Low- and high-density lipoproteins as mitogenic factors for vascular smooth muscle cells: individual, additive and synergistic effects

T-Cadherin and the Ratio of Its Ligands as Predictors of Carotid Atherosclerosis: A Pilot Study

In the cardiovascular system, atherogenic low-density lipoproteins (LDL) and the protective hormone adiponectin bind to the same receptor, T-cadherin. In this study, we tested the hypothesis that the ratio of circulating LDL to high-molecular weight (HMW) adiponectin could predict the development of atherosclerosis. Using enzyme-linked immunosorbent assay, we measured the level of circulating HMW adiponectin in the blood of donors together with ultrasound measuring of intima-media thickness (IMT) of carotid arteries. Single-nucleotide polymorphisms in the T-cadherin gene were identified using polymerase chain reaction. We found that carotid artery IMT is inversely correlated with the level of HMW in male subjects. We also found that the G allele of rs12444338 SNP in the T-cadherin gene correlates with a lower level of circulating T-cadherin and thinner IMT and therefore could be considered as an atheroprotective genotype. Despite our data, we could not provide direct evidence for the initial study hypothesis. However, we did uncover an important correlation between circulating T-cadherin and thinner carotid IMT.

Designing a multicellular organotypic 3D liver model with a detachable, nanoscale polymeric Space of Disse

The design of in vitro models that mimic the stratified multicellular hepatic microenvironment continues to be challenging. Although several in vitro hepatic cultures have been shown to exhibit liver functions, their physiological relevance is limited due to significant deviation from in vivo cellular composition. We report the assembly of a novel three-dimensional (3D) organotypic liver model incorporating three different cell types (hepatocytes, liver sinusoidal endothelial cells, and Kupffer cells) and a polymeric interface that mimics the Space of Disse. The nanoscale interface is detachable, optically transparent, derived from self-assembled polyelectrolyte multilayers, and exhibits a Young's modulus similar to in vivo values for liver tissue. Only the 3D liver models simultaneously maintain hepatic phenotype and elicit proliferation, while achieving cellular ratios found in vivo. The nanoscale detachable polymeric interfaces can be modulated to mimic basement membranes that exhibit a wide range of physical properties. This facile approach offers a versatile new avenue in the assembly of engineered tissues. These results demonstrate the ability of the tri-cellular 3D cultures to serve as an organotypic hepatic model that elicits proliferation and maintenance of phenotype and in vivo-like cellular ratios.

Antioxidant and cytoprotective properties of high-density lipoproteins in vascular cells

Beside their key role in the regulation of cholesterol homeostasis, HDL exhibit antioxidant and anti-inflammatory properties that participate to their general antiatherogenic effect. The purpose of this review is to summarize the recent findings on antioxidant activity and cytoprotective cell signalling elicited by HDL against oxidized LDL and proatherogenic agents in vascular cells. HDL exhibit an antioxidant activity efficient to prevent LDL oxidation, or to inactivate newly formed lipid oxidation products. The antioxidant ability of HDL is due to the apoprotein moiety and to the presence of associated enzymes, paraoxonase and PAF-Acetyl Hydrolase. HDL prevent the intracellular oxidative stress and the inflammatory response elicited by oxidized LDL (ox-LDL), by inhibiting the NFkappaB signalling pathway, and the subsequent inflammatory events (expression of adhesion molecules, recruitment and proliferation of mononuclear cells within the vascular wall). HDL prevent ox-LDL-mediated cell activation and proliferation, this being also attributed to the presence in HDL of sphingosine-1 phosphate which modulates the migration and survival of vascular cells. Lastly, HDL inhibit apoptosis elicited by ox-LDL in vascular cells. Recent evidences indicate that, beside their strong antiatherogenic properties, HDL could exert their protective effect in diseases generally associated to inflammatory events.

Human Urotensin-II Potentiates the Mitogenic Effect of Mildly Oxidized Low-Density Lipoprotein on Vascular Smooth Muscle Cells: Comparison with Other Vasoactive Agents and Hydrogen Peroxide

Human urotensin-II (U-II) is the most potent vasoactive peptide identified to date, and may be involved in hypertension and atherosclerosis. We investigated the effects of the interactions between U-II or other vasoactive agents and mildly oxidized low-density lipoprotein (mox-LDL) or hydrogen peroxide (H2O2) on the induction of vascular smooth muscle cell (VSMC) proliferation. Growth-arrested rabbit VSMCs were incubated with vasoactive agents (U-II, endothelin-1, angiotensin-II, serotonin, or thromboxane-A2) in the presence or absence of mox-LDL or H2O2. [3H]Thymidine incorporation into DNA was measured as an index of VSMC proliferation. On interaction with mox-LDL or H2O2, U-II induced the greatest increase in [3H]thymidine incorporation among these vasoactive agents. A low concentration of U-II (10 nmol/l) enhanced the potential mitogenic effect of low concentrations of mox-LDL (120 to 337%) and H2O2 (177 to 226%). U-II at 50 nmol/l showed the maximal mitogenic effect (161%), which was abolished by G protein inactivator (GDP-beta-S), c-Src tyrosine kinase inhibitor (radicicol), protein kinase C (PKC) inhibitor (Ro31-8220), extracellular signal-regulated kinase (ERK) kinase inhibitor (PD98059), or Rho kinase inhibitor (Y27632). Mox-LDL at 5 microg/ml showed the maximal mitogenic effect (211%), which was inhibited by free radical scavenger (catalase), intracellular and extracellular antioxidants (N-acetylcysteine and probucol), nicotinamide adenine dinucleotide phosphate oxidase inhibitor (diphenylene iodonium), or c-Jun N-terminal kinase (JNK) inhibitor (SP600125). These results suggested that U-II acts in synergy with mox-LDL in inducing VSMC DNA synthesis at the highest rate among these vasoactive agents. Activation of the G protein/c-Src/PKC/ERK and Rho kinase pathways by U-II together with the redox-sensitive JNK pathway by mox-LDL may explain the synergistic interaction between these agents.

LDL induces intracellular signalling and cell migration via atypical LDL-binding protein T-cadherin

Cadherins are a superfamily of adhesion molecules that mediate Ca(2+)-dependent cell-cell adhesion. T-cadherin (T-cad), a unique glycosylphosphatidylinositol-anchored member of the cadherin superfamily, was initially identified by immunoblotting of vascular cell membranes as an atypical low affinity low density lipoprotein (LDL)-binding protein. It is not known whether this heterophilic interaction is physiologically relevant. Expression of T-cadherin is upregulated in vascular cells during atherosclerosis, restenosis and tumour angiogenesis, conditions characterized by enhanced cell migration and growth. Elevated levels of serum low density lipoproteins (LDL), which result in cholesterol accumulation in vascular wall, is a widely accepted risk factor in atherosclerosis development. Additionally to its metabolic effects, LDL can produce hormone-like effects in a number of cell types. This study has utilized HEK293 cells and L929 cells stably transfected with T-cadherin cDNA to investigate T-cad-dependent responses to LDL. Stable expression of T-cad in both HEK293 and L929 cells results in significantly (p < 0.05) elevated specific surface binding of [I125]-LDL. Compared with mock-transfectants, cells expressing T-cad exhibit significantly (p < 0.01) enhanced LDL-induced mobilization of intracellular Ca(2+)-stores and a significantly (p < 0.01) increased migration toward an LDL gradient (0.1% BSA + 60 microg/ml LDL) in Boyden chamber migration assay. Thus LDL-binding to T-cad is capable of activating physiologically relevant intracellular signaling and functional responses.

Tetrahydrocortisol-apolipoprotein A-I complex specifically interacts with eukaryotic DNA and GCC elements of genes

Tetrahydrocortisol stimulates DNA and protein biosynthesis in hepatocytes only when it enters the complex with apolipoprotein A-I. Tetrahydrocortisol-apolipoprotein A-I (THC-apoA-I) complex specifically interacts with eukaryotic DNA isolated from rat liver. In the process of interaction, rupture of hydrogen bonds between the pairs of nitrous bases occurs with the formation of single-stranded DNA structures. In such state DNA forms complexes with DNA-dependent RNA-polymerase. The most probable site of binding the tetrahydrocortisol-apolipoprotein A-I complex with DNA is the sequence of CC(GCC)(n) type entering the structure of many genes, among them the structure of human apolipoprotein A-I gene. Oligonucleotide of this type has been synthesized. Association constant (K(ass)) of it with tetrahydrocortisol-apolipoprotein A-I complex was shown to be 1.66 x 10(6)M(-1). Substitution of tetrahydrocortisol for cortisol in the complex results in a considerable decrease of K(ass). It was assumed that in the GC-pairs of the given sequence tetrahydrocortisol itself participates in the formation of hydrogen bonds with cytosine, favoring their rupture with complementary base-guanine.

Antioxidant N-acetylcysteine inhibits vasoactive agents-potentiated mitogenic effect of mildly oxidized LDL on vascular smooth muscle cells

Mildly oxidized LDL (mox-LDL) has been shown to induce monocyte-endothelial interactions and vascular smooth muscle cell (VSMC) proliferation, key events in the formation of the atherosclerotic lesion. Growth factors and vasoactive peptides are also thought to play a major role in atherogenesis. We examined the interaction between mox-LDL and well-known vasoactive agents such as serotonin (5-HT), angiotensin II (Ang-II), endothelin-1 (ET-1), or urotensin II (U-II) in inducing DNA synthesis in VSMCs. Growth-arrested VSMCs were incubated with different concentrations of native LDL, mox-LDL, or highly oxidized LDL (ox-LDL) with 5-HT, Ang-II, ET-1, or U-II in the absence or presence of N-acetylcysteine (NAC), an intracellular free radical scavenger. DNA synthesis in VSMCs was examined by [3H]thymidine incorporation into cellular DNA. Mox-LDL and ox-LDL stimulated [3H]thymidine incorporation with a maximal effect at 5 microg/ml (211%, 154%), which values were significantly greater than that for native LDL (128%). 5-HT, Ang-II, ET-1, or U-II also stimulated [3H]thymidine incorporation in a dose-dependent manner. 5-HT had a maximal stimulatory effect at a concentration of 50 micromol/l (205%), Ang-II at 1.75 micromol/l (202%), ET-1 at 0.1 micromol/l (205%), and U-II at 0.05 micromol/l (161%). When added together, mox-LDL (100 ng/ml)-induced [3H]thymidine incorporation was potentiated by low concentrations of 5-HT (1 micromol/l), Ang-II (0.5 micromol/l), ET-1 (1 nmol/l), or U-II (10 nmol/l) (114% to 330%, 325%, 338%, or 345%, respectively). Synergistic interactions of mox-LDL with 5-HT, Ang-II, ET-1, or U-II were significantly inhibited by NAC (400 micromol/l). Our results suggest that mild oxidation of LDL may enhance its atherogenic potential and exert a synergistic interaction with vasoactive agents in inducing DNA synthesis via the generation of reactive oxygen species in VSMCs.

HDL and arteriosclerosis: beyond reverse cholesterol transport

The inverse correlation between serum levels of high density lipoprotein (HDL) cholesterol and the risk of coronary heart disease, the protection of susceptible animals from atherosclerosis by transgenic manipulation of HDL metabolism, and several potentially anti-atherogenic in vitro-properties have made HDL metabolism an interesting target for pharmacological intervention in atheroslcerosis. We have previously reviewed the concept of reverse cholesterol transport, which describes both the metabolism and the classic anti-atherogenic function of HDL (Arterioscler. Thromb. Vasc. Biol. 20 2001 13). We here summarize the current understanding of additional biological, potentially anti-atherogenic properties of HDL. HDL inhibits the chemotaxis of monocytes, the adhesion of leukocytes to the endothelium, endothelial dysfunction and apoptosis, LDL oxidation, complement activation, platelet activation and factor X activation but also stimulates the proliferation of endothelial cells and smooth muscle cells, the synthesis of prostacyclin and natriuretic peptide C in endothelial cells, and the activation of proteins C and S. These anti-inflammatory, anti-oxidative, anti-aggregatory, anti-coagulant, and pro-fibrinolytic activities are exerted by different components of HDL, namley apolipoproteins, enzymes, and even specific phospholipids. This complexity further emphasizes that changes in the functionality of HDL rather than changes of plasma HDL-cholesterol levels determine the anti-atherogenicity of therapeutic alterations of HDL metabolism.

β-Migrating Very Low Density Lipoprotein (βVLDL) Activates Smooth Muscle Cell Mitogen-activated Protein (MAP) Kinase via G Protein-coupled Receptor-mediated Transactivation of the Epidermal Growth Factor (EGF) Receptor

This study examined the premise that the atherogenic lipoprotein, beta-migrating very low density lipoprotein (betaVLDL), might activate the mitogen-activated protein (MAP) kinases ERK1/ERK2, thereby contributing to the induction of smooth muscle cell proliferation in atherosclerosis. The data show that betaVLDL activates rabbit smooth muscle cell ERK1/ERK2. Interestingly, ERK1/ERK2 activation is mediated by G protein-coupled receptors that transactivate the epidermal growth factor (EGF) receptor. betaVLDL-induced MAP kinase activation depends on Ras and Src activity as well as protein kinase C. The inhibition of lysosomal degradation of betaVLDL has no effect on ERK1/ERK2 activation. The contribution of betaVLDL-induced activation of ERK1/ERK2 to smooth muscle cell proliferation was also explored. betaVLDL induces expression of egr-1 and c-fos mRNA. Despite its ability to stimulate early gene expression, betaVLDL alone is unable to inspire quiescent cells into S phase. When added in conjunction with EGF, however, stimulation of [(3)H]thymidine incorporation into DNA and an increase in histone gene expression are observed. Moreover, betaVLDL plus EGF synergistically induce cyclin D1 expression and down-regulate p27(KIP1) expression. The addition of either betaVLDL or EGF stimulates a robust activation of ERK1/ERK2, but the addition of both agents simultaneously sustains the activation for a longer time period. Inhibition of MAP kinase kinase, pertussis toxin-sensitive G proteins, the EGF receptor, or protein kinase C blocks betaVLDL plus EGF-induced proliferation, demonstrating that activation of the betaVLDL-induced signaling pathway results in smooth muscle cell proliferation.

Evidence that lipoproteins are carriers of bioactive factors

We recently demonstrated that the mitogenic effect of LDL (100 microg/mL) as well as its early intracellular signaling pathway are mediated by a pertussis-toxin (PTX)-sensitive G(i) protein-coupled receptor that is independent from its classical receptor and involves activation of extracellular response kinases (ERK1/2) (also known as p44(mapk)/p42(mapk)). In the present study we examined whether LDL-adherent factors may be responsible for some of the effects of LDL. The term "signaling activity" is used to characterize fractions that cause an increase in intracellular free Ca(2+) concentration or stimulate ERK1/2 and c-fos mRNA expression. LDL, HDL, and VLDL stimulate ERK1/2 with the following order of potency: LDL>HDL>VLDL. After delipidation of LDL with chloroform/methanol/water mixtures a PTX-sensitive signaling activity was found in one fraction arbitrarily called LDL-F. After further analysis of LDL-F compounds by high pressure liquid chromatography, a PTX-sensitive signaling activity was detected only in the fraction with a retention time of 33 minutes (arbitrarily called LDL-F33). Similarly, after separation of sphingosine-1-phosphate (SPP) and sphingosylphosphorylcholine (SPC) by high pressure liquid chromatography, a PTX-sensitive signaling activity was found in the fractions 33 and 33 to 35, respectively. These findings demonstrate that the effects of LDL-F33 are mimicked by similar fractions collected from SPP/SPC, hence suggesting that these LDL-adherent molecules are possibly closely related to SPP/SPC. A PTX-sensitive signaling activity was also detected in HDL and HDL-F33. Therefore, LDL and other lipoproteins may function as carriers for bioactive phospholipids thereby contributing to the development of coronary artery disease. Our findings support a new research concept that may contribute in elucidating cellular mechanisms promoting coronary artery disease.

Oxidized lipoproteins increase reactive oxygen species formation in microglia and astrocyte cell lines

Lipoproteins exist in the central nervous system and surrounding vasculature possibly mediating effects upon cells in the brain during times of oxidative stress or compromised blood-brain barrier. The focus of the present study was to determine the effect of unmodified and oxidatively modified lipoproteins on astrocytes and microglia. Application of oxidized low-density lipoprotein resulted in an increase in DCF fluorescence, which was inhibited by pretreatment with antioxidants, consistent with the formation of reactive oxygen species (ROS). Low-density at concentrations below 20 microg/ml likewise increased ROS formation. Because ROS are associated with numerous astrocyte and microglia activities including proliferation, activation, and cytokine production it is possible that lipoproteins may mediate such effects on glial cells in the central nervous system.

Opposing actions of native and oxidized lipoprotein on motor neuron-like cells

Lipoproteins are present in the central nervous system and surrounding vasculature and possibly mediate effects relevant to neuronal physiology and pathology. To determine the effects of lipoproteins on motor neurons, native low density lipoproteins (LDL) and oxidized LDL (oxLDL) were applied to a motor neuron cell line. Oxidized LDL, but not native LDL, resulted in a dose- and time-dependent increase in reactive oxygen species and neuron death. Oxidized LDL-induced toxicity was attenuated by a calcium chelator, antioxidants, caspase inhibitors, and inhibitors of macromolecular synthesis. In addition to being nontoxic, application of native LDL attenuated reactive oxygen species formation and neuron loss following glucose deprivation injury. Together, these data demonstrate a possible neuroprotective role for unmodified lipoproteins and suggest oxidized lipoproteins may amplify oxidative stress and neuron loss.

High-density lipoprotein: multipotent effects on cells of the vasculature

The epidemiological evidence showing a strong inverse correlation between the level of plasma high-density lipoprotein (HDL) and the incidence of heart disease suggests that HDL has a protective effect against cardiovascular disease. The mechanism of this protective effect has been the raison d'etre for much research. The ability of HDL to mediate cholesterol efflux from peripheral tissues has been used to explain the cardioprotective effect of HDL. However, there is little direct evidence to suggest that in subjects with low plasma levels of HDL the rate of cholesterol efflux from peripheral tissues is significantly reduced. This observation suggested that HDL may be mediating its protective effect through other mechanisms. This review provides an account of the burgeoning evidence that HDL has many effects on cellular processes, in addition to the effects on cholesterol efflux, and will illustrate the multipotency of this lipoprotein.

Identification of an atypical lipoprotein-binding protein from human aortic smooth muscle as T-cadherin

We have previously described an atypical lipoprotein-binding protein of about 105 kDa (p105) in membranes of vascular smooth muscle cells (VSMCs) that is distinct from currently known lipoprotein receptors. In the present work we have developed a procedure for purification of p105 from human aortic media. Partial sequencing of purified protein has revealed identity of p105 with human T-cadherin. Anti-peptide antisera raised against human T-cadherin recognized a protein spot corresponding to the purified p105 on two-dimensional Western blots. The antisera also inhibited LDL binding to p105 on ligand blots. We conclude that the 105 kDa lipoprotein-binding protein present in human VSMCs is T-cadherin, an unusual glycosylphosphatidylinositol-anchored member of the cadherin family of cell-cell adhesion proteins.

Stimulation of mitogen activated protein kinase by LDL and oxLDL in human U-937 macrophage-like cells

Mitogen activated protein kinase in extracts of U-937 macrophage-like cells was stimulated by LDL and oxLDL. A maximum value (161% of the basal phosphotransferase activity) was obtained after 6 min exposure to oxidized LDL (27 microgram/ml) using APRTPGGRR peptide substrate. The activatory effect was more pronounced (LDL 181%, oxLDL 201%) when MAPK of stimulated cells was immunoprecipitated with anti-p42MAPK antibodies and phosphotransferase activity was assayed in immune complexes. Stimulation produced by oxLDL was inhibited by poly I, fucoidan, dextran sulfate and by the MAPKK inhibitor PD 098059 but not by PMA-mediated depletion of PKC or by pre-treatment with chloroquine or with pertussis toxin. These results suggest a direct mitogenic effect of LDL which, in the case of oxLDL, is dependent on scavenger receptor ligation but not on G-protein mediated or PKC-dependent signal transduction.