Rho activity can alter the translation of p27 mRNA and is important for RasV12-induced transformation in a manner dependent on p27 status

Rho GTPases Signaling in Zebrafish Development and Disease

Cells encounter countless external cues and the specificity of their responses is translated through a myriad of tightly regulated intracellular signals. For this, Rho GTPases play a central role and transduce signals that contribute to fundamental cell dynamic and survival events. Here, we review our knowledge on how zebrafish helped us understand the role of some of these proteins in a multitude of in vivo cellular behaviors. Zebrafish studies offer a unique opportunity to explore the role and more specifically the spatial and temporal dynamic of Rho GTPases activities within a complex environment at a level of details unachievable in any other vertebrate organism.

p27kip1 expression limits H-Ras-driven transformation and tumorigenesis by both canonical and non-canonical mechanisms

The tumor suppressor protein p27Kip1 plays a pivotal role in the control of cell growth and metastasis formation.Several studies pointed to different roles for p27Kip1 in the control of Ras induced transformation, although no explanation has been provided to elucidate these differences. We recently demonstrated that p27kip1 regulates H-Ras activity via its interaction with stathmin.Here, using in vitro and in vivo models, we show that p27kip1 is an important regulator of Ras induced transformation. In H-RasV12 transformed cells, p27kip1 suppressed cell proliferation and tumor growth via two distinct mechanisms: 1) inhibition of CDK activity and 2) impairment of MT-destabilizing activity of stathmin. Conversely, in K-Ras4BV12 transformed cells, p27kip1 acted mainly in a CDK-dependent but stathmin-independent manner.Using human cancer-derived cell lines and primary breast and sarcoma samples, we confirmed in human models what we observed in mice.Overall, we highlight a pathway, conserved from mouse to human, important in the regulation of H-Ras oncogenic activity that could have therapeutic and diagnostic implication in patients that may benefit from anti-H-Ras therapies.

CRMP4 Inhibits Bone Formation by Negatively Regulating BMP and RhoA Signaling

We identified the neuroprotein collapsing response mediator protein-4 (CRMP4) as a noncanonical osteogenic factor that regulates the differentiation of mouse bone marrow skeletal stem cells (bone marrow stromal stem cells [mBMSCs]) into osteoblastic cells. CRMP4 is the only member of the CRMP1-CRMP5 family to be expressed by mBMSCs and in osteoprogenitors of both adult mouse and human bones. In vitro gain-of-function and loss-of-function of CRMP4 in murine stromal cells revealed its inhibitory effect on osteoblast differentiation. In addition, Crmp4-deficient mice (Crmp4^-/- ) displayed a 40% increase in bone mass, increased mineral apposition rate, and bone formation rate, compared to wild-type controls. Increased bone mass in Crmp4^-/- mice was associated with enhanced BMP2 signaling and BMP2-induced osteoblast differentiation in Crmp4^-/- osteoblasts (OBs). Furthermore, Crmp4^-/- OBs exhibited enhanced activation of RhoA/focal adhesion kinase (FAK) signaling that led to cytoskeletal changes with increased cell spreading. In addition, Crmp4^-/- OBs exhibited increased cell proliferation that was mediated via inhibiting cyclin-dependent kinase inhibitor 1B, p27^Kip1 and upregulating cyclin D1 expression which are targets of RhoA signaling pathway. Our findings identify CRMP4 as a novel negative regulator of osteoblast differentiation. © 2016 American Society for Bone and Mineral Research.

Rewiring of the apoptotic TGF-β-SMAD/NFκB pathway through an oncogenic function of p27 in human papillary thyroid cancer

Papillary thyroid carcinoma (PTC), the most frequent thyroid cancer, is characterized by low proliferation but no apoptosis, presenting frequent lymph-node metastasis. Papillary thyroid carcinoma overexpress transforming growth factor-beta (TGF-β). In human cells, TGF-β has two opposing actions: antitumoral through pro-apoptotic and cytostatic activities, and pro-tumoral promoting growth and metastasis. The switch converting TGF-β from a tumor-suppressor to tumor-promoter has not been identified. In the current study, we have quantified a parallel upregulation of TGF-β and nuclear p27, a CDK2 inhibitor, in samples from PTC. We established primary cultures from follicular epithelium in human homeostatic conditions (h7H medium). TGF-β-dependent cytostasis occurred in normal and cancer cells through p15/CDKN2B induction. However, TGF-β induced apoptosis in normal and benign but not in carcinoma cultures. In normal thyroid cells, TGF-β/SMAD repressed the p27/CDKN1B gene, activating CDK2-dependent SMAD3 phosphorylation to induce p50 NFκB-dependent BAX upregulation and apoptosis. In thyroid cancer cells, oncogene activation prevented TGF-β/SMAD-dependent p27 repression, and CDK2/SMAD3 phosphorylation, leading to p65 NFκB upregulation which repressed BAX, induced cyclin D1 and promoted TGF-β-dependent growth. In PTC samples from patients, upregulation of TGF-β, p27, p65 and cyclin D1 mRNA were significantly correlated, while the expression of the isoform BAX-β, exclusively transcribed in apoptotic cells, was negatively correlated. Additionally, combined ERK and p65 NFκB inhibitors reduced p27 expression and potentiated apoptosis in thyroid cancer cells while not affecting survival in normal thyroid cells. Our results therefore suggest that the oncoprotein p27 reorganizes the effects of TGF-β in thyroid cancer, explaining the slow proliferation but lack of apoptosis and metastatic behavior of PTC.

Exploitation of pleiotropic actions of statins by using tumour-targeted delivery systems

Statins are drugs traditionally used to lower cholesterol levels in blood. At concentrations 100- to 500-fold higher than those needed for reaching cholesterol lowering activity, they have anti-tumour activity. This anti-tumour activity is based on statins pleiotropic effects derived from their ability to inhibit the mevalonate synthesis and include anti-proliferative, pro-apoptotic, anti-angiogenic, anti-inflammatory, anti-metastatic actions and modulatory effects on intra-tumour oxidative stress. Thus, in this review, we summarise the possible pleiotropic actions of statins involved in tumour growth inhibition. Since the administration of these high doses of statins is accompanied by severe side effects, targeted delivery of statins seems to be the appropriate strategy for efficient application of statins in oncology. Therefore, we also present an overview of the current status of targeted delivery systems for statins with possible utilisation in oncology.

Hydrostatic pressure promotes the proliferation and osteogenic/chondrogenic differentiation of mesenchymal stem cells: The roles of RhoA and Rac1

Our previous studies have shown that hydrostatic pressure can serve as an active regulator for bone marrow mesenchymal stem cells (BMSCs). The current work further investigates the roles of cytoskeletal regulatory proteins Ras homolog gene family member A (RhoA) and Ras-related C3 botulinum toxin substrate 1 (Rac1) in hydrostatic pressure-related effects on BMSCs. Flow cytometry assays showed that the hydrostatic pressure promoted cell cycle initiation in a RhoA- and Rac1-dependent manner. Furthermore, fluorescence assays confirmed that RhoA played a positive and Rac1 displayed a negative role in the hydrostatic pressure-induced F-actin stress fiber assembly. Western blots suggested that RhoA and Rac1 play central roles in the pressure-inhibited ERK phosphorylation, and Rac1 but not RhoA was involved in the pressure-promoted JNK phosphorylation. Finally, real-time polymerase chain reaction (PCR) experiments showed that pressure promoted the expression of osteogenic marker genes in BMSCs at an early stage of osteogenic differentiation through the up-regulation of RhoA activity. Additionally, the PCR results showed that pressure enhanced the expression of chondrogenic marker genes in BMSCs during chondrogenic differentiation via the up-regulation of Rac1 activity. Collectively, our results suggested that RhoA and Rac1 are critical to the pressure-induced proliferation and differentiation, the stress fiber assembly, and MAPK activation in BMSCs.

MicroRNA-mediated transformation by the Kaposi's sarcoma-associated herpesvirus Kaposin locus

The human oncogenic Kaposi's sarcoma-associated herpesvirus (KSHV) expresses a set of ∼20 viral microRNAs (miRNAs). miR-K10a stands out among these miRNAs because its entire stem-loop precursor overlaps the coding sequence for the Kaposin (Kap) A/C proteins. The ectopic expression of KapA has been reported to lead to transformation of rodent fibroblasts. However, these experiments inadvertently also introduced miR-K10a, which raises the question whether the transforming activity of the locus could in fact be due to miR-K10a expression. To answer this question, we have uncoupled miR-K10a and KapA expression. Our experiments revealed that miR-K10a alone transformed cells with an efficiency similar to that when it was coexpressed with KapA. Maintenance of the transformed phenotype was conditional upon continued miR-K10a but not KapA protein expression, consistent with its dependence on miRNA-mediated changes in gene expression. Importantly, miR-K10a taps into an evolutionarily conserved network of miR-142-3p targets, several of which are expressed in 3T3 cells and are also known inhibitors of cellular transformation. In summary, our studies of miR-K10a serve as an example of an unsuspected function of an mRNA whose precursor is embedded within a coding transcript. In addition, our identification of conserved miR-K10a targets that limit transformation will point the way to a better understanding of the role of this miRNA in KSHV-associated tumors.

IMPORTANCE

Kaposi's sarcoma-associated herpesvirus (KSHV) is a human tumor virus. The viral Kaposin locus has known oncogenic potential, which has previously been attributed to the encoded KapA protein. Here we show that the virally encoded miR-K10a miRNA, whose precursor overlaps the KapA-coding region, may account for the oncogenic properties of this locus. Our data suggest that miR-K10a mimics the cellular miRNA miR-142-3p and thereby represses several known inhibitors of oncogenic transformation. Our work demonstrates that functional properties attributed to a coding region may in fact be carried out by an embedded noncoding element and sheds light on the functions of viral miR-K10a.

Cell-Cycle Gene Expression in Lovastatin-Induced Medulloblastoma Apoptosis

Background:

3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase is a key rate-limiting enzyme in the mevalonate pathway, which generates precursors both for cholesterol biosynthesis and for the production of nonsteroidal mevalonate derivatives that are involved in a number of growth-regulatory processes. We have reported that lovastatin, a competitive inhibitor of HMG-CoA reductase, not only inhibits medulloblastoma proliferation in vitro, but also induces near-complete cell death via apoptosis. The mechanism of this phenomenon is unclear. Possible involvement of changes in expression of certain cell-cycle related genes led us to study some of them in more detail.

Methods:

Medulloblastoma cell lines were exposed in vitro to lovastatin, and the effects of gene expression changes were studied using RT-PCR, antisense oligonucleotide, DNA electrophoresis and Western blotting analysis.

Results:

1) Levels of total Ras gene mRNA and individual Ras gene mRNA are stable in lovastatin treatment in all examined medulloblastoma cell lines. 2) Blocking c-myc gene over-expression does not enhance medulloblastoma cell sensitivity to lovastatin. 3) Following lovastatin treatment, p16 expression exhibits no change, but pronounced increases of p27KIP1 protein are observed in all examined cell lines. Lovastatin induces pronounced increases of p21WAF1 protein only in Daoy and UW228, but not in D283 Med and D341 Med. 4) Following lovastatin treatment, increased p53 protein is detected only in D341 Med, and bax protein is unchanged in all cell lines.

Conclusion:

Lovastatin-induced growth inhibition and apoptosis in medulloblastoma are not dependent on the regulation of Ras and c-myc gene expression, but may be mediated by p27KIP1 gene expression. Lovastatin-induced apoptosis in medulloblastoma is probably p53 independent, but p53 and p21WAF1 gene expression may also mediate anti-proliferative effects of lovastatin on specific medulloblastoma cell lines.

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.

Cell type-specific signaling function of RhoA GTPase: lessons from mouse gene targeting

RhoA GTPase is a key intracellular regulator of actomyosin dynamics and other cell functions, including adhesion, proliferation, survival, and gene expression. Most of our knowledge of RhoA signaling function is from studies in immortalized cell lines utilizing inhibitors or dominant mutant overexpression, both of which are limited in terms of specificity, dosage, and clonal variation. Recent mouse gene targeting studies of rhoA and its regulators/effectors have revealed cell type-specific signaling mechanisms in the context of mammalian physiology. The new knowledge may present therapeutic opportunities for the rational targeting of RhoA signaling-mediated pathophysiologies.

Transcriptional repression of Bmp2 by p21(Waf1/Cip1) links quiescence to neural stem cell maintenance

Relative quiescence and self renewal are defining features of adult stem cells, but their potential coordination remains unclear. Subependymal neural stem cells (NSCs) lacking cyclin-dependent kinase (CDK) inhibitor (CKI) 1a (p21) exhibit rapid expansion that is followed by their permanent loss later in life. Here we demonstrate that transcription of the gene encoding bone morphogenetic protein 2 (Bmp2) in NSCs is under the direct negative control of p21 through actions that are independent of CDK. Loss of p21 in NSCs results in increased levels of secreted BMP2, which induce premature terminal differentiation of multipotent NSCs into mature non-neurogenic astrocytes in an autocrine and/or paracrine manner. We also show that the cell-nonautonomous p21-null phenotype is modulated by the Noggin-rich environment of the subependymal niche. The dual function that we describe here provides a physiological example of combined cell-autonomous and cell-nonautonomous functions of p21 with implications in self renewal, linking the relative quiescence of adult stem cells to their longevity and potentiality.

Crosstalk of Ras and Rho: activation of RhoA abates Kras-induced liver tumorigenesis in transgenic zebrafish models

RAS and Rho small GTPases are key molecular switches that control cell dynamics, cell growth and tissue development through their distinct signaling pathways. Although much has been learnt about their individual functions in both cell and animal models, the physiological and pathophysiological consequences of their signaling crosstalk in multi-cellular context in vivo remain largely unknown, especially in liver development and liver tumorigenesis. Furthermore, the roles of RhoA in RAS-mediated transformation and their crosstalk in vitro remain highly controversial. When challenged with carcinogens, zebrafish developed liver cancer that resembles the human liver cancer both molecularly and histopathologically. Capitalizing on the growing importance and relevance of zebrafish (Danio rerio) as an alternate cancer model, we have generated liver-specific, Tet-on-inducible transgenic lines expressing oncogenic Kras(G12V), RhoA, constitutively active RhoA(G14V) or dominant-negative RhoA(T19N). Double-transgenic lines expressing Kras(G12V) with one of the three RhoA genes were also generated. Based on quantitative bioimaging and molecular markers for genetic and signaling aberrations, we showed that the induced expression of oncogenic Kras during early development led to liver enlargement and hepatocyte proliferation, associated with elevated Erk phosphorylation, activation of Akt2 and modulation of its two downstream targets, p21Cip and S6 kinase. Such an increase in liver size and Akt2 expression was augmented by dominant-negative RhoA(T19N), but was abrogated by the constitutive-active RhoA(G14V). Consequently, induced expression of the oncogenic Kras in adult transgenic fish led to the development of hepatocellular carcinomas. Survival studies further revealed that the co-expression of dominant-negative RhoA(T19N) with oncogenic Kras increased the mortality rate compared with the other single or double-transgenic lines. This study provides evidence of the previously unappreciated signaling crosstalk between Kras and RhoA in regulating liver overgrowth and liver tumorigenesis. Our results also implicate that activating Rho could be beneficial to suppress the Kras-induced liver malignancies.

Increased number of multi-oocyte follicles (MOFs) in juvenile p27Kip1 mutant mice: potential role of granulosa cells

STUDY QUESTION

Why are female mice that lack a functional p27 protein infertile?

SUMMARY ANSWER

The absence of a functional p27 leads to a dramatic increase in the number of multi-oocyte follicles (MOFs) in juvenile female mice; p27 would promote the individualization of follicles favoring the development of fertile eggs.

WHAT IS KNOWN ALREADY

p27-/- female mice are infertile. p27 suppresses excessive follicular endowment and activation and promotes follicular atresia in mice.

MATERIALS AND METHODS

Ovaries from wild type (WT) and p27Kip1 mutant mice aged 2, 4 and 12 weeks were subjected to immunohistochemistry/immunofluorescence. The slides with whole organs serially sectioned were scanned and examined by image analysis.

MAIN RESULTS AND THE ROLE OF CHANCE

Compared with WT, p27Kip1 mutant pre-pubertal mice had a greater number of oocytes, a greater number of growing follicles and a greater number of MOFs. These differences were statistically significant (P < 0.05), particularly in the case of MOFs (P > 0.001). The unusually large number of MOFs in juvenile p27-deficient mice is a novel observation. In WT mice p27 protein remains present in the oocyte nucleus but gradually decreases in the ooplasm during follicular growth, while granulosa cells show dynamic, follicle stage-related changes.

LIMITATIONS, REASONS FOR CAUTION

These results have been obtained in mice and they cannot be directly extrapolated to humans.

WIDER IMPLICATIONS OF THE FINDINGS

The dramatic increase in the numbers of MOFs in juvenile p27 mutants has not been previously reported. The number of MOFs declines sharply as the mice become sexually mature, pointing to their negative selection. These findings open a new approach to the study of sterility.

STUDY FUNDING/COMPETING INTERESTS

This study has been funded by the Basque Government, Dept. of Health grant 2007111063 and Dept. of Industry (Saiotek) grant S-PC11UN008. Jairo Perez-Sanz was the recipient of a grant from Fundación Jesús de Gangoiti Barrera. The authors have no conflicts of interest to declare.

Statins and cancer: current and future prospects

Statins are inhibitors of 3-hydroxy-methylglutaryl (HMG) CoA reductase. They exhibit effects beyond cholesterol reduction, including anticancer activity. This review presents the effects of statins in vitro and their possible molecular anticancer mechanisms and critically discusses the data regarding the role of statins in cancer prevention. Finally, this review focuses on the use of statins combined with other chemotherapeutics to increase the effectiveness of cancer treatments. Despite rare and inconclusive clinical data, the preclinical results strongly suggest that such combined treatment could be a promising new strategy for the treatment of certain tumor types.

p27kip1 protein levels reflect a nexus of oncogenic signaling during cell transformation

SV40 small t-antigen (ST) collaborates with SV40 large T-antigen (LT) and activated rasv12 to promote transformation in a variety of immortalized human cells. A number of oncogenes or the disruption of the general serine-threonine phosphatase protein phosphatase 2A (PP2A) can replace ST in this paradigm. However, the relationship between these oncogenes and PP2A activity is not clear. To address this, we queried the connectivity of these molecules in silico. We found that p27 was connected to each of those oncogenes that could substitute for ST. We further determined that p27 loss can substitute for the expression of ST during transformation of both rodent and human cells. Conversely, knock-in cells expressing the degradation-resistant S10A and T187A mutants of p27 were resistant to the transforming activities of ST. This suggests that p27 is an important target of the tumor-suppressive effects of PP2A and likely an important target of the multitude of cellular oncoproteins that emulate the transforming function of ST.

Loss of protein-tyrosine phosphatase α (PTPα) increases proliferation and delays maturation of oligodendrocyte progenitor cells

Tightly controlled termination of proliferation determines when oligodendrocyte progenitor cells (OPCs) can initiate differentiation and mature into myelin-forming cells. Protein-tyrosine phosphatase α (PTPα) promotes OPC differentiation, but its role in proliferation is unknown. Here we report that loss of PTPα enhanced in vitro proliferation and survival and decreased cell cycle exit and growth factor dependence of OPCs but not neural stem/progenitor cells. PTPα(-/-) mice have more oligodendrocyte lineage cells in embryonic forebrain and delayed OPC maturation. On the molecular level, PTPα-deficient mouse OPCs and rat CG4 cells have decreased Fyn and increased Ras, Cdc42, Rac1, and Rho activities, and reduced expression of the Cdk inhibitor p27Kip1. Moreover, Fyn was required to suppress Ras and Rho and for p27Kip1 accumulation, and Rho inhibition in PTPα-deficient cells restored expression of p27Kip1. We propose that PTPα-Fyn signaling negatively regulates OPC proliferation by down-regulating Ras and Rho, leading to p27Kip1 accumulation and cell cycle exit. Thus, PTPα acts in OPCs to limit self-renewal and facilitate differentiation.

Hyaluronan regulates cell behavior: a potential niche matrix for stem cells

Hyaluronan is a linear glycosaminoglycan that has received special attention in the last few decades due to its extraordinary physiological functions. This highly viscous polysaccharide is not only a lubricator, but also a significant regulator of cellular behaviors during embryogenesis, morphogenesis, migration, proliferation, and drug resistance in many cell types, including stem cells. Most hyaluronan functions require binding to its cellular receptors CD44, LYVE-1, HARE, layilin, and RHAMM. After binding, proteins are recruited and messages are sent to alter cellular activities. When low concentrations of hyaluronan are applied to stem cells, the proliferative activity is enhanced. However, at high concentrations, stem cells acquire a dormant state and induce a multidrug resistance phenotype. Due to the influence of hyaluronan on cells and tissue morphogenesis, with regards to cardiogenesis, chondrogenesis, osteogenesis, and neurogenesis, it is now been utilized as a biomaterial for tissue regeneration. This paper summarizes the most important and recent findings regarding the regulation of hyaluronan in cells.

Upstream molecular signaling pathways of p27(Kip1) expression in human breast cancer cells in vitro: differential effects of 4-hydroxytamoxifen and deficiency of either D-(+)-glucose or L-leucine

BACKGROUND

The objective of this study was to investigate whether the levels of glucose or certain amino acids could regulate the expression of a cell cycle repressor protein p27(Kip1), thereby dictating the risk of cancer in either obesity or caloric/dietary restriction. Previously, we identified and reported four different upstream molecular signaling pathways of p27 expression in human breast cancer cells. We called these four pathways as pathway #1, #2, #3 and #4. We found that 4-hydroxytamoxifen - but not tamoxifen - up-regulated the expression of p27 using pathway #1 which consisted mainly of receptor tyrosine kinases and mTORC1. We now investigate, using 4-hydroxytamoxifen as a reference anti-cancer agents, whether (a) the moderate increase in the concentration of D-(+)-glucose could down-regulate and, conversely, (b) the deficiency of D-(+)-glucose or certain L-amino acids could up-regulate the expression of p27 in these cells using pathway #2 which consists mainly of AMPK and mTORC1.

RESULTS

Using human MDA-MB-231 breast cancer cells in vitro, these hypotheses were tested experimentally by performing p27-luciferase reporter transfection assays and western immunoblot analyses. The results obtained are consistent with these hypotheses. Furthermore, the results indicated that, although 4-hydroxytamoxifen used primarily pathway #1 to down-regulate the phosphorylation of 4E-BP1 and up-regulate the expression of p27, it also secondarily down-regulated the phosphorylation of S6K1. In contrast, the deficiency of D-(+)-glucose or L-leucine used primarily pathway #2 to down-regulate the phosphorylation of S6K1, but they also secondarily down-regulated the phosphorylation of 4E-BP1 and up-regulated the expression of p27. Finally, deficiency of D-(+)-glucose or L-leucine - but not 4-hydroxytamoxifen - up-regulated the expression of mitochondrial ATP5A and SIRT3.

CONCLUSIONS

(a) 4-Hydroxitamoxifen used primarily pathway #1 to up-regulate the expression of p27. (b) Moderate increase in the concentration of D-(+)-glucose used primarily pathway #2 to down-regulate the expression of p27. (c) Deficiency of D-(+)-glucose or L-leucine also used primarily pathway #2 to up-regulate the expression of p27. (d) Deficiency of D-(+)-glucose or L-leucine - but not 4-hydroxytamoxifen - up-regulated the expression of mitochondrial ATP5A in the Complex V of respiratory oxidation-phosphorylation chain and mitochondrial SIRT3. The SIRT3 is one of the seven mammalian anti-aging as well as anti-metabolic sirtuins.

Rho kinase inhibition by fasudil attenuates cyclosporine-induced kidney injury

It has been shown that the inhibition of the Rho/Rho kinase (ROCK) pathway prevents tubulointerstitial fibrosis and ameliorates renal function in various progressive renal disorders. The present study was to determine whether fasudil, a ROCK inhibitor, has a protective effect on cyclosporine A (CsA)-induced nephropathy. Male Sprague-Dawley rats were treated with CsA (n = 10, 20 mg · kg(-1) day(-1) s.c.), CsA + fasudil (n = 10, 3 mg · kg(-1) day(-1) i.p.), or vehicle alone (n = 10) for 28 days. Fasudil cotreatment ameliorated CsA-induced changes and restored renal function. CsA decreased the expression of endothelial nitric-oxide synthase and increased inducible nitric-oxide synthase/3-nitrotyrosine in the kidney. Accordingly, there was infiltration of inflammatory cells and up-regulation of inflammatory cytokines. Fasudil also significantly suppressed the expression of transforming growth factor-β1, Smad signaling, and subsequent epithelial-to-mesenchymal processes. In addition, fasudil augmented p27(kip1) expression and decreased the number of proliferating cell nuclear antigen-positive cells. In another series of experiments using HK-2 cells in culture, fasudil also suppressed CsA-induced increases in mitogen-activated protein kinase phosphorylation. CsA induced expression of p53, the degree of which was attenuated by fasudil in association with decreases of proapoptotic markers such as Bad, Bax, and total/cleaved caspase-3. These results suggest that inhibition of the Rho/ROCK pathway attenuates CsA-induced nephropathy through the suppression of the induction of inflammatory, apoptotic, and fibrogenic factors, along with inhibition of Smad, mitogen-activated protein kinases, and nitric oxide signaling pathways.

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.

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.

Heparin inhibits pulmonary artery smooth muscle cell proliferation through guanine nucleotide exchange factor-H1/RhoA/Rho kinase/p27

Ras homolog gene family member A (RhoA) through Rho kinase kinase (ROCK), one of its downstream effectors, regulates a wide range of cell physiological functions, including vascular smooth muscle cell (SMC) proliferation, by degrading cyclin-dependent kinase inhibitor, p27. Our previous studies found that heparin inhibition of pulmonary artery SMC (PASMC) proliferation and pulmonary hypertension was dependent on p27 up-regulation. To investigate whether ROCK, a regulator of p27, is involved in regulation of heparin inhibition of PASMC proliferation, we analyzed ROCK expression in the lungs from mice and from human PASMCs exposed to hypoxia, and investigated the effect of ROCK expression in vitro by RhoA cDNA transfection. We also investigated the effect of guanine nucleotide exchange factor (GEF)-H1, an upstream regulator of RhoA, on heparin inhibition of PASMC proliferation by GEF-H1 cDNA transfection. We found that: (1) hypoxia increased ROCK expression in mice and PASMCs; (2) overexpression of RhoA diminished the inhibitory effect of heparin on PASMC proliferation and down-regulated p27 expression; and (3) overexpression of GEF-H1 negated heparin inhibition of PASMC proliferation, which was accompanied by increased GTP-RhoA and decreased p27. This study demonstrates that the RhoA/ROCK pathway plays an important role in heparin inhibition on PASMC proliferation, and reveals that heparin inhibits PASMC proliferation through GEF-H1/RhoA/ROCK/p27 signaling pathway, by down-regulating GEF-H1, RhoA, and ROCK, and then up-regulating p27.

Bone marrow mesenchymal stem cells modulate the expression of RhoA and P27 in hepatic stellate cells

AIM: To investigate the effects of bone marrow mesenchymal stem cells (MSCs) on the mRNA and protein expression of RhoA (Ras homolog gene family member A) and P27 in hepatic stellate cells (HSCs) and explore the mechanisms how MSCs regulate cell cycle progression of HSCs.

METHODS: MSCs were isolated from rat bone marrow and propagated in culture flasks. Meanwhile, HSCs and fibroblasts were thawed and passaged. An indirect co-culture system between MSCs/fibroblasts and HSCs was established using a Transwell membrane system (diameter: 24 mm; pore size: 0.4 μm). HSCs were randomly divided into three groups: blank control group (HSCs alone), negative control group (HSCs plus fibroblasts), and experimental group (HSCs plus MSCs). Cell proliferation was tested by WST-8 assay. Cell-cycle phases were determined by flow cytometry. The mRNA and protein expression of RhoA and P27 in HSCs was determined by reverse transcription-polymerase chain reaction (RT-PCR) and Western blot, respectively.

RESULTS: After 24 h of co-culture, the reduced rate of cell proliferation in the experimental group was significantly higher than those in the blank control group and negative control group co-culture(both P < 0.01). Flow cytometry analysis showed that, after 12 hours of co-culture, the percentage of HSCs in the G₀/G₁ phase in the experimental group was significantly higher than those in the two control groups (both P < 0.01), while the percentage of HSCs in the S phase in the experimental group was significantly lower than those in the two control groups (both P < 0.01). After 12 h of co-culture, the expression level of RhoA mRNA in the experimental group was significantly lower than those in the two control groups (both P < 0.01), whereas the expression level of P27 mRNA showed no significant differences between the experimental group and the two control groups (both P > 0.05). The expression level of RhoA protein in the experimental group was significantly lower than those in the two control groups (both P < 0.01), whereas the expression level of P27 in the experimental group was significantly higher than those in the two control groups (both P < 0.01). No correlation was noted between the expression of RhoA and P27 mRNAs (r = 0.105). However, a negative correlation was noted between the expression of RhoA and P27 proteins (r = -0.943, P < 0.01).

CONCLUSION: MSCs inhibit the proliferation of HSCs possibly by modulating the RhoA-P27 pathway to alter cell cycle progression of HSCs. The upregulation of P27 protein may be due to the downregulation of RhoA activity.

The cytoskeleton and cancer

Cancer is a disease in which many of the characteristics of normal cell behavior are lost or perturbed. Uncontrolled cell proliferation and inappropriate cell survival are common features of all cancers, but in addition defects in cellular morphogenesis that lead to tissue disruption, the acquisition of inappropriate migratory and invasive characteristics and the generation of genomic instability through defects in mitosis also accompany progression of the disease. This volume is focused on the actin and microtubule cytoskeletons, key players that underpin these cellular processes. Actin and tubulin form highly versatile, dynamic polymers that are capable of organizing cytoplasmic organelles and intracellular compartments, defining cell polarity and generating both pushing and contractile forces. In the cell cycle, these two cytoskeletal structures drive chromosomal separation and cell division. During morphogenesis, they determine cell shape and polarity, and promote stable cell-cell and cell-matrix adhesions through their interactions with cadherins and integrins, respectively. Finally, during cell migration they generate protrusive forces at the front and retraction forces at the rear. These are all aspects of cell behavior than often go awry in cancer. This volume brings together those interested in understanding the contribution of the actin and microtubule cytoskeletons to the cell biology of cancer.

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.

miR-181a regulates cap-dependent translation of p27(kip1) mRNA in myeloid cells

p27(kip1) (p27) is a cell cycle inhibitor and tumor suppressor whose expression is tightly regulated in the cell. Translational control of p27 mRNA has emerged as a prominent mechanism to regulate p27 expression during differentiation, quiescence, and cancer progression. The microRNAs miR-221 and miR-222 repress p27 expression in various cancer cells, and this repression promotes tumor cell proliferation. In addition, the presence of an internal ribosome entry site in the 5' untranslated region (UTR) of p27 mRNA has been reported. Here, we show that p27 mRNA is translated via a cap-dependent mechanism in HeLa and HL60 cells and that the previously reported IRES activity can be attributed to cryptic promoters in the sequence corresponding to the p27 5' UTR. Furthermore, cap-dependent translation of p27 mRNA is repressed by miR-181a in undifferentiated HL60 cells. Repression by miR-181a is relieved during differentiation of HL60 into monocyte-like cells, allowing the accumulation of p27, which is necessary to fully block cell cycle progression and reach terminal differentiation. These results identify miR-181a as a regulator of p27 mRNA translation during myeloid cell differentiation.

Structure and activity of the internal ribosome entry site within the human p27 Kip1 5'-untranslated region

The cyclin dependent kinase inhibitor p27(Kip1) is a key cell cycle regulatory protein that is often downregulated in cancer cells. The cellular levels of p27(Kip1) are regulated, in part, through translational control mechanisms. The 5'-UTR of the p27(Kip1) mRNA is known to harbor an IRES that may facilitate expression of p27(Kip1) under conditions of stress such as loss of cell adhesion or growth factor and nutrient deprivation. The results presented here further characterize the p27(Kip1) 5'-UTR and its IRES activity. We confirm that the major transcription start site of the p27(Kip1) gene produces an mRNA with a 5'-UTR of approximately 472 nucleotides. Other minor transcripts are also observed but the 472 nucleotide 5'-UTR displays the highest IRES activity. A structural model for the 472 nucleotide 5'-UTR was derived from nuclease digestion patterns coupled with MFOLD secondary structural prediction software. These results indicate that the 5'-UTR has significant secondary structure but also contains a large single-stranded loop that extends from nucleotides -31 to -66 relative to the start codon. Mapping of the ribosome entry window indicates that the ribosome is recruited to this single-stranded loop. The single-stranded loop also includes a U-rich sequence that has previously been shown to bind several proteins, including HuR. This is significant because HuR has previously been shown to inhibit p27(Kip1) IRES activity and cause downregulation of endogenous p27(Kip1) protein levels. Thus HuR may inhibit IRES activity by blocking the ribosome entry site.

CDK inhibitors: cell cycle regulators and beyond

First identified as cell cycle inhibitors mediating the growth inhibitory cues of upstream signaling pathways, the cyclin-CDK inhibitors of the Cip/Kip family p21Cip1, p27Kip1, and p57Kip2 have emerged as multifaceted proteins with functions beyond cell cycle regulation. In addition to regulating the cell cycle, Cip/Kip proteins play important roles in apoptosis, transcriptional regulation, cell fate determination, cell migration and cytoskeletal dynamics. A complex phosphorylation network modulates Cip/Kip protein functions by altering their subcellular localization, protein-protein interactions, and stability. These functions are essential for the maintenance of normal cell and tissue homeostasis, in processes ranging from embryonic development to tumor suppression.

The Cdk inhibitor p27 in human cancer: prognostic potential and relevance to anticancer therapy

The cyclin-dependent kinase (Cdk) inhibitor p27 (also known as KIP1) regulates cell proliferation, cell motility and apoptosis. Interestingly, the protein can exert both positive and negative functions on these processes. Diverse post-translational modifications determine the physiological role of p27. Phosphorylation regulates p27 binding to and inhibition of cyclin-Cdk complexes, its localization and its ubiquitin-mediated proteolysis. In cancers, p27 is inactivated through impaired synthesis, accelerated degradation and by mislocalization. Moreover, studies in several tumour types indicate that p27 expression levels have both prognostic and therapeutic implications.

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.

Deficiency of the NHE1 gene prevents hypoxia-induced pulmonary hypertension and vascular remodeling

RATIONALE

Our previous studies found that Na(+)/H(+) exchanger (NHE) activity played an essential role in pulmonary artery smooth muscle cell (PASMC) proliferation and in the development of hypoxia-induced pulmonary hypertension and vascular remodeling. Other investigators recently observed increased expression of the NHE isoform 1 (NHE1) gene in rodents with pulmonary hypertension induced by hypoxia. However, a causal role for the NHE1 gene in pulmonary hypertension has not been determined.

OBJECTIVES

To determine the causal role of the NHE1 gene in pulmonary hypertension and vascular remodeling.

METHODS

We used NHE1-null mice to define the role of the NHE1 gene in the development of pulmonary hypertension and remodeling induced by hypoxia and to delineate the NHE1 regulatory pathway.

MEASUREMENTS AND MAIN RESULTS

After 2 weeks of exposure to hypoxia, in contrast to wild-type hypoxic littermates, there was no significant increase in right ventricular systolic pressure, in the ratio of right ventricular to left ventricular plus septal weight [RV/(LV + S)], or in medial wall thickness of the pulmonary arterioles in homozygous mice (NHE1(-/-)). There was a significant decrease in Rho kinase (ROCK1 and ROCK2) expression, accompanied by an increase in p27 expression in NHE1(-/-) mice.

CONCLUSIONS

Our study demonstrated that deficiency of the NHE1 gene prevented the development of hypoxia-induced pulmonary hypertension and vascular remodeling in mice and revealed a novel regulatory pathway associated with NHE1 signaling.

Effect of hypoxia and Beraprost sodium on human pulmonary arterial smooth muscle cell proliferation: the role of p27kip1

BACKGROUND

Hypoxia induces the proliferation of pulmonary arterial smooth muscle cell (PASMC) in vivo and in vitro, and prostacyclin analogues are thought to inhibit the growth of PASMC. Previous studies suggest that p27kip1, a kind of cyclin-dependent kinase inhibitor, play an important role in the smooth muscle cell proliferation. However, the mechanism of hypoxia and the subcellular interactions between p27kip1 and prostacyclin analogues in human pulmonary arterial smooth muscle cell (HPASMC) are not fully understood.

METHODS

We investigated the role of p27kip1 in the ability of Beraprost sodium (BPS; a stable prostacyclin analogue) to inhibit the proliferation of HPASMC during hypoxia. To clarify the biological effects of hypoxic air exposure and BPS on HPASMC, the cells were cultured in a hypoxic chamber under various oxygen concentrations (0.1-21%). Thereafter, DNA synthesis was measured as bromodeoxyuridine (BrdU) incorporation, the cell cycle was analyzed by flow cytometry with propidium iodide staining. The p27kip1 mRNA and protein expression and it's stability was measured by real-time RT-PCR and Western blotting. Further, we assessed the role of p27kip1 in HPASMC proliferation using p27kip1 gene knockdown using small interfering RNA (siRNA) transfection.

RESULTS

Although severe hypoxia (0.1% oxygen) suppressed the proliferation of serum-stimulated HPASMC, moderate hypoxia (2% oxygen) enhanced proliferation in accordance with enhanced p27kip1 protein degradation, whereas BPS suppressed HPASMC proliferation under both hypoxic and normoxic conditions by suppressing p27kip1 degradation with intracellular cAMP-elevation. The 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP), a cAMP analogue, had similar action as BPS in the regulation of p27kip1. Moderate hypoxia did not affect the stability of p27kip1 protein expression, but PDGF, known as major hypoxia-induced growth factors, significantly decreased p27kip1 protein stability. We also demonstrated that BPS and 8-Br-cAMP suppressed HPASMC proliferation under both hypoxic and normoxic conditions by blocking p27kip1 mRNA degradation. Furthermore, p27kip1 gene silencing partially attenuated the effects of BPS and partially restored hypoxia-induced proliferation.

CONCLUSION

Our study suggests that moderate hypoxia induces HPASMC proliferation, which is partially dependent of p27kip1 down-regulation probably via the induction of growth factors such as PDGF, and BPS inhibits both the cell proliferation and p27kip1 mRNA degradation through cAMP pathway.

Cooperation between p27 and p107 during endochondral ossification suggests a genetic pathway controlled by p27 and p130

Pocket proteins and cyclin-dependent kinase (CDK) inhibitors negatively regulate cell proliferation and can promote differentiation. However, which members of these gene families, which cell type they interact in, and what they do to promote differentiation in that cell type during mouse development are largely unknown. To identify the cell types in which p107 and p27 interact, we generated compound mutant mice. These mice were null for p107 and had a deletion in p27 that prevented its binding to cyclin-CDK complexes. Although a fraction of these animals survived into adulthood and looked similar to single p27 mutant mice, a larger number of animals died at birth or within a few weeks thereafter. These animals displayed defects in chondrocyte maturation and endochondral bone formation. Proliferation of chondrocytes was increased, and ectopic ossification was observed. Uncommitted mouse embryo fibroblasts could be induced into the chondrocytic lineage ex vivo, but these cells failed to mature normally. These results demonstrate that p27 carries out overlapping functions with p107 in controlling cell cycle exit during chondrocyte maturation. The phenotypic similarities between p107(-/-) p27(D51/D51) and p107(-/-) p130(-/-) mice and the cells derived from them suggest that p27 and p130 act in an analogous pathway during chondrocyte maturation.

The effects of PACAP on neural cell proliferation

PACAP and its receptors are expressed in growth zones of the brain. By stimulating PAC(1)-receptors PACAP can enhance, as well as reduce, the proliferation rate in a cell-type dependent manner. PACAP can enhance the proliferation rate by activating phospholipase C and protein kinase C, although other signal transduction pathways may also be responsible. PACAP can suppress proliferation by inhibiting protein complexes of the cyclins D and E with the cyclin-dependent kinases 4/6 and 2, respectively, which are necessary for entry into the cell cycle. PACAP seems to exert these inhibitory effects by acting via the Sonic hedgehog glycoprotein and the small GTPase RhoA. Also, the activation of a cyclin-dependent kinase inhibitor has been suggested. The signal transduction pathways mediating the effects of PACAP on proliferation are discussed.

Dibutyryl cAMP stimulates the proliferation of SH-SY5Y human neuroblastoma cells by up-regulating Skp2 protein

PURPOSE

We previously found that the proliferation of SH-SY5Y neuroblastoma cells is stimulated when cAMP is up-regulated by stable expression of stimulatory G protein. Therefore, this study was performed to investigate the mechanism whereby cAMP stimulates the proliferation of SH-SY5Y cells.

METHODS

To investigate the effect of cAMP on cellular proliferation, SH-SY5Y neuroblastoma cells were treated with dibutyryl cAMP (dbcAMP), and then cell growth, thymidine incorporation and cell cycle phase distribution were analyzed. The expression and the activity of the molecules that regulate cell cycle progression were monitored by Western blot, RT-PCR, and kinase activity assay.

RESULTS

Treatment with dbcAMP produced a biphasic effect on cellular proliferation; especially treatment with low concentration of dbcAMP (0.5 mM) showed a higher cellular proliferation rate and promoted G1/S transition in cell cycle. The dbcAMP (0.5 mM) treatment increased CDK2 activity, and it significantly decreased p27Kip1 expression with a decreased half-life of p27Kip1 protein. Moreover, dbcAMP (0.5 mM) increased the protein level and the stability of Skp2 with a concomitant decrease in its ubiquitination.

CONCLUSIONS

cAMP up-regulates Skp2 protein by reducing its degradation probably through decreasing the ubiquitination of Skp2, which might result in accelerated degradation of p27Kip1, increase in CDK2 activity, and stimulation of SH-SY5Y cell proliferation in sequence.

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.

Rho GTPases: biochemistry and biology

Approximately one percent of the human genome encodes proteins that either regulate or are regulated by direct interaction with members of the Rho family of small GTPases. Through a series of complex biochemical networks, these highly conserved molecular switches control some of the most fundamental processes of cell biology common to all eukaryotes, including morphogenesis, polarity, movement, and cell division. In the first part of this review, we present the best characterized of these biochemical pathways; in the second part, we attempt to integrate these molecular details into a biological context.

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.

Effects of HRAS oncogene on cell cycle progression in a cervical cancer-derived cell line

BACKGROUND

Human papillomavirus (HPV) infection is the most prevalent factor in anogenital cancers. However, epidemiological surveys and molecular data indicate that viral presence is not enough to induce cervical cancer, suggesting that cellular factors could play a key role. One of the most important genes involved in cancer development is the RAS oncogene, and activating mutations in this gene have been associated with HPV infection and cervical neoplasia. Thus, we determined the effect of HRAS oncogene expression on cell proliferation in a cell line immortalized by E6 and E7 oncogenes.

METHODS

HPV positive human cervical carcinoma-derived cell lines (HeLa), previously transfected with the HRAS oncogene or the empty vector, were used. We first determined the proliferation rate and cell cycle profile of these cells by using flow cytometry and BrdU incorporation assays. In order to determine the signaling pathway regulated by HRAS and implicated in the alteration of proliferation of these cells, we used specific chemical inhibitors to inactivate the Raf and PI3K pathways.

RESULTS

We observed that HeLa cells stably transfected with oncogenic HRAS progressed faster than control cells on the cell cycle by reducing their G1 phase. Additionally, HRAS overexpression accelerated the G1/S transition. Specific chemical inhibitors for PI3K and MEK activities indicated that both PI3K/AKT and RAF/MEK/ERK pathways are involved in the HRAS oncogene-induced reduction of the G1 phase.

CONCLUSIONS

Our results suggest that the HRAS oncogene could play an important role in the development of cervical cancer, in addition to the presence of HPV, by reducing the G1 phase and accelerating the G1/S transition of infected cells.

PDGF-BB Regulates p27 Expression through ERK-dependent RNA Turn-over inVascular Smooth MuscleCells

Cyclin-dependent kinase inhibitor p27, a critical determinant for cell cycle progression, is an important regulation target of mitogenic signals during arterial injury. In this study, we show in rat aortic smooth muscle cells that PDGF-BB down-regulated p27 protein and mRNA in an ERK-dependent mechanism. Inhibition of ERK, but not other subtypes of the mitogen-activated protein kinase family, prevented the reduction of p27 protein and mRNA. Conversely, direct activation of ERK via adenovirus-mediated expression of a constitutively active form of MEK led to a reduction of p27 protein and mRNA, further supporting the central role of ERK in regulation of p27 expression. Rapamycin, which potently inhibited PDGF-induced activation of p70 S6 kinase as well as proliferation of smooth muscle cells, did not alter the expression of p27. To delineate the molecular mechanism underlying the p27 down-regulation, we examined the effect of PDGF-BB on p27 promoter activity as well as mRNA stability. Stimulation with PDGF-BB significantly shortened the half-life of p27 mRNA without affecting its promoter activity. To further understand the PDGF-stimulated p27 mRNA turnover, we inserted the 5'- and/or 3'-untranslated regions of p27 cDNA into a non-PDGF-responsive luciferase gene. Only those chimeric genes that contained the 3'-untranslated region responded to PDGF-BB with reduced expression. Moreover, inhibition of ERK completely prevented the effect of PDGF on the chimera expression. In summary, our data suggest that p27 is down-regulated by PDGF-BB in vascular smooth muscle cells through an ERK-dependent posttranscriptional mechanism.

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.

Regulation of the cytoskeleton: an oncogenic function for CDK inhibitors?

Cyclin-dependent kinase inhibitors (CKIs) are well known inhibitors of cell proliferation. Their activity is disrupted in many tumour types. Recent studies show that some of these proteins have interesting alternative functions, acting in the cytoplasm to regulate Rho signalling and thereby controlling cytoskeletal organization and cell migration. The upregulation of CKIs in the cytoplasm of many cancer cells indicates that although loss of nuclear CKIs is important for cancer cell proliferation, gain of cytoplasmic CKI function might be involved in tumour invasion and metastasis.