Rho regulates p21(CIP1), cyclin D1, and checkpoint control in mammary epithelial cells

PD-1 Independent Role of PD-L1 in Triple-Negative Breast Cancer Progression

Triple-negative breast cancer (TNBC) is a type of breast malignancy characterized by a high proliferative rate and metastatic potential leading to treatment failure, relapse, and poor prognosis. Therefore, efforts are continuously being devoted to understanding its biology and identifying new potential targets. Programmed death-ligand 1 (PD-L1) is an immunosuppressive protein that inactivates T cells by binding to the inhibitory receptor programmed death-1 (PD-1). PD-L1 overexpression in cancer cells contributes to immune evasion and, subsequently, poor survival and prognosis in several cancers, including breast cancer. Apart from its inhibitory impact on T cells, this ligand is believed to have an intrinsic role in cancer cells. This study was performed to clarify the PD-1 independent role of PD-L1 in TNBC MDA-MB-231 cells by knocking out the PD-L1 using three designs of CRISPR-Cas9 lentiviral particles. Our study revealed that PD-L1 knockout significantly inhibited MDA-MB-231 cell proliferation and colony formation in vitro and tumor growth in the chick embryo chorioallantoic membrane (CAM) model in vivo. PD-L1 knockout also decreased the migration and invasion of MDA-MB-231 cells in vitro. We have shown that PD-L1 knockout MDA-MB-231 cells have low levels of p-Akt and p-ERK in addition to some of their downstream proteins, c-Fos, c-Myc, p21, survivin, and COX-2. Furthermore, PD-L1 knockout significantly decreased the expression of Snail and RhoA. This study shows the intrinsic role of PD-L1 in TNBC independently of its binding to PD-1 receptors on T cells. It may pave the way for developing novel therapeutic strategies using PD-L1 inhibitors alone and in combination to treat TNBC more effectively.

Pleiotropic effects of statins: A focus on cancer

The statin drugs ('statins') potently inhibit hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase by competitively blocking the active site of the enzyme. Statins decrease de novo cholesterol biosynthesis and thereby reduce plasma cholesterol levels. Statins exhibit "pleiotropic" properties that are independent of their lipid-lowering effects. For example, preclinical evidence suggests that statins inhibit tumor growth and induce apoptosis in specific cancer cell types. Furthermore, statins show chemo-sensitizing effects by impairing Ras family GTPase signaling. However, whether statins have clinically meaningful anti-cancer effects remains an area of active investigation. Both preclinical and clinical studies on the potential mechanisms of action of statins in several cancers have been reviewed in the literature. Considering the contradictory data on their efficacy, we present an up-to-date summary of the pleiotropic effects of statins in cancer therapy and review their impact on different malignancies. We also discuss the synergistic anti-cancer effects of statins when combined with other more conventional anti-cancer drugs to highlight areas of potential therapeutic development.

Rho GTPases: Big Players in Breast Cancer Initiation, Metastasis and Therapeutic Responses

Rho GTPases, a family of the Ras GTPase superfamily, are key regulators of the actin cytoskeleton. They were originally thought to primarily affect cell migration and invasion; however, recent advances in our understanding of the biology and function of Rho GTPases have demonstrated their diverse roles within the cell, including membrane trafficking, gene transcription, migration, invasion, adhesion, survival and growth. As these processes are critically involved in cancer initiation, metastasis and therapeutic responses, it is not surprising that studies have demonstrated important roles of Rho GTPases in cancer. Although the majority of data indicates an oncogenic role of Rho GTPases, tumor suppressor functions of Rho GTPases have also been revealed, suggesting a context and cell-type specific function for Rho GTPases in cancer. This review aims to summarize recent progresses in our understanding of the regulation and functions of Rho GTPases, specifically in the context of breast cancer. The potential of Rho GTPases as therapeutic targets and prognostic tools for breast cancer patients are also discussed.

Focus on Cdc42 in Breast Cancer: New Insights, Target Therapy Development and Non-Coding RNAs

Breast cancer is the most common malignant tumors in females. Although the conventional treatment has demonstrated a certain effect, some limitations still exist. The Rho guanosine triphosphatase (GTPase) Cdc42 (Cell division control protein 42 homolog) is often upregulated by some cell surface receptors and oncogenes in breast cancer. Cdc42 switches from inactive guanosine diphosphate (GDP)-bound to active GTP-bound though guanine-nucleotide-exchange factors (GEFs), results in activation of signaling cascades that regulate various cellular processes such as cytoskeletal changes, proliferation and polarity establishment. Targeting Cdc42 also provides a strategy for precise breast cancer therapy. In addition, Cdc42 is a potential target for several types of non-coding RNAs including microRNAs and lncRNAs. These non-coding RNAs is extensively involved in Cdc42-induced tumor processes, while many of them are aberrantly expressed. Here, we focus on the role of Cdc42 in cell morphogenesis, proliferation, motility, angiogenesis and survival, introduce the Cdc42-targeted non-coding RNAs, as well as present current development of effective Cdc42-targeted inhibitors in breast cancer.

Binding and inhibition of the ternary complex factor Elk-4/Sap1 by the adapter protein Dok-4

The adapter protein Dok-4 (downstream of kinase-4) has been reported as both an activator and inhibitor of Erk and Elk-1, but lack of knowledge about the identity of its partner molecules has precluded any mechanistic insight into these seemingly conflicting properties. We report that Dok-4 interacts with the transactivation domain of Elk-4 through an atypical phosphotyrosine-binding domain-mediated interaction. Dok-4 possesses a nuclear export signal and can relocalize Elk-4 from nucleus to cytosol, whereas Elk-4 possesses two nuclear localization signals that restrict interaction with Dok-4. The Elk-4 protein, unlike Elk-1, is highly unstable in the presence of Dok-4, through both an interaction-dependent mechanism and a pleckstrin homology domain-dependent but interaction-independent mechanism. This is reversed by proteasome inhibition, depletion of endogenous Dok-4 or lysine-to-arginine mutation of putative Elk-4 ubiquitination sites. Finally, Elk-4 transactivation is potently inhibited by Dok-4 overexpression but enhanced by Dok-4 knockdown in MDCK renal tubular cells, which correlates with increased basal and EGF-induced expression of Egr-1, Fos and cylcinD1 mRNA, and cell proliferation despite reduced Erk activation. Thus, Dok-4 can target Elk-4 activity through multiple mechanisms, including binding of the transactivation domain, nuclear exclusion and protein destabilization, without a requirement for inhibition of Erk.

High expression of GTPase regulator associated with the focal adhesion kinase (GRAF) is a favorable prognostic factor in acute myeloid leukemia

BACKGROUND

GRAF is a recognized tumor suppressor gene that was found inactivated in AML. However, the prognostic role of a GRAF transcript has not been studied in patients with AML.

METHODS

In this study, we investigated the expression of the GRAF transcript by real time quantitative PCR in 60 AML patients and 30 healthy age and sex matched controls.

RESULTS

GRAF expression was significantly lower in patients with AML when compared to controls (P=0.008). There were no significant differences in clinical features, FAB subtypes and cytogenetic risk subgroups between patients with high and low GRAF expression levels. Kaplan-Meier analysis showed that patients with high GRAF expression had longer overall survival (OS). Multivariate analysis revealed that, besides WBC count, GRAF expression was also an independent prognostic factor for AML.

CONCLUSION

We provide evidence that high GRAF expression is a favorable prognostic marker in patients with AML.

Rho GTPases modulate malignant transformation of tumor cells

Rho GTPases are involved in the acquisition of all the hallmarks of cancer, which comprise 6 biological capabilities acquired during the development of human tumors. The hallmarks include proliferative signaling, evading growth suppressors, resisting cell death, enabling replicative immortality, inducing angiogenesis, and activating invasion and metastasis programs, as defined by Hanahan and Weinberg. (1) Controlling these hallmarks are genome instability and inflammation. Emerging hallmarks are reprogramming of energy metabolism and evading immune destruction. To give a different view to the readers, we will not be focusing on invasion, metastasis, or cytoskeletal remodeling, but we will review here how Rho GTPases contribute to other hallmarks of cancer with a special emphasis on malignant transformation.

The Ras-related protein, Rap1A, mediates thrombin-stimulated, integrin-dependent glioblastoma cell proliferation and tumor growth

Rap1 is a Ras family GTPase with a well documented role in ERK/MAP kinase signaling and integrin activation. Stimulation of the G-protein-coupled receptor PAR-1 with thrombin in human 1321N1 glioblastoma cells led to a robust increase in Rap1 activation. This response was sustained for up to 6 h and mediated through RhoA and phospholipase D (PLD). Thrombin treatment also induced a 5-fold increase in cell adhesion to fibronectin, which was blocked by down-regulating PLD or Rap1A or by treatment with a β1 integrin neutralizing antibody. In addition, thrombin treatment led to increases in phospho-focal adhesion kinase (tyrosine 397), ERK1/2 phosphorylation and cell proliferation, which were significantly inhibited in cells treated with β1 integrin antibody or Rap1A siRNA. To assess the role of Rap1A in tumor formation in vivo, we compared growth of 1321N1 cells stably expressing control, Rap1A or Rap1B shRNA in a mouse xenograft model. Deletion of Rap1A, but not of Rap1B, reduced tumor mass by >70% relative to control. Similar observations were made with U373MG glioblastoma cells in which Rap1A was down-regulated. Collectively, these findings implicate a Rap1A/β1 integrin pathway, activated downstream of G-protein-coupled receptor stimulation and RhoA, in glioblastoma cell proliferation. Moreover, our data demonstrate a critical role for Rap1A in glioblastoma tumor growth in vivo.

Aberrant expression of p53, p21, cyclin D1, and Bcl2 and their clinicopathological correlation in ampullary adenocarcinoma

Previous studies on the molecular alterations in ampullary adenocarcinoma (AA) are limited, and little is known about their clinical implications. The objective of this study is to examine the expression of p53, p21, cyclin D1, and Bcl2 and their clinical significance in patients with AA. Tissue microarrays were constructed using archival tissue from 92 patients with AA who underwent pancreaticoduodenectomy at our institution. Each tumor was sampled in triplicate with a 1.0-mm punch from representative areas. The expression of p53, p21, cyclin D1, and Bcl2 was evaluated by immunohistochemistry, and the staining results were correlated with clinicopathological features and survival. Among 92 cases studied, overexpression of p53, p21, cyclin D1, and Bcl2 was observed in 58.7%, 39.2%, 71.7%, and 5.4% of tumors, respectively. Patients whose tumor showed high level of cyclin D1 expression had higher risk of disease recurrence (P = .02) and worse recurrence-free and overall survivals after pancreaticoduodenectomy than did those with no or low cyclin D1 expression (P = .027 and P = .02, respectively). In multivariate analysis, cyclin D1 expression was an independent prognostic factor for both recurrence-free and overall survival (P < .05). However, there was no significant correlation between p53, p21, or Bcl2 expression and survival (P > .05). Our study showed that p53, p21, and cyclin D1, but not Bcl2, are frequently overexpressed in AAs. Cyclin D1 overexpression is associated with increased risk of disease recurrence and worse survival in patients with AA after resection.

SUMOylation of RhoGDIα is required for its repression of cyclin D1 expression and anchorage-independent growth of cancer cells

Selective activation of Rho GTPase cascade requires the release of Rho from RhoGDI (GDP-dissociation inhibitors) complexes. Our previous studies identified RhoGDIα SUMOylation at Lys-138 and its function in the regulation of cancer cell invasion. In the current study, we demonstrate that RhoGDIα SUMOylation has a crucial role in the suppression of cancer cell anchorage-independent growth as well as the molecular mechanisms underlying this suppression. We found that ectopic expression of RhoGDIα resulted in marked inhibition of an anchorage-independent growth with induction of G0/G1 cell cycle arrest, while point mutation of RhoGDIα SUMOylation at residue Lys-138 (K138R) abrogated this growth suppression and G0/G1 cell cycle arrest in cancer cells. Further studies showed that SUMOylation at Lys-138 was critical for RhoGDIα down-regulation of cyclin D1 protein expression and that MEK1/2-Erk was a specific downstream target of SUMOylated RhoGDIα for its inhibition of C-Jun/AP-1 cascade, cyclin d1 transcription, and cell cycle progression. These results strongly demonstrate that SUMOylated RhoGDIα suppressed C-Jun/AP-1-dependent transactivation specifically via targeting MEK1/2-Erk, subsequently leading to the down-regulation of cyclin D1 expression and anti-cancer activity. Our results provide new mechanistic insights into the understanding of essential role of SUMOylation at Lys-138 in RhoGDIα's biological function.

Rho isoform-specific interaction with IQGAP1 promotes breast cancer cell proliferation and migration

We performed a proteomics screen for Rho isoform-specific binding proteins to clarify the tumor-promoting effects of RhoA and C that contrast with the tumor-suppressive effects of RhoB. We found that the IQ-motif-containing GTPase-activating protein IQGAP1 interacts directly with GTP-bound, prenylated RhoA and RhoC, but not with RhoB. Co-immunoprecipitation of IQGAP1 with endogenous RhoA/C was enhanced when RhoA/C were activated by epidermal growth factor (EGF) or transfection of a constitutively active guanine nucleotide exchange factor (GEF). Overexpression of IQGAP1 increased GTP-loading of RhoA/C, while siRNA-mediated depletion of IQGAP1 prevented endogenous RhoA/C activation by growth factors. IQGAP1 knockdown also reduced the amount of GTP bound to GTPase-deficient RhoA/C mutants, suggesting that IQGAP enhances Rho activation by GEF(s) or stabilizes Rho-GTP. IQGAP1 depletion in MDA-MB-231 breast cancer cells blocked EGF- and RhoA-induced stimulation of DNA synthesis. Infecting cells with adenovirus encoding constitutively active RhoA(L63) and measuring absolute amounts of RhoA-GTP in infected cells demonstrated that the lack of RhoA(L63)-induced DNA synthesis in IQGAP1-depleted cells was not due to reduced GTP-bound RhoA. These data suggested that IQGAP1 functions downstream of RhoA. Overexpression of IQGAP1 in MDA-MB-231 cells increased DNA synthesis irrespective of siRNA-mediated RhoA knockdown. Breast cancer cell motility was increased by expressing a constitutively-active RhoC(V14) mutant or overexpressing IQGAP1. EGF- or RhoC-induced migration required IQGAP1, but IQGAP1-stimulated migration independently of RhoC, placing IQGAP1 downstream of RhoC. We conclude that IQGAP1 acts both upstream of RhoA/C, regulating their activation state, and downstream of RhoA/C, mediating their effects on breast cancer cell proliferation and migration, respectively.

Targeting of syndecan-1 by microRNA miR-10b promotes breast cancer cell motility and invasiveness via a Rho-GTPase- and E-cadherin-dependent mechanism

microRNAs are small endogenous noncoding RNAs, which post-transcriptionally regulate gene expression. In breast cancer, overexpression of the transmembrane heparan sulfate proteoglycan syndecan-1, a predicted target of the oncomiR miR-10b, correlates with poor clinical outcome. To investigate the potential functional relationship of miR-10b and syndecan-1, MDA-MB-231 and MCF-7 breast cancer cells were transiently transfected with pre-miR-10b, syndecan-1 siRNA or control reagents, respectively. Altered cell behavior was monitored by proliferation, migration and invasion chamber assays, and time-lapse video microscopy. miR-10b overexpression induced post-transcriptional downregulation of syndecan-1, as demonstrated by quantitative real-time PCR (qPCR), flow cytometry, and 3'UTR luciferase assays, resulting in increased cancer cell migration and matrigel invasiveness. Syndecan-1 silencing generated a copy of this phenotype. Adhesion to fibronectin and laminin and basal cell proliferation was increased. Syndecan-1 coimmunoprecipitated with focal adhesion kinase, which showed increased activation upon syndecan-1 depletion. Affymetrix screening and confirmatory qPCR and Western blotting analysis of syndecan-1-deficient cells revealed upregulation of ATF-2, COX-2, cadherin-11, vinculin, actin γ 2, MYL9, transgelin-1, RhoA/C, matrix metalloproteinase 2 (MMP2) and heparanase, and downregulation of AML1/RUNX1, E-cadherin, CLDN1, p21WAF/CIP, cyclin-dependent kinase 6, TLR-4, PAI1/2, Collagen1alpha1, JHDM1D, Mpp4, MMP9, matrilin-2 and ANXA3/A10. Video microscopy demonstrated massively increased Rho kinase-dependent motility of syndecan-1-depleted cells, which displayed increased filopodia formation. We conclude that syndecan-1 is a novel target of the oncomiR miR-10b. Rho-GTPase-dependent modulation of cytoskeletal function and downregulation of E-cadherin expression are identified as relevant effectors of the miR-10b-syndecan-1 axis, which emerges as a promising target for the development of new therapeutic approaches for breast cancer.

Casein kinase iγ2 impairs fibroblasts actin stress fibers formation and delays cell cycle progression in g1

Actin cytoskeleton remodeling is under the regulation of multiple proteins with various activities. Here, we demonstrate that the γ2 isoform of Casein Kinase I (CKIγ2) is part of a novel molecular path regulating the formation of actin stress fibers. We show that overexpression of CKIγ2 in fibroblasts alters cell morphology by impairing actin stress fibers formation. We demonstrate that this is concomitant with increased phosphorylation of the CDK inhibitor p27^Kip and lower levels of activated RhoA, and is dependent on CKIγ2 catalytic activity. Moreover, we report that roscovitine, a potent inhibitor of cyclin-dependent kinases, including Cdk5, decreases p27^Kip protein levels and restores actin stress fibers formation in CKIγ2 overexpressing cells, suggesting the existence of a CKIγ2-Cdk5-p27^Kip-RhoA pathway in regulating actin remodeling. On the other hand, we also show that in a manner independent of its catalytic activity, CKIγ2 delays cell cycle progression through G1. Collectively our findings reveal that CKIγ2 is a novel player in the control of actin cytoskeleton dynamics and cell proliferation.

Abnormal methylation of GRAF promoter Chinese patients with acute myeloid leukemia

The epigenetic disturbances are recognized as an alternative mechanism contributing to the pathogenesis of acute myeloid leukemia (AML). GTPase regulator associated with focal adhesion kinase (GRAF), a putative tumor suppressor gene, was revealed with mutations and promoter methylation in AML and myelodysplastic syndrome. In this study, we investigated the methylation status of GRAF promoter in Chinese AML patients. Aberrant methylation of GRAF promoter was detected in 66.7% (88/132) of the cases analyzed. The methylation of GRAF gene could be detected in all FAB subtypes and in all cytogenetic risk groups. There were no significant differences in clinical features, FAB subtypes and cytogenetic risk groups between patients with and without GRAF methylation. GRAF transcript was significantly lower in AML group compared to controls (3.30 vs 56.06, P<0.001). Both patients with methylated GRAF gene and those without methylated GRAF gene had significantly lower GRAF transcript than controls (P<0.001). Furthermore, GRAF transcript was significantly lower in patients with methylated GRAF than those without methylated GRAF (1.64 vs 6.42, P=0.005). These findings suggest that the hypermethylation of GRAF promoter might be one of early events in the development of AML.

Conversion of Helicobacter pylori CagA from senescence inducer to oncogenic driver through polarity-dependent regulation of p21

The Helicobacter pylori CagA bacterial oncoprotein plays a critical role in gastric carcinogenesis. Upon delivery into epithelial cells, CagA causes loss of polarity and activates aberrant Erk signaling. We show that CagA-induced Erk activation results in senescence and mitogenesis in nonpolarized and polarized epithelial cells, respectively. In nonpolarized epithelial cells, Erk activation results in oncogenic stress, up-regulation of the p21(Waf1/Cip1) cyclin-dependent kinase inhibitor, and induction of senescence. In polarized epithelial cells, CagA-driven Erk signals prevent p21(Waf1/Cip1) expression by activating a guanine nucleotide exchange factor-H1-RhoA-RhoA-associated kinase-c-Myc pathway. The microRNAs miR-17 and miR-20a, induced by c-Myc, are needed to suppress p21(Waf1/Cip1) expression. CagA also drives an epithelial-mesenchymal transition in polarized epithelial cells. These findings suggest that CagA exploits a polarity-signaling pathway to induce oncogenesis.

GTPase regulator associated with the focal adhesion kinase (GRAF) transcript was down-regulated in patients with myeloid malignancies

BACKGROUND

GTPase regulator associated with the focal adhesion kinase (GRAF), a putative tumor suppressor gene, is found inactivated in hematopoietic malignancies by either genetic or epigenetic abnormalities. However, the expression level of GRAF gene has not yet been studied in leukemia. The aim of this study was to investigate the expression level of GRAF gene in those patients with myeloid malignancies including acute myeloid leukemia (AML), myelodysplastic syndrome (MDS) and chronic myeloid leukemia (CML).

METHODS

The expression levels of GRAF transcript were determined in 94 patients using real-time quantitative PCR (RQ-PCR). Clinical and laboratory data of these patients were collected and analyzed.

RESULTS

The significantly decreased level of GRAF transcript was observed in three myeloid malignancies compared to controls. Within AML, there was no difference in the level of GRAF transcript among different FAB subtypes (P > 0.05). Difference was not observed in the amount of GRAF mRNA between CML at chronic phase and controls. As CML progressed, GRAF transcript significantly decreased. In MDS, three cases with 5q deletion had lower GRAF transcript than four without 5q deletion (median 0.76 vs 2.99) (P > 0.05).

CONCLUSION

our results demonstrate that the GRAF transcript is decreased in myeloid malignancies.

MST kinases monitor actin cytoskeletal integrity and signal via c-Jun N-terminal kinase stress-activated kinase to regulate p21Waf1/Cip1 stability

As well as providing a structural framework, the actin cytoskeleton plays integral roles in cell death, survival, and proliferation. The disruption of the actin cytoskeleton results in the activation of the c-Jun N-terminal kinase (JNK) stress-activated protein kinase (SAPK) pathway; however, the sensor of actin integrity that couples to the JNK pathway has not been characterized in mammalian cells. We now report that the mammalian Ste20-like (MST) kinases mediate the activation of the JNK pathway in response to the disruption of the actin cytoskeleton. One consequence of actin disruption is the JNK-mediated stabilization of p21(Waf1/Cip1) (p21) via the phosphorylation of Thr57. The expression of MST1 or MST2 was sufficient to stabilize p21 in a JNK- and Thr57-dependent manner, while the stabilization of p21 by actin disruption required MST activity. These data indicate that, in addition to being components of the Salvador-Warts-Hippo tumor suppressor network and binding partners of c-Raf and the RASSF1A tumor suppressor, MST kinases serve to monitor cytoskeletal integrity and couple via the JNK SAPK pathway to the regulation of a key cell cycle regulatory protein.

RhoA regulates G1-S progression of gastric cancer cells by modulation of multiple INK4 family tumor suppressors

RhoA, a member of the Rho GTPase family, has been extensively studied in the regulation of cytoskeletal dynamics, gene transcription, cell cycle progression, and cell transformation. Overexpression of RhoA is found in many malignancies and elevated RhoA activity is associated with proliferation phenotypes of cancer cells. We reported previously that RhoA was hyperactivated in gastric cancer tissues and suppression of RhoA activity could partially reverse the proliferation phenotype of gastric cancer cells, but the underlying mechanism has yet to be elucidated. It has been reported that RhoA activation is crucial for the cell cycle G(1)-S procession through the regulation of Cip/Kip family tumor suppressors in benign cell lines. In this study, we found that selective suppression of RhoA or its effectors mammalian Diaphanous 1 and Rho kinase (ROCK) by small interfering RNA and a pharmacologic inhibitor effectively inhibited proliferation and cell cycle G(1)-S transition in gastric cancer lines. Down-regulation of RhoA-mammalian Diaphanous 1 pathway, but not RhoA-ROCK pathway, caused an increase in the expression of p21(Waf1/Cip1) and p27(Kip1), which are coupled with reduced expression and activity of CDK2 and a cytoplasmic mislocalization of p27(Kip1). Suppression of RhoA-ROCK pathway, on the other hand, resulted in an accumulation of p15(INK4b), p16(INK4a), p18(INK4c), and p19(INK4d), leading to reduced expression and activities of CDK4 and CDK6. Thus, RhoA may use two distinct effector pathways in regulating the G(1)-S progression of gastric cancer cells.

Cellular functions of GEF-H1, a microtubule-regulated Rho-GEF: is altered GEF-H1 activity a crucial determinant of disease pathogenesis?

The Rho guanine nucleotide exchange factor GEF-H1 is uniquely regulated by microtubule binding and is crucial in coupling microtubule dynamics to Rho-GTPase activation in a variety of normal biological situations. Here, we review the roles of GEF-H1 in epithelial barrier permeability, cell motility and polarization, dendritic spine morphology, antigen presentation, leukemic cell differentiation, cell cycle regulation, and cancer. GEF-H1 might also contribute to pathophysiological signaling involved in leukemias, and in cancers associated with mutated p53 tumor suppressor gene, epithelial and endothelial cell dysfunction, infectious disease, and cardiac hypertrophy. We suggest that GEF-H1 could be a novel therapeutic target in multiple human diseases.

Emodin-induced generation of reactive oxygen species inhibits RhoA activation to sensitize gastric carcinoma cells to anoikis

RhoA is a critical signaling molecule regulating a variety of cellular processes, such as cytoskeletal organization, adhesion, and apoptosis. It is recently considered responsive to reactive oxygen species (ROS). Nevertheless, how RhoA regulates anoikis, a detachment-initiated apoptosis, and how this regulation is affected by ROS are not clear. The present study investigated the role of RhoA in apoptosis/anoikis in gastric cancer cells and the changes of RhoA and anoikis under oxidative stress. Immunohistochemistry showed that RhoA expression was upregulated in the primary gastric carcinoma compared with normal gastric mucosa. Overactivation of RhoA by transfection with the V14RhoA mutant prevented gastric cancer line SGC-7901 cells from arsenic-induced apoptosis and conferred anoikis resistance through, at least in part, promoting formations of F-actin fibers and focal adhesion. Oxidative stress caused by emodin, an ROS producer, in combination with arsenic trioxide (ATO) led to RhoA inactivation that triggered structural disruption of focal adhesion complex and eventually resulted in anoikis, and these effects could be partially reversed by antioxidant N-acetylcysteine (NAC). In conclusion, activation of RhoA is required for the maintenance of anoikis resistance phenotype of gastric cancer cells, and oxidative stress might be a therapeutic strategy for the inhibition of RhoA in cancer cells.

The fusion protein of respiratory syncytial virus triggers p53-dependent apoptosis

Infection with respiratory syncytial virus (RSV) frequently causes inflammation and obstruction of the small airways, leading to severe pulmonary disease in infants. We show here that the RSV fusion (F) protein, an integral membrane protein of the viral envelope, is a strong elicitor of apoptosis. Inducible expression of F protein in polarized epithelial cells triggered caspase-dependent cell death, resulting in rigorous extrusion of apoptotic cells from the cell monolayer and transient loss of epithelial integrity. A monoclonal antibody directed against F protein inhibited apoptosis and was also effective if administered to A549 lung epithelial cells postinfection. F protein expression in epithelial cells caused phosphorylation of tumor suppressor p53 at serine 15, activation of p53 transcriptional activity, and conformational activation of proapoptotic Bax. Stable expression of dominant-negative p53 or p53 knockdown by RNA interference inhibited the apoptosis of RSV-infected A549 cells. HEp-2 tumor cells with low levels of p53 were not sensitive to RSV-triggered apoptosis. We propose a new model of RSV disease with the F protein as an initiator of epithelial cell shedding, airway obstruction, secondary necrosis, and consequent inflammation. This makes the RSV F protein a key target for the development of effective postinfection therapies.

Elevated small GTPase activation influences the cell proliferation signaling control in Niemann-Pick type C fibroblasts

Niemann-Pick type C (NPC) disease is characterized at the cellular level by the intracellular accumulation of free cholesterol. We have previously identified a similar phenotype in cystic fibrosis (CF) cell models that results in the activation of the small GTPase RhoA. The hypothesis of this study was that NPC cells would also exhibit an increase in small GTPase activation. An examination of the active, GTP-bound form of GTPases revealed a basal increase in the content of the active-form Ras and RhoA small GTPases in NPC fibroblasts compared to wt controls. To assess whether this increase in GTP-bound Ras and RhoA manifests a functional outcome, the expression of the proliferation control proteins p21/waf1 and cyclin D were examined. Consistent with increased GTPase signaling, p21/waf1 expression is reduced and cyclin D expression is elevated in NPC fibroblasts. Interestingly, cell growth rate is not altered in NPC fibroblasts compared to wt cells. However, NPC sensitivity to statin treatment is reversed by addition of the isoprenoid geranylgeranyl pyrophosphate (GGPP), a modifier of RhoA. It is concluded that Ras and RhoA basal activation is elevated in NPC fibroblasts and has an impact on cell survival pathways.

The protective role of a small GTPase RhoE against UVB-induced DNA damage in keratinocytes

RhoE, a p53 target gene, was identified as a critical factor for the survival of human keratinocytes in response to UVB. The Rho family of GTPases regulates many aspects of cellular behavior through alterations to the actin cytoskeleton, acting as molecular switches cycling between the active, GTP-bound and the inactive, GDP-bound conformations. Unlike typical Rho family proteins, RhoE (also known as Rnd3) is GTPase-deficient and thus expected to be constitutively active. In this study, we investigated the response of cultured human keratinocyte cells to UVB irradiation. RhoE protein levels increase upon exposure to UVB, and ablation of RhoE induction through small interfering RNA resulted in a significant increase in apoptosis and a reduction in the levels of the pro-survival targets p21, Cox-2, and cyclin D1, as well as an increase of reactive oxygen species levels when compared with control cells. These data indicate that RhoE is a pro-survival factor acting upstream of p38, JNK, p21, and cyclin D1. HaCat cells expressing small interfering RNA to p53 indicate that RhoE functions independently of its known associates, p53 and Rho-associated kinase I (ROCK I). Targeted expression of RhoE in epidermis using skin-specific transgenic mouse model resulted in a significant reduction in the number of apoptotic cells following UVB irradiation. Thus, RhoE induction counteracts UVB-induced apoptosis and may serve as a novel target for the prevention of UVB-induced photodamage regardless of p53 status.

Rho GTPases and cell cycle control

Members of the Rho family of small GTPases are crucial regulators of biological responses in eukaryotic cells, including cytoskeletal dynamics, cell motility and cell cycle progression. In the present review, we summarize our current understanding of the role of Rho proteins in cell cycle control, highlighting the contribution of specific members of the Rho family and their downstream targets to the regulation of key elements from the core cell cycle machinery, mostly involved in the G1/S transition.

The Rho GTPase effector ROCK regulates cyclin A, cyclin D1, and p27Kip1 levels by distinct mechanisms

The members of the Rho GTPase family are well known for their regulation of actin cytoskeletal structures. In addition, they influence progression through the cell cycle. The RhoA and RhoC proteins regulate numerous effector proteins, with a central and vital signaling role mediated by the ROCK I and ROCK II serine/threonine kinases. The requirement for ROCK function in the proliferation of numerous cell types has been revealed by studies utilizing ROCK-selective inhibitors such as Y-27632. However, the mechanisms by which ROCK signaling promotes cell cycle progression have not been thoroughly characterized. Using a conditionally activated ROCK-estrogen receptor fusion protein, we found that ROCK activation is sufficient to stimulate G1/S cell cycle progression in NIH 3T3 mouse fibroblasts. Further analysis revealed that ROCK acts via independent pathways to alter the levels of cell cycle regulatory proteins: cyclin D1 and p21(Cip1) elevation via Ras and the mitogen-activated protein kinase pathway, increased cyclin A via LIM kinase 2, and reduction of p27(Kip1) protein levels. Therefore, the influence of ROCK on cell cycle regulatory proteins occurs by multiple independent mechanisms.

An MLCK-dependent window in late G1 controls S phase entry of proliferating rodent hepatocytes via ERK-p70S6K pathway

We show that MLCK (myosin light chain kinase) plays a key role in cell cycle progression of hepatocytes: either chemical inhibitor ML7 or RNA interference led to blockade of cyclin D1 expression and DNA replication, providing evidence that MLCK regulated S phase entry. Conversely, inhibition of RhoK by specific inhibitor Y27632 or RhoK dominant-negative vector did not influence progression in late G1 and S phase entry. Inhibition of either MLCK or RhoK did not block ERK1/2 phosphorylation, whereas MLCK regulated ERK2-dependent p70S6K activation. In addition, DNA synthesis was reduced in hepatocytes treated with p70S6K siRNA, demonstrating the key role played by the kinase in S phase entry. Interestingly, after the G1/S transition, DNA replication in S phase was no longer dependent on MLCK activity. We strengthened this result by ex vivo experiments and evidenced an MLCK-dependent window in late G1 phase of regenerating liver after two-thirds partial hepatectomy. In conclusion, our results underline an MLCK-dependent restriction point in G1/S transition, occurring downstream of ERK2 through the regulation of p70S6K activation, and highlighting a new signaling pathway critical for hepatocyte proliferation.

Stability of p21Waf1/Cip1 CDK inhibitor protein is responsive to RhoA-mediated regulation of the actin cytoskeleton

The proto-oncogene Ras GTPase stimulates transcription of p21Waf1/Cip1 (p21), which is repressed by the RhoA GTPase. We previously showed that Ras also elevates p21 protein levels by reducing its proteasome-mediated degradation. Therefore, we investigated whether RhoA also influenced p21 protein degradation. Pulse-chase analysis of p21 protein stability revealed that inhibitors of Rho function, which disrupt filamentous actin (F-actin), drastically slowed p21 degradation. Direct F-actin disruption mimicked Rho inhibition to stabilize p21. We found that Rho inhibition, or F-actin disruption, activated the JNK stress-activated protein kinase, and that interfering with JNK signalling, but not p38, abrogated p21 stabilization by Rho inhibition or F-actin-disrupting drugs. In addition, Ras-transformation led to increased constitutive JNK activity that contributed to the elevated p21 protein levels. These data suggest that p21 stability is influenced by a mechanism that monitors F-actin downstream of Rho, and which acts through a pathway involving activation of JNK. These results may have significant implications for therapies that target Rho-signalling pathways to induce p21-mediated cell-cycle arrest.

Integrin-dependent signal transduction regulating cyclin D1 expression and G1 phase cell cycle progression

Integrins and growth factor receptors coordinately regulate proliferation in nontransformed cells. Coordinate signaling from these receptors controls the activation of the G1 phase cyclin-dependent kinases, largely by regulating levels of cyclin D1 and p27(kip1). Induction of cyclin D1 is one of the best understood examples of an integrin/growth factor receptor-regulated G1 phase target. This review focuses on the integrin-dependent signal transduction events that regulate the expression of cyclin D1 during G1 phase.

Differential involvement of the actin cytoskeleton in differentiation and mitogenesis of thyroid cells: inactivation of Rho proteins contributes to cyclic adenosine monophosphate-dependent gene expression but prevents mitogenesis

In thyroid epithelial cells, TSH via cAMP induces a rounding up of the cells associated with actin stress fiber disruption, expression of differentiation genes and cell cycle progression. Here we have evaluated the role of small G proteins of the Rho family and their impact on the actin cytoskeleton in these different processes in primary cultures of canine thyrocytes. TSH and forskolin, but not growth factors, rapidly inactivated RhoA, Rac1, and Cdc42, as assayed by detection of GTP-bound forms. Using toxins that inactivate Rho proteins (toxin B, C3 exoenzyme) or activate them [cytotoxic necrotizing factor 1 (CNF1)], in comparison with disruption of the actin cytoskeleton by dihydrocytochalasin B (DCB) or latrunculin, two unexpected conclusions were reached: 1) inactivation of Rho proteins by cAMP, by disorganizing actin microfilaments and inducing cell retraction, could be necessary and sufficient to mediate at least part of the cAMP-dependent induction of thyroglobulin and thyroid oxidases, but only partly necessary for the induction of Na(+)/I(-) symporter and thyroperoxidase; 2) as indicated by the effect of their inhibition by toxin B and C3, some residual activity of Rho proteins could be required for the induction by cAMP-dependent or -independent mitogenic cascades of DNA synthesis and retinoblastoma protein (pRb) phosphorylation, through mechanisms targeting the activity, but not the stimulated assembly, of cyclin D3-cyclin-dependent kinase 4 complexes. However, at variance with current concepts mostly derived from fibroblast models, DNA synthesis induction and cyclin D3-cyclin-dependent kinase 4 activation were resistant to actin depolymerization by dihydrocytochalasin B in canine thyrocytes, which provides a first such example in a normal adherent cell.

Multifaceted role of Rho proteins in angiogenesis

The Rho family of GTPases is part of the Ras superfamily. The Rho, Rac, and Cdc42 members of the family are present in mammalian cells and have been the subject of attention of researchers due to their vast spectrum of functions. Rac 1, Cdc42, and RhoA are well-known for their role in the regulation of the actin cytoskeleton in promoting the formation of lamellipodia, filopodia, and stress fibers, respectively. The Rho proteins also participate in the control of cell growth, motility, cell-cell adhesions, morphogenesis, cytoskeletal dynamics, and cellular trafficking. The mechanisms for eliciting these functions have become clearer during the last decade. Concordant with their roles in multiple processes of cellular control, the Rho proteins have been shown to be involved in tumor growth, progression, metastasis, and now angiogenesis.

Small G-protein RhoE is underexpressed in prostate cancer and induces cell cycle arrest and apoptosis

BACKGROUND

RhoE/Rnd3, a recently described novel member of the Rho GTPases family, was discussed as a possible antagonist of the RhoA protein that stimulates cell cycle progression and is overexpressed and/or overactivated in prostate cancer. We investigated the expression of RhoE and its role in cell cycle regulation and apoptosis in the human prostate.

METHODS

RhoE expression in cell lines and tissue specimens was assessed by immunoblot analysis, real-time PCR (RT-PCR), and immunohistochemistry. To elucidate RhoE effects on the prostate, RhoE was cloned and overexpressed in DU-145 prostate cancer. Cell cycle modulation and apoptosis was investigated by immunoblot and FACS analysis.

RESULTS

Immunoblot analysis showed a strong RhoE signal in both, benign epithelial and stromal cells. In contrast, almost no protein was detected in various prostate cancer cells. On RT-PCR and microarray analysis, RhoE mRNA expression was significantly reduced in malignant tissue when compared to benign samples. RhoE immunostaining was strong in benign tissue, especially in prostate epithelial cells, whereas it was minimal or absent in malignant tissue. Forced RhoE overexpression in a prostate cancer cell line inhibits the expression of two proteins essential for G2/M transition, namely CDC2 and cyclin B1, and induces G2/M arrest. In addition, apoptotic cell death as measured by a cleavage product of caspase 3 is significantly increased in RhoE-overexpressing cells.

CONCLUSION

In conclusion, our findings suggest RhoE as a tumor suppressor gene that is downregulated early in the development of prostate cancer.

Binding of GEF-H1 to the Tight Junction-Associated Adaptor Cingulin Results in Inhibition of Rho Signaling and G1/S Phase Transition

The activity of Rho GTPases is carefully timed to control epithelial proliferation and differentiation. RhoA is downregulated when epithelial cells reach confluence, resulting in inhibition of signaling pathways that stimulate proliferation. Here we show that GEF-H1/Lfc, a guanine nucleotide exchange factor for RhoA, directly interacts with cingulin, a junctional adaptor. Cingulin binding inhibits RhoA activation and signaling, suggesting that the increase in cingulin expression in confluent cells causes downregulation of RhoA by inhibiting GEF-H1/Lfc. In agreement, RNA interference of GEF-H1 or transfection of GEF-H1 binding cingulin mutants inhibit G1/S phase transition of MDCK cells, and depletion of cingulin by regulated RNA interference results in irregular monolayers and RhoA activation. These results indicate that forming epithelial tight junctions contribute to the downregulation of RhoA in epithelia by inactivating GEF-H1 in a cingulin-dependent manner, providing a molecular mechanism whereby tight junction formation is linked to inhibition of RhoA signaling.

Multi-lobulation of the nucleus in prolonged S phase by nuclear expression of Chk tyrosine kinase

Chk tyrosine kinase phosphorylates Src-family tyrosine kinases and suppresses their kinase activity. We recently showed that Chk localizes to the nucleus as well as the cytoplasm and inhibits cell proliferation. To investigate the role of nuclear Chk in proliferation, various Chk mutants were constructed and expressed. Nuclear localization of Chk-induced dynamic multi-lobulation of the nucleus and prolonged S phase of the cell cycle. The N-terminal domain of Chk and a portion of its kinase domain but not the kinase activity were responsible for induction of the multi-lobulation. Cell sorting analysis revealed that nuclear multi-lobulated cells were enriched in late S phase. Multi-lobulated nuclei were surrounded with lamin B1 that was particularly concentrated in concave regions of the nuclei. Furthermore, treatment with nocodazole or taxol disrupted multi-lobulation of the nucleus. These results suggest that nuclear multi-lobulation in late S phase, which is dependent on polymerization and depolymerization of microtubules, may be involved in nuclear Chk-induced inhibition of proliferation.

Reversal of the malignant phenotype of gastric cancer cells by inhibition of RhoA expression and activity

PURPOSE

The small GTPase RhoA has been implicated in the regulation of cell morphology, motility, and transformation, but the role of RhoA protein in the carcinogenesis of gastric cancer remains unclear. In the present study, we have analyzed the expression status of the RhoA protein in human gastric cancer cells and tissues and investigated the possible involvement of RhoA in regulating the malignant phenotype of gastric cancer cells.

EXPERIMENTAL DESIGN

RhoA expression was analyzed by immunohistochemistry and Western blot in gastric cancer tissues and cell lines. The RhoA-specific small interfering RNA (siRNA) vector was designed and constructed. We examined the role of RhoA in the malignant phenotype of gastric cancer cells by using siRNA knockdown and dominant-negative RhoA mutant suppression of endogenous RhoA activity.

RESULTS

RhoA was found frequently overexpressed in gastric cancer tissues and cells compared with normal tissues or gastric epithelial cells. RhoA-specific siRNA could specifically and stably reduce RhoA expression up to 90% in AGS cells. Both RhoA-specific siRNA and dominant-negative RhoA expressions could significantly inhibit the proliferation and tumorigenicity of AGS cells and enhance chemosensitivity of the cancer cells to Adriamycin and 5-fluorouracil.

CONCLUSION

RhoA may play a critical role in the carcinogenesis of gastric cancer, and the interference of RhoA expression and/or activity could provide a novel avenue in reversing the malignant phenotype of gastric cancer cells.

RhoE inhibits cell cycle progression and Ras-induced transformation

Rho GTPases are major regulators of cytoskeletal dynamics, but they also affect cell proliferation, transformation, and oncogenesis. RhoE, a member of the Rnd subfamily that does not detectably hydrolyze GTP, inhibits RhoA/ROCK signaling to promote actin stress fiber and focal adhesion disassembly. We have generated fibroblasts with inducible RhoE expression to investigate the role of RhoE in cell proliferation. RhoE expression induced a loss of stress fibers and cell rounding, but these effects were only transient. RhoE induction inhibited cell proliferation and serum-induced S-phase entry. Neither ROCK nor RhoA inhibition accounted for this response. Consistent with its inhibitory effect on cell cycle progression, RhoE expression was induced by cisplatin, a DNA damage-inducing agent. RhoE-expressing cells failed to accumulate cyclin D1 or p21(cip1) protein or to activate E2F-regulated genes in response to serum, although ERK, PI3-K/Akt, FAK, Rac, and cyclin D1 transcription was activated normally. The expression of proteins that bypass the retinoblastoma (pRb) family cell cycle checkpoint, including human papillomavirus E7, adenovirus E1A, and cyclin E, rescued cell cycle progression in RhoE-expressing cells. RhoE also inhibited Ras- and Raf-induced fibroblast transformation. These results indicate that RhoE inhibits cell cycle progression upstream of the pRb checkpoint.

Rho-modifying C3-like ADP-ribosyltransferases

C3-like exoenzymes comprise a family of seven bacterial ADP-ribosyltransferases, which selectively modify RhoA, B, and C at asparagine-41. Crystal structures of C3 exoenzymes are available, allowing novel insights into the structure-function relationships of these exoenzymes. Because ADP-ribosylation specifically inhibits the biological functions of the low-molecular mass GTPases, C3 exoenzymes are established pharmacological tools to study the cellular functions of Rho GTPases. Recent studies, however, indicate that the functional consequences of C3-induced ADP-ribosylation are more complex than previously suggested. In the present review the basic properties of C3 exoenzymes are briefly summarized and new findings are reviewed.

Deregulation of the Rho GTPase, Rac1, suppresses cyclin-dependent kinase inhibitor p21(CIP1) levels in androgen-independent human prostate cancer cells

Abnormally suppressed levels of cyclin-dependent kinase inhibitors (CKIs) are associated with aggressive androgen-independent prostate cancer and contribute to uncontrolled proliferation. The androgen-independent human prostate cancer cell lines, LNCaP-104R1, ALVA31 and PC-3, express low levels of the CKI, p21(CIP1), compared to the less-malignant, androgen-dependent LNCaP cells. We investigated the mechanism underlying this suppression by examining the role of Rho GTPases, signaling proteins that play important roles in cell cycle progression, at least in part through regulation of CKIs. Inhibition of Rac1 induced p21 expression in androgen-independent lines but had no effect on the higher p21 levels characteristic of LNCaP cells. This induction of p21 was functionally significant as evidenced by inhibition of cyclin-dependent kinase 2 activity and decreased cell proliferation. Conversely, overexpression of constitutively active Rac1 suppressed the higher p21 levels seen in LNCaP cells. Thus, Rac1 activity is both necessary and sufficient for suppression of p21 in prostate cancer cells. Furthermore, Rac1 activity was significantly higher in all three androgen-independent cell lines compared to LNCaP cells. Thus in three models of aggressive human prostate cancer, hyperactivity of Rac1 corresponds to suppressed levels of p21. These results are unique in describing a role for Rac1 in p21 regulation and may implicate the Rac1 signaling pathway as a potential therapeutic target for controlling prostate cancer cell growth following progression to androgen independence.

Altered Rho GTPase signaling pathways in breast cancer cells

The Rho family of GTPases have emerged as key players in regulating a diverse set of biological activities including actin organization, focal complex/adhesion assembly, cell motility, cell polarity, gene transcription and cell-cycle progression. Some Rho GTPases and their signaling components are overexpressed and/or are hyperactive in breast cancer and recent studies have shown a requirement for Rho GTPases in breast cancer cell metastasis in vivo. Herein we describe the contribution of Rho GTPase to the malignant phenotype of breast cancer cells and the role of these pathways as potential targets for breast cancer therapy. Rho GTPases promote cell-cycle progression through cyclin D1, and cyclin D1 in turn reduces cellular adhesion and promotes migration, an example of 'inside-out' signaling by cyclin D1. As cyclin D1 overexpression correlates with metastatic cancer, the 'inside-out' signaling function of cyclin D1 to promote cell migration may represent a useful new therapeutic target.

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.

Distinct activities of the alpha-catenin family, alpha-catulin and alpha-catenin, on beta-catenin-mediated signaling

Alpha-catenin, an integral part of cadherin-catenin adhesion complexes, is a major binding partner of beta-catenin, a key component of the Wnt pathway, which activates T-cell factor (TCF)/lymphoid enhancer factor (LEF) transcription and is often upregulated in cancers. Recently, we identified an alpha-catenin-related protein, alpha-catulin, whose function is poorly understood, as part of a Rho GTPase signaling complex. Here, based on evidence suggesting that alpha-catulin may associate with a beta-catenin fraction, we investigated the role of alpha-catenin family members in beta-catenin-mediated signals. Expression of the full length or a 103-residue region of alpha-catenin strongly inhibits the induction of the TCF/LEF-responsive TOPFLASH reporter in HEK293T cells expressing activated beta-catenin or in cancer cells with constitutively upregulated Wnt signaling, whereas alpha-catulin expression had no effect. Interestingly, alpha-catulin expression attenuates the activation of the cyclin D1 promoter, a target of Wnt pathway signals. Alpha-catulin appears to inhibit Ras-mediated signals to the cyclin D1 promoter, rather than beta-catenin signals, and the synergy between Ras and beta-catenin required to fully activate this promoter. Data suggesting the involvement of Rho in this response are presented and discussed. These results suggest a novel function for alpha-catulin and imply that alpha-catenin and alpha-catulin have distinct activities that downregulate, respectively, beta-catenin and Ras signals converging on the cyclin D1 promoter.

Raf and RhoA Cooperate to Transform Intestinal Epithelial Cells and Induce Growth Resistance to Transforming Growth Factor β

Although unregulated activation of the Ras/Raf/mitogen-activated protein kinase kinase/Erk signaling pathway is believed to be a central mechanism by which many cell types undergo oncogenic transformation, recent studies indicate that activation of Raf kinase by oncogenic Ras is not sufficient to cause tumorigenic transformation in intestinal epithelial cells. Thus, identification of signaling proteins and pathways that interact with Raf to transform intestinal epithelial cells may be critical for understanding aberrant growth control in the intestinal epithelium. Functional interactions between Raf and the small GTPase RhoA were studied in RIE-1 cells overexpressing both activated Raf(22W) and activated RhoA(63L). Double transfectants were morphologically transformed, formed colonies in soft agar, grew in nude mice, overexpressed cyclin D1 and cyclooxygenase-2 (COX-2), and were resistant to growth inhibition by transforming growth factor (TGF) β. RIE-Raf and RIE-RhoA single transfectants showed none of these characteristics. Expression of a dominant-negative RhoA(N19) construct in RIE-Ras(12V) cells was associated with markedly reduced COX-2 mRNA, COX-2 protein, and prostaglandin E2 levels when compared with RIE-Ras(12V) cells transfected with vector alone. However, no change in transformed morphology, growth in soft agar, cyclin D1 expression, TGFα expression, or TGFβ sensitivity was observed. In summary, coexpression of activated Raf and RhoA induces transformation and TGFβ resistance in intestinal epithelial cells. Although blockade of RhoA signaling reverses certain well-described characteristics of RIE-Ras cells, it is insufficient to reverse the transformed phenotype and restore TGFβ sensitivity. Blockade of additional Rho family members or alternate Ras effector pathways may be necessary to fully reverse the Ras phenotype.

Rho Proteins and Cancer

The Rho family of GTPases has been intensively studied for their roles in signal transduction processes leading to cytoskeletal-dependent responses, including cell migration and phagocytosis. In addition, they are important regulators of cell cycle progression and affect the expression of a number of genes, including those for matrix-degrading proteases implicated in cancer invasion. So far, the expression of some Rho family members has been found to be increased in some human cancers, and some cancer-associated mutations in Rho family regulators have been characterized. This makes Rho protein signalling pathways attractive targets for cancer therapy. However, there is little evidence so far from animal studies to define if and how Rho proteins contribute to cancer cell proliferation, survival, invasion and metastasis.

Rho GTPases as Key Transducers of Proliferative Signals in G1 Cell Cycle Regulation

Mitogenic growth factor- and integrin-dependent signaling pathways cooperate to control the proliferation of nontransformed cells. As integral mediators of these networks, the Rho family of GTPases play a pivotal role in G1 cell cycle progression, primarily through regulation of cyclin D1 expression, as well as the levels of the cyclin-dependent kinase inhibitors p21cip1 and p27kip1. Such dual control of both the critical positive and negative regulators of G1 progression make the Rho GTPases prime candidates to target the autonomous proliferation which typifies cancer cells. Cyclin D1 has been identified as an important oncogene and the cdk inhibitors as tumor suppressors in human breast carcinogenesis. Evidence pointing to the potential role of Rho-dependent pathways and their interaction with oncogenic Ras in contributing to such cell cycle abnormalities that characterize human breast cancer is also presented.

RhoA stimulates IEC-6 cell proliferation by increasing polyamine-dependent Cdk2 activity

Although RhoA plays an important role in cell proliferation and in Ras transformation in fibroblasts and mammary epithelial cells, its role in intestinal epithelial cells (IEC) is unknown. In a previous study (Ray RM, Zimmerman BJ, McCormack SA, Patel TB, and Johnson LR. Am J Physiol Cell Physiol 276: C684-C691, 1999), we showed that polyamine depletion [dl-alpha-difluoromethylornithine (DFMO) treatment] strongly inhibits the proliferation of IEC. In this report, we examined the effect of RhoA on IEC-6 cell proliferation and whether polyamine depletion inhibits cell proliferation in the presence of constitutively active RhoA. Constitutively active RhoA and vector-transfected IEC-6 cell lines were grown in the presence or absence of DFMO, which causes polyamine depletion by inhibiting ornithine decarboxylase, the first rate-limiting step in polyamine synthesis. Constitutively active RhoA significantly increased the rate of cell proliferation. These cells also lost contact inhibition and formed conspicuous foci when they were fully confluent. Decreased p21Waf1/Cip1 expression and increased cyclin-dependent kinase (Cdk2) mRNA levels and activity accompanied the increased proliferation. The inhibition of p21Waf1/Cip1 was independent of p53. There was no activation of the Ras-Raf-MEK-ERK pathway in the RhoA-transfected cell line. Polyamine depletion totally prevented the effect of activated RhoA on IEC-6 cell proliferation, focus formation, and Cdk2 expression. The stability of mRNA and protein for Cdk2 and p21Waf1/Cip1 in V14-RhoA cells was not significantly different from that of vector-transfected cells. In conclusion, RhoA activation decreased p21Waf1/Cip1 expression and increased basal and serum-induced ornithine decarboxylase activity, Cdk2 expression, Cdk2 protein, and Cdk2 activity, leading to the stimulation of IEC proliferation and transformation. Polyamine depletion totally prevented RhoA's effect on proliferation by decreasing Cdk2 expression and activity.

Differentiation-dependent expression of signal transducers and activators of transcription (STATs) might modify responses to growth factors in the cancers of the head and neck

Overexpression of the epidermal growth factor receptor (EGFR) in the cancers of the head and neck is well demonstrated. In addition, copy numbers of the EGFR mRNA were significantly higher in poorly differentiated tumors than in tumors that had a differentiated phenotype. Studies by others also showed that the constitutively activated signal transducer and activator of transcription-3 (STAT3), but not STAT1, is required for EGFR-mediated cell growth. Our aim was to reveal if STAT expression is differentiation-dependent and thus, might respond to exogenous stimuli in a differentiation-dependent manner. Both reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry revealed that expression of STAT1 was high in well/moderately differentiated tumors in vivo. In contrast, STAT3 was expressed in poorly differentiated tumors. In vitro experiments showed that differentiated primary oral keratinocytes expressed higher levels of STAT1, but lower levels of STAT3 than did their undifferentiated counterparts. Epidermal growth factor treatment of oral keratinocytes with various degrees of differentiation showed the maximal induction of cyclin D1 in undifferentiated cells. Our findings suggest that the level of differentiation might modulate the outcome of EGFR signaling (i.e. cyclin D1 transcription), due to the differentiation-associated intracellular balance of transcriptional regulators (STAT1 versus STAT3).

Cell growth and metastasis in pancreatic cancer: is Vav the Rho'd to activation?

The best-known family of low molecular weight GTP-binding proteins is Ras, owing to their high incidence of gain of function mutations in a variety of human cancers including pancreatic cancer. Unlike Ras, no activating mutations have been observed thus far for Rho family GTP-binding proteins in cancer, yet there is increasing evidence that overexpression of Rho family members and/or dysregulation of the GDP-->GTP cycle play an important role in cancer development and progression. The activation of Rho family GTPases downstream of cell surface receptors results in the induction of several intracellular signaling cascades that have been shown to impact on such diverse cellular responses as reorganization of the actin cytoskeleton, gene transcription, cell survival, and cell proliferation. One family of guanine nucleotide exchange factors (GEFs) that have the potential to couple the activation of Rho family members to upstream growth factor receptor tyrosine kinases (RTKs) is the Vav family of proto-oncogenes. Recent experimental evidence has implicated Vav in the regulation of numerous Rho-mediated pathways downstream of RTKs and other cell surface receptors. In this review, we will discuss our current understanding of how Vav proteins are regulated, and how Vav and their target GTP-binding proteins participate in tumorigenesis.