Searching new targets for anticancer drug design: the families of Ras and Rho GTPases and their effectors

Construction of an miRNA-regulated drug-pathway network reveals drug repurposing candidates for myasthenia gravis

Myasthenia gravis (MG) is a rare debilitating autoimmune neuromuscular disorder. Many studies have focused on the mechanism and treatment strategies of MG. However, the exact pathogenesis of MG and effective treatment strategies remain unclear. Recent studies have indicated that microRNAs (miRNAs or miRs) can regulate the pathological pathways of MG, suggesting their potential role in novel treatments. In the present study, we created a comprehensive catalog of experimentally confirmed MG risk genes and miRNAs by manually mining published literature and public databases. Based on these genes and miRNAs, we identified 41 MG risk pathways and 105 approved drugs that can affect these pathways. Some important MG-related pathways, such as hsa04060 (cytokine-cytokine receptor interaction) and hsa05200 (pathway in cancer), were found to be regulated by MG risk miRNAs and drugs. Furthermore, we constructed an miRNA-regulated drug-pathway network and identified miRNAs and drugs that synergistically regulate key MG pathways and biological processes. We developed a drug repurposing strategy to identify 25 drug repurposing candidates for MG; several of these drugs, such as rituximab, adalimumab, sunitinib, and muromonab, have the potential to be novel MG treatment drugs. This study provides novel insight into the pathogenesis of MG and potential drug candidates for MG were identified.

Rac Regulates Giardia lamblia Encystation by Coordinating Cyst Wall Protein Trafficking and Secretion

Encystation of the common intestinal parasite Giardia lamblia involves the production, trafficking, and secretion of cyst wall material (CWM). However, the molecular mechanism responsible for the regulation of these sequential processes remains elusive. Here, we examined the role of GlRac, Giardia’s sole Rho family GTPase, in the regulation of endomembrane organization and cyst wall protein (CWP) trafficking. Localization studies indicated that GlRac is associated with the endoplasmic reticulum (ER) and the Golgi apparatus-like encystation-specific vesicles (ESVs). Constitutive GlRac signaling increased levels of the ER marker PDI2, induced ER swelling, reduced overall CWP1 production, and promoted the early maturation of ESVs. Quantitative analysis of cells expressing constitutively active hemagglutinin (HA)-tagged GlRac (HA-Rac^CA) revealed fewer but larger ESVs than control cells. Consistent with the phenotype of premature maturation of ESVs in HA-Rac^CA-expressing cells, constitutive GlRac signaling resulted in increased CWP1 secretion and, conversely, morpholino depletion of GlRac blocked CWP1 secretion. Wild-type cells unexpectedly secreted large quantities of CWP1 into the medium, and free CWP1 was used cooperatively during cyst formation. These results, in part, could account for the previously reported observation that G. lamblia encysts more efficiently at high cell densities. These studies of GlRac show that it regulates encystation at several levels, and our findings support its coordinating role as a regulator of CWP trafficking and secretion. The central role of GlRac in regulating membrane trafficking and the cytoskeleton, both of which are essential to Giardia parasitism, further suggests its potential as a novel target for drug development to treat giardiasis.

The encystation process is crucial for the transmission of giardiasis and the life cycle of many protists. Encystation for Giardia lamblia involves the assembly of a protective cyst wall via sequential production, trafficking, and secretion of cyst wall material. However, the regulatory pathways that coordinate cargo maturation and secretion remain unknown. Here, we asked whether the signaling activities of G. lamblia’s single Rho family GTPase, GlRac, might have a regulatory role in the encystation process. We show that GlRac localizes to endomembranes and its signaling activities regulate the production of cyst wall protein 1 (CWP1), the maturation of encystation-specific vesicles (ESVs), and secretion of CWP1. We also show that secreted CWP1 is available for the development of cysts at the population level, a finding that in part could explain why Giardia encystation proceeds more efficiently at high cell densities.

Russell's viper venom affects regulation of small GTPases and causes nuclear damage

Russell's viper with its five sub-species is found throughout the Indian subcontinent. Its venom is primarily hemotoxic. However, its envenomation causes damage to several physiological systems. The present work was aimed to study the dose and time dependent cytotoxic effects of Russell's viper venom (RVV) on human A549 cells grown in vitro. Time dependent changes have been observed in cellular morphology following exposure to RVV. Presence of stress granules, rounding-off of the cells, and formation of punctate structure and loss of cell-cell contact characterized the cellular effects. Fluorescence microscopic studies revealed that apoptotic cell population increased on exposure to RVV. Further to understand the mechanism of these effects, status of small GTPase (smGTPases) expression were studied by Western blot and RT-PCR; as smGTPases play pivotal roles in deciding the cellular morphology, polarity, cell movement and overall signaling cascade. It was shown for the first time that expression patterns of Rac, Rho and CDC42 genes are altered on exposure to RVV. Similarly, significant difference in the expression pattern of HSP70 and p53 at the mRNA levels were noted. Our results confirmed that RVV induces apoptosis in A549 cells; this was further confirmed by AO/EtBr staining as well as caspase-3 assay. All experiments were compared using RVV unexposed cells. We propose for the first time that RVV induces morphological changes in human A549 cells through modulation of smGTPase expression and affects the cellular-nuclear architecture which in turn interferes in proliferation and migration of these cells along with apoptosis.

Functional cross-talk between ras and rho pathways: a Ras-specific GTPase-activating protein (p120RasGAP) competitively inhibits the RhoGAP activity of deleted in liver cancer (DLC) tumor suppressor by masking the catalytic arginine finger

The three deleted in liver cancer genes (DLC1-3) encode Rho-specific GTPase-activating proteins (RhoGAPs). Their expression is frequently silenced in a variety of cancers. The RhoGAP activity, which is required for full DLC-dependent tumor suppressor activity, can be inhibited by the Src homology 3 (SH3) domain of a Ras-specific GAP (p120RasGAP). Here, we comprehensively investigated the molecular mechanism underlying cross-talk between two distinct regulators of small GTP-binding proteins using structural and biochemical methods. We demonstrate that only the SH3 domain of p120 selectively inhibits the RhoGAP activity of all three DLC isoforms as compared with a large set of other representative SH3 or RhoGAP proteins. Structural and mutational analyses provide new insights into a putative interaction mode of the p120 SH3 domain with the DLC1 RhoGAP domain that is atypical and does not follow the classical PXXP-directed interaction. Hence, p120 associates with the DLC1 RhoGAP domain by targeting the catalytic arginine finger and thus by competitively and very potently inhibiting RhoGAP activity. The novel findings of this study shed light on the molecular mechanisms underlying the DLC inhibitory effects of p120 and suggest a functional cross-talk between Ras and Rho proteins at the level of regulatory proteins.

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.

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.

Differential role of human choline kinase alpha and beta enzymes in lipid metabolism: implications in cancer onset and treatment

BACKGROUND

The Kennedy pathway generates phosphocoline and phosphoethanolamine through its two branches. Choline Kinase (ChoK) is the first enzyme of the Kennedy branch of synthesis of phosphocholine, the major component of the plasma membrane. ChoK family of proteins is composed by ChoKalpha and ChoKbeta isoforms, the first one with two different variants of splicing. Recently ChoKalpha has been implicated in the carcinogenic process, since it is over-expressed in a variety of human cancers. However, no evidence for a role of ChoKbeta in carcinogenesis has been reported.

METHODOLOGY/PRINCIPAL FINDINGS

Here we compare the in vitro and in vivo properties of ChoKalpha1 and ChoKbeta in lipid metabolism, and their potential role in carcinogenesis. Both ChoKalpha1 and ChoKbeta showed choline and ethanolamine kinase activities when assayed in cell extracts, though with different affinity for their substrates. However, they behave differentially when overexpressed in whole cells. Whereas ChoKbeta display an ethanolamine kinase role, ChoKalpha1 present a dual choline/ethanolamine kinase role, suggesting the involvement of each ChoK isoform in distinct biochemical pathways under in vivo conditions. In addition, while overexpression of ChoKalpha1 is oncogenic when overexpressed in HEK293T or MDCK cells, ChoKbeta overexpression is not sufficient to induce in vitro cell transformation nor in vivo tumor growth. Furthermore, a significant upregulation of ChoKalpha1 mRNA levels in a panel of breast and lung cancer cell lines was found, but no changes in ChoKbeta mRNA levels were observed. Finally, MN58b, a previously described potent inhibitor of ChoK with in vivo antitumoral activity, shows more than 20-fold higher efficiency towards ChoKalpha1 than ChoKbeta.

CONCLUSION/SIGNIFICANCE

This study represents the first evidence of the distinct metabolic role of ChoKalpha and ChoKbeta isoforms, suggesting different physiological roles and implications in human carcinogenesis. These findings constitute a step forward in the design of an antitumoral strategy based on ChoK inhibition.

Subcellular localization of APMCF1 and its biological significance of expression pattern in normal and malignant human tissues

Background

APMCF1 is a novel human gene first cloned from apoptotic MCF-7 cells. Our previous study found ectogenic APMCF1 could induce G1 arrest in hepatocarcinoma cell line HHCC. In order to search its broad expression profile for further understanding of its mechanism in tumor, we investigated a subcellular location of APMCF1 and performed an immunohistochemistry study including various tumor and normal tissues. Discovery from the expression characterization of AMPCF1 may have applicability in the analysis of its biological function in tumor.

Methods

We investigated subcellular localization of APMCF1 by transient transfection in green monkey kidney epithelial cells (COS-7) with a fusion protein vector pEGFP-APMCF1 and detected expression profile in a broad range of normal and malignant human tissues via tissue microarray (TMA) by immunohistochemistry with polyclonal antibody first produced in our laboratory.

Results

EGFP-APMCF1 was generally localized in the cytoplasm of COS-7 cell. Positive staining of APMCF1 was found in liver, lung, breast, colon, stomach, esophagus and testis, exhibited a ubiquitous expression pattern while its expression was up-regulated in tumor tissues compared with corresponding normal tissues. Normal brain neuron cells also showed expression of APMCF1, but negative in gliocyte cells and glioma. Both the normal and tumor tissues of ovary were absent of APMCF1 expression. Positive immunostaining for APMCF1 with large samples in liver, colon, esophagus, lung and breast carcinomas were 96% (51/53), 80% (44/55), 57% (30/53), 58% (33/57) and 34% (16/47) respectively.

Conclusion

These results revealed a cytoplastic expression pattern of APMCF1 and up-regulated in tumour tissues suggesting APMCF1 may have potential relationship with oncogenesis. The data presented should serve as a useful reference for further studies of APMCF1 functions in tumorigenesis and might provide a potential anti-tumor target.

Proteomic analysis of human NK-92 cells after NK cell-mediated cytotoxicity against K562 cells

To better understand the natural killer (NK) cell cytotoxicity mechanism at the proteome level, we comparatively analyzed the proteome of the human NK-92 cells which participate in NK cell-mediated cytotoxicity assay and that of control cells. Soluble proteins were separated by two-dimensional gel electrophoresis (2-DE), 75 protein spots were found to be reproducibly differentially expressed between control and cytotoxic human NK-92 cells. A total of 60 different proteins were unequivocally identified by MALDI-TOF MS coupled with database interrogation; 37 proteins were up-regulated, whereas 23 proteins were down-regulated. Western blotting analysis of heat shock protein 60 (HSP60) and cathepsin W verified their proteome results. Some of identified proteins are involved in NK-92 cytotoxicity, which is consistent with the literature. In addition, we modeled the pathway networks between differentially expressed proteins and cellular processes of secretion and exocytosis through PathwayStudio software. The results of this study help to provide insight into the molecular mechanism of NK cell cytotoxicity.

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.

Identification of TRIO-GEFD1 chemical inhibitors using the yeast exchange assay

BACKGROUND INFORMATION

Rho GTPases are involved in many biological processes and participate in cancer development. Their activation is catalysed by exchange factors [RhoGEFs (Rho GTPase guanine nucleotide-exchange factor)] of the Dbl family. RhoGEFs display proto-oncogenic features, thus appearing as candidate targets for anticancer drugs. Dominant-negative Rho GTPase mutants have been widely used to block RhoGEF signalling. However, these tools suffer from limitations, due to the high number of RhoGEFs and the complex mechanisms that control Rho GTPase activation.

RESULTS

RhoG-T17N is a poor inhibitor of its exchange factor TRIO-GEFD1 (first exchange domain of the exchange factor TRIO) in vivo: although it binds to TRIO-GEFD1, RhoG-T17N does not block the downstream signalling. Using the yeast exchange assay, we show that in the presence of TRIO-GEFD1, RhoG-T17N can bind to its effectors, which illustrates how negative mutants may produce misleading interpretations and emphasizes the need for new types of RhoGEF inhibitors. In that prospect, we adapted the yeast exchange assay method to identify RhoGEF inhibitors. Using this novel approach, we screened a 3500-chemical-compound library and identified a potential inhibitor of TRIO-GEFD1. This molecule inhibited TRIO-GEFD1 in vitro. Among the chemical analogues of this compound, we identified two molecules with better inhibitory activity. The three TRIO-GEFD1 inhibitors had no effect on ARHGEF17 and ARNO [ARF (ADP-ribosylation factor) nucleotide-binding-site opener], two exchange factors for RhoA and Arf1 respectively.

CONCLUSIONS

The development of RhoGEF inhibitors appears as a valuable tool for the study of Rho GTPase signalling pathways. The yeast exchange assay adaptation we present here is suitable to screen for chemical or peptide libraries and identify candidate inhibitors.

Molecular biology of breast cancer

Although Breast Cancer (BC) has been considered for many years as a single entity with a common management and treatment, it is actually a extremely heterogeneous disease which includes at least 4 or 5 very different subtypes. The first step in the recognition of the heterogeneity of BC was the demonstration of the presence of functional hormonal receptors (HR) in nearly two thirds of breast cancer specimens. This finding, which established a first classification of BC in two clear subtypes (HR-positive and HR-negative) was followed by the demonstration of many other differential features. The her2/neu gene alteration, present in nearly 20% of BC tumors, is probably the most relevant of them, but certainly not the only one. The development of new technologies and, in particular, the use of complementary DNA (cDNA) microarrays will allow us now the simultaneous analysis of thousands of genes and the establishment of new, more refined BC subtypes based on gene expression profiles/genetic fingerprints. This review discusses the practical applications of molecular analysis of BC, which can be classified in four categories: 1. Establishment of a new molecular taxonomy of breast cancer. 2. Definition of prognostic factors/prognostic indexes based on molecular/genetic peculiarities. 3. Prediction of response to diverse antitumoral treatments. 4. Identification of molecular targets that allows the development of new tailored antitumor treatments.

Inhibition of motile and invasive properties of ovarian cancer cells by ASODN against Rho-associated protein kinase

p160ROCK, a kinase effector of Rho GTPase mediating RhoA-induced assembly of focal adhesions and stress fibers, plays an important role in the invasive process of various tumor cells. The purpose of this study was to investigate the role of p160ROCK in the invasive behaviors of human ovarian cancer cells and their metastasis. Transfection with a dominant-active form of p160ROCK mutant (p160ROCKDelta 3) enhanced cell migration and invasion of ovarian cancer cells, while antisense oligodeoxynucleotide (ASODN) against p160ROCK inhibited the motile and invasive properties of the cells. Our data suggested that p160ROCK was involved in ovarian cancer cell invasion and metastasis by facilitating cancer cell migration, and that p160ROCK might be a potential new effective target for preventing metastasis of ovarian cancer.

The Rho-kinase pathway regulates angiotensin II-induced renal damage

BACKGROUND

Angiotensin II (AngII) is a key factor in the pathogenesis of renal damage. AngII via AngII type 1 receptors activates several intracellular signaling systems, including the small guanosine triphosphatase Rho and its downstream effector Rho-dependent serine-threonine kinase (Rho-kinase). The Rho/Rho-kinase pathway contributes to inflammatory and proliferative changes observed in cardiovascular diseases. However, the data on renal diseases are scarce. The aim of this study was to investigate the effect of Rho-kinase inhibition in AngII-induced renal damage.

METHODS

We used the model of systemic AngII infusion into normal rats (100 ng/kg per minute; subcutaneous osmotic minipumps), and some animals were treated with the Rho-kinase inhibitor Y-27632 (30 mg/kg per day). In the kidneys of these animals, we evaluated renal lesions, transcription factor activity (by electrophoretic mobility shift assay), and messenger RNA (by polymerase chain reaction) and protein expression levels (by Western blot and/or immunohistochemistry) of proinflammatory and profibrotic factors.

RESULTS

Rats infused with AngII for three days present renal inflammatory cell infiltration and slight tubular damage, which were diminished by treatment with the Rho-kinase inhibitor Y-27632. AngII activates nuclear factor-kappaB and causes overexpression of proinflammatory factors, including cytokines (tumor necrosis factor alpha) and chemokines (monocyte chemotactic protein-1), and of profibrotic factors (connective tissue growth factor). Treatment of AngII-infused rats with Y-27632 decreases the upregulation of these proinflammatory and profibrotic mediators.

CONCLUSION

These data demonstrate that the Rho-kinase pathway is involved in renal damage caused by AngII through the regulation of proinflammatory and profibrotic mediators. These results suggest that inhibition of the Rho-kinase pathway represents a novel therapy for renal diseases associated with local AngII generation.

Rho GTPase expression in tumourigenesis: evidence for a significant link

Rho proteins belong to the small GTPases superfamily. They function as molecular switches that, in response to diverse stimuli, control key signaling and structural aspects of the cell. Although early studies proposed a role for Rho GTPases in cellular transformation, this effect was underestimated due to the fact that no genetic mutations affecting Rho-encoding genes were found in tumors. Recently, it has become evident that Rho GTPases participate in the carcinogenic process by either overexpression of some of the members of the family with oncogenic activity, downmodulation of other members with suggested tumor suppressor activity, or by alteration of upstream modulators or downstream effectors. Thus, alteration of the levels of expression of different members of the family of Rho GTPases has been detected in many types of human tumors leading to a great interest in the cellular effects elicited by these oncoproteins. This essay reviews the current evidence of dysregulation of Rho signaling by overexpression in human tumors.

Choline Kinase Is a Novel Oncogene that Potentiates RhoA-Induced Carcinogenesis

Choline kinase is overexpressed in human breast, lung, colorectal, and prostate tumors, a finding that suggests the involvement of this enzyme in carcinogenesis. Here we show that overexpression of choline kinase induce oncogenic transformation of human embryo kidney fibroblasts and canine epithelial Madin-Darby canine kidney cells. Choline kinase lays downstream of RhoA signaling and is activated through ROCK kinase, one of the best-characterized RhoA effectors. In keeping with this, coexpression of RhoA and choline kinase potentiates both anchorage independent growth and tumorigenesis. Finally, choline kinase-mediated transformation is sensitive to MN58b, a well-characterized specific choline kinase inhibitor. These results provide the definitive evidence that choline kinase has oncogenic properties and that choline kinase inhibition constitutes a novel valid antitumor strategy.

Cdc42/Rac1 participates in the control of telomerase activity in human nasopharyngeal cancer cells

Telomerase, a specialized ribonucleoprotein reverse transcriptase that directs the synthesis of telomeric DNA, is repressed in normal human somatic cells, but is activated in most cancers. Little is known concerning how telomerase activity is activated and maintained in cancer cells. We have previously shown that protein kinase C-zeta (PKC zeta) controls telomerase activity in nasopharyngeal cancer (NPC) cells. Since PKC zeta activity is known to be modulated by Cdc42/Rac1, we investigated the effects of inhibiting Cdc42 and Rac1 on the telomerase activity of NPC-076 cells. Treatment of NPC cells with antisense oligonucleotides against Cdc42 or Rac1 produced an inhibition of telomerase activity. Similarly, transient expression of dominant-negative mutants of Cdc42 or Rac1, but not the wild-type Cdc42 or Rac1, also produced an inhibition of telomerase activity in NPC cells. This inhibition of telomerase activity is not associated with a transcriptional down-regulation of hTERT, the key regulator of telomerase. We suggest that Cdc42/Rac1 participates in the posttranscriptional control of telomerase activity in NPC cells.

Rho GTPases: potential candidates for anticancer therapy

Low molecular weight Rho GTPases are proteins that, in response to diverse stimuli, control key cellular processes such as cell proliferation, apoptosis, lipid metabolism, cytoarchitecture, adhesion, migration, cell polarity, and transcriptional regulation. The high incidence of overexpression of some members of the Rho family of GTPases in human tumors suggests that these proteins are important in the carcinogenic process, and therefore potential candidates for a therapeutic intervention. In recent years, the characterization of downstream effectors to Rho GTPases has increased our understanding of the general cellular effects that permit aberrant proliferation and motility of tumor cells. In addition, several transcription factors have been identified to play important roles at various levels of Rho-induced tumorigenesis. Accordingly, drugs that specifically alter Rho signaling display antineoplastic properties both at the level of tumor growth and tumor metastasis. In this review, a brief summary of the progress made in understanding the biological functions elicited by Rho GTPases that contribute to tumor biology will be made. In addition, a description of new drugs available targeted to specific elements of Rho signaling with antineoplastic or antimetastatic activity is included.

Functional analysis of Ras in Colletotrichum trifolii

Ras is a small monomeric GTP binding protein that transduces signals for growth and differentiation of eukaryotic organisms. Previously, a unique ras gene, designated Ct-ras, was cloned from the alfalfa fungal phytopathogen, Colletotrichum trifolii. Expression of Ct-Ras in mouse fibroblast cells (NIH3T3) demonstrated that Ct-ras is functionally similar to the mammalian ras genes since activating mutations of Ct-ras caused oncogenic phenotypes in nu/nu mice, including tumors. In C. trifolii, activated 'oncogenic' Ras (Val2) induced abnormal hyphal proliferation, defects in polarized growth and significantly reduced differentiation such as conidiation and appressorium formation in a nutrient dependent manner. Gene disruption of ct-ras was lethal. To further evaluate the function of Ct-Ras in C. trifolii, three different approaches were used: overexpression of cytosolic Ras by CAAX box deletion; expression of dominant negative Ct-RasT22N; and antisense ct-ras expression. Results showed that suppression of Ct-Ras activity significantly decreases fungal germination frequencies and hyphal growth rates. Taken together, these data suggest involvement of Ct-Ras in regulation of fungal cell growth and differentiation.

The polybasic region of Ras and Rho family small GTPases: a regulator of protein interactions and membrane association and a site of nuclear localization signal sequences

Many small GTPases in the Ras and Rho families have a C-terminal polybasic region (PBR) comprised of multiple lysines or arginines. The PBR controls diverse functions of these small GTPases, including their ability to associate with membranes, interact with specific proteins, and localize in subcellular compartments. Different signaling pathways mediated by Ras and Rho family members may converge when the small GTPases are directed by their PBRs to shared binding sites in specific proteins or at cell membranes. The PBR promotes the interactions of small GTPases with SmgGDS, which is a nucleocytoplasmic shuttling protein that stimulates guanine nucleotide exchange by small GTPases. The PBR of Rac1 was recently found to have a functional nuclear localization signal (NLS) sequence, which enhances the nuclear accumulation of protein complexes containing SmgGDS and Rac1. Sequence analysis demonstrates that canonical NLS sequences (K-K/R-x-K/R) are present in the PBRs of additional Ras and Rho family members, and are evolutionarily conserved across several phyla. These findings suggest that the PBR regulates the nucleocytoplasmic shuttling of some Ras and Rho family members when they are in protein complexes that are too large to diffuse through nuclear pores. These diverse functions of the PBR indicate its critical role in signaling by Ras and Rho family GTPases.

LEK1 is a potential inhibitor of pocket protein-mediated cellular processes

LEK1, a member of the LEK family of proteins, is ubiquitously expressed in developing murine tissues. Our current studies are aimed at identifying the role of LEK1 during cell growth and differentiation. Little is known about the function of LEK proteins. Recent studies in our laboratory have focused on the characterization of the LEK1 atypical Rb-binding domain that is conserved among all LEK proteins. Our findings suggest that LEK1 potentially functions as a universal regulator of pocket protein activity. Pocket proteins exhibit distinct expression patterns during development and function to regulate cell cycle, apoptosis, and tissue-specific gene expression. We show that LEK1 interacts with all three pocket proteins, p107, p130, and pRb. Additionally, this interaction occurs specifically between the LEK1 Rb-binding motif and the "pocket domain" of Rb proteins responsible for Rb association with other targets. Analyses of the effects of disruption of LEK1 protein expression by morpholino oligomers demonstrate that LEK1 depletion decreases cell proliferation, disrupts cell cycle progression, and induces apoptosis. Given its expression in developing cells, its association with pocket proteins, and its effects on proliferation, cell cycle, and viability of cells, we suggest that LEK1 functions in a similar manner to phosphorylation to disrupt association of Rb proteins with appropriate binding targets. Thus, the LEK1/Rb interaction serves to retain cells in a pre-differentiative, actively proliferative state despite the presence of Rb proteins during development. Our data suggest that LEK1 is unique among LEK family members in that it specifically functions during murine development to regulate the activity of Rb proteins during cell division and proliferation. Furthermore, we discuss the distinct possibility that a yet unidentified splice variant of the closely related human CENP-F, serves a similar function to LEK1 in humans.

Transcription of the platelet-derived growth factor A-chain gene

The molecular mechanisms which control the transcription of growth factor genes underlie such diverse biological processes as embryonic development, cellular differentiation and wound healing. Moreover, disruption of these controls is implicated in the development and progression of a wide variety of human diseases, including cancer, atherosclerosis and fibrotic disease. This review highlights progress made in the study of the gene encoding platelet-derived growth factor A-chain (PDGF-A) from the perspective of its normal patterns of expression, as well as possible mechanisms leading to dysregulation and disease. A particular focus has been placed on the identification and characterization of specific DNA elements, DNA-binding proteins and other aspects of transcriptional regulation involved in activation and repression of the human PDGF-A promoter.

ROCK and nuclear factor-kappaB-dependent activation of cyclooxygenase-2 by Rho GTPases: effects on tumor growth and therapeutic consequences

Rho GTPases are overexpressed in a variety of human tumors contributing to both tumor proliferation and metastasis. Recently, several studies demonstrate an essential role of transcriptional regulation in Rho GTPases-induced oncogenesis. Herein, we demonstrate that RhoA, Rac1, and Cdc42 promote the expression of cyclooxygenase-2 (COX-2) at the transcriptional level by a mechanism that is dependent on the transcription factor nuclear factor-kappaB (NF-kappaB), but not Stat3, a transcription factor required for RhoA-induced tumorigenesis. With respect to RhoA, this effect is dependent on ROCK, but not PKN. Treatment of RhoA-, Rac1-, and Cdc42-transformed epithelial cells with Sulindac and NS-398, two well-characterized nonsteroid antiinflammatory drugs (NSAIDs), results in growth inhibition as determined by cell proliferation assays. Accordingly, tumor growth of RhoA-expressing epithelial cells in syngeneic mice is strongly inhibited by NS-398 treatment. The effect of NSAIDs over RhoA-induced tumor growth is not exclusively dependent on COX-2 because DNA-binding of NF-kappaB is also abolished upon NSAIDs treatment, resulting in complete loss of COX-2 expression. Finally, treatment of RhoA-transformed cells with Bay11-7083, a specific NF-kappaB inhibitor, leads to inhibition of cell proliferation. We suggest that treatment of human tumors that overexpress Rho GTPases with NSAIDs and drugs that target NF-kappaB could constitute a valid antitumoral strategy.

Relative molecular similarity in selected chemical carcinogens and the nucleoside triphosphate chain

Several markers of cell toxicity are useful as screening tests for epigenetic carcinogens. The direct effects of chemicals on ATPase and GTPase function are pertinent to the early stages of carcinogenesis. Interference with triphosphate-diphosphate exchange mechanisms may result from the interaction of carcinogens with the substrate triphosphate chain. To investigate this hypothesis, a computational chemistry programme is used in this study to investigate molecular similarity in ATPase inhibitors, carcinogens and tumour promoters, in relation to the nucleoside triphosphate chain. The results show that atoms in the investigated molecular structures superimpose on sets of oxygen atoms in the triphosphate chain with interatomic distances < 0.3A. Relative molecular similarity to the substrate triphosphate chain is discussed in terms of the established inhibitory properties of carcinogens/tumour promoters on ATPase function, the carcinogen/ tumour promoting properties of ATPase inhibitors and the prediction of carcinogenic activity from chemical structure.

STAT5a activation mediates the epithelial to mesenchymal transition induced by oncogenic RhoA

The involvement of Rho GTPases in signal transduction pathways leading to transcription activation is one of the major roles of this family of GTPases. Thus, the identification of transcription factors regulated by Rho GTPases and the understanding of the mechanisms of their activation and its biological outcome are of great interest. Here, we provide evidence that Rho GTPases modulate Stat5a, a transcription factor of the family of signal transducers and activators of transcription. RhoA triggers tyrosine phosphorylation (Y696) of Stat5a via a JAK2-dependent mechanism and promotes DNA-binding activity of Stat5a. Tyrosine phosphorylation of Stat5a is also stimulated physiologically by lysophosphatidic acid (LPA) in a Rho-dependent manner. Simultaneously, RhoA reduces serine phosphorylation of Stat5a at both serine residues S726 and S780, resulting in a further increase of activity as defined by mutagenesis experiments. Furthermore, serine dephosphorylation of Stat5a by RhoA does not take place by down-modulation of either JNK1, MEK1, or p38 MAP kinases, as determined by transfection experiments or chemical inhibition of both MEK1, p38, and JNK serine kinases. Thus, RhoA regulates Stat5a via tyrosine phosphorylation and via a yet to be determined novel down-modulating pathway that involves serine dephosphorylation. Finally, we provide evidence for a role of Stat5a in RhoA-induced epithelial-to-mesenchymal transition with concomitant increase in vimentin expression, E-cadherin down-regulation, and cell motility.

GTP-binding proteins in cell survival and demise: the emerging picture in the pancreatic beta-cell

It is widely believed that guanine nucleotide-binding regulatory proteins (G-proteins) play central roles as "molecular switches" in a variety of cellular processes ranging from signal transduction to protein and vesicle trafficking. To achieve these regulatory functions, G-proteins form complexes with a wide range of effector molecules whose activities are altered upon interaction with the G-protein. These effector molecules can be either soluble or membrane bound, and it is likely that some are localized to secretory granules where they direct the movement, docking, and fusion of granules during exocytosis. The effector molecules regulated by G-proteins are diverse and include phospholipases, protein kinases, protein phosphatases, ion channels, adenylate cyclases, cytoskeletal elements, as well as secretory vesicle and plasma membrane-associated fusion-proteins. The majority of studies performed in the pancreatic beta-cell have focused on the role of G-proteins in the regulation of insulin secretion, whereas very little attention has been focused on their potential involvement in other cellular processes. Such studies have identified and implicated both heterotrimeric (comprising alpha, beta, and gamma subunits) and monomeric (low molecular mass) G-proteins in the regulation of insulin secretion, but intriguing recent evidence has also begun to emerge which favors the view that they may be involved in the maintenance of beta-cell viability. In the present commentary, we will review this evidence and discuss the current understanding of the role of G-proteins in the life and death of the beta-cell.

Simultaneous tyrosine and serine phosphorylation of STAT3 transcription factor is involved in Rho A GTPase oncogenic transformation

Stats (signal transducers and activators of transcription) are latent cytoplasmic transcription factors that on a specific stimulus migrate to the nucleus and exert their transcriptional activity. Here we report a novel signaling pathway whereby RhoA can efficiently modulate Stat3 transcriptional activity by inducing its simultaneous tyrosine and serine phosphorylation. Tyrosine phosphorylation is exerted via a member of the Src family of kinases (SrcFK) and JAK2, whereas the JNK pathway mediates serine phosphorylation. Furthermore, cooperation of both tyrosine as well as serine phosphorylation is necessary for full activation of Stat3. Induction of Stat3 activity depends on the effector domain of RhoA and correlates with induction of both Src Kinase-related and JNK activities. Activation of Stat3 has biological implications. Coexpression of an oncogenic version of RhoA along with the wild-type, nontransforming Stat3 gene, significantly enhances its oncogenic activity on human HEK cells, suggesting that Stat3 is an essential component of RhoA-mediated transformation. In keeping with this, dominant negative Stat3 mutants or inhibition of its tyrosine or serine phosphorylation completely abrogate RhoA oncogenic potential. Taken together, these results indicate that Stat3 is an important player in RhoA-mediated oncogenic transformation, which requires simultaneous phosphorylation at both tyrosine and serine residues by specific signaling events triggered by RhoA effectors.

Inhibition of ChoK is an efficient antitumor strategy for Harvey-, Kirsten-, and N-ras-transformed cells

An increasing amount of evidence suggests that elevated PCho levels are related to the transforming properties of the H-Ras oncoprotein. Based on these observations, we have designed an antitumor strategy using choline kinase, the enzyme responsible of PCho production, as a novel target for drug discovery. However, little relationship between this lipid-related pathway and the other two Ras members, N- and K-ras, has been established. Since N- and K-ras are the most frequently mutated ras genes in human tumors, we have analyzed the PC-PLD/ChoK pathway and the sensitivity to ChoK inhibition of all three ras-transformed cells. Here we demonstrate that transformation by the three Ras oncoproteins results in increased levels of PCho to a similar extent, resulting from a similar constitutive increase of ChoK activity. As well, sensitivity to choline kinase inhibitors as antiproliferative drugs is similar in cell lines transformed by each of the three ras oncogenes, being in all cases higher than parental, nontransformed cells. In addition, H, K and N-ras-induced alterations in PC metabolism is discussed. These results indicate that ChoK can be used as a general target for anticancer drug design against Ras-dependent tumorigenesis.

Rho signals to cell growth and apoptosis

Ras and Rho GTPases are among the best studied signaling molecules in molecular biology. Essential cellular processes, such as cell growth, lipid metabolism, cytoarchitecture, membrane trafficking, transcriptional regulation, apoptosis, and response to genotoxic agents, are directly modulated by different members of this superfamily of proteins. Not until recently have we begun to understand the physiological implications of Ras and Rho GTPases, linking them to processes such as embryonic development, tissue remodeling, tumorigenesis and metastasis. In this sense, uncontrolled activation, due to overexpression of different members of the Rho family in a variety of tissues, leads to uncontrolled proliferation and invasiveness of human tumors. In this review, an attempt to briefly integrate recent findings in transcriptional regulation by Rho GTPases in the context of carcinogenesis and metastasis as well as apoptosis is made.