Dominique Boivin

Green tea catechins as novel antitumor and antiangiogenic compounds.

The concept of cancer prevention by use of naturally occuring substances that could be included in the diet is under investigation as a practical approach towards reducing cancer incidence, and therefore the mortality and morbidity associated with this disease. Tea, which is the most popularly consumed beverage aside from water, has been particularly associated with decreased risk of various proliferative diseases such as cancer and atherosclerosis in humans. Various studies have provided evidence that polyphenols are the strongest biologically active agents in green tea. Green tea polyphenols (GTPs) mainly consist of catechins (3-flavanols), of which (-)-epigallocatechin gallate is the most abundant and the most extensively studied. Recent observations have raised the possibility that green tea catechins, in addition to their antioxidative properties, also affect the molecular mechanisms involved in angiogenesis, extracellular matrix degradation, regulation of cell death and multidrug resistance. This article will review the effects and the biological activities of green tea catechins in relation to these mechanisms, each of which plays a crucial role in the development of cancer in humans. The extraction of polyphenols from green tea, as well as their bioavailability, are also discussed since these two important parameters affect blood and tissue levels of the GTPs and consequently their biological activities. In addition, general perspectives on the application of dietary GTPs as novel antiangiogenic and antitumor compounds are also presented.

The flavonols quercetin, kaempferol, and myricetin inhibit hepatocyte growth factor-induced medulloblastoma cell migration.

Medulloblastoma, the most common malignant brain tumor in children, is a highly metastatic disease, with up to 30% of children having evidence of disseminated disease at presentation. Recently, the hepatocyte growth factor (HGF) and its receptor, the tyrosine kinase Met, have emerged as key components of human medulloblastoma growth and metastasis, suggesting that inhibition of this pathway may represent an attractive target for the prevention and treatment of this disease. Using immunoblotting procedures, we observed that the dietary-derived flavonols quercetin, kaempferol, and myricetin inhibited HGF/Met signaling in a medulloblastoma cell line (DAOY), preventing the formation of actin-rich membrane ruffles and resulting in the inhibition of Met-induced cell migration in Boyden chambers. Furthermore, quercetin and kaempferol also strongly diminished HGF-mediated Akt activation. Interestingly, the inhibitory effects of quercetin on the tyrosine kinase receptor Met [half-maximal inhibitory effect (IC(50)) of 12 micromol/L] or on the Met-induced activation of Akt (IC(50) of 2.5 micromol/L) occurred at concentrations achievable through dietary approaches. These results highlight quercetin, kaempferol, and myricetin as dietary-derived inhibitors of Met activity and suggest that this inhibitory effect may contribute to the chemopreventive properties of these molecules.

The stem cell marker CD133 (prominin-1) is phosphorylated on cytoplasmic tyrosine-828 and tyrosine-852 by Src and Fyn tyrosine kinases.

CD133 (prominin-1) is a transmembrane glycoprotein expressed at the surface of normal and cancer stem cells, progenitor cells, rod photoreceptor cells, and a variety of epithelial cells. Although CD133 is widely used as a marker of various somatic and putative cancer stem cells, its contribution to fundamental properties of stem cells such as self-renewal and differentiation remains unknown. CD133 contains a short C-terminal cytoplasmic domain with five tyrosine residues, including a consensus tyrosine phosphorylation site that has not yet been investigated. In this study, we show that CD133 is phosphorylated in human medulloblastoma D283 and Daoy cells, in a Src family kinase-dependent manner. The cytoplasmic domain of CD133 is tyrosine phosphorylated in Daoy cells overexpressing Src and Fyn tyrosine kinases, as well as in vitro using recombinant proteins. Deletion of the C-terminal cytoplasmic domain of CD133 considerably reduced its phosphorylation by Src. To identify the tyrosine phosphorylation sites in CD133, we used matrix-assisted laser desorption/ionization quadrupole time-of-flight (MALDI Q-TOF) and liquid chromatography tandem mass spectrometry (LC-MS/MS). Analysis of tyrosine-phosphorylated CD133 by mass spectrometry and site-directed mutagenesis identified tyrosine-828 and the nonconsensus tyrosine-852 as the major tyrosine phosphorylation sites both in vitro and in intact cells. Identification of CD133 as a substrate for Src-family tyrosine kinases suggests that the cytoplasmic domain of CD133 might play an important role in the regulation of its functions.

c-Met activation in medulloblastoma induces tissue factor expression and activity : effects on cell migration.

Met, the receptor for hepatocyte growth factor (HGF), is a receptor tyrosine kinase that has recently emerged as an important contributor to human neoplasia. In physiological and pathological conditions, Met triggers various cellular functions related to cell proliferation, cell migration and the inhibition of apoptosis, and also regulates a genetic program leading to coagulation. Since medulloblastomas (MBs) express high levels of tissue factor (TF), the main initiator of blood coagulation, we therefore examined the link between Met and TF expression in these pediatric tumors. We observed that stimulation of the MB cell line DAOY with HGF led to a marked increase of TF expression and procoagulant activity, in agreement with analysis of clinical MB tumor specimens, in which tumors expressing high levels of Met also showed high levels of TF. The HGF-dependent increase in TF expression and activity required Src family kinases and led to the translocation of TF to actin-rich structures at the cell periphery, suggesting a role of the protein in cell migration. Accordingly, addition of physiological concentrations of the TF activator factor VIIa (FVII) to HGF-stimulated DAOY cells promoted a marked increase in the migratory potential of these cells. Overall, these results suggest that HGF-induced activation of the Met receptor results in TF expression by MB cells and that this event probably contribute to tumor proliferation by enabling the formation of a provisional fibrin matrix. In addition, TF-mediated non-hemostatic functions, such as migration toward FVIIa, may also play a central role in MB aggressiveness.

The Antiangiogenic Agent Neovastat (Æ-941) Stimulates Tissue Plasminogen Activator Activity

The plasminogen activator/plasmin system represents a key component of the proteolytic machinery underlying angiogenesis. In this work, we investigated the effect of Neovastat (Æ-941), a naturally occurring multifunctional antiangiogenic agent that is currently in Phase III clinical trials, on tissue and urokinase plasminogen activator activities. We found that in vitro, Neovastat at 100μg/ml markedly stimulates t-PA-mediated plasmin generation, while it slightly inhibits the generation of plasmin mediated by uPA. The stimulatory effect of Neovastat on t-PA activity was markedly increased by a heat treatment, resulting in a 15-fold increase in the rate of activation of plasminogen. Neovastat did not directly stimulate the activity of t-PA or plasmin towards exogenous substrates, suggesting that its effect requires the presence of plasminogen. Accordingly, kinetic analysis showed that Neovastat increases both the kcat of t-PA as well as its affinity for plasminogen by 10-fold. The stimulation of t-PA activity by Neovastat was also correlated with a direct interaction of Neovastat with plasminogen as monitored by the surface plasmon resonance technology. Overall, these results identify Neovastat as a potent stimulator of t-PA-dependent activation of plasminogen, further emphasizing its pleiotropic mechanism of action on several molecular events involved in angiogenesis.

Tissue factor mediates the HGF/Met-induced anti-apoptotic pathway in DAOY medulloblastoma cells

The classical treatment scheme for medulloblastoma (MB) is based on a tri-therapy approach consisting of surgical tumor resection, craniospinal axis radiation and chemotherapy. With current treatments relying mainly on non-specific cytotoxic therapy, a better understanding of the mechanisms underlying resistance to these treatments is important in order to improve their effectiveness. In this study, we report that stimulation of DAOY with HGF resulted in the protection of these cells against etoposide-induced apoptosis, this anti-apoptotic effect being correlated with an increase in the expression of tissue factor (TF), the initiator of the extrinsic pathway of coagulation. HGF-mediated protection from apoptosis was abolished by a c-Met inhibitor as well as by siRNA-mediated reduction of TF levels, implying a central role of Met-dependent induction of TF expression in this process. Accordingly, stimulation of DAOY with FVIIa, the physiological ligand of TF, also resulted in a significant protection from etoposide-mediated cytotoxicity. Overall, our results suggest the participation of the haemostatic system to drug resistance in MB and may thus provide novel therapeutic approaches for the treatment of these tumors.

Inhibition of HUVEC tubulogenesis by hederacolchiside-A1 is associated with plasma membrane cholesterol sequestration and activation of the Ha-Ras/MEK/ERK cascade.

PurposeNeoangiogenesis is critical to cancer proliferation and metastasis and constitutes an attractive target for cancer therapy. It has previously been demonstrated that hederacolchiside-A1 (HCol-A1), a triterpenoid saponin from Hedera colchica Koch, has antimelanoma potential. The goal of this study was to evaluate, in vitro, if in addition to its tumoricidal effect on melanoma cells, HCol-A1 might affect endothelial cell network formation.MethodsWe investigated whether HCol-A1 affects matrigel-induced tubulogenesis and inhibits the viability (WST-1 assay) of human umbilical vein endothelial cells (HUVECs). To provide structure-activity relationships (SAR), studies were conducted on HCol-A1, oleanolic acid and hederacolchiside A (HCol-A), a triterpenoid saponin which possess the same sugar sequence as Hcol-A1. Plasma membrane cholesterol sequestration was studied by labelling with [3H]cholesterol and assayed with HCol-A1-cholesterol complexes. HCol-A1 signalling was investigated using immunoassays.ResultsIn contrast to HCol-A and oleanolic acid, HCol-A1 inhibited matrigel-induced angiogenesis at micromolar concentration. Plasma membrane cholesterol sequestration was found to be critical for this activity. Activation of the Ras/MEK/ERK cascade appears to be one of the mechanisms by which Hcol-A1 affects HUVEC network formation. The predominant activation of the Ha-Ras isoform, which decreases HUVEC-tolerance to apoptosis, might contribute to the high susceptibility of this cell line to HCol-A1.ConclusionSince cholesterol sequestration affects cell confluence-dependent remodelling of endothelial membranes and vascular endothelial growth factor receptor-2 activity, these results raise the possibility that Hcol-A1 might slow-down cancer proliferation and metastasis in vivo by inhibiting critical aspects of neoangiogenesis. Further in vivo studies are needed to verify this hypothesis.

Tyrosine protein kinase activity in renal brush-border membranes.

Tyrosine protein kinase (TPK) activity was detected in rat renal brush-border membranes (BBM) using poly(Glu80Na,Tyr20) as a substrate. Maximal TPK activity required prior detergent dispersion of the membranes with 0.05% Triton X-100 and the presence of vanadate, a potent inhibitor of phosphotyrosine protein phosphatases, in the phosphorylation medium. Optimal conditions for measurement of TPK activity were 10 mM of MgCl2 and MnCl2, at 30 degrees C and pH 7.0. TPK activity was inhibited by genistein, with a IC50 value of 15 microM, while no inhibition was observed in the presence of 1-(5-isoquinolinesulfonyl)-2-methyl-piperazine dihydrochloride (H7), an inhibitor of serine-threonine kinases. TPK activity was enriched 4-fold in the BBM fraction relative to cortex homogenate. It was co-enriched with BBM enzyme markers, but not with those of the basolateral membrane (BLM). The endogenous substrates of TPK in brush-border and basolateral membranes were determined by Western blot analysis using an antiphosphotyrosine monoclonal antibody (PY20). Various phosphotyrosine-containing proteins were found in the BBM (31, 34, 46, 50, 53, 72, 90, 118 and 170 kDa) and in the BLM (37, 48, 50, 53, 72, 90, 130 and 170 kDa). Addition of exogenous insulin receptor to BBM and BLM increased the phosphorylation of most of the substrates. Solubilization of the TPK activity from BBM with 0.5% CHAPS and subsequent gel filtration on Superdex 75 yielded two peaks of tyrosine protein kinase activity with apparent molecular masses of 49 and 66 kDa. These results provide evidence for a non-receptor TPK activity associated with the renal tubular luminal membrane.

Essential arginine residues in isoprenylcysteine protein carboxyl methyltransferase

We used specific amino acid modifying reagents to characterize the isoprenylcysteine carboxyl methyltransferase in kidney membranes. The enzyme was inactivated by reagents specific for arginine, histidine, cysteine, and tryptophan residues. Protection by the product and inhibitor S-adenosyl-L-homocysteine was observed for arginine modification by phenylglyoxal and tryptophan modification by N-bromosuccinimide. We focused on modification by phenylglyoxal, a highly specific modifier of arginine residues. The inactivation of methyltransferase by phenylglyoxal follows pseudo-first-order kinetics and the order of the reaction, n, with respect to phenylglyoxal was 1.2. The inactivation increased with the alkalinity of the preincubation medium and was maximal at pH 10. Kinetic analysis showed that the K(m) for S-adenosyl-L-methionine is not significantly affected by treatment with phenylglyoxal but that the Vmax is reduced p-Hydroxyphenylglyoxal, a more hydrophilic derivative of phenylglyoxal, was a less potent inactivator of methyltransferase than phenylglyoxal, suggesting that arginine residues modified are in a hydrophobic environment. The methyltransferase is protected from phenylglyoxal modification by S-adenosyl-L-homocysteine but not S-adenosyl-L-methionine, sinefungin, N-acetyl-S-farnesyl-L-cysteine, or farnesylthioacetate. The arginine residue modified may thus be located either at the active site or at another additional binding site for S-adenosyl-L-homocysteine. These results indicate that arginine residues are essential for the enzymatic activity of isoprenylcysteine carboxyl methyltransferase.

Functional size of C-terminal protein carboxyl methyltransferase from kidney basolateral plasma membranes.

The functional sizes of the C-terminal isoprenylcysteine protein carboxyl methyltransferase (PCMT) from kidney cortex basolateral plasma membranes and yeast membranes have been estimated by the radiation inactivation and fragmentation method. Attempts to solubilize the methyltransferase with detergents were unsuccessful as they resulted in the irreversible denaturation of its enzymatic activity. The radiation inactivation sizes of the methyltransferases were 98 and 24 kDa for kidney and yeast, respectively. Kinetic experiments showed that irradiation affects the Vmax of the reaction but not the apparent Km for either S-adenosyl-L-methionine and N-acetyl farnesylcysteine. The functional size reported here for the kidney membrane is about 4-times larger than the size predicted for the Saccharomyces cerevisiae C-terminal PCMT deduced from the nucleotide sequence of its gene (28 kDa). These results suggest that mammalian methyltransferase has a functional size different from that of the yeast; tetramerization of monomers is one possible hypothesis for this difference.