Confluence of vascular endothelial cells induces cell cycle exit by inhibiting p42/p44 mitogen-activated protein kinase activity

Differential expression of EDF-1 and endothelial nitric oxide synthase by proliferating, quiescent and senescent microvascular endothelial cells

Endothelial Differentiation-related Factor (EDF)-1 is a low molecular weight polypeptide downregulated in endothelial cells exposed to HIV-1-Tat or the phorbol ester TPA. EDF-1 acts in the cytosol as a calmodulin binding protein, and in the nucleus as a transcriptional coactivator. Here, we show that EDF-1 is downregulated in non-proliferating microvascular endothelial cells. Indeed, both quiescence and senescence reduce the levels of EDF-1 and this is due to protein degradation through the proteasome. We also describe a different subcellular localization of EDF-1 which is mainly nuclear in senescent 1G11 cells. Since (i) endothelial nitric oxide (NO) seems to play a role in endothelial proliferation and (ii) NO is an important mediator involved in the control of vascular tone, inflammatory responses and angiogenesis, it is noteworthy that senescence downregulates the expression and the activity of endothelial nitric oxide synthase (eNOS) in microvascular endothelial cells. On the contrary, quiescence does not affect NOS expression and activity. The modulation of EDF-1 in microvascular endothelial cells might offer new insights into the molecular events involved in angiogenesis and in microvascular dysfunctions in the elderly.

Cellular quiescence induced by contact inhibition or serum withdrawal in C3H10T1/2 cells

Either confluence or serum withdrawal may cause growth arrest of cultured non-transformed cells. Here, we compared sparsely populated and confluent C3H10T1/2 cells with and without serum-containing medium. The following proliferation-relevant end points were examined: cell-cycle distribution, Ki-67 antigen presence, the level of the von Hippel-Lindau (VHL) protein, and gene expression, determined using a microarray approach. In sparse/logarithmic cultures, the fraction of cells in G(0)/G(1) phase increased from 55 to 85% following serum withdrawal. Moreover, the fraction of Ki-67 positive cells dropped from 89 to 47%. In confluent cultures, the majority of cells (80%) were in G(0)/G(1) phase and only 25-30% were Ki-67 positive, regardless of serum presence. In both serum-deprived and contact-inhibited cultures, significant and distinct changes in gene expression were observed. Serum deprivation of sparsely cultured cells resulted in significant over-expression of several transcription factors, while confluent cells showed elevated expression of genes coding for Wnt6, uPar, Tdag51, Egr1, Ini1a and Mor1. These results indicate that contact inhibition and serum withdrawal lead to cellular quiescence through distinct genetic and molecular mechanisms.

17 beta-estradiol transiently disrupts adherens junctions in endothelial cells

Interendothelial junctions are important regulators of endothelial cell functions such as migration and proliferation, major features in angiogenesis, and endothelial cell monolayer wound healing. 17beta-estradiol regulates these functions in vivo and in vitro and also increases endothelial monolayer permeability as it results from impaired monolayer integrity and intercellular adhesion. We hypothesized that 17beta-estradiol affects these cell adhesion-dependent functions in endothelial cells by targeting the adherens junction complex. Here, we show that 17beta-estradiol increases uterine microvascular endothelial cell monolayer permeability and transiently redistributes interendothelial junction-forming proteins in endothelial cells. Concomitantly, adherens junction proteins are disconnected from the cytoskeleton and alpha-catenin, which links VE-cadherin to the cytoskeleton, is redistributed from the membrane and the adherens junction complex. Furthermore, 17beta-estradiol increased tyrosine phosphorylation of the adherens junction complex. These effects were inhibited by the estrogen receptor antagonist ICI 182,780 but could be provoked using non-cell membrane-permeable 17beta-estradiol-BSA in all cells tested, including EA.hy 926 cells, which have been shown unable to stimulate 17beta-estradiol-dependent gene transcription. Additionally, 17beta-estradiol treatment enhanced the angiogenic effect of vascular endothelial growth factor in an in vitro angiogenesis model, as a potential implication of the adherens junction disruption. Cotreatment with the Src-family kinase inhibitor PP2 prevented the redistribution and phosphorylation of the adherens junction proteins. Taken together, our data show that adherens junctions in endothelial cells are a downstream target of membrane-associated 17beta-estradiol signaling, possibly through Src-family kinases.

Compatibility of mammalian cells on surfaces of poly(dimethylsiloxane)

This paper describes the influence of the composition of poly(dimethylsiloxane) (PDMS) on the attachment and growth of several different types of mammalian cells: primary human umbilical artery endothelial cells (HUAECs), transformed 3T3 fibroblasts (3T3s), transformed osteoblast-like MC3T3-E1 cells, and HeLa (transformed epithelial) cells. Cells grew on PDMS having different ratios of base to curing agent: 10:1 (normal PDMS, PDMSN), 10:3 (PDMSCA), and 10:0.5 (PDMSB). They were also grown on "extracted PDMS" (normal PDMS that has reduced quantities of low molecular-weight oligomers, PDMSN,EX) and normal PDMS that had been extracted and then oxidized (PDMSN,EX,OX); all surfaces were exposed to a solution of fibronectin prior to cell attachment. Generally, fibronectin-coated PDMS is a suitable substrate for culturing mammalian cells. Compatibility of cells on some surfaces, however, was dependent on the cell type: PDMSN,EX,OX caused cell detachment of 3T3 fibroblasts and MC3T3-E1 cells, and PDMSCA caused detachment of HUAECs and HeLa cells. Growth of cells on PDMSN, PDMSN,EX, and PDMSB was comparable to growth on tissue culture-treated polystyrene for most of the cell types. All cells grew at similar rates on PDMS substrates regardless of the stiffness of the substrate, for substrates having Young's moduli ranging from E=0.60 +/- 0.04 to 2.6 +/- 0.2 MPa (for PDMSB and PDMSN,EX, respectively).

The tubulin-binding agent combretastatin A-4-phosphate arrests endothelial cells in mitosis and induces mitotic cell death

The tubulin-binding agent combretastatin A-4-phosphate (CA-4-P), rapidly disrupts the vascular network of tumors leading to secondary tumor cell death. In vitro, CA-4-P destabilizes microtubules and causes endothelial cell death. In this study we analyze the mechanisms by which CA-4-P induces the death of proliferating endothelial cells. We demonstrate that at >/=7.5 nmol/L, CA-4-P damages mitotic spindles, arrests cells at metaphase, and leads to the death of mitotic cells with characteristic G(2)/M DNA content. Mitotic arrest was associated with elevated levels of cyclin B1 protein and p34(cdc2) activity. Inhibition of p34(cdc2) activity by purvalanol A caused mitotic-arrested cells to rapidly exit mitosis, suggesting that sustained p34(cdc2) activity was responsible for metaphase arrest. Pharmacological prevention of entry into mitosis protected cells from undergoing cell death, further establishing the link between mitosis and cell death induction by CA-4-P. CA-4-P-mediated cell death shared characteristics of apoptosis but was independent of caspase activation suggesting the involvement of a non-caspase pathway(s). These data suggest that induction of apoptosis in endothelial cells by CA-4-P is associated with prolonged mitotic arrest. Therefore, by activating cell death pathways, CA-4-P, in addition to being an effective anti-vascular agent, may also interfere with regrowth of blood vessels in the tumor.

Akt phosphorylation is not sufficient for insulin-like growth factor-stimulated myogenin expression but must be accompanied by down-regulation of mitogen-activated protein kinase/extracellular signal-regulated kinase phosphorylation

IGF-I has a unique biphasic effect on skeletal muscle differentiation. Initially, IGF-I inhibits expression of myogenin, a skeletal muscle-specific regulatory factor essential for myogenesis. Subsequently, IGF-I switches to stimulating expression of myogenin. The mechanisms that mediate this switch in IGF action are incompletely understood. Several laboratories have demonstrated that the phosphatidylinositol-3-kinase/Akt signaling pathway is essential for myogenic differentiation and have suggested that this pathway mediates IGF-I stimulation of myogenin mRNA expression, an early critical step in the differentiation process. These studies, however, did not address concurrent Akt and MAPK/ERK1/2 phosphorylation, the latter of which is also known to regulate myogenic differentiation. In the present study in rat L6E9 muscle cells, we have manipulated ERK1/2 phosphorylation with either an upstream inhibitor or activator and examined concurrent levels of Akt and ERK1/2 phosphorylation and of myogenin mRNA expression in response to treatment with IGF-I. We find that even in the presence of phosphorylated Akt, it is only when ERK1/2 phosphorylation is inhibited that IGF-I can stimulate myogenin mRNA expression. Thus, although Akt phosphorylation may be necessary, it is not sufficient for induction of myogenic differentiation by IGF-I and must be accompanied by a decrease in ERK1/2 phosphorylation.

Regulation of c-Jun N-terminal Kinase and p38 Kinase Pathways in Endothelial Cells

The rapid and transient induction of E-selectin gene expression by inflammatory tumor necrosis factor (TNF)-alpha in endothelial cells is mediated by signaling pathways which involve c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) kinase pathways. To explore this regulation, we first observed that in the continuous presence of cytokine TNF, activation of JNK-1 in both nuclear and cytoplasmic compartments peaked at 15-30 min, with activity returning to uninduced levels by 60 min. Phosphorylation of both the p38 kinase and its molecular target, the nuclear transcription factor, activating transcription factor-2, were transient after TNF-alpha or interleukin (IL)-1beta induction. However, cycloheximide treatment prolonged the TNF-alpha-induced JNK-1 kinase activity beyond 60 min, suggesting that protein synthesis is required to limit this signaling cascade. We investigated the possible role of the dual-specificity phosphatases MAPK phosphatase (MKP)-1 and MKP-2 in limiting cytokine-induced MAPK signaling. Maximum induction of MKP-1 mRNA and nuclear protein levels by TNF-alpha or IL-1beta were noted at 60 min and their expression correlated with the termination of JNK kinase activity, whereas nuclear levels of MKP-2 were not significantly affected by treatment with TNF-alpha or IL-1beta. Transient overexpression of MKP-1 demonstrated significant specific inhibition of E-selectin promoter activity consistent with a regulatory role for dual-specificity phosphatases. Inhibition of MKP-1 expression through the use of small interfering RNAs prolonged the cytokine-induced p38 and JNK kinase phosphorylation. Our results suggest that endogenous inhibitors of the MAPK cascade, such as the dual-specificity phosphatases like MKP-1 may be important for the postinduction repression of MAPK activity and E-selectin transcription in endothelial cells. Thus, these inhibitors may play an important role in limiting the inflammatory effects of TNF-alpha and IL-1beta.

Genome-wide analysis of the endothelial transcriptome under short-term chronic hypoxia

We have utilized serial analysis of gene expression (SAGE) to analyze the temporal response of human aortic endothelial cells (HAECs) to short-term chronic hypoxia at the level of transcription. Primary cultures of HAECs were exposed to 1% O2hypoxia for 8 and 24 h and compared with identical same passage cells cultured under standard (5% CO2-95% air) conditions. A total of 121,446 tags representing 37,096 unique tags were sequenced and genes whose expression levels were modulated by hypoxia identified by novel statistical analyses. Hierarchical clustering of genes displaying statistically significant hypoxia-responsive alterations in expression revealed temporal modulation of a number of major functional gene families including those encoding heat shock factors, glycolytic enzymes, extracellular matrix factors, cytoskeletal factors, apoptotic factors, cell cycle regulators and angiogenic factors. Within these families we documented the coordinated modulation of both previously known hypoxia-responsive genes, numerous genes whose expressions have not been previously shown to be altered by hypoxia, tags matching uncharacterized UniGene entries and entirely novel tags with no UniGene match. These preliminary data, which indicate a reduction in cell cycle progression, elevated metabolic stress and increased cytoskeletal remodeling under acute hypoxic stress, provide a foundation for further analyses of the molecular mechanisms underlying the endothelial response to short-term chronic hypoxia.

Evidence for ERK1/2 phosphorylation controlling contact inhibition of proliferation in Madin-Darby canine kidney epithelial cells

Increasing cell density arrests epithelial cell proliferation by a process termed contact inhibition. We investigated mechanisms of contact inhibition using a model of contact-inhibited epithelial cells. Hepatocyte growth factor (HGF) treatment of contact-inhibited Madin-Darby canine kidney (MDCK) cells stimulated cell proliferation and increased levels of phosphorylated ERK1/2 (phospho-ERK1/2) and cyclin D1. MEK inhibitors PD-98059 and U0126 inhibited these HGF-dependent changes, indicating the dependence on phosphorylation of ERK1/2 during HGF-induced loss of contact inhibition. In relation to contact-inhibited high-density cells, low-density MDCK cells proliferated and had higher levels of phospho-ERK1/2 and cyclin D1. PD-98059 and U0126 inhibited low-density MDCK cell proliferation. Trypsinization of high-density MDCK cells immediately increased phospho-ERK1/2 and was followed by a transient increase in cyclin D1 levels. Reformation of cell junctions after trypsinization led to decreases in phospho-ERK1/2 and cyclin D1 levels. High-density MDCK cells express low levels of both cyclin D1 and phospho-ERK1/2, and treatment of these cells with fresh medium containing HGF but not fresh medium alone for 6 h increased phospho-ERK1/2 and cyclin D1 levels compared with cells without medium change. These data provide evidence that HGF abrogates MDCK cell contact inhibition by increasing ERK1/2 phosphorylation and levels of cyclin D1. These results suggest that in MDCK cells, contact inhibition of cell proliferation in the presence of serum occurs by cell density-dependent regulation of ERK1/2 phosphorylation.

Common alterations in gene expression and increased proliferation in recurrent acute myeloid leukemia

Recurrent disease following high-dose chemotherapy is a major problem in patients with acute myeloid leukemia (AML). To identify its characteristics, we performed expression profiling in blasts from untreated AML and relapse, using a specific cDNA microarray comprising 4128 genes generated by cDNA subtraction supplemented with cancer-associated genes. Expression analysis of 18 AML bone marrow specimens showed that recurrent AML is commonly associated with the mRNA expression changes in a set of 58 genes. Increased cellular proliferation was indicated by the overexpression of the transferrin receptor, proliferating cell nuclear antigen, and G1 cyclins. An immunohistochemical study for Ki-67-positive blasts in 18 paired bone marrow biopsy samples confirmed a highly significant (P<0.0001) increase in the proliferation fraction at relapse. In addition, we found enhanced activation of the RAF/MEK/ERK cascade as mRNAs of MKP-1, c-jun, c-fos, and egr-1 were significantly increased at relapse. Immunohistochemistry and immunoblotting analyses for biphosphorylated ERK1/2 protein provide additional evidence for enhanced activation of the RAF/MEK/ERK pathway. The degree of increase is significantly correlated with the increased proliferation. Furthermore, the genes identified provide a rationale for further studies on predictive diagnosis and therapeutic intervention.

Effector ExoU from the type III secretion system is an important modulator of gene expression in lung epithelial cells in response to Pseudomonas aeruginosa infection

Pseudomonas aeruginosa is an important pathogen in immunocompromised patients and secretes a diverse set of virulence factors that aid colonization and influence host cell defenses. An important early step in the establishment of infection is the production of type III-secreted effectors translocated into host cells by the bacteria. We used cDNA microarrays to compare the transcriptomic response of lung epithelial cells to P. aeruginosa mutants defective in type IV pili, the type III secretion apparatus, or in the production of specific type III-secreted effectors. Of the 18,000 cDNA clones analyzed, 55 were induced or repressed after 4 h of infection and could be classified into four different expression patterns. These include (i) host genes that are induced or repressed in a type III secretion-independent manner (32 clones), (ii) host genes induced specifically by ExoU (20 clones), and (iii) host genes induced in an ExoU-independent but type III secretion dependent manner (3 clones). In particular, ExoU was essential for the expression of immediate-early response genes, including the transcription factor c-Fos. ExoU-dependent gene expression was mediated in part by early and transient activation of the AP1 transcription factor complex. In conclusion, the present study provides a detailed insight into the response of epithelial cells to infection and indicates the significant role played by the type III virulence mechanism in the initial host response.

Modulation of VE-cadherin and PECAM-1 mediated cell-cell adhesions by mitogen-activated protein kinases

Endothelial cell transition from a differentiated, quiescent phenotype to a migratory, proliferative phenotype is essential during angiogenesis. This transition is dependent on alterations in the balanced production of stimulatory and inhibitory factors, which normally keep angiogenesis in check. Activation of MAPK/ERKs is essential for endothelial cell migration and proliferation. However, its role in regulation of endothelial cell adhesive mechanisms requires further delineation. Here, we show that sustained activation of MAPK/ERKs results in disruption of cadherin-mediated cell-cell adhesion, down-regulation of PECAM-1 expression, and enhanced cell migration in microvascular endothelial cells. Expression of a constitutively active MEK-1 in mouse brain endothelial (bEND) cells resulted in down-regulation of VE-cadherin and catenins expression concomitant with down-regulation of PECAM-1 expression. In contrast, inhibition of MEK-1 restored parental morphology, cadherin/catenins expression and localization. These data are further supported by our observation that sustained activation of MAPK/ERKs in phorbol myristate acetate incubated HUVEC lead to disruption of cadherin-mediate cell-cell interactions and enhanced capillary formation on Matrigel. Thus, sustained activation of MAPK/ERKs plays an important role in disruption of cell-cell adhesion and migration of endothelial cells.

Functional overlap and cooperativity among alphav and beta1 integrin subfamilies during skin angiogenesis

Angiogenesis requires endothelial cell survival and proliferation, which depend upon cytokine stimulation together with integrin-mediated cell adhesion to extracellular matrix; however, the question of which specific integrins are the best targets for suppressing neovascularization is controversial and unresolved. Therefore, we designed experiments to compare contributions of individual integrins from both the alphav and beta1 integrin subfamilies. With immobilized antibodies, we determined that adhesion through integrins alpha1beta1, alpha2beta1, alphavbeta3, and alphavbeta5 each individually supported dermal microvascular endothelial cell survival. Also, substratum coated with collagen I (which binds alpha1beta1 and alpha2beta1) and vitronectin (which binds alphavbeta3 and alphavbeta5) each supported survival. Importantly, substratum coated with combinations of collagen I and vitronectin were most effective at promoting survival, and survival on three-dimensional collagen I gels was strongly enhanced by vitronectin. Vascular endothelial growth factor activation of the p44/p42 mitogen-activated protein kinase pathway, which is required for angiogenesis, was supported by adhesion through either alpha1beta1, alpha2beta1, alphavbeta3, or alphavbeta5, and pharmacologic inhibition of this pathway blocked proliferation and suppressed survival. Therefore, these studies establish that the alpha1beta1, alpha2beta1, alphavbeta3, and alphavbeta5 integrins each support dermal microvascular endothelial cell viability, and that each collaborate with vascular endothelial growth factor to support robust activation of the mitogen-activated protein kinase pathway which mediates both proliferation and survival. Moreover, survival is supported most significantly by extracellular matrices, which engage all of these integrins in combination. Consistent with important complementary and overlapping functions, combined antagonism of these integrins provided superior inhibition of angiogenesis in skin, indicating that multiplicity of integrin involvement should be considered in designing strategies for controlling neovascularization.

Contact inhibition of VEGF-induced proliferation requires vascular endothelial cadherin, beta-catenin, and the phosphatase DEP-1/CD148

Confluent endothelial cells respond poorly to the proliferative signals of VEGF. Comparing isogenic endothelial cells differing for vascular endothelial cadherin (VE-cadherin) expression only, we found that the presence of this protein attenuates VEGF-induced VEGF receptor (VEGFR) 2 phosphorylation in tyrosine, p44/p42 MAP kinase phosphorylation, and cell proliferation. VE-cadherin truncated in beta-catenin but not p120 binding domain is unable to associate with VEGFR-2 and to induce its inactivation. beta-Catenin-null endothelial cells are not contact inhibited by VE-cadherin and are still responsive to VEGF, indicating that this protein is required to restrain growth factor signaling. A dominant-negative mutant of high cell density-enhanced PTP 1 (DEP-1)//CD148 as well as reduction of its expression by RNA interference partially restore VEGFR-2 phosphorylation and MAP kinase activation. Overall the data indicate that VE-cadherin-beta-catenin complex participates in contact inhibition of VEGF signaling. Upon stimulation with VEGF, VEGFR-2 associates with the complex and concentrates at cell-cell contacts, where it may be inactivated by junctional phosphatases such as DEP-1. In sparse cells or in VE-cadherin-null cells, this phenomenon cannot occur and the receptor is fully activated by the growth factor.

Loss of von Hippel-Lindau protein causes cell density dependent deregulation of CyclinD1 expression through hypoxia-inducible factor

Loss of the von Hippel-Lindau gene (VHL) expression ca-uses deregulation of contact inhibition of cell growth, which might be one of the bases of the tumor suppressor function of VHL. Here we show that this function of the VHL gene product (pVHL) depends on cell autonomous events. To identify the target gene of pVHL, which is directly involved in the contact inhibition, we compared the gene expression profile between VHL-deficient renal carcinoma 786-O cells and those infected with an adenovirus vector encoding VHL. In addition to known pVHL-regulated genes, such as vascular endothelial growth factor and carbonic anhydrase, we found cyclinD1 as a new target of pVHL at a high cell density. In VHL-expressing cells (VHL (+) cells), the cyclinD1 mRNA expression level diminishes at a high cell density, while it remains at a relatively high level in VHL-deficient cells (VHL (-) cells). The cyclinD1 expression level was also abnormally high in VHL (-) cells at a high cell density. Consequently, the phosporylation level of the retinoblastoma (Rb) protein remained high in these cells, whereas there was no phosporylated Rb in VHL (+) cells under the contact inhibition. The abnormal expression of cyclinD1 at a high cell density was observed even in VHL (+) cells under the hypoxic state. Moreover, ectopic expression of a HIF mutant resistant to pVHL-mediated proteolysis causes the abnormal cyclinD1 expression in VHL (+) cells. Taken together, these observations indicate that VHL is required for the downregulation of cyclinD1 at a high cell density through HIF.

Survival pathways regulating the apoptosis induced by tumour necrosis factor-alpha in primary cultured bovine endothelial cells

The aim of the present study was to identify biochemical pathways driving the resistance of endothelial cells to apoptosis induced by tumour necrosis factor-alpha (TNF). (1) Although nuclear factor-kappa B (NF-kappaB) was activated by TNF, its inhibition by MG-132 failed to sensitize these cells. (2) The activation of protein kinase C (PKC) by phorbol ester completely abolished the TNF-induced cell death. (3) The phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin (Wo) triggered apoptosis and enhanced the TNF-induced cell death. (4) The MEK inhibitor PD98059 did not affect the TNF-induced apoptotic process. (5) The p38 is activated by TNF and its inhibition by SB203580 sensitized the cells to TNF. This is correlated with the inhibition of phosphorylation of heat-shock protein of 27 kDa (HSP27). These results indicate that TNF activates NF-kappaB, which does not drive any anti-apoptotic response, and p38, which plays an anti-apoptotic function probably through HSP27 phosphorylation. Moreover, PKC and PI3K are involved in the control of survival pathways.

EphrinA1 inactivates integrin-mediated vascular smooth muscle cell spreading via the Rac/PAK pathway

Interactions between the Eph receptor tyrosine kinase and ephrin ligands transduce short-range signals regulating axon pathfinding, development of the cardiovascular system, as well as migration and spreading of neuronal and non-neuronal cells. Some of these effects are believed to be mediated by alterations in actin dynamics. The members of the small Rho GTPase family elicit various effects on actin structures and are probably involved in Eph receptor-induced actin modulation. EphrinA1 is proposed to contribute to angiogenesis as it is strongly expressed at sites of neovascularization. Moreover, angiogenic factors induce the expression of ephrinA1 in endothelial cells. In this study, using rat vascular smooth muscle cells (VSMCs), we investigated the contribution of the small Rho GTPases in ephrinA1-induced integrin inactivation. EphrinA1 did not significantly affect early adhesion of VSMCs on purified laminin or fibronectin, but strongly impaired cell spreading. The Rho kinase inhibitor Y-27632 partly reversed the ephrinA1 effect, suggesting involvement of Rho in this model. However, inhibition of RhoA synthesis with short interfering (si)RNA had a modest effect, suggesting that RhoA plays a limited role in ephrinA1-mediated inhibition of spreading in VSMCs. The ephrinA1-mediated morphological alterations correlated with inhibition of Rac1 and p21-activated kinase 1 (PAK1) activity, and were antagonized by the expression of a constitutively active Rac mutant. Moreover, repression of Rac1 synthesis with siRNA amplifies the ephrinA1-induced inhibition of spreading. Finally, sphingosine-1-phosphate (S1P), a lipid mediator known to inhibit Rac activation in VSMCs amplifies the ephrinA1 effect. In conclusion, our results emphasize the role of the Rac/PAK pathway in ephrinA1-mediated inhibition of spreading. In this way, ephrinA1, alone or in synergy with S1P, can participate in blood vessel destabilization, a prerequisite for angiogenesis.

MAPKs mediate S phase arrest induced by vanadate through a p53-dependent pathway in mouse epidermal C141 cells

Mitogen-activated protein (MAP) kinases play an important role in mediation of the signal transduction pathway in cellular response to genotoxic stress. Cell growth arrest is considered as an early stage in response to the genotoxic stress. p53 is well-known as a tumor suppression gene involved in both cell growth arrest and apoptosis. The present study investigated the involvement of MAP kinases in vanadate-induced cell growth arrest and the relationship of p53. DNA content analysis showed that vanadate-induced S phase arrest is time- and dose-dependent in p53 wild-type C141 cells but not in p53-deficient C141 cells. Western blotting results indicated that vanadate caused an inactivation of p-cdk2 at Thr160, which is an important kinase for the progression of S phase, and an increase in expression of p21, which is a key for S phase arrest. In p53-deficient cells, vanadate did not induce any observable change in p21 or p-cdk2 level. In addition, vanadate up-regulated phospho-p38 and ERK, two members of MAP kinases. At the same time, vanadate increased the p53 activity as measured by luciferase assay. Addition of PD98059 and SB202190, inhibitors of ERK and p38, respectively, decreased vanadate-induced S phase arrest, reduced p21 levels, restored activation of p-cdk2, and decreased p53 activity. The study demonstrated that vanadate-induced S phase arrest is mediated by both ERK and p38 in a p53-dependent pathway.

A highly specific isolation of rat sinusoidal endothelial cells by the immunomagnetic bead method using SE-1 monoclonal antibody

BACKGROUND/AIMS

To develop a specific isolation method of hepatic sinusoidal endothelial cells (SEC), we applied the immunomagnetic method using a monoclonal antibody (SE-1) that recognizes a membranous antigen expressed only in rat SEC.

METHODS

Cells were isolated by incubating mixed non-parenchymal cells, which were obtained by collagenase digestion of the liver, with SE-1-conjugated superparamagnetic polystyrene beads. The conventional Percoll method was also performed in parallel to compare with the immunomagnetic method. The isolated cells were cultured on glass coverslips coated with type I collagen in the presence of various growth factors for 6 days.

RESULTS

Approximately 98% of the isolated cells were positive for SE-1 and the contamination of Kupffer cells or stellate cells was less than 1%. The purity was significantly better than that obtained by the Percoll method. The cultured cells showed typical SEC features, such as sieve plates and uptake of acetylated low-density lipoprotein. Although the cells continuously underwent apoptotic cell death after 2 days, they started robust cell growth after 3 days and were well maintained during the culture period.

CONCLUSIONS

Our simple and specific isolation method enables us to culture SEC with high purity and should be useful for the biological analysis of SEC.

MPP+ increases alpha-synuclein expression and ERK/MAP-kinase phosphorylation in human neuroblastoma SH-SY5Y cells

Alpha-synuclein is a brain presynaptic protein that is linked to familiar early onset Parkinson's disease and it is also a major component of Lewy bodies in sporadic Parkinson's disease and other neurodegenerative disorders. Alpha-synuclein expression increases in substantia nigra of both MPTP-treated rodents and non-human primates, used as animal models of parkinsonism. Here we describe an increase in alpha-synuclein expression in a human neuroblastoma cell line, SH-SY5Y, caused by 5-100 microM MPP+, the active metabolite of MPTP, which induces apoptosis in SH-SY5Y cells after a 4-day treatment. We also analysed the activation of the MAPK family, which is involved in several cellular responses to toxins and stressing conditions. Parallel to the increase in alpha-synuclein expression we observed activation of MEK1,2 and ERK/MAPK but not of SAPK/JNK or p38 kinase. The inhibition of the ERK/MAPK pathway with U0126, however, did not affect the increase in alpha-synuclein. The highest increase in alpha-synuclein (more than threefold) in 4-day cultures was found in adherent cells treated with low concentrations of MPP+ (5 microM). Inhibition of ERK/MAPK reduced the damage caused by MPP+. We suggest that alpha-synuclein increase and ERK/MAPK activation have a prominent role in the cell mechanisms of rescue and damage, respectively, after MPP+ -treatment.

Vascular endothelial growth factor induces SHC association with vascular endothelial cadherin: a potential feedback mechanism to control vascular endothelial growth factor receptor-2 signaling

Vascular endothelial (VE)-cadherin is endothelium specific, mediates homophilic adhesion, and is clustered at intercellular junctions. VE-cadherin is required for normal development of the vasculature in the embryo and for angiogenesis in the adult. Here, we report that VE-cadherin is associated with VE growth factor (VEGF) receptor-2 (VEGFR-2) on the exposure of endothelial cells to VEGF. The binding parallels receptor phosphorylation on tyrosine residues, which is maximal at 5 minutes and then declines within 30 minutes. Tyrosine phosphorylation of VE-cadherin was maximal at 30 minutes after the addition of the growth factor. At this time point, the protein could be coimmunoprecipitated with the adaptor protein Shc. Pull-down experiments with different Shc domains and mutants of the VE-cadherin cytoplasmic tail have shown that Shc binds to the carboxy-terminal domain of the VE-cadherin tail through its Src homology 2 domain (SH2). We found that Shc phosphorylation lasts longer in endothelial cells carrying a targeted null mutation in the VE-cadherin gene than in VE-cadherin-positive cells. These data suggest that VE-cadherin expression exerts a negative effect on Shc phosphorylation by VEGFR-2. We speculate that VE-cadherin binding to Shc promotes its dephosphorylation through associated phosphatases.

The tumor vascular targeting agent combretastatin A-4-phosphate induces reorganization of the actin cytoskeleton and early membrane blebbing in human endothelial cells

Combretastatin A-4-phosphate (CA-4-P) is a tubulin-binding compound currently in clinical trial as a tumor vascular-targeting agent. In endothelial cells, CA-4-P is known to cause microtubule depolymerization, but little is known about its subsequent effects on cell morphology and function. Here, we demonstrate that within minutes of endothelial cell exposure to CA-4-P, myosin light chain (MLC) was phosphorylated, leading to actinomyosin contractility, assembly of actin stress fibers, and formation of focal adhesions. These cytoskeletal alterations appeared to be a consequence of Rho activation, as they were abolished by either the Rho inhibitor C3 exoenzyme or Rho-kinase inhibitor Y-27632. In response to CA-4-P, some cells rapidly assumed a blebbing morphology in which F-actin accumulated around surface blebs, stress fibers misassembled into a spherical network surrounding the cytoplasm, and focal adhesions appeared malformed. Blebbing was associated with decreased cell viability and could be inhibited by Rho/Rho-kinase inhibitors or by blocking the CA-4-P-mediated activation of stress-activated protein kinase-2/p38. The extracellular-regulated kinases 1 and 2 (ERK-1/2) were shown to protect against blebbing since blebbing was attenuated on ERK-1/2 stimulation and was up-regulated by specific inhibition of ERK-1/2 activation. The use of MLC kinase (MLCK) and myosin adenosine triphosphatase inhibitors led us to propose a role for MLCK and myosin activity independent of MLC phosphorylation in regulating the blebbing process. CA-4-P-mediated contractility and blebbing were associated with a Rho-dependent increase in monolayer permeability to dextrans, suggesting that such functional changes may be important in the rapid response of the tumor endothelium to CA-4-P in vivo.

Early stimulation and late inhibition of extracellular signal-regulated kinase 1/2 phosphorylation by IGF-I: a potential mechanism mediating the switch in IGF-I action on skeletal muscle cell differentiation

IGF-I has a unique biphasic effect on skeletal muscle cell differentiation. Initially, IGF-I inhibits differentiation and promotes proliferation of skeletal myoblasts. Subsequently, IGF-I switches to stimulating differentiation of these cells. The mechanisms responsible for this switch in IGF action remain unknown. We have examined the role of extracellular signal-regulated kinase (Erk)1/2 signaling in mediating the early inhibitory and late stimulatory effects of IGF-I on the gene expression of myogenin, a skeletal muscle-specific transcription factor essential for myogenic differentiation. We find that, concurrent with its early inhibitory and late stimulatory effects on myogenin mRNA, IGF-I has a biphasic but opposite effect on phosphorylation of Erk1/2: initially, IGF-I increases and subsequently decreases the phosphorylation of Erk1/2 in comparison to untreated cells. Cotreatment with an inhibitor of Erk1/2 activation prevents the early IGF-I-stimulation of Erk1/2 phosphorylation and partially reverses IGF-I-inhibition of myogenin mRNA. Conversely, preventing the late IGF-I-induced decrease in Erk1/2 phosphorylation blocks IGF-I-stimulation of myogenin mRNA. Our data indicate that the time-dependent, opposing effects of IGF-I on skeletal muscle cell differentiation are mediated, at least in part, by biphasic but opposite effects on activation of the Erk1/2 MAPK signaling pathway.

The alpha(1)beta(1) and alpha(2)beta(1) integrins provide critical support for vascular endothelial growth factor signaling, endothelial cell migration, and tumor angiogenesis

Angiogenesis is a complex process, involving functional cooperativity between cytokines and endothelial cell (EC) surface integrins. In this study, we investigated the mechanisms through which the alpha(1)beta(1) and alpha(2)beta(1) integrins support angiogenesis driven by vascular endothelial growth factor (VEGF). Dermal microvascular EC attachment through either alpha(1)beta(1) or alpha(2)beta(1) supported robust VEGF activation of the Erk1/Erk2 (p44/42) mitogen-activated protein kinase signal transduction pathway that drives EC proliferation. Haptotactic EC migration toward collagen I was dependent on alpha(1)beta(1) and alpha(2)beta(1) as was VEGF-stimulated chemotaxis of ECs in a uniform collagen matrix. Consistent with the functions of alpha(1)beta(1) and alpha(2)beta(1) in supporting signal transduction and EC migration, antibody antagonism of either integrin resulted in potent inhibition of VEGF-driven angiogenesis in mouse skin. Moreover, combined antagonism of alpha(1)beta(1) and alpha(2)beta(1) substantially reduced tumor growth and angiogenesis of human squamous cell carcinoma xenografts. Collectively, these studies identify critical collaborative functions for the alpha(1)beta(1) and alpha(2)beta(1) integrins in supporting VEGF signal transduction, EC migration, and tumor angiogenesis.

Emerging targets: molecular mechanisms of cell contact-mediated growth control

Contact inhibition of cell proliferation evokes a unique cellular program of growth arrest compared with stress, age, or other physical constraints. The last decade of research on genes activated by cell-cell contact has uncovered features of transmembrane signaling, cytoskeletal reorganization, and transcriptional control that initiate and maintain a quiescent phenotype. This review will focus on mechanisms controlling contact inhibition of cell proliferation, highlighting specific gene expression responses that are activated by cell-cell contact. Although a temporal framework for imposition of these mechanisms has not yet been well described, contact inhibition of cell proliferation clearly requires their coordinated function. Novel targets for intervention in proliferative disorders are emerging from these studies.

Endothelial atheroprotective and anti-inflammatory mechanisms

Atherosclerosis preferentially occurs in areas of turbulent flow and low fluid shear stress, whereas laminar flow and high shear stress are atheroprotective. Inflammatory cytokines, such as tumor necrosis factor-alpha (TNF), have been shown to stimulate expression of endothelial cell (EC) genes that may promote atherosclerosis. Recent data suggest that steady laminar flow decreases EC apoptosis and blocks TNF-mediated EC activation. EC apoptosis is likely important in the process termed "plaque erosion" that leads to platelet aggregation. Steady laminar flow inhibits EC apoptosis by preventing cell cycle entry, by increasing antioxidant mechanisms (e.g., superoxide dismutase), and by stimulating nitric oxide-dependent protective pathways that involve enzymes PI3-kinase and Akt. Conversely, our laboratory has identified nitric oxide-independent mechanisms that limit TNF signal transduction. TNF regulates gene expression in EC, in part, by stimulating mitogen-activated protein kinases (MAPK) which phosphorylate transcription factors. We hypothesized that fluid shear stress modulates TNF effects on EC by inhibiting TNF-mediated activation of MAP kinases. To test this hypothesis, we determined the effects of steady laminar flow (shear stress = 12 dynes/cm2) on TNF-stimulated activity of two MAP kinases: extracellular signal regulated kinase (ERK1/2) and c-Jun N-terminal kinase (JNK). Flow alone stimulated ERK1/2 activity, but decreased JNK activity compared to static controls. TNF (10 ng/ml) alone activated both ERK1/2 and JNK maximally at 15 minutes in human umbilical vein EC (HUVEC). Pre-exposing HUVEC for 10 minutes to flow inhibited TNF activation of JNK by 46%, but it had no significant effect on ERK1/2 activation. Incubation of EC with PD98059, a specific mitogen-activated protein kinase kinase inhibitor, blocked the flow-mediated inhibition of TNF activation of JNK. Flow-mediated inhibition of JNK was unaffected by 0.1 mM L-nitroarginine, 100 pM 8-bromo-cyclic GMP, or 100 microM 8-bromo-cyclic AMP. Transfection studies with dominant negative constructs of the protein kinase MEK1 and MEK5 suggested an important role for BMK1 in flow-mediated regulation of TNF signals. In summary, the atheroprotective effects of steady laminar flow on the endothelium involve multiple synergistic mechanisms.

Glyoxal and methylglyoxal induce lyoxal and methyglyoxal induce aggregation and inactivation of ERK in human endothelial cells

Increased production of glyoxal (GO) and methylglyoxal (MGO) under oxidative stress is harmful to the cells. In this study, we examined the early signaling effect of GO/MGO on cultured human umbilical vein endothelial cells. Both GO and MGO induced tyrosine phosphorylation and aggregation of a number of cellular proteins. Aggregation occurred mainly for cell surface proteins such as Flk-1 and VE-cadherin, but barely for the majority of intracellular proteins. Interestingly, however, GO/MGO caused both aggregation and dephosphorylation of intracellular phospho-ERK for inactivation. This phospho-ERK dephosphorylation was mediated by orthovanadate-sensitive phosphatase activity accompanying chemical recruitment of MKP-1 to the aggregated phospho-ERK. Evidence was provided that GO/MGO upregulated MKP-1 activity that in turn dephosphorylated possibly co-aggregated phospho-ERK efficiently for inactivation. These results together suggest that GO and MGO trigger a novel pathway for chemical reaction-mediated downregulation of ERK.

Quiescence, cell viability, apoptosis and necrosis of smooth muscle cells using different growth inhibitors

Smooth muscle cells and endothelial cells play an important role in cardiovascular diseases and may therefore be a potential target for gene therapy. Most in vitro experiments are performed using proliferating cell cultures. Nevertheless, non-dividing cells would represent more realistic in vivo conditions for gene therapy. Therefore, a simple method to achieve physiologically quiescence in cell cultures is needed for experiments. Growth to confluence is sufficient for endothelial cells to reach quiescence, in contrast to smooth muscle cells. Alternative techniques were investigated to achieve quiescence for smooth muscle cells. N-acetyl-cysteine, heparin, aphidicolin and serum-free medium are known inhibitors of smooth muscle cell proliferation and were tested for cell viability, necrosis and apoptosis. The inhibition status was evaluated counting cells in a cell counter. Toxicity, necrosis and apoptosis were determined using FACS analysis. Then, smooth muscle cells and endothelial cells were transfected with plasmid containing the beta-galactosidase gene using liposomes. Analysis of gene expression in transfected cells included a quantitative beta-galactosidase assay and X-gal staining. Growth inhibition was achieved with all agents tested. Using N-acetyl-cysteine, only slightly reduced growth rates were observed. Aphidicolin stopped cell growth almost immediately, but demonstrated enhanced toxicity. The amount of apoptotic and necrotic cells was lowest using heparin in the presence of foetal calf serum. Transfection experiments using stationary cultures of smooth muscle cells using heparin or aphidicolin demonstrated 5-10-fold lower transfection rates compared to transfected proliferating cell cultures serving as controls. Transfection experiments using stationary cultures of endothelial cells using growth inhibition through confluence demonstrated 40-fold lower transfection rates than transfected proliferating cell cultures. Transfer efficiency was much lower in endothelial cells compared to smooth muscle cells. In conclusion, quiescent cells simulate more realistically the in vivo situation and may therefore represent a better model for future in vivo experiments based on in vitro findings.

Fluid shear stress inhibits TNF-alpha activation of JNK but not ERK1/2 or p38 in human umbilical vein endothelial cells: Inhibitory crosstalk among MAPK family members

Atherosclerosis preferentially occurs in areas of turbulent flow and low fluid shear stress, whereas laminar flow and high shear stress are atheroprotective. Inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha) and IL-1 stimulate expression of endothelial cell (EC) genes that may promote atherosclerosis. TNF-alpha and IL-1 regulate gene expression in ECs, in part, by stimulating mitogen-activated protein kinases (MAPK), which phosphorylate transcription factors. We hypothesized that steady laminar flow inhibits cytokine-mediated activation of MAPK in EC. To test this hypothesis, we determined the effects of flow (shear stress = 12 dynes/cm(2)) on TNF-alpha and IL-1-stimulated activity of three MAPK in human umbilical vein ECs (HUVEC): extracellular signal-regulated kinase (ERK1/2), p38, and c-Jun N-terminal kinase (JNK). Flow alone stimulated ERK1/2 and p38 activity but decreased JNK activity compared with static controls. TNF-alpha or IL-1 alone activated ERK1/2, p38, and JNK maximally at 15 min in HUVEC. Preexposing HUVEC for 10 min to flow inhibited TNF-alpha and IL-1 activation of JNK by 46% and 49%, respectively, but had no significant effect on ERK1/2 or p38 activation. Incubation of HUVEC with PD98059, which inhibits flow-mediated ERK1/2 activation, prevented flow from inhibiting cytokine activation of JNK. Phorbol 12-myristate 13-acetate, which strongly activates ERK1/2, also inhibited TNF-alpha activation of JNK. These findings indicate that fluid shear stress inhibits TNF-alpha-mediated signaling events in HUVEC via the activation of the ERK1/2 signaling pathway. Inhibition of TNF-alpha signal transduction represents a mechanism by which steady laminar flow may exert atheroprotective effects on the endothelium.

Modulation of cyclin D1 and its signaling components by the phorbol ester TPA and the tyrosine phosphatase inhibitor vanadate

The mechanism by which 12-O-tetradecanoylphorbol-13-acetate (TPA) triggers cell-cycle progression at G1 phase in mouse embryonic fibroblast C3H 10T1/2 cells was examined. TPA treatment resulted in a temporary induction of cyclin D1 peaking at 9 h post stimulation. PD98059 (10 microM), the specific inhibitor of MAPK kinase, completely blocked TPA-stimulated cyclin D1 induction and DNA synthesis, confirming that MAPK activation plays an essential role in TPA-stimulated cell-cycle progression. Although both PKCalpha and PKCepsilon are expressed in C3H 10T1/2 cells, inhibitor studies suggest that PKCepsilon activation is required for the activation of MEK/MAPK signal transduction cascade. p70s6K, an important kinase involved in the regulation of protein synthesis and cell-cycle progression, has been reported to be activated through a PKC-dependent pathway (TPA-activatable) in addition to a PI3K-dependent pathway. Here, we demonstrate for the first time that TPA-stimulated MAPK activation is essential for the phosphorylation of several key residues involved in the activation of p70s6K, namely, thr389, thr421, and ser424. Vanadate, the tyrosine phosphatase inhibitor, triggered a sustained elevation of the level of active MAPK. However, corresponding to a rapid loss of cyclin D1 protein, vanadate treatment resulted in a significant shut out of 3H-thymidine incorporation into DNA regardless of TPA cotreatment. Vanadate treatment also led to the increase of active MEK, increased phosphorylation of p70s6K at thr389, thr421, and ser424 yet without activation of PKB. These data suggest that vanadate can selectively perturb the activation of signaling components which raises the interesting issue as to how vanadate downregulates the cyclin D1 level.

Signaling angiogenesis via p42/p44 MAP kinase and hypoxia

Angiogenesis is associated with a number of pathological situations. In this study, we have focused our attention on the role of p42/p44 MAP (mitogen-activated protein) kinases and hypoxia in the control of angiogenesis. We demonstrate that p42/p44 MAP kinases play a pivotal role in angiogenesis by exerting a determinant action at three levels: i) persistent activation of p42/p44 MAP kinases abrogates apoptosis; ii) p42/p44 MAP kinase activity is critical for controlling proliferation and growth arrest of confluent endothelial cells; and iii) p42/p44 MAP kinases promote VEGF (vascular endothelial growth factor) expression by activating its transcription via recruitment of the AP-2/Sp1 (activator protein-2) complex on the proximal region (-88/-66) of the VEGF promoter and by direct phosphorylation of hypoxia-inducible factor 1 alpha (HIF-1 alpha). HIF-1 alpha plays a crucial role in the control of HIF-1 activity, which mediates hypoxia-induced VEGF expression. We show that oxygen-regulated HIF-1 alpha protein levels are not affected by intracellular localization (nucleus versus cytoplasm). Finally, we propose a model which suggests an autoregulatory feedback mechanism controlling HIF-1 alpha and therefore HIF-1-dependent gene expression.

Paxillin alpha and Crk-associated substrate exert opposing effects on cell migration and contact inhibition of growth through tyrosine phosphorylation

Protein tyrosine phosphorylation accompanies and is essential for integrin signaling. We have shown that tyrosine phosphorylation of paxillin alpha and Crk-associated substrate (p130(Cas)) is a prominent event on integrin activation in normal murine mammary gland epithelial cells. Tyrosine phosphorylation of p130(Cas) has been demonstrated to facilitate cell migration. We show here that tyrosine phosphorylation of paxillin alpha acts to reduce haptotactic cell migrations as well as transcellular invasive activities in several different experimental cell systems, whereas tyrosine phosphorylation of p130(Cas) exerts opposing effects to those of paxillin alpha. Each of the phosphorylation-null mutants acts as a dominant negative for each phenotype. Moreover, we found that overexpression of paxillin alpha reduced the cell saturation density of normal murine mammary gland cells, whereas overexpression of p130(Cas) increased it. These effects also seemed to depend on tyrosine phosphorylation events. Cell growth rates and morphologies at growing phases were not significantly altered, nor were cells transformed. Addition of epidermal growth factor increased saturation density of the paxillin alpha-overexpressing cells, whereas no further increment was observed in p130(Cas)-overexpressing cells. We propose that tyrosine phosphorylation of paxillin alpha and p130(Cas) exerts opposing effects on several integrin-mediated cellular events, possibly through different signaling pathways.

Signaling angiogenesis via p42/p44 MAP kinase cascade

Vascular endothelial growth factor (VEGF), a potent agonist secreted by virtually all cells, controls migration and division of vascular endothelial cells. Disruption of one VEGF allele in mice has revealed a dramatic lethal effect in early embryogenesis, suggesting a key role in vasculogenesis. We analyzed the regulation of VEGF mRNA in normal and transformed CCL39 fibroblasts and then dissected the VEGF promoter to identify the signaling pathway(s) controlling the activation of this promoter in response to growth factors, oncogenes, and hypoxic stress. We demonstrated that the p42/p44 MAP kinase signaling cascade controls VEGF expression at least at two levels. In normoxic conditions, MAPKs activate the VEGF promoter at the proximal (-88/-66) region where Sp-1/AP-2 factors bind. Activation of p42/p44 MAPKs is sufficient to turn on VEGF mRNA. At low O2 tension, hypoxia inducible factor-1 alpha (HIF-1 alpha), a limiting factor rapidly stabilized and phosphorylated, plays a key role in the expression of several genes including VEGF. We demonstrated that p42/p44MAPKs stoichiometrically phosphorylate HIF-1 alpha in vitro and that HIF-1-dependent VEGF gene expression is strongly enhanced by the exclusive activation of p42/p44MAPKs. Finally, we demonstrated that the regulation of p42/p44MAPK activity is critical for controlling proliferation and growth arrest of vascular endothelial cells at confluency. These results point to at least three major targets of angiogenesis where p42/p44 MAP kinases exert a determinant action.

Reentry into the cell cycle of contact-inhibited vascular endothelial cells by a phosphatase inhibitor. Possible involvement of extracellular signal-regulated kinase and phosphatidylinositol 3-kinase

Vascular endothelial cells are unique in that they exit from the cell cycle when they come into contact with each other. Although the phenomenon is called "contact inhibition," little is known about the cellular mechanisms involved. Here we show that the phosphatase inhibitor sodium orthovanadate (SOV) induced the reentry of contact-inhibited human umbilical vascular endothelial cells (HUVECs) into the cell cycle and that reentry was associated with activation of the extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI 3-K)/Akt pathways. SOV stimulated [(3)H]thymidine uptake of contact-inhibited HUVECs in a time- and dose-dependent manner. SOV-induced increase in [(3)H]thymidine uptake was significantly inhibited by the mitogen-activated protein kinase kinase inhibitor PD98059 and by the PI 3-K inhibitor LY294002. SOV also stimulated the expression of cyclin D1, cyclin E, and cyclin A, and the activity of CDK2 kinase, whereas it decreased the expression of p27(kip1). In marked contrast, growth media alone did not induce these changes. Furthermore, these SOV-induced changes were abolished by pretreatment with PD98059 and LY294002. SOV stimulated phosphorylation of ERK and Akt in contact-inhibited HUVECs, while growth media alone did not. This phosphorylation was associated with inhibition of phosphatase activity in the cells. Finally, overexpression of high cell density-enhanced protein tyrosine phosphatase 1 inhibited c-fos and cyclin A promoter activity. Taken together, our results suggest that in contact-inhibited HUVECs, increased phosphatase activity suppressed the ERK and PI 3-K/Akt pathways, resulting in exit from the cell cycle by down-regulation of cyclin D1, cyclin E, and cyclin A and by up-regulation of p27(kip1).

p70 S6 kinase-mediated protein synthesis is a critical step for vascular endothelial cell proliferation

In this work, we analyzed the role of the PI3K-p70 S6 kinase (S6K) signaling cascade in the stimulation of endothelial cell proliferation. We found that inhibitors of the p42/p44 MAPK pathway (PD98059) and the PI3K-p70 S6K pathway (wortmannin, Ly294002, and rapamycin) all block thymidine incorporation stimulated by fetal calf serum in the resting mouse endothelial cell line 1G11. The action of rapamycin can be generalized, since it completely inhibits the mitogenic effect of fetal calf serum in primary endothelial cell cultures (human umbilical vein endothelial cells) and another established capillary endothelial cell line (LIBE cells). The inhibitory effect of rapamycin is only observed when the inhibitor is added at the early stages of G(0)-G(1) progression, suggesting an inhibitory action early in G(1). Rapamycin completely inhibits growth factor stimulation of protein synthesis, which perfectly correlates with the inhibition of cell proliferation. In accordance with its inhibitory action on protein synthesis, activation of cyclin D1 and p21 proteins by growth factors is also blocked by preincubation with rapamycin. Expression of a p70 S6K mutant partially resistant to rapamycin reverses the inhibitory effect of the drug on DNA synthesis, indicating that rapamycin action is via p70 S6K. Thus, in vascular endothelial cells, activation of protein synthesis via p70 S6K is an essential step for cell cycle progression in response to growth factors.