RhoA stimulates p27(Kip) degradation through its regulation of cyclin E/CDK2 activity

The Novel Small Molecule BTB Inhibits Pro-Fibrotic Fibroblast Behavior though Inhibition of RhoA Activity

Idiopathic pulmonary fibrosis (IPF) is a progressive, chronic, interstitial lung disease with a poor prognosis. Although specific anti-fibrotic medications are now available, the median survival time following diagnosis remains very low, and new therapies are urgently needed. To uncover novel therapeutic targets, we examined how biochemical properties of the fibrotic lung are different from the healthy lung. Previous work identified lactate as a metabolite that is upregulated in IPF lung tissue. Importantly, inhibition of the enzyme responsible for lactate production prevents fibrosis in vivo. Further studies revealed that fibrotic lesions of the lung experience a significant decline in tissue pH, likely due to the overproduction of lactate. It is not entirely clear how cells in the lung respond to changes in extracellular pH, but a family of proton sensing G-protein coupled receptors has been shown to be activated by reductions in extracellular pH. This work examines the expression profiles of proton sensing GPCRs in non-fibrotic and IPF-derived primary human lung fibroblasts. We identify TDAG8 as a proton sensing GPCR that is upregulated in IPF fibroblasts and that knockdown of TDAG8 dampens myofibroblast differentiation. To our surprise, BTB, a proposed positive allosteric modulator of TDAG8, inhibits myofibroblast differentiation. Our data suggest that BTB does not require TDAG8 to inhibit myofibroblast differentiation, but rather inhibits myofibroblast differentiation through suppression of RhoA mediated signaling. Our work highlights the therapeutic potential of BTB as an anti-fibrotic treatment and expands upon the importance of RhoA-mediated signaling pathways in the context of myofibroblast differentiation. Furthermore, this works also suggests that TDAG8 inhibition may have therapeutic relevance in the treatment of IPF.

Proximal-tubule molecular relay from early Protein diaphanous homolog 1 to late Rho-associated protein kinase 1 regulates kidney function in obesity-induced kidney damage

The small GTPase protein RhoA has two effectors, ROCK (Rho-associated protein kinase 1) and mDIA1 (Protein diaphanous homolog 1), which cooperate reciprocally. However, temporal regulation of RhoA and its effectors in obesity-induced kidney damage remains unclear. Here, we investigated the role of RhoA activation in the proximal tubules at the early and late stages of obesity-induced kidney damage. In mice, a three week high-fat diet induced proximal tubule hypertrophy and damage without increased albuminuria, and RhoA/mDIA1 activation without ROCK activation. Conversely, a 12- week high-fat diet induced proximal tubule hypertrophy, proximal tubule damage, increased albuminuria, and RhoA/ROCK activation without mDIA1 elevation. Proximal tubule hypertrophy resulting from cell cycle arrest accompanied by downregulation of the multifunctional cyclin-dependent kinase inhibitor p27Kip1 was elicited by RhoA activation. Mice overexpressing proximal tubule-specific and dominant-negative RHOA display amelioration of high-fat diet-induced kidney hypertrophy, cell cycle abnormalities, inflammation, and renal impairment. In human proximal tubules cells, mechanical stretch mimicking hypertrophy activated ROCK, which triggered inflammation. In human kidney samples from normal individuals with a body mass index of about 25, proximal tubule cell size correlated with body mass index, proximal tubule cell damages, and mDIA1 expression. Thus, RhoA activation in proximal tubules is critical for the initiation and progression of obesity-induced kidney damage. Hence, the switch in the downstream RhoA effector in proximal tubule represents a transition from normal to pathogenic kidney adaptation and to body weight gain, leading to obesity-induced kidney damage.

α-Tubulin Acetyltransferase Is a Novel Target Mediating Neurite Growth Inhibitory Effects of Chondroitin Sulfate Proteoglycans and Myelin-Associated Glycoprotein

Damage to the CNS results in neuronal and axonal degeneration, and subsequent neurological dysfunction. Endogenous repair in the CNS is impeded by inhibitory chemical and physical barriers, such as chondroitin sulfate proteoglycans (CSPGs) and myelin-associated glycoprotein (MAG), which prevent axon regeneration. Previously, it has been demonstrated that the inhibition of axonal histone deacetylase-6 (HDAC6) can promote microtubule α-tubulin acetylation and restore the growth of CSPGs- and MAG-inhibited axons. Since the acetylation of α-tubulin is regulated by two opposing enzymes, HDAC6 (deacetylation) and α-tubulin acetyltransferase-1 (αTAT1; acetylation), we have investigated the regulation of these enzymes downstream of a growth inhibitory signal. Our findings show that exposure of primary mouse cortical neurons to soluble CSPGs and MAG substrates cause an acute and RhoA-kinase-dependent reduction in α-tubulin acetylation and αTAT1 protein levels, without changes to either HDAC6 levels or HDAC6 activity. The CSPGs- and MAG-induced reduction in αTAT1 occurs primarily in the distal and middle regions of neurites and reconstitution of αTAT1, either by Rho-associated kinase (ROCK) inhibition or lentiviral-mediated αTAT1 overexpression, can restore neurite growth. Lastly, we demonstrate that CSPGs and MAG signaling decreases αTAT1 levels posttranscriptionally via a ROCK-dependent increase in αTAT1 protein turnover. Together, these findings define αTAT1 as a novel potential therapeutic target for ameliorating CNS injury characterized by growth inhibitory substrates that are prohibitive to axonal regeneration.

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.

[Effects of RhoA silencing on proliferation of tongue squamous cancer cells]

OBJECTIVE

This study investigated the effect of RhoA silencing through RNA interference on proliferation and growth of tongue cancer cells, as well as explored the possible mechanisms of this effect.

METHODS

SSC-4 tongue cancer cells were cultured in vitro and then transfected with small interfering RNA to knock down RhoA expression. The tested cells were divided into three groups: experimental group (experimental group 1: transfected with RhoA-siRNA-1; experi-mental group 2: transfected with RhoA-siRNA-2), negative control group (transfected by random sequence NC-siRNA), and blank control group (transfected with Lipofectamine). The expression levels of RhoA mRNA were respectively measured by quantitative real-time polymerase chain reaction and western blot assay. Moreover, the expression levels of cyclin D1, p21, and p27 and RhoA protein were evaluated by Western blot assay. Proliferation and growth potentiality were analyzed through evaluation of doubling times and methyl thiazolyl tetrazolium assessment.

RESULTS

The expression levels of RhoA gene and protein of experimental groups significantly decreased following siRNA transfection compared with those in the negative and blank control groups. The expression of cyclin D1 decreased significantly and that of p21 and p27 increased significantly. The doubling time was extended and the growth potentiality decreased.

CONCLUSIONS

The results indicated that RhoA silencing can inhibit proliferation of tongue cancer cells, whereas RhoA affects cell proliferation by regulating the cell cycle pathway. Thus, RhoA is a potential target in gene therapy for tongue cancer.

Statin-induced depletion of geranylgeranyl pyrophosphate inhibits cell proliferation by a novel pathway of Skp2 degradation

Statins, such as lovastatin, can induce a cell cycle arrest in the G1 phase. This robust antiproliferative activity remains intact in many cancer cells that are deficient in cell cycle checkpoints and leads to an increased expression of CDK inhibitor proteins p27Kip1 and p21Cip1. The molecular details of this statin-induced growth arrest remains unclear. Here we present evidence that lovastatin can induce the degradation of Skp2, a subunit of the SCFSkp2 ubiquitin ligase that targets p27Kip1 and p21Cip1 for proteasomal destruction. The statin-induced degradation of Skp2 is cell cycle phase independent and does not require its well characterised degradation pathway mediated by APC/CCdh1- or Skp2 autoubiquitination. An N-terminal domain preceding the F-box of Skp2 is both necessary and sufficient for its statin mediated degradation. The degradation of Skp2 results from statin induced depletion of geranylgeranyl isoprenoid intermediates of cholesterol biosynthesis. Inhibition of geranylgeranyl-transferase-I also promotes APC/CCdh1- independent degradation of Skp2, indicating that de-modification of a geranylgeranylated protein triggers this novel pathway of Skp2 degradation.

VEGF-A/NRP1 stimulates GIPC1 and Syx complex formation to promote RhoA activation and proliferation in skin cancer cells

Neuropilin-1 (NRP1) has been identified as a VEGF-A receptor. DJM-1, a human skin cancer cell line, expresses endogenous VEGF-A and NRP1. In the present study, the RNA interference of VEGF-A or NRP1 suppressed DJM-1 cell proliferation. Furthermore, the overexpression of the NRP1 wild type restored shNRP1-treated DJM-1 cell proliferation, whereas NRP1 cytoplasmic deletion mutants did not. A co-immunoprecipitation analysis revealed that VEGF-A induced interactions between NRP1 and GIPC1, a scaffold protein, and complex formation between GIPC1 and Syx, a RhoGEF. The knockdown of GIPC1 or Syx reduced active RhoA and DJM-1 cell proliferation without affecting the MAPK or Akt pathway. C3 exoenzyme or Y27632 inhibited the VEGF-A-induced proliferation of DJM-1 cells. Conversely, the overexpression of the constitutively active form of RhoA restored the proliferation of siVEGF-A-treated DJM-1 cells. Furthermore, the inhibition of VEGF-A/NRP1 signaling upregulated p27, a CDK inhibitor. A cell-penetrating oligopeptide that targeted GIPC1/Syx complex formation inhibited the VEGF-A-induced activation of RhoA and suppressed DJM-1 cell proliferation. In conclusion, this new signaling pathway of VEGF-A/NRP1 induced cancer cell proliferation by forming a GIPC1/Syx complex that activated RhoA to degrade the p27 protein.

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.

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.

RHOA-FAK is a required signaling axis for the maintenance of KRAS-driven lung adenocarcinomas

Non-small cell lung cancer (NSCLC) often expresses mutant KRAS together with tumor-associated mutations of the CDKN2A locus, which are associated with aggressive, therapy-resistant tumors. Here, we unravel specific requirements for the maintenance of NSCLC that carries this genotype. We establish that the extracellular signal-regulated kinase (ERK)/RHOA/focal adhesion kinase (FAK) network is deregulated in high-grade lung tumors. Suppression of RHOA or FAK induces cell death selectively in mutant KRAS;INK4A/ARF-deficient lung cancer cells. Furthermore, pharmacologic inhibition of FAK caused tumor regression specifically in the high-grade lung cancer that developed in mutant Kras;Cdkn2a-null mice. These findings provide a rationale for the rapid implementation of genotype-specific targeted therapies using FAK inhibitors in patients with cancer.

SIGNIFICANCE

Targeted therapies are effective for only a small fraction of patients with cancer. We report that FAK inhibitors exert potent antitumor effects in NSCLCs that express mutant KRAS in association with INK4A/ARF deficiency. These results reveal a novel genotype-specific vulnerability of cancer cells that can be exploited for therapeutic purposes.

Mevalonate metabolism regulates Basal breast cancer stem cells and is a potential therapeutic target

There is increasing evidence that breast tumors are organized in a hierarchy, with a subpopulation of tumorigenic cancer cells, the cancer stem cells (CSCs), which sustain tumor growth. The characterization of protein networks that govern CSC behavior is paramount to design new therapeutic strategies targeting this subpopulation of cells. We have sought to identify specific molecular pathways of CSCs isolated from 13 different breast cancer cell lines of luminal or basal/mesenchymal subtypes. We compared the gene expression profiling of cancer cells grown in adherent conditions to those of matched tumorsphere cultures. No specific pathway was identified to be commonly regulated in luminal tumorspheres, resulting from a minor CSC enrichment in tumorsphere passages from luminal cell lines. However, in basal/mesenchymal tumorspheres, the enzymes of the mevalonate metabolic pathway were overexpressed compared to those in cognate adherent cells. Inhibition of this pathway with hydroxy-3-methylglutaryl CoA reductase blockers resulted in a reduction of breast CSC independent of inhibition of cholesterol biosynthesis and of protein farnesylation. Further modulation of this metabolic pathway demonstrated that protein geranylgeranylation (GG) is critical to breast CSC maintenance. A small molecule inhibitor of the geranylgeranyl transferase I (GGTI) enzyme reduced the breast CSC subpopulation both in vitro and in primary breast cancer xenografts. We found that the GGTI effect on the CSC subpopulation is mediated by inactivation of Ras homolog family member A (RHOA) and increased accumulation of P27(kip1) in the nucleus. The identification of protein GG as a major contributor to CSC maintenance opens promising perspectives for CSC targeted therapy in basal breast cancer.

RhoD participates in the regulation of cell-cycle progression and centrosome duplication

We have previously identified a Rho protein, RhoD, which localizes to the plasma membrane and the early endocytic compartment. Here, we show that a GTPase-deficient mutant of RhoD, RhoDG26V, causes hyperplasia and perturbed differentiation of the epidermis, when targeted to the skin of transgenic mice. In vitro, gain-of-function and loss-of-function approaches revealed that RhoD is involved in the regulation of G1/S-phase progression and causes overduplication of centrosomes. Centriole overduplication assays in aphidicolin-arrested p53-deficient U2OS cells, in which the cell and the centrosome cycles are uncoupled, revealed that the effects of RhoD and its mutants on centrosome duplication and cell cycle are independent. Enhancement of G1/S-phase progression was mediated via Diaph1, a novel effector of RhoD, which we have identified using a two-hybrid screen. These results indicate that RhoD participates in the regulation of cell-cycle progression and centrosome duplication.

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.

Identification and characterization of mechanism of action of P61-E7, a novel phosphine catalysis-based inhibitor of geranylgeranyltransferase-I

Small molecule inhibitors of protein geranylgeranyltransferase-I (GGTase-I) provide a promising type of anticancer drugs. Here, we first report the identification of a novel tetrahydropyridine scaffold compound, P61-E7, and define effects of this compound on pancreatic cancer cells. P61-E7 was identified from a library of allenoate-derived compounds made through phosphine-catalyzed annulation reactions. P61-E7 inhibits protein geranylgeranylation and blocks membrane association of geranylgeranylated proteins. P61-E7 is effective at inhibiting both cell proliferation and cell cycle progression, and it induces high p21(CIP1/WAF1) level in human cancer cells. P61-E7 also increases p27(Kip1) protein level and inhibits phosphorylation of p27(Kip1) on Thr187. We also report that P61-E7 treatment of Panc-1 cells causes cell rounding, disrupts actin cytoskeleton organization, abolishes focal adhesion assembly and inhibits anchorage independent growth. Because the cellular effects observed pointed to the involvement of RhoA, a geranylgeranylated small GTPase protein shown to influence a number of cellular processes including actin stress fiber organization, cell adhesion and cell proliferation, we have evaluated the significance of the inhibition of RhoA geranylgeranylation on the cellular effects of inhibitors of GGTase-I (GGTIs). Stable expression of farnesylated RhoA mutant (RhoA-F) results in partial resistance to the anti-proliferative effect of P61-E7 and prevents induction of p21(CIP1/WAF1) and p27(Kip1) by P61-E7 in Panc-1 cells. Moreover, stable expression of RhoA-F rescues Panc-1 cells from cell rounding and inhibition of focal adhesion formation caused by P61-E7. Taken together, these findings suggest that P61-E7 is a promising GGTI compound and that RhoA is an important target of P61-E7 in Panc-1 pancreatic cancer cells.

A novel knowledge-driven systems biology approach for phenotype prediction upon genetic intervention

Deciphering the biological networks underlying complex phenotypic traits, e.g., human disease is undoubtedly crucial to understand the underlying molecular mechanisms and to develop effective therapeutics. Due to the network complexity and the relatively small number of available experiments, data-driven modeling is a great challenge for deducing the functions of genes/proteins in the network and in phenotype formation. We propose a novel knowledge-driven systems biology method that utilizes qualitative knowledge to construct a Dynamic Bayesian network (DBN) to represent the biological network underlying a specific phenotype. Edges in this network depict physical interactions between genes and/or proteins. A qualitative knowledge model first translates typical molecular interactions into constraints when resolving the DBN structure and parameters. Therefore, the uncertainty of the network is restricted to a subset of models which are consistent with the qualitative knowledge. All models satisfying the constraints are considered as candidates for the underlying network. These consistent models are used to perform quantitative inference. By in silico inference, we can predict phenotypic traits upon genetic interventions and perturbing in the network. We applied our method to analyze the puzzling mechanism of breast cancer cell proliferation network and we accurately predicted cancer cell growth rate upon manipulating (anti)cancerous marker genes/proteins.

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.

The deubiquitinating enzyme USP17 is highly expressed in tumor biopsies, is cell cycle regulated, and is required for G1-S progression

Ubiquitination is a reversible posttranslational modification that is essential for cell cycle control, and it is becoming increasingly clear that the removal of ubiquitin from proteins by deubiquitinating enzymes (DUB) is equally important. In this study, we have identified high levels of the DUB USP17 in several tumor-derived cell lines and primary lung, colon, esophagus, and cervix tumor biopsies. We also report that USP17 is tightly regulated during the cell cycle in all the cells examined, being abundantly evident in G(1) and absent in S phase. Moreover, regulated USP17 expression was necessary for cell cycle progression because its depletion significantly impaired G(1)-S transition and blocked cell proliferation. Previously, we have shown that USP17 regulates the intracellular translocation and activation of the GTPase Ras by controlling Ras-converting enzyme 1 (RCE1) activation. RCE1 also regulates the processing of other proteins with a CAAX motif, including Rho family GTPases. We now show that USP17 depletion blocks Ras and RhoA localization and activation. Moreover, our results confirm that USP17-depleted cells have constitutively elevated levels of the cyclin-dependent kinase inhibitors p21(cip1) and p27(kip1), known downstream targets of Ras and RhoA signaling. These observations clearly show that USP17 is tightly regulated during cell division and that its expression is necessary to coordinate cell cycle progression, and thus, it may be considered a promising novel cancer therapeutic target.

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.

Rho-associated kinase (ROCK) inhibition reverses low cell activity on hydrophobic surfaces

Hydrophobic polymers do not offer an adequate scaffold surface for cells to attach, migrate, proliferate, and differentiate. Thus, hydrophobic scaffolds for tissue engineering have traditionally been physicochemically modified to enhance cellular activity. However, modifying the surface by chemical or physical treatment requires supplementary engineering procedures. In the present study, regulation of a cell signal transduction pathway reversed the low cellular activity on a hydrophobic surface without surface modification. Inhibition of Rho-associated kinase (ROCK) by Y-27632 markedly enhanced adhesion, migration, and proliferation of osteoblastic cells cultured on a hydrophobic polystyrene surface. ROCK inhibition regulated cell-cycle-related molecules on the hydrophobic surface. This inhibition also decreased expression of the inhibitors of cyclin-dependent kinases such as p21(cip1) and p27(kip1) and increased expression of cyclin A and D. These results indicate that defective cellular activity on the hydrophobic surface can be reversed by the control of a cell signal transduction pathway without physicochemical surface modification.

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.

The rapid activation of N-Ras by alpha-thrombin in fibroblasts is mediated by the specific G-protein Galphai2-Gbeta1-Ggamma5 and occurs in lipid rafts

alpha-thrombin is a potent mitogen for fibroblasts and initiates a rapid signal transduction pathway leading to the activation of Ras and the stimulation of cell cycle progression. While the signaling events downstream of Ras have been studied in significant detail and appear well conserved across many species and cell types, the precise molecular events beginning with thrombin receptor activation and leading to the activation of Ras are not as well understood. In this study, we examined the immediate events in the rapid response to alpha-thrombin, in a single cell type, and found that an unexpected degree of specificity exists in the pathway linking alpha-thrombin to Ras activation. Specifically, although IIC9 cells express all three Ras isoforms, only N-Ras is rapidly activated by alpha-thrombin. Further, although several Galpha subunits associate with PAR1 and are released following stimulation, only Galpha(i2) couples to the rapid activation of Ras. Similarly, although IIC9 cells express many Gbeta and Ggamma subunits, only a subset associates with Galpha(i2), and of those, only a single Gbetagamma dimer, Gbeta(1)gamma(5), participates in the rapid activation of N-Ras. We then hypothesized that co-localization into membrane microdomains called lipid rafts, or caveolae, is at least partially responsible for this degree of specificity. Accordingly, we found that all components localize to lipid rafts and that disruption of caveolae abolishes the rapid activation of N-Ras by alpha-thrombin. We thus report the molecular elucidation of an extremely specific and rapid signal transduction pathway linking alpha-thrombin stimulation to the activation of Ras.

Blockade of protein geranylgeranylation inhibits Cdk2-dependent p27Kip1 phosphorylation on Thr187 and accumulates p27Kip1 in the nucleus: implications for breast cancer therapy

We describe the design of a potent and selective peptidomimetic inhibitor of geranylgeranyltransferase I (GGTI), GGTI-2418, and its methyl ester GGTI-2417, which increases the levels of the cyclin-dependent kinase (Cdk) inhibitor p27(Kip1) and induces breast tumor regression in vivo. Experiments with p27(Kip1) small interfering RNA in breast cancer cells and p27(Kip1) null murine embryonic fibroblasts demonstrate that the ability of GGTI-2417 to induce cell death requires p27(Kip1). GGTI-2417 inhibits the Cdk2-mediated phosphorylation of p27(Kip1) at Thr187 and accumulates p27(Kip1) in the nucleus. In nude mouse xenografts, GGTI-2418 suppresses the growth of human breast tumors. Furthermore, in ErbB2 transgenic mice, GGTI-2418 increases p27(Kip1) and induces significant regression of breast tumors. We conclude that GGTIs' antitumor activity is, at least in part, due to inhibiting Cdk2-dependent p27(Kip1) phosphorylation at Thr187 and accumulating nuclear p27(Kip1). Thus, GGTI treatment might improve the poor prognosis of breast cancer patients with low nuclear p27(Kip1) levels.

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 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.

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.

The cyclopentenone 15-deoxy-delta(12,14)-prostaglandin J2 inhibits G1/S transition and retinoblastoma protein phosphorylation in immortalized lymphocytes from Alzheimer's disease patients

Epidemiologic studies indicated that non-steroidal anti-inflammatory drugs (NSAIDs) might prevent or delay the clinical features of Alzheimer disease (AD). The pharmacological activity of NSAIDs is generally attributed to inhibition of cyclooxygenase and peroxisome proliferator-activated receptor gamma (PPARgamma) activation. Based on the antineoplastic and apoptotic effects of PPARgamma activation in a number of cell types, we hypothesized that NSAIDs could protect neurons by controlling the regulation of cell cycle. Recent work suggests that uncoordinated expression of cell cycle molecules and perturbation of cell cycle checkpoints may be one of the mechanisms by which post-mitotic neurons die. Since cell cycle dysfunction is not restricted to neurons in AD, we found it interesting to study the role of PPARgamma activation on cell proliferation in immortalized lymphocytes from AD patients. We report here that 15-deoxy-delta(12,14)-prostaglandin J2 (15d-PGJ2), but not NSAIDs or thiazolidinediones inhibited the serum-mediated enhancement of cell proliferation in AD by blocking the events critical for G1/S transition. The cyclopentenone induced a partial inhibition of retinoblastoma protein phosphorylation and increased levels of the CDK inhibitor p27kip1.

Pituitary Adenylyl Cyclase-activating Polypeptide 38 Reduces Astroglial Proliferation by Inhibiting the GTPase RhoA

Pituitary adenylyl cyclase-activating polypeptide 38 (PACAP38) plays an important role in the proliferation and differentiation of neural cells. In the present study, we have investigated how PACAP38 inhibits the proliferation of cultured neocortical astroglial cells. When applied to synchronized cells during the G(1) phase of the cell cycle, PACAP38 diminished the subsequent nuclear uptake of bromodeoxyuridine. When applied for 2 days, it reduced the cell number. PACAP38 did not exert its antiproliferative effect by activating protein kinase A. It also did not reduce the activity of mitogen-activated protein kinases essential for G(1) phase progression. Instead, PACAP38 acted on a member of the Rho family of small GTPases. It reduced the activity of RhoA as was shown with a Rhotekin pull-down assay. The decrease in endogenous RhoA activity induced by treatment of the cells with C3 exotoxin or by expression of dominant negative RhoA also reduced the nuclear uptake of bromodeoxyuridine. In contrast, expression of constitutively active RhoA prevented the effect of PACAP38. Our data show a novel signal transduction pathway by which the neuropeptide influences cell proliferation.

Basic fibroblast growth factor exhibits dual and rapid regulation of cyclin D1 and p27 to stimulate proliferation of rat cerebral cortical precursors

While extracellular signals play a major role in brain neurogenesis, little is known about the cell cycle machinery underlying mitogen stimulation of precursor proliferation. Current models suggest that the D cyclins function as primary sensors of extracellular mitogens. Here we define the mechanisms by which basic fibroblast growth factor (bFGF) stimulates cortical precursors, with particular attention to the responses of cell cycle promitogenic and antimitogenic regulators. bFGF produced a 4-fold increase in DNA synthesis and a 3-fold rise in bromodeoxyuridine labeling, suggesting that the factor promotes the G1/S transition. There was also a 3-fold increase in cyclin-dependent kinase 2 (CDK2) kinase activity, which is critical for S phase entry. CDK2 activation was apparently cyclin E dependent, since only its protein and mRNA levels were elevated at 24 h, whereas CDK2, p27KIP1 and p57KIP2 levels were unaltered. Late G1 phase CDK2/cyclin E activity depends on early G1 D cyclin function. Indeed, cyclin D1, but not cyclin D3, was upregulated selectively at 8 h after bFGF treatment, a time when cyclin E was unchanged. The sequential activation of cyclin D1 and cyclin E supports the idea that cyclin E gene transcription is regulated by cyclin-D/CDK4/6-mediated pRb phosphorylation and subsequent E2F transcription factor release. However, in addition to increased D1 cyclin, we unexpectedly detected a 75% reduction in p27KIP1 protein at 8 h, suggesting that both pro- and antimitogenic regulators are targets of extracellular mitogens during brain development.

Rho kinase-induced nuclear translocation of ERK1/ERK2 in smooth muscle cell mitogenesis caused by serotonin

There is now considerable evidence supporting a mitogenic action of serotonin (5-HT) on vascular smooth muscle cells (SMC) that might participate in pulmonary hypertension (PH). Our previous studies have demonstrated that 5-HT-induced proliferation depends on the generation of reactive oxygen species and activation of extracellular signal-regulated kinase (ERK) 1/ERK2. Activation of Rho kinase (ROCK) in SMC also may be important in PH. We undertook the present study to assess the role of Rho A/ROCK and its possible relation to ERK1/ERK2 in 5-HT-induced pulmonary artery SMC proliferation. We found that this stimulation of SMC proliferation requires Rho A/ROCK as inhibition with Y27632, a ROCK inhibitor, or dominant negative (DN) mutant Rho A blocks 5-HT-induced proliferation, cyclin D1 expression, phosphorylation of Elk, and the DNA binding of transcription factors, Egr-1 and GATA-4. 5-HT activated ROCK, and the activation was blocked by GR 55562 and GR127935, 5-HT 1B/1D receptor antagonists, but not by serotonin transport (SERT) inhibitors. Activation of Rho kinase by 5-HT was independent of activation of ERK1/ERK2, and 5-HT activated ERK1/ERK2 independently of ROCK. Treatment of SMC with Y27632 and expression of DNRho A in cells blocked translocation of ERK1/ERK2 to the cellular nucleus. Depolymerization of actin with cytochalasin D (CD) and latrunculin B (latB) failed to block the translocation of ERK, suggesting that the actin cytoskeleton does not participate in the translocation. The studies show for the first time to our knowledge combinational action of SERT and a 5-HT receptor in SMC growth and Rho A/ROCK participation in 5-HT receptor 1B/1D-mediated mitogenesis of vascular SMCs through an effect on cytoplasmic to nuclear translocation of ERK1/ERK2.

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.

Role of RhoA, mDia, and ROCK in cell shape-dependent control of the Skp2-p27kip1 pathway and the G1/S transition

Cell shape-dependent control of cell-cycle progression underlies the spatial differentials of growth that drive tissue morphogenesis, yet little is known about how cell distortion impacts the biochemical signaling machinery that is responsible for growth control. Here we show that the Rho family GTPase, RhoA, conveys the "cell shape signal" to the cell-cycle machinery in human capillary endothelial cells. Cells accumulating p27(kip1) and arrested in mid G(1) phase when spreading were inhibited by restricted extracellular matrix adhesion, whereas constitutively active RhoA increased expression of the F-box protein Skp2 required for ubiquitination-dependent degradation of p27(kip1) and restored G(1) progression in these cells. Studies with dominant-negative and constitutively active forms of mDia1, a downstream effector of RhoA, and with a pharmacological inhibitor of ROCK, another RhoA target, revealed that RhoA promoted G(1) progression by altering the balance of activities between these two downstream effectors. These data indicate that signaling proteins such as mDia1 and ROCK, which are thought to be involved primarily in cytoskeletal remodeling, also mediate cell growth regulation by coupling cell shape to the cell-cycle machinery at the level of signal transduction.

p27Kip1 modulates cell migration through the regulation of RhoA activation

The tumor suppressor p27(Kip1) is an inhibitor of cyclin/cyclin-dependent kinase (CDK) complexes and plays a crucial role in cell cycle regulation. However, p27(Kip1) also has cell cycle-independent functions. Indeed, we find that p27(Kip1) regulates cell migration, as p27(Kip1)-null fibroblasts exhibit a dramatic decrease in motility compared with wild-type cells. The regulation of motility by p27(Kip1) is independent of its cell-cycle regulatory functions, as re-expression of both wild-type p27(Kip1) and a mutant p27(Kip1) (p27CK(-)) that cannot bind to cyclins and CDKs rescues migration of p27(-/-) cells. p27(-/-) cells have increased numbers of actin stress fibers and focal adhesions. This is reminiscent of cells in which the Rho pathway is activated. Indeed, active RhoA levels were increased in cells lacking p27(Kip1). Moreover, inhibition of ROCK, a downstream effector of Rho, was able to rescue the migration defect of p27(-/-) cells in response to growth factors. Finally, we found that p27(Kip1) binds to RhoA, thereby inhibiting RhoA activation by interfering with the interaction between RhoA and its activators, the guanine-nucleotide exchange factors (GEFs). Together, the data suggest a novel role for p27(Kip1) in regulating cell migration via modulation of the Rho pathway.

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.

p27 deregulation in breast cancer: prognostic significance and implications for therapy

p27 is a key regulator of G1-to-S phase progression. It prevents premature activation of cyclin E-cdk2 in G1 and promotes the assembly and activation of D-type cyclin-cdks. While the p27 gene is rarely mutated in human cancers, the action of p27 is impaired in breast and other human cancers through accelerated p27 proteolysis, sequestration by cyclin D-cdks, and by p27 mislocalization in tumor cell cytoplasm. Reduced p27 protein is strongly associated with high histopathologic tumor grade, reflecting a lack of tumor differentiation. Loss of p27 is also an indicator of poor patient outcome in a majority of breast cancer studies, including node negative disease. The broad application of p27 in the clinical evaluation of breast cancer prognosis will require a consensus on methods of tumor fixation, staining, and scoring. This review will focus on mechanisms of p27 regulation in normal cells and how deregulation of p27 may arise in breast and other human cancers. The prognostic significance of p27 in human breast cancer and the possible therapeutic implications of these findings will also be reviewed.

Enhanced insulin signaling via Shc in human breast cancer

Insulin is a mild mitogen and has been shown to potentiate mitogenic influence of other growth factors. Because hyperinsulinemia and/or overexpression of insulin receptors have been linked to development, progression, and outcome of breast cancer, we attempted to evaluate the mechanism of these associations. We have compared the expression of insulin receptors and the magnitude of insulin signaling in breast tumors and adjacent normal mammary tissue samples obtained from 20 patients. We observed that insulin binding more than doubled in the tumors as compared with the normal tissue (P <.01 by paired t test). Insulin signaling to Shc, judged by the magnitude of its phosphorylation, was also significantly enhanced in the tumors. In contrast, the phosphorylation of the insulin-receptor substrate-1 (IRS-1), Akt, and mitogen-activated protein (MAP) kinase were identical in the tumorous and normal mammary tissues. Finally, tumors displayed significantly increased amounts of farnesylated p21 Ras and geranylgeranylated Rho-A (P <.01), consistent with Shc-dependent activation of farnesyl (FTase) and geranylgeranyl transferases (GGTase) in the tumor tissue. We conclude that the mechanism of the mitogenic influence of insulin in breast cancer may include increased expression of insulin receptors, preferential hyperphosphorylation of Shc, and increased amounts of prenylated p21 Ras and Rho-A in tumor tissue as compared with adjacent normal mammary tissue.

Expression of cyclin E renders cyclin D-CDK4 dispensable for inactivation of the retinoblastoma tumor suppressor protein, activation of E2F, and G1-S phase progression

The activation of CDK2-cyclin E in late G1 phase has been shown to play a critical role in retinoblastoma protein (pRb) inactivation and G1-S phase progression of the cell cycle. The phosphatidylinositol 3-OH-kinase inhibitor LY294002 has been shown to block cyclin D1 accumulation, CDK4 activity and, thus, G1 progression in alpha-thrombin-stimulated IIC9 cells (Chinese hamster embryonic fibroblasts). Our previous results show that expression of cyclin E rescues S phase progression in alpha-thrombin-stimulated IIC9 cells treated with LY294002, arguing that cyclin E renders CDK4 activity dispensable for G1 progression. In this work we investigate the ability of alpha-thrombin-induced CDK2-cyclin E activity to inactivate pRb in the absence of prior CDK4-cyclin D1 activity. We report that in the absence of CDK4-cyclin D1 activity, CDK2-cyclin E phosphorylates pRb in vivo on at least one residue and abolishes pRb binding to E2F response elements. We also find that expression of cyclin E rescues E2F activation and cyclin A expression in cyclin D kinase-inhibited, alpha-thrombin-stimulated cells. Furthermore, the rescue of E2F activity, cyclin A expression, and DNA synthesis by expression of E can be blocked by the expression of either CDK2(D145N) or RbDeltaCDK, a constitutively active mutant of pRb. However, restoring four known cyclin E-CDK2 phosphorylation sites to RbDeltaCDK renders it susceptible to inactivation in late G1, as assayed by E2F activation, cyclin A expression, and S phase progression. These data indicate that CDK2-cyclin E, without prior CDK4-cyclin D activity, can phosphorylate and inactivate pRb, activate E2F, and induce DNA synthesis.

Bcl2 retards G1/S cell cycle transition by regulating intracellular ROS

Bcl2's antiapoptotic function is regulated by phosphorylation. Bcl2 also regulates cell cycle progression, but the molecular mechanism is unclear. Bcl2 is functionally expressed in mitochondria where it can act as an antioxidant that may regulate intracellular reactive oxygen species (ROS). Since ROS have been reported to act as second messengers in cell signaling, we tested whether Bcl2 phosphorylation regulates ROS and cell cycle progression. G1 --> S transition and ROS levels were measured in cells expressing either the gain of function phosphomimetic Bcl2 mutants S70E and T69E/S70E/S87E (EEE) or the nonphosphorylatable and survival-deficient mutants S70A and T69A/S70A/S87A (AAA). Expression of S70E and EEE but not the A-containing Bcl2 mutants retards G1 --> S transition by 35% to 50% and significantly slows cell growth in association with reduced levels of intracellular ROS. In addition to expression of the phosphomimetic Bcl2 mutants, either interleukin-3 withdrawal or treatment of cells with the antioxidant pyrrolidine dithiocarbamate (PDTC) also reduces intracellular ROS levels in association with up-regulation of p27 and accumulation of cells in G0/G1. Retardation of G1 --> S transition can be overridden by directly adding H2O2 to the cells in a mechanism that involves down-regulation of p27 and activation of Cdk2. Thus, Bcl2 may regulate G1 --> S transition by a novel signaling mechanism that couples regulation of intracellular ROS with p27 and Cdk2. Furthermore, phosphorylation of Bcl2 may functionally link its antiapoptotic, cell cycle retardation, and antioxidant properties.

Deregulation of p27 by oncogenic signaling and its prognostic significance in breast cancer

p27 is a key regulator of progression from G1 to S phase. Although the gene encoding p27 is rarely mutated in human cancers, p27 is functionally inactivated in a majority of human cancers through accelerated p27 proteolysis, through sequestration by cyclin D-cyclin-dependent kinase complexes and by cytoplasmic mislocalization. Here we review mechanisms whereby oncogenic activation of receptor tyrosine kinase and Ras pathways lead to accelerated p27 proteolysis and p27 mislocalization in cancer cells. The prognostic significance of p27 in human breast cancer is also reviewed.

CD44: a new means to inhibit acute myeloid leukemia cell proliferation via p27Kip1

Acute myeloid leukemia (AML) is sustained by the extensive proliferation of leukemic stem and progenitor cells, which give rise to the population of leukemic blasts with defective differentiation and low proliferative capacity. We have recently shown that ligation of CD44, a cell surface molecule present on AML cells, with specific monoclonal antibodies (mAbs) inhibits their proliferation. However, its mechanism has not been investigated yet. Here, using the NB4 cell line as a model of proliferating human AML cells, and the A3D8 mAb to ligate CD44, we show for the first time that CD44 ligation stabilizes the cyclin-dependent kinase inhibitor p27(Kip1) (p27) protein, resulting in increased association with cyclin E/Cdk2 complexes and inhibition of their kinase activity. Moreover, using a p27 antisense vector, we provide direct evidence that p27 is the main mediator of cell growth arrest by CD44. CD44 ligation also leads to p27 accumulation in THP-1, KG1a, and HL60 cell lines and in primary leukemic cells, suggesting that this process is general in AML. Taken together, our present results suggest that CD44 is a new and efficient means to increase the expression of p27 in AML cells. Considering that elevated expression of p27 is a factor of good prognosis in AML, these results provide a new basis for developing CD44-targeted therapy in AML.

Additive effects of tamoxifen and the farnesyl transferase inhibitor FTI-277 on inhibition of MCF-7 breast cancer cell-cycle progression

The efficacy of tamoxifen in the hormonal therapy of breast cancer is well established, but therapeutic resistance is inevitable. FTIs are a new class of anticancer drugs that are in phase III clinical evaluation. Since the mechanisms of action of these 2 classes of drugs are different, we tested the combination of tamoxifen and FTI-277 on inhibiting proliferation of hormone-dependent MCF-7 human breast cancer cells. An additive effect on cell proliferation was demonstrated, accompanied by an additive G(0)/G(1) arrest. The major effect of the combination of the 2 drugs was to maintain p21(waf/cip1) at an intermediate level, higher than that observed in the presence of tamoxifen alone. This was associated with an additive effect on inactivation of cyclin E-Cdk2 complexes and decreased phosphorylation of pRb and p130 pocket proteins. These effects were accompanied by increased association of 2 CDIs, p27(kip1) and p21(waf/cip1), with cyclin E-Cdk2 complexes. These data demonstrate that the additive effect is likely predominantly due to the recruitment of p27(kip1) and, to a lesser extent, p21(waf/cip1) into the cyclin E-Cdk2 complexes. Together, these results suggest that the combination of FTI and tamoxifen may increase the antitumor effect of either drug alone in breast cancer.

Cdc42 promotes G1 progression through p70 S6 kinase-mediated induction of cyclin E expression

The Rho family GTPase Cdc42 is recognized for its role in cellular proliferation and transformation. However, the mechanism by which it promotes cell cycle progression has remained undefined. Using an inducible expression system, we show that constitutively active Cdc42 (Cdc42V12) is sufficient by itself to induce anchorage-independent but not mitogen-independent growth in NIH3T3 cells. However, Cdc42V12 markedly accelerates activation of cyclin E-Cdk2 in response to mitogen. These effects were highly specific, as the kinetics of cyclin D-Cdk4 activation was unaltered. Cdc42V12 promotes Cdk2 activation by selectively inducing cyclin E expression without affecting other regulatory proteins such as the p27 Cdk inhibitor or Cdc25A. Furthermore, Cdc42V12 was able to activate a reporter gene driven by the cyclin E promoter in the absence of exogenous mitogen or adhesion. Cyclin E induction was sensitive to rapamycin but not inhibitors of mitogen-activated protein kinases, implicating p70 S6 kinase (p70S6k) as the relevant mediator. Consistent with this notion, wild type and constitutively active alleles of p70S6k were sufficient to activate the cyclin E promoter. In sum, these studies provide novel insights into the mechanism by which Cdc42 promotes G1 progression.

CrkII induces serum response factor activation and cellular transformation through its function in Rho activation

CrkII belongs to the adaptor protein family that plays a crucial role in signal transduction. In order to better understand the biological functions of CrkII, we focused on the regulation of gene expression by CrkII. Various transcriptional control elements were examined for their activation by CrkII-expression, and we found that CrkII selectively activates the serum response element (SRE), a transcriptional control element of immediate-early genes. This SRE activation induced by CrkII-overexpression was mediated by the serum response factor (SRF) via Rho. Indeed, we confirmed that the amount of activated Rho was increased in the CrkII-expressing cells. Moreover, we showed that when overexpressed, CrkII induces the cellular transformation of NIH 3T3 cells and that a dominant negative mutant of Rho suppresses this transformation, strongly suggesting that activation of Rho is essential for the transforming activity by CrkII. Furthermore, we also found that CrkII and Galpha12, a member of the heterotrimeric G proteins, synergistically activates Rho as well as the SRF, and that an SH3 mutant of CrkII can inhibit the Galpha12-induced activation of SRF. These results strongly suggest that CrkII is involved in the activation of Rho and SRF by Galpha12. Our study provides strong evidence that Rho activation plays a crucial role in CrkII-mediated signals to induce gene expression and cellular transformation.

The HMG-CoA reductase inhibitor cerivastatin enhances the nitric oxide bioavailability of the endothelium

The aim of this study was to investigate whether the HMG-CoA reductase inhibitor cerivastatin alters the nitric oxide (NO) bioavailability of porcine aortic endothelial cell cultures and of native porcine coronary endothelium, after short-term (minutes) and long-term (24-hour) treatment with cerivastatin (electrochemical NO sensor). NO-synthase expression (Western blot, ELISA) and activity (3H-arginine assay) after cerivastatin treatment were determined. Furthermore, the authors investigated whether cerivastatin modulates an angiotensin II (10 micromol/L; 4 hours) induced reactive oxygen species (ROS) release from intact vessels (lucigenin-enhanced chemiluminescence-assay). Acute addition of cerivastatin induced a concentration-dependent NO release from endothelial cell cultures that could be blocked by the NO-synthase inhibitor N-monomethyl-arginine. A long-term incubation with cerivastatin also resulted in a concentration-dependent, significantly enhanced basal NO bioavailability (approximately 3-fold increase at 10 nmol/L cerivastatin) that could be partly reversed by a coincubation with mevalonate. No enhanced endothelial NO-synthase expression or increased NO-synthase activity was detected after long-term treatment with cerivastatin (24 hours). However, cerivastatin induced a significant concentration-dependent inhibition of the angiotensin II-induced ROS release from native endothelial cells of porcine coronary arteries. Therefore, there seems to be an acute and a long-term effect of cerivastatin that results in enhanced endothelial NO bioavailability.