Contribution of double-stranded RNA-activated protein kinase toward antiproliferative actions of heparin on vascular smooth muscle cells

Neutrophil TLR4 and PKR are targets of breast cancer cell glycosaminoglycans and effectors of glycosaminoglycan-induced APRIL secretion

A proliferation-inducing ligand (APRIL), which induces survival and migration signals and tumor growth, is commonly observed in breast cancer tissues but is not often expressed in breast cancer cells themselves. Here, we examined whether breast cancer cells induce APRIL secretion from neutrophils, which are frequently recruited into the breast tumor microenvironment. We found that breast cancer cells do stimulate neutrophils to secrete APRIL through their glycosaminoglycans. Breast cancer cells depleted of heparan sulfate or chondroitin sulfate glycosaminoglycans lose their ability to induce APRIL secretion from neutrophils, and heparan sulfate and chondroitin sulfate can induce secretion that is comparable to that of breast cancer cell-induced secretion. While stimulation of the RNA-activated protein kinase (PKR) is sufficient to induce neutrophil APRIL secretion, both PKR and the toll-like receptor 4 (TLR4) are required for breast cancer cell glycosaminoglycan-induced secretion as separate and specific inhibition of TLR4 or PKR completely prevents the process, suggesting that breast cancer cell glycosaminoglycans target neutrophil TLR4 and PKR to trigger APRIL secretion. Thus, apart from the putative role of cell surface heparan sulfate in binding APRIL that leads to cell growth, we demonstrate that heparan sulfate, as well as chondroitin sulfate plays a novel role in promoting neutrophil secretion of APRIL that could lead to further cell growth. We propose that breast cancer cells take advantage of the neutrophil recruitment to the tumor microenvironment through the dual role of heparan sulfate as cell surface receptor or docking molecule for APRIL and as a ligand that induces neutrophil APRIL secretion to promote their own growth.

Inhibition of PKR impairs angiogenesis through a VEGF pathway

Peripheral artery disease (PAD) is a common clinical problem, and its pathophysiological mechanisms are incompletely understood. Double-stranded RNA-activated protein kinase (PKR) is a ubiquitously expressed serine/threonine protein kinase. Although PKR has been reported in antivirus and the immune system, the role of PKR in vascular function, especially in angiogenesis, is still unclear. PKR(-/-) mice were used in our experiments. Blood flow recovery was significantly delayed in PKR(-/-) vs. WT mice (Laser Doppler detection, n = 9, P < 0.01), accompanied by 34% reduced CD31-positive stain in ischemic muscle 28 days after procedure (immunohistochemistry, n = 9, P < 0.05). PKR expression decreased in the first 12 h and increased to peak at 24 h in human umbilical vein endothelial cells (HUVECs) in response to hypoxia (Western blot analyses, n = 3, P < 0.05). Accordingly, phospho-PKR expression increased in HUVECs 24 h after treatment with hypoxia (Western blot analyses, n = 3, P < 0.05). Inhibition of PKR (siRNA transfection) reduced microtubule formation (Matrigel tube formation, n = 3, vs. control siRNA, P < 0.05) and migration (wound healing, n = 3, vs. control siRNA, P < 0.05) by 33 and 59%, respectively. Vascular endothelial growth factor (VEGF) expression in ischemic muscle from PKR(-/-) mice was significantly decreased by 54% 1 day after procedure (n = 3, P < 0.05, vs. WT) and by 63% 7 days after procedure (n = 3, P < 0.01, vs. WT), respectively. At the same time, VEGF expression in HUVECs decreased by 21% (n = 3, P < 0.05, PKR siRNA vs. control siRNA). These findings demonstrate that PKR mediates angiogenesis through a VEGF pathway, which may form the basis for future intervention of PAD.

Identification of pro- and anti-proliferative oligosaccharides of heparins

Heparins, unfractionated heparin (UFH) and low molecular weight heparins (LMWHs), are heterogeneous mixtures of anticoagulant and non-anticoagulant oligosaccharides. In addition to their well-known anticoagulant effect, heparins have shown to mediate a wide range of non-anticoagulant effects, including the modulation of cellular growth. However, contradictory results have been reported with regard to their effects on cellular proliferation, with some studies suggesting anti-proliferative while others indicating pro-proliferative effects. This study investigated the proliferation of human colonic epithelial cancer cells in the presence of UFH and LMWHs (enoxaparin and dalteparin). In our experimental setting, all heparins caused a dose-dependent reduction in cellular growth, which correlated well with the induction of cell cycle arrest in the G₁ phase and which was not associated with significant changes in cell viability. The effects on cellular proliferation of 14 different oligosaccharides of enoxaparin obtained through ion-exchange chromatography were also assessed. Surprisingly, only two oligosaccharides showed distinctive anti-proliferative effects while the majority of oligosaccharides actually stimulated proliferation. Interestingly, the smallest oligosaccharide devoid of any anticoagulant activity showed the strongest anti-proliferative effect. Notably, heparins are currently standardised only according to their anticoagulant activity but not based on other non-anticoagulant properties. Our results indicate that slight differences in the composition of heparins' non-anticoagulant oligosaccharides, due to different origins of material and preparation methods, have the potential to cause diverse effects and highlight the need for additional characterisation of non-anticoagulant activities.

STAT1 requirement for PKR-induced cell cycle arrest in vascular smooth muscle cells in response to heparin

Interferons (IFNs) are a family of cytokines that exhibit antiviral, antiproliferative, and immunomodulatory properties. PKR (protein kinase, RNA activated) is of central importance in mediating the antiproliferative actions of IFNs. Our research has established that PKR inhibits vascular smooth muscle cell (VSMC) proliferation by regulating G1 to S transition. Many cardiovascular diseases result from complications of atherosclerosis, a chronic and progressive inflammatory condition often characterized by excessive proliferation of VSMC. Thus, an effective method for inhibiting VSMC proliferation is likely to arrest atherosclerosis and restenosis at early stages. Our research establishes that PKR activation in VSMC leads to a G1 arrest brought about by an inhibition of cyclin-dependent kinase 2 (Cdk2) activity by p27(kip1). In quiescent VSMC, p27(kip1) levels are high and when stimulated by serum/growth factors, p27(kip1) levels drop by destabilization of the protein. Under conditions that lead to activation of PKR, there is a marked inhibition of p27(kip1) down-regulation due to increased stability of p27(kip1) protein. In order to understand the mechanism of heparin-induced stabilization of p27(kip1) in VSMC, we examined the involvement of the Signal Transducer and Activator of Transcription-1 (STAT1), which is an important player in mediating antiproliferative effects of IFNs. Our results demonstrate that PKR overexpression in VSMC leads to an increase in p27(kip1) protein levels and this increase requires the catalytic activity of PKR. PKR activation induced by antiproliferative agent heparin leads to phosphorylation of STAT1 on serine 727, which is essential for the cell cycle block. STAT1 null VSMCs are largely defective in heparin-induced cell cycle arrest and in PKR null cells the STAT1 phosphorylation in response to heparin was absent. These results establish that heparin causes STAT1 phosphorylation on serine 727 via activation of PKR and that this event is required for the G1 arrest in VSMC.

The mechanisms of nadroparin-mediated inhibition of proliferation of two human lung cancer cell lines

OBJECTIVES

Clinical data suggest that heparin treatment improves survival of lung cancer patients, but the mechanisms involved are not fully understood. We investigated whether low molecular weight heparin nadroparin, directly affects lung cancer cell population growth in conventionally cultured cell lines.

MATERIALS AND METHODS

A549 and CALU1 cells' viability was assessed by MTT and trypan blue exclusion assays. Cell proliferation was assessed using 5-bromo-2-deoxyuridine incorporation. Apoptosis and cell-cycle distribution were analysed by flow cytometry; cyclin B1, Cdk1, p-Cdk1 Cdc25C, p-Cdc25C and p21 expressions were analysed by western blotting. mRNA levels were analysed by real time RT-PCR.

RESULTS

Nadroparin inhibited cell proliferation by 30% in both cell lines; it affected the cell cycle in A549, but not in CALU-1 cells, inducing arrest in the G(2) /M phase. Nadroparin in A549 culture inhibited cyclin B1, Cdk1, Cdc25C and p-Cdc25C, while levels of p-Cdk1 were elevated; p21 expression was not altered. Dalteparin caused a similar reduction in A549 cell population growth; however, it did not alter cyclin B1 expression as expected, based on previous reports. Fondaparinux caused minimal inhibition of A549 cell population growth and no effect on either cell cycle or cyclin B1 expression.

CONCLUSIONS

Nadroparin inhibited proliferation of A549 cells by inducing G(2) /M phase cell-cycle arrest that was dependent on the Cdc25C pathway, whereas CALU-1 cell proliferation was halted by as yet not elucidated modes.

The chemotherapeutic drug 5-fluorouracil promotes PKR-mediated apoptosis in a p53-independent manner in colon and breast cancer cells

The chemotherapeutic drug 5-FU is widely used in the treatment of a range of cancers, but resistance to the drug remains a major clinical problem. Since defects in the mediators of apoptosis may account for chemo-resistance, the identification of new targets involved in 5-FU-induced apoptosis is of main clinical interest. We have identified the ds-RNA-dependent protein kinase (PKR) as a key molecular target of 5-FU involved in apoptosis induction in human colon and breast cancer cell lines. PKR distribution and activation, apoptosis induction and cytotoxic effects were analyzed during 5-FU and 5-FU/IFNα treatment in several colon and breast cancer cell lines with different p53 status. PKR protein was activated by 5-FU treatment in a p53-independent manner, inducing phosphorylation of the protein synthesis translation initiation factor eIF-2α and cell death by apoptosis. Furthermore, PKR interference promoted a decreased response to 5-FU treatment and those cells were not affected by the synergistic antitumor activity of 5-FU/IFNα combination. These results, taken together, provide evidence that PKR is a key molecular target of 5-FU with potential relevance in the clinical use of this drug.

Molecular regulation of contractile smooth muscle cell phenotype: implications for vascular tissue engineering

The molecular regulation of smooth muscle cell (SMC) behavior is reviewed, with particular emphasis on stimuli that promote the contractile phenotype. SMCs can shift reversibly along a continuum from a quiescent, contractile phenotype to a synthetic phenotype, which is characterized by proliferation and extracellular matrix (ECM) synthesis. This phenotypic plasticity can be harnessed for tissue engineering. Cultured synthetic SMCs have been used to engineer smooth muscle tissues with organized ECM and cell populations. However, returning SMCs to a contractile phenotype remains a key challenge. This review will integrate recent work on how soluble signaling factors, ECM, mechanical stimulation, and other cells contribute to the regulation of contractile SMC phenotype. The signal transduction pathways and mechanisms of gene expression induced by these stimuli are beginning to be elucidated and provide useful information for the quantitative analysis of SMC phenotype in engineered tissues. Progress in the development of tissue-engineered scaffold systems that implement biochemical, mechanical, or novel polymer fabrication approaches to promote contractile phenotype will also be reviewed. The application of an improved molecular understanding of SMC biology will facilitate the design of more potent cell-instructive scaffold systems to regulate SMC behavior.

Low-concentration heparin suppresses ionomycin-activated CAMK-II/EGF receptor- and ERK-mediated signaling in mesangial cells

Heparin and endogenous heparinoids inhibit the proliferation of smooth muscle cells, including renal mesangial cells; multiple effects on signaling pathways are well established, including effects on PKC, Erk, and CaMK-II. Many studies have used heparin at concentrations of 100 microg/ml or higher, whereas endogenous concentrations of heparinoids are much lower. Here we report the effects of low-concentration (1 microg/ml) heparin on activation of several kinases and subsequent induction of the c-fos gene in mesangial cells in response to the calcium ionophore, ionomycin, in the absence of serum factors. Ionomycin rapidly increases the phosphorylation of CaMK-II (by 30 s), and subsequently of the EGF receptor (EGFR), c-Src, and Erk 1/2. Low-dose heparin suppresses the ionomycin-dependent phosphorylation of EGFR, c-Src, and Erk 1/2, but not of CaMK-II, whereas inhibition of activated CaMK-II reduces phosphorylation of EGFR, c-Src, and Erk. Our data support a mechanism whereby heparin acts at the cell surface to suppress downstream targets of CaMK-II, including EGFR, leading in turn to a decrease in Erk- (but not c-Src-) dependent induction of c-fos.

The effects of heparin releasing hydrogels on vascular smooth muscle cell phenotype

Poly(ethylene glycol) diacrylate (PEGDA) hydrogel scaffolds were engineered to promote contractile smooth muscle cell (SMC) phenotype via controlled release of heparin. The scaffold design was evaluated by quantifying the effects of free heparin on SMC phenotype, engineering hydrogels to provide controlled release of heparin, and synthesizing cell-adhesive, heparin releasing hydrogels to promote contractile SMC phenotype. Heparin inhibited SMC proliferation and up-regulated expression of contractile SMC phenotype markers, including smooth muscle alpha-actin, calponin, and SM-22alpha, in a dose-dependent fashion (6 microg/ml to 3.2mg/ml). Heparin release from PEGDA hydrogels was controlled by altering PEGDA molecular weight (MW 1000-6000) and concentration at polymerization (10-30% w/w), yielding release profiles ranging from hours to weeks in duration. Heparin released from PEGDA gels, formulated for optimized heparin loading and release kinetics (30% w/w PEGDA, MW 3000), stimulated SMCs to up-regulate contractile marker mRNA. A cell-instructive scaffold construct was prepared by polymerizing a thin hydrogel film, with pendant RGD peptides for cell attachment, over the optimized hydrogel depots. SMCs seeded on these constructs had elevated levels of contractile marker mRNA after 3 d of culture compared with SMCs on control constructs. These results indicate that RGD-modified, heparin releasing PEGDA gels can act as cell-instructive scaffolds that promote contractile SMC phenotype.

Interaction of human tRNA-dihydrouridine synthase-2 with interferon-induced protein kinase PKR

PKR is an interferon (IFN)-induced protein kinase, which is involved in regulation of antiviral innate immunity, stress signaling, cell proliferation and programmed cell death. Although a low amount of PKR is expressed ubiquitously in all cell types in the absence of IFNs, PKR expression is induced at transcriptional level by IFN. PKR's enzymatic activity is activated by its binding to one of its activators. Double-stranded (ds) RNA, protein activator PACT and heparin are the three known activators of PKR. Activation of PKR in cells leads to a general block in protein synthesis due to phosphorylation of eIF2α on serine 51 by PKR. PKR activation is regulated very tightly in mammalian cells and a prolonged activation of PKR leads to apoptosis. Thus, positive and negative regulation of PKR activation is important for cell viability and function. The studies presented here describe human dihydrouridine synthase-2 (hDUS2) as a novel regulator of PKR. We originally identified hDUS2 as a protein interacting with PACT in a yeast two-hybrid screen. Further characterization revealed that hDUS2 also interacts with PKR through its dsRNA binding/dimerization domain and inhibits its kinase activity. Our results suggest that hDUS2 may act as a novel inhibitor of PKR in cells.

Heparin suppresses lipid raft-mediated signaling and ligand-independent EGF receptor activation

Heparin is well known to suppress vascular smooth muscle cell (VSMC) proliferation, and attempts to exploit this therapeutically have led to recognition of multiple pathways for heparin's anti-mitogenic actions. At low concentrations (ca. 1 microg.ml(-1)), these suppressive effects may reflect physiological activities of endogenous heparan sulfates, and appear to be rapid responses to extracellular or cell surface-associated heparin. Because heparin has been shown to influence expression of caveolin proteins, and caveolae/lipid rafts are critical structures modulating cell signaling, we examined the effect of heparin on signaling involving cholesterol-rich membrane microdomains. The VSMC line PAC-1 activates the MAP kinase Erk in response to the cholesterol-sequestering agents methyl-beta-cyclodextrin and nystatin. This follows a temporal sequence that involves Ras-GTP activation of MEK, and is independent of PKC, Src, and PI3 kinase. However, ligand-independent phosphorylation of the EGF receptor (EGFR) by removal of cholesterol precedes Ras activation, and the EGFR kinase inhibitor AG1478 blocks Erk phosphorylation, supporting occurrence of the signaling sequence EGFR-Ras-MEK-Erk. Phosphorylation of EGFR occurs predominantly in caveolin-rich microdomains as identified by Western blotting of fractions from density gradient centrifugation of membranes prepared under detergent-free conditions. In these situations, heparin inhibits phosphorylation of EGFR on the Src-dependent site Tyr(845), but not the autophosphorylation of Tyr(1173), and decreases Ras activation and Erk phosphorylation. We conclude that heparin can suppress Erk signaling in VSMC with effects on site-specific phosphorylation of EGFR localized in caveolin-enriched lipid rafts.

In vivo and in vitro degradation of heparan sulfate (HS) proteoglycans by HPR1 in pancreatic adenocarcinomas. Loss of cell surface HS suppresses fibroblast growth factor 2-mediated cell signaling and proliferation

Heparan sulfate proteoglycans (HSPGs) function as a co-receptor for heparin-binding growth factors, such as fibroblast growth factors (FGFs) and heparin-bound epidermal growth factor (HB-EGF). The HS side chain of HSPGs can be cleaved by HPR1 (heparanase-1), an endoglycosidase that is overexpressed in many types of malignancies. In the present study, we demonstrated that HPR1 expression in pancreatic adenocarcinomas inversely correlated with the presence of heparan sulfate (HS) in the basement membrane. In vitro cell culture study revealed that cell surface HS levels inversely correlated with HPR1 activity in five pancreatic cancer cell lysates and their conditioned media. Heparin and PI-88, two HPR1 inhibitors, were able to increase cell surface HS levels in PANC-1 cells in a dose-dependent manner. The ability of HPR1 to degrade cell surface HS was confirmed by showing that cell surface HS levels were increased in HT1080 cells stably transfected with the HPR1 antisense gene but was decreased in the cells overexpressing HPR1. Further studies showed that PI-88 and heparin were able to stimulate PANC-1 cell proliferation in the absence or presence of exogenous FGF2, whereas exogenous HPR1 was able to inhibit PANC-1 cell proliferation in a dose-dependent manner. Modulation of PANC-1 cell proliferation by HPR1 or HPR1 inhibitors corresponded with the inhibition or activation of the mitogen-activated protein kinase. Our results suggest that HPR1 expressed in pancreatic adenocarcinomas can suppress the proliferation of pancreatic tumor cells in response to the growth factors that require HSPGs as their co-receptors.

Cyclin-Dependent Kinase Inhibitor p27 Kip1 , But Not p21 WAF1/Cip1 , Is Required for Inhibition of Hypoxia-Induced Pulmonary Hypertension and Remodeling by Heparin in Mice

Heparin has growth inhibitory effects on pulmonary artery smooth muscle cell (PASMC) in vitro and in vivo. However, the mechanism has not been fully defined. In this study, we investigated the role of cyclin-dependent kinase inhibitors, p21(WAF1/cip1) (p21) and p27Kip1 (p27), in the inhibitory effect of heparin on PASMC proliferation in vitro and on hypoxia-induced pulmonary hypertension in vivo using p21 and p27-null mice. In vitro, loss of the p27 gene negated the inhibitory effect of heparin on PASMC proliferation, but p21 was not critical for this inhibition. In vivo, heparin significantly inhibited the development of hypoxia-induced pulmonary hypertension and remodeling, as evidenced by decreased right ventricular systolic pressure, ratio of right ventricular weight to left ventricle plus septum weight, and percent wall thickness of pulmonary artery, in p21(+/+), p21(-/-), p27(+/+), and p27(+/-), but not in p27(-/-) mice. We also observed that hypoxia decreased p27 expression significantly in mouse lung, which was restored by heparin. Heparin inhibited Ki67 proliferative index in terminal bronchial vessel walls in p27(+/+) and p27(+/-), but not in p27(-/-) mice exposed to hypoxia. Therefore, we conclude that the cyclin-dependent kinase inhibitor p27, but not p21, is required for the inhibition of hypoxic pulmonary vascular remodeling by heparin.

Regulation of vascular smooth muscle proliferation by heparin: inhibition of cyclin-dependent kinase 2 activity by p27(kip1)

Uncontrolled proliferation of vascular smooth muscle cells (VSMCs) contribute to intimal hyperplasia during atherosclerosis and restenosis. Heparin is an antiproliferative agent for VSMCs and has been shown to block VSMC proliferation both in tissue culture systems and in animals. Despite the well documented antiproliferative actions of heparin, its cellular targets largely remain unknown. In an effort to characterize the mechanism of the antiproliferative property of heparin, we have analyzed the effect of heparin on cell cycle in VSMC. Our results indicate that the heparin-induced block in G(1) to S phase transition is imposed by p27(kip1)-mediated inhibition of cyclin-dependent kinase 2 activity. Further analysis of p27(kip1) mRNA levels showed that the increase in p27(kip1) protein levels in heparin-treated VSMC occurs at posttranscriptional levels. We present evidence that heparin causes stabilization of p27(kip1) protein during G(1) phase and thereby prevents activation of cyclin-dependent kinase 2.

Identification of the heparin-binding domains of the interferon-induced protein kinase, PKR

PKR is an interferon-induced serine-threonine protein kinase that plays an important role in the mediation of the antiviral and antiproliferative actions of interferons. PKR is present at low basal levels in cells and its expression is induced at the transcriptional level by interferons. PKR's kinase activity stays latent until it binds to its activator. In the case of virally infected cells, double-stranded (ds) RNA serves as PKR's activator. The dsRNA binds to PKR via two copies of an evolutionarily conserved motif, thus inducing a conformational change, unmasking the ATP-binding site and leading to autophosphorylation of PKR. Activated PKR then phosphorylates the alpha-subunit of the protein synthesis initiation factor 2 (eIF2alpha) thereby inducing a general block in the initiation of protein synthesis. In addition to dsRNA, polyanionic agents such as heparin can also activate PKR. In contrast to dsRNA-induced activation of PKR, heparin-dependent PKR activation has so far remained uncharacterized. In order to understand the mechanism of heparin-induced PKR activation, we have mapped the heparin-binding domains of PKR. Our results indicate that PKR has two heparin-binding domains that are nonoverlapping with its dsRNA-binding domains. Although both these domains can function independently of each other, they function cooperatively when present together. Point mutations created within these domains rendered PKR defective in heparin-binding, thereby confirming their essential role. In addition, these mutants were defective in kinase activity as determined by both in vitro and in vivo assays.

Coils and tubes releasing heparin. Studies on a new vascular graft prototype

Coiled metallic guidewires find widespread use, for instance in interventional cardiology. It is known that release of heparin from the surface of guidewires is advantageous to prevent formation of thrombotic emboli. New coiled tubular structures, having larger inner and outer diameter as compared to guidewires, are presented. In theory these tubes can be used as interposition vascular grafts. Ten coiled tubes with an internal diameter of 690 microm were made. Five different adherent polymeric coatings with increasing hydrophilicity were used. Five tubes contained heparin in the coating and the other five were unheparinised controls. The five tubes containing heparin were studied with respect to heparin release in vitro (amount released, kinetics), and immobilised heparin that is exposed at the surface. All tubes were studied with a direct cell contact assay using 3T3 mouse fibroblast cells, a dynamic thrombin generation test, and endothelial cell growth onto the coils. It was found that the heparinised tubes lead to very little thrombin formation. It is argued that this is due to heparin that is immobilised and exposed at the inner surface of such tubes. Furthermore the coils showed to be cytocompatible and endothelial cells adhere and proliferate well onto the coils. This concept is believed to hold promise for further development of small vascular grafts.

The carboxy-terminal, M3 motifs of PACT and TRBP have opposite effects on PKR activity

PKR is an interferon(IFN)-induced, serine-threonine protein kinase, which plays a crucial role in IFN's antiviral and antiproliferative actions. The three known activators of PKR are dsRNA, heparin, and PACT. PACT activates PKR by direct protein-protein interaction in response to cellular stress. The human TAR (trans-activating region)-binding protein (TRBP), which is very homologous to PACT, also interacts with PKR, leading to an inhibition of PKR activity. Since these two highly homologous proteins have opposite effects on PKR, we examined if they interact with PKR differently by assaying their interaction with various point mutants of PKR. Our results indicate that TRBP and PACT interact with PKR through the same residues, and no differences were identified in these two interactions. Domain swap experiments between PACT and TRBP indicated that the inhibitory effects of TRBP on PKR activity are mediated through its carboxy-terminal residues, which contain TRBP's third dsRNA-binding motif.