoC activation, as well as clues regarding how it mediates distinct cellular responses relative to those induced by RhoA, we have undertaken a structural comparison of RhoC in its GDP-bound (signaling-inactive) state versus its GTP-bound (signaling-active) state as induced by the nonhydrolyzable GTP analogues, guanosine 5'-(beta,gamma-iminotriphosphate) (GppNHp) and guanosine 5'-(3-O-thiotriphosphate) (GTPgammaS). Interestingly, we find that GppNHp-bound RhoC only shows differences in its switch II domain, relative to GDP-bound RhoC, whereas GTPgammaS-bound RhoC exhibits differences in both its switch I and switch II domains. Given that each of the nonhydrolyzable GTP analogues is able to promote the binding of RhoC to effector proteins, these results suggest that RhoC can undergo at least two conformational transitions during its conversion from a signaling-inactive to a signaling-active state, similar to what has recently been proposed for the H-Ras and M-Ras proteins. In contrast, the available X-ray structures for RhoA suggest that it undergoes only a single conformational transition to a signaling-active state. These and other differences regarding the changes in the switch domains accompanying the activation of RhoA and RhoC provide plausible explanations for the functional specificity exhibited by the two GTPases.

Simvastatin suppresses self-renewal of mouse embryonic stem cells by inhibiting RhoA geranylgeranylation

Statins, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, were originally developed to lower cholesterol. Their pleiotropic (or cholesterol-independent) effects at the cellular and molecular levels are highly related to numerous cellular functions, such as proliferation and differentiation. However, they are hardly studied in embryonic stem cells. In this study, we evaluated the effects of statins on mouse ESCs (J1, D3, and RW.4) to enhance our understanding of the molecular basis of ESC self-renewal. Treatment of ESCs with simvastatin, mevastatin, atorvastatin, or pravastatin induced morphological change and decreased cell proliferation. We observed that the use of simvastatin was most effective in all three ESCs. Loss of ESC self-renewal by simvastatin was determined by marked downregulation of ESC markers alkaline phosphatase, Oct4, Nanog, Rex-1, and SSEA-1. Simvastatin effects were selectively reversed by either mevalonate or its metabolite geranylgeranyl pyrophosphate (GGPP) but not by cholesterol or farnesyl pyrophosphate. These results suggest that simvastatin effects were mainly derived from depletion of intracellular pools of GGPP, the substrate required for the geranylgeranylation. Using this approach, we found that GGPP, a derivative of the mevalonate pathway, is critical for ESC self-renewal. Furthermore, we identified that simvastatin selectively blocked cytosol-to-membrane translocalization of RhoA small guanosine triphosphate-binding protein, known to be the major target for geranylgeranylation, and lowered the levels of Rho-kinase (ROCK)2 protein in ESCs. In addition, simvastatin downregulated the ROCK activity, and this effect was reversed by addition of GGPP. Our data suggest that simvastatin, independently of its cholesterol-lowering properties, impairs the ESC self-renewal by modulating RhoA/ROCK-dependent cell-signaling.

Dominant-negative alpha-subunit of farnesyl- and geranyltransferase inhibits glucose-stimulated, but not KCl-stimulated, insulin secretion in INS 832/13 cells

The majority of small G-proteins undergo posttranslational modifications (e.g., isoprenylation) at their C-terminal cysteine residues. Such modifications increase their hydrophobicity, culminating in translocation of the modified proteins to their relevant membranous sites for interaction with their respective effectors. Previously, we reported glucose-dependent activation and membrane association of Rac1 in INS 832/13 cells. We also demonstrated modulatory roles for Rac1/GDP dissociation inhibitor in glucose-stimulated insulin secretion (GSIS) in INS 832/13 cells, further affirming roles for Rac1 in GSIS. Herein, we demonstrate that geranylgeranyltransferase inhibitor-2147 (GGTI-2147), an inhibitor of protein prenylation, markedly increased cytosolic accumulation of Rac1 and elicited significant inhibition of GSIS from INS 832/13 cells. In the current study, we also examined the localization of protein prenyltransferases (PPTases) and regulation of GSIS by PPTases in INS 832/13 cells. Western blot analyses indicated that the regulatory alpha-subunit and the structural beta-subunit of PPTase holoenzyme are predominantly cytosolic in their distribution. Overexpression of an inactive mutant of the regulatory alpha-subunit of PPTase markedly attenuated glucose- but not KCl-induced insulin secretion from INS 832/13 cells. Together, our findings provide the first evidence for the regulation of GSIS by PPTase in INS 832/13 cells. Furthermore, they support our original hypothesis that prenylation of specific G-proteins may be necessary for GSIS.

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.

Rho GTPases and Leukocyte Adhesion Receptor Expression and Function in Endothelial Cells

Rho family GTPases are key signal transducers that regulate cell adhesion and migration and a variety of other cellular responses, including changes in gene expression. In this review, we discuss how Rho GTPases regulate signaling by endothelial cell receptors involved in leukocyte extravasation. First, Rho GTPases affect the expression of some leukocyte adhesion molecules on endothelial cells, such as intracellular adhesion molecule-1 and E-selectin, that can be induced by proinflammatory mediators, hypoxia, or shear stress. Second, Rho GTPases are activated by engagement of several leukocyte adhesion receptors and contribute to both early morphological changes and subsequent alterations in gene expression. Rho GTPases are therefore candidate targets for inhibiting leukocyte transendothelial migration in heart disease and chronic inflammatory disorders.

The 3-hydroxy-3-methylglutaryl co-enzyme A reductase inhibitor pravastatin enhances neurite outgrowth in hippocampal neurons

Epidemiological studies demonstrate a relationship between statin [3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor] usage and reduced risk of developing Alzheimer's disease. To determine whether statins affect neuronal development, we treated cultured rat hippocampal neurons with pravastatin. After 4-48 h of treatment, pravastatin significantly increased the number of neurites produced by each cell and caused a corresponding increase in levels of the membrane phospholipid phosphatidylcholine. Pravastatin treatment also significantly increased neurite length and branching but did not affect cellular cholesterol levels. Co-incubation with mevalonate, but not cholesterol, abolished the stimulatory effect of pravastatin on neurite outgrowth. Treatment of neurons with isoprenoids also abolished the effect of pravastatin on neurite growth, suggesting that pravastatin may stimulate neuritogenesis by preventing isoprenylation of signaling molecules such as the Rho family of small GTPases. A specific inhibitor of geranylgeranylation, but not farnesylation, mimicked the stimulatory effect of pravastatin on neuritogenesis. Pravastatin treatment significantly decreased levels of membrane-associated RhoA. These data suggest that pravastatin treatment increases neurite outgrowth and may do so via inhibiting the activity of geranylgeranylated proteins such as RhoA.

Combining prenylation inhibitors causes synergistic cytotoxicity, apoptosis and disruption of RAS-to-MAP kinase signalling in multiple myeloma cells

The high incidence of activating RAS mutations, coupled with accumulating evidence linking RAS to multiple myeloma (MM) pathogenesis, indicate that novel therapies utilising inhibitors of RAS prenylation and signalling may be successful in the management of this disease. While preclinical studies investigating prenylation inhibitors, such as lovastatin, farnesyltransferase inhibitors (FTI) and geranylgeranyltransferase inhibitors (GGTI), have been promising, recent phase I/II clinical trials with FTI R115777 were disappointing, suggesting resistance to FTI monotherapy. To address this issue, the effects of FTI, GGTI and lovastatin alone and in combination were analysed in MM cell lines and primary cells. FTI treatment blocked H-RAS processing, but was ineffective at inhibiting K- and N-RAS prenylation because of alternative geranylgeranylation of these isoforms. However, combinations of FTI and GGTI or lovastatin were found to synergistically inhibit MM cell proliferation, migration, K- and N-RAS processing, RAS-to-mitogen-activated protein kinase signalling and to induce apoptosis. In contrast to FTI, lovastatin and some GGTI were found to cause intracellular accumulation of Rho proteins. Our results suggest that clinical efficacy of prenylation inhibitors in MM are limited by alternative prenylation of several small G-proteins, such as RhoB, K- and N-RAS. Furthermore, strategies combining FTI with GGTI or statins may provide greater efficacy in MM treatment.

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.

Down-Regulation of Na+/Ca2+Exchanger by Fluvastatin in Rat Cardiomyoblast H9c2 Cells: Involvement of RhoB in Na+/Ca2+Exchanger mRNA Stability

We investigated the effect of fluvastatin (Flv), an HMG-CoA reductase inhibitor, on Na(+)/Ca(2+) exchanger 1 (NCX1) expression in H9c2 cardiomyoblasts. Reverse transcriptase-polymerase chain reaction analyses revealed that Flv decreased NCX1 mRNA in a concentration- and time-dependent manner and NCX1 protein. This effect of Flv was caused by the inhibition of HMG-CoA reductase, because Flv did not affect the NCX1 mRNA in the presence of mevalonate. Flv-induced down-regulation of NCX1 mRNA was also cancelled by farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP), suggesting an involvement of small G-proteins. However, overexpression of neither constitutive active RhoA nor Ras affected NCX1 mRNA. In contrast, intracellular expression of C3 toxin, a specific inhibitor of Rho family proteins, decreased NCX1 mRNA, suggesting that Flv decreases NCX1 mRNA by inhibiting a signaling pathway of Rho family proteins other than RhoA. On the other hand, lysophosphatidylcholine (LPC), an activator of Rho signaling, increased both NCX1 mRNA and protein in a C3 toxin-sensitive manner. Western blot analyses revealed that membrane-associated RhoB, which is isoprenylated by either FPP or GGPP, was decreased by Flv but was increased by LPC. Selective inhibition of gene expression by short interfering RNA duplex showed that RhoB but not RhoA is involved in the regulation of NCX1 mRNA and protein. When transcription was blocked by 5,6-dichlorobenzimidazole riboside, the NCX1 mRNA stability was decreased by Flv. Long-term treatment of rat with Flv in vivo also down-regulated the cardiac NCX1 mRNA. These results suggest that a RhoB-mediated signaling pathway regulates cardiac NCX1 levels by controlling the NCX1 mRNA stability.

Protein and m-RNA expression of farnesyl-transferases, RhoA and RhoB in rat liver hepatocytes: action of perillyl alcohol and vitamin A in vivo

We analysed the action, in rats in vivo, of the protein isoprenylation inhibitor perillyl alcohol (POH) and that of vitamin A, alone or in association, on m-RNA and protein expression of farnesyltransferases (FTases alpha and beta subunits) and their protein substrates RhoA and RhoB, in isolated hepatocytes. Combined administration of POH and vitamin A induced a sharp decrease in FTase alpha protein after 96 h, suggesting an involvement not only of farnesyltransferases but also of geranylgeranyltransferases, which share the FTase alpha protein. FTase beta protein did not decrease. POH plus vitamin A, in contrast with POH or vitamin A alone, induced a decrease in RhoB protein, probably because of different cleavages. No modification was observed in RhoA protein. Vitamin A alone increased RhoB m-RNA and protein expression. As one of the functions of RhoB is cell polarisation, these data support our previous hypothesis of a polarised transport of vitamin A from hepatocytes to hepatic stellate cells. As the behaviours of m-RNAs and proteins in this study were often different, cytoplasmic metabolic pathways must be considered for the parameters studied. The behaviour of Rho B, which is thought to have an antioncogene function, is discussed in view of its isoprenylated forms in the membranes. These preliminary findings stress the need, when studying the association of two isoprenoids in cancer therapy, to consider normal as well as tumour-bearing animals.

Clostridium difficile toxin A induces expression of the stress-induced early gene product RhoB

Clostridium difficile toxin A monoglucosylates the Rho family GTPases Rho, Rac, and Cdc42. Glucosylation leads to the functional inactivation of Rho GTPases and causes disruption of the actin cytoskeleton. A cDNA microarray revealed the immediate early gene rhoB as the gene that was predominantly up-regulated in colonic CaCo-2 cells after treatment with toxin A. This toxin A effect was also detectable in epithelial cells such as HT29 and Madin-Darby canine kidney cells, as well as NIH 3T3 fibroblasts. The expression of RhoB was time-dependent and correlated with the morphological changes of cells. The up-regulation of RhoB was approximately 15-fold and was based on the de novo synthesis of the GTPase because cycloheximide completely inhibited the toxin A effect. After 8 h, a steady state was reached, with no further increase in RhoB. The p38 MAPK inhibitor SB202190 reduced the expression of RhoB, indicating a participation of the p38 MAPK in this stress response. Surprisingly, newly formed RhoB protein was only partially glucosylated by toxin A, sparing a pool of potentially active RhoB, as checked by sequential C3bot-catalyzed ADP-ribosylation. A pull-down assay in fact revealed a significant amount of active RhoB in toxin A-treated cells that was not present in control cells. We demonstrate for the first time that toxin A has not only the property to inactivate the GTPases RhoA, Rac1, and Cdc42 by glucosylation, but it also has the property to generate active RhoB that likely contributes to the overall picture of toxin treatment.

Statins potentiate caspase-3 activity in immortalized murine neurons

Statins are lipid-lowering drugs that have been shown to reduce atherosclerotic cardiovascular morbidity and mortality. However, there is growing evidence from epidemiological studies that long-term treatment with statins has unwanted effects on extrahepatic tissue and increases the risk for neuropathy. To investigate underlying molecular mechanisms we analyzed whether statins influence the activity of caspase-3 in immortalized neurons. Lovastatin and mevastatin are not able to activate caspase-3 but they strongly potentiate its activity when apoptotic signal transduction is initiated by staurosporine. The increase in caspase-3 activity after coincubation with statins and staurosporine was paralleled by an increase in the protein level of the pro-apoptotic GTPase RhoB. Our data provide evidence that statins enhance neuronal apoptosis and therefore give reasons for a careful evaluation when patients with neurological diseases are treated with these drugs.

Cerivastatin inhibits proliferation of interleukin-1β-induced rat mesangial cells by enhanced formation of nitric oxide

The antiproliferative effect of statins on mesangial cells could represent a new therapeutic approach in glomerulonephritis. We studied in rat mesangial cells whether the antiproliferative action of cerivastatin on mesangial cells may be mediated by mesangial nitric oxide (NO) formation due to the inducible NO synthase (iNOS) or by induction of cyclooxygenase-2. Mesangial cells were stimulated with interleukin-1 beta and treated with cerivastatin for 24 h. Cell proliferation was examined by bromodeoxy-uridine (BrdU) incorporation, and nitrite and prostaglandin production was measured in supernatants as a means for iNOS or cyclooxygenase-2 activity. iNOS and cyclooxygenase-2 expression was quantified by Northern and Western blot analyses. Cerivastatin (0.0625 microM) significantly inhibited DNA synthesis in interleukin-1 beta-stimulated mesangial cells without altering cell viability. Interleukin-1 beta-induced nitrite production was twofold increased by 0.05 microM cerivastatin, and this effect could be reversed by addition of 100 microM mevalonate. iNOS mRNA levels increased sixfold (33% of maximum) in cerivastatin-treated mesangial cells as compared with vehicle-treated controls (3.5% of maximum). iNOS and cyclooxygenase-2 protein expression increased threefold (iNOS: 2.77+/-0.53/cyclooxygenase-2: 3.49+/-1.25). The NOS inhibitors N-methyl-L-arginine (L-NMMA) and L-N6-(1-iminoethyl)lysine (L-NIL) reversed the antiproliferative effect of cerivastatin. The cyclooxygenase-2 inhibitor celecoxib did not alter DNA synthesis and iNOS or cyclooxygenase-2 expression, but blocked prostacyclin production in interleukin-1 beta and cerivastatin-treated mesangial cells. In conclusion, cerivastatin increased cytokine-induced iNOS and cyclooxygenase-2 expression, thus constituting NO-regulated growth inhibition of mesangial cells.

Protein farnesylation in plants--conserved mechanisms but different targets

Protein farnesylation has an important role in the regulation of plant development and signal transduction, but the exact function of this modification is not well understood. The identification of protein farnesyltransferase substrates, together with the genetic analysis of mutants that are deficient in protein farnesylation, should significantly increase our knowledge of this form of protein modification in plants.