Induction of Growth-related Metabolism in Human Vascular Smooth Muscle Cells by Low Density Lipoprotein

Oxidized LDL stimulates mitogen-activated protein kinases in smooth muscle cells and macrophages

It has been proposed that oxidized LDL is more atherogenic than native LDL. However, the mechanisms by which native LDL and oxidized LDL alter function of cells in the vessel wall remain undefined. A signal transduction pathway that mediates many changes in cell function is the mitogen-activated protein (MAP) kinase cascade. We therefore examined the effect of native LDL and oxidized LDL on MAP kinase activity in cultured vascular smooth muscle cells (VSMC), endothelial cells, and macrophages by using an in-gel-kinase assay and anti-phosphotyrosine MAP kinase antibodies. Native LDL and LDL oxidized by the addition of Cu2+ (Cu(2+)-oxidized LDL) stimulated MAP kinase in a time- and dose-dependent manner in baboon and rat VSMC but not in bovine endothelial cells. Cu(2+)-oxidized LDL stimulated MAP kinase in human monocyte-derived macrophages, but the effect was much greater in cells cultured for 7 days compared with 1 day, suggesting dynamic regulation of the cellular response to oxidized LDL. In rat VSMC, the maximal MAP kinase response to Cu(2+)-oxidized LDL was significantly greater than the response to native LDL. Cu(2+)-oxidized LDL was more potent, with half-maximal activation at 15 micrograms/mL versus 30 micrograms/mL for native LDL. Stimulation of MAP kinase appeared to involve protein kinase C, since phorbol ester pretreatment for 24 hours blocked MAP kinase activation. Oxidation of LDL by other methods showed that activation of MAP kinase was not well correlated with lipid peroxides or aldehydes, suggesting that other components present in oxidized LDL were responsible. The active moiety appeared to be lipid based on extraction of oxidized LDL with organic solvents. These data indicate that LDL stimulates MAP kinase in VSMC, oxidation of LDL potentiates the effect, a lipid moiety is involved, and Cu(2+)-oxidized LDL activation of MAP kinase is cell-type specific. These findings suggest a role for MAP kinase in the pathways by which oxidized LDL contributes to altered cellular function associated with atherogenesis.

Lipoprotein-induced prostacyclin production in endothelial cells and effects of lipoprotein modification

Although lipoprotein modification has been implicated in atherogenesis, the effect of modified forms of lipoproteins on vascular cell function has not been fully resolved. We have investigated lipoprotein-induced prostaglandin production by macrovascular endothelial cells. This study delineates early responses of endothelial cells after exposure to native and modified forms of the lipoproteins. Modification of lipoproteins by oxidation or glycation significantly affected the capacity of lipoproteins to induce prostacyclin (PGI2) production by bovine aortic endothelial cells (BAEC). Modified low-density lipoprotein (LDL) increased PGI2 production in the short term (up to 24 h), but oxidized LDL caused an inhibition of PGI2-producing capacity in longer term incubations (48-72 h). Glycated (Glc) high-density lipoprotein 3 (HDL3) caused higher production of PGI2 in the short term (4-24 h) but reached similar levels as HDL3 over time. Glycation of high-density lipoprotein 2 had no effect on the PGI2-producing capacity of the lipoprotein. Thus modification of the lipoproteins affects their potential to induce PGI2 production in endothelial cells, and this may have an influence on vascular function in disease states such as diabetes and atherosclerosis. Although the changes appear to contradict data from long-term in vivo studies, these results from in vitro studies may reflect the situation in very early lesion development. GlcLDL, while causing an increase in endothelial cell PGI2 production, may be involved in compromised endothelial function, since GlcLDL is prone to oxidation.

Cellular signaling and proliferative action of AVP in mesangium of SHR: effect of low density lipoprotein

The present study was undertaken to determine whether low density lipoprotein (LDL) modulates the cellular action of arginine vasopressin (AVP) in cultured glomerular mesangial cells of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The AVP-induced cellular signal transduction, including inositol 1,4,5-trisphosphate (IP3) production, fura-2 intracellular calcium measurements and cellular alkalinization, was significantly greater in cells of SHR than those of WKY. This is based on an increase in AVP V1 receptor number in cells of the SHR. Also, the AVP activation of mitogen-activated protein (MAP) kinase and [3H]thymidine incorporation was significantly exaggerated in cells of SHR compared with those of WKY. LDL at a concentration of 10 micrograms/ml augmented the cellular signaling and proliferative action of AVP in cells of WKY, but not in those of SHR. Since [3H]AVP receptor binding was not affected by the LDL pretreatment, LDL modulates the signal transduction between a location distal to the AVP receptors and proximal from the production of IP3 and diacylglycerol. These results indicate that an increase in AVP receptor capacity has a profound effect on the AVP-induced cellular signaling and proliferation, and that LDL has a slight alteration on the action of AVP in glomerular mesangial cells of SHR.

Studies on the influence of insulin on cyclic adenosine monophosphate in human vascular smooth muscle cells: dependence on cyclic guanosine monophosphate and modulation of catecholamine effects

Insulin increases both cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP) in human vascular smooth muscle cells (hVSMC) and attenuates noradrenaline-induced vasoconstriction. In the present study, we aimed at investigation in hVSMC: 1) the interrelationships between insulin-induced increases of cGMP and cAMP; 2) the insulin effect on the catecholamine modulation of cAMP. Catecholamines cause both vasoconstriction and vasodilation. Vasoconstriction is attributable to the reduced synthesis of cAMP in hVSMC through alpha 2-adrenoceptors and to direct effects on calcium fluxes through alpha 1-adrenoceptors; vasodilation is attributable to the increased synthesis of cAMP through beta-adrenoceptors. In the present study, we determined the influence of insulin on cAMP in hVSMC incubated with or without: a) the inhibitor of guanylate cyclase methylene blue or the inhibitor of nitric oxide synthase NG-monomethyl-L-arginine (L-NMMA); b) the beta-adrenergic agonists isoproterenol and salbutamol; c) the physiological catecholamines noradrenaline and adrenaline; d) noradrenaline+the beta-adrenergic antagonist propranolol or the alpha 2-adrenergic antagonist yohimbine; e) noradrenaline+methylene blue of L-NMMA. We demonstrated that: 1) the inhibition of the insulin-induced cGMP synthesis blunts the insulin-induced increase of cAMP; 2) insulin induces a significant increase of cAMP also in the presence of isoproterenol, salbutamol, noradrenaline and adrenaline: the combined effects of insulin and catecholamines were additive in some, but not in all the experiments; 3) insulin enhances the cAMP concentrations induced by noradrenaline also in the presence of alpha 2- or beta-adrenergic antagonists; 4) in the presence of methylene blue or L-NMMA insulin does not modify the noradrenaline effects on cAMP.

Impaired radial artery compliance in normotensive subjects with familial hypercholesterolemia

Hypercholesterolemia impairs arteriolar dilatation, but whether the vascular abnormalities accompanying this condition include large artery function is unknown. We addressed this issue in 13 normotensive subjects with familial hypercholesterolemia (serum cholesterol 401.6 +/- 16.9 mg/dl, mean +/- S.E., FHC) and no evidence of atherosclerotic lesions, in whom radial artery (RA) diameter and blood pressure (BP) were measured beat to beat by an echotracking and a Finapres device, respectively. RA compliance (RAC) was derived from the diameter/BP relationship and expressed over the systo-diastolic BP range, both at baseline and after a 12-min brachial artery occlusion. RAC was expressed also as the area under the RAC/BP curve divided for pulse BP. Measurements included maximal forearm blood flow (plethysmography) and minimal forearm vascular resistance (FVR) which were obtained from the values following the 12-min brachial arterial occlusion. Data were collected before and after 6- and 24-month lipid lowering treatment (simvastatin 40 mg/day). Ten age-matched normotensive normocholesterolemic healthy subjects (N) served as controls. Compared to N, baseline RAC was strikingly reduced in FHC (-53.5%, P < 0.01). After ischemia RAC increased significantly and markedly in N (+38.7, P < 0.01), while only a modest and non-significant increase was observed in FHC. Minimal FVR was markedly higher in FHC than in N (3.5 +/- 0.9 vs 1.6 +/- 0.1 units, P < 0.01). In FHC (7 subjects) RAC remained unchanged after 6 months of lipid lowering treatment, but increased markedly (+55.2%, p < 0.05) when treatment was prolonged to 24 months. Lipid lowering treatment also reduced minimal FVR, the effect being significant both after 6 and after 24 months. No changes in RAC and minimal FVR were seen after 6 months in controls. Thus, in subjects with a marked increase in serum cholesterol due to FHC, not only arteriolar dilatation, but also RAC and distensibility are markedly impaired. This impairment can be favourably affected by an effective lipid lowering treatment of long duration.

Induction of human endothelial cell growth by mildly oxidized low density lipoprotein

The aim of the present study was to examine whether endothelial growth could be modulated by mildly oxidized low density lipoprotein. When human endothelial cells were cultured in the presence of mildly oxidized low density lipoprotein (1 microgram/ml), a significant induction of endothelial cell growth was observed, whereas native low or high density lipoprotein were ineffective. Further, treatment of endothelial cells with mildly oxidized low density lipoprotein modulated the expression of cytokines and growth factors which may be relevant in atherogenesis. Endothelial cells chronically exposed to mildly oxidized low density lipoprotein underwent a more rapid onset of cellular senescence. Since senescence is associated with endothelial dysfunction, the novel finding showing that mildly oxidized low density lipoprotein induces endothelial cell growth may be relevant in the development and evolution of the atherosclerotic lesions.

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.

Oxidized low-density lipoprotein stimulates protein kinase C (PKC) and induces expression of PKC-isotypes via prostaglandin-H-synthase in P388D1 macrophage-like cells

Treatment of cells with LPS-free oxLDL significantly enhanced protein kinase C (PKC) activity in cell extracts from P388D1 macrophage-like cells as determined by phosphorylation of histone H1 or Ac-MBP[4-14] substrate peptide. This effect was abolished by the PKC inhibitors H-7 and bisindolylmaleimide I while pertussis toxin failed to block stimulation. The phosphotransferase activity was also increased by acetylated LDL (acLDL) and maleylated albumin (malBSA), the oxLDL effect was inhibited by chloroquine which also blocked oxLDL-induced stimulation of tyrosine kinase activity. Marginal stimulation of PKC activity was observed when lipid extracts from oxLDL were used, indicating that uptake via scavenger receptors (SR) is mandatory. Polyinosinic acid (poly I) exhibited a concentration-dependent inhibition of the oxLDL-induced effect suggesting that SR II/I but not CD36 interactions are critical to PKC activation. Modified (lipo)proteins increased the concentration of diacylglycerol and differentially affected the levels of individual PKC isoenzymes predominantly in the cytosolic fraction. Changes of activity induced by oxLDL could be primarily assigned to alterations of the activities and levels of the isoenzymes beta and delta. Treatment with oxLDL, acLDL, and malBSA was also accompanied by increased production of prostaglandins as well as by an enhanced level of cyclooxygenase 2 (COX 2) as determined by Western blot analysis. Effects (correction) of oxLDL on PKC activity/expression was suppressed by the cyclooxygenase, 2,2-dimethyl-6-(4-chlorophenyl)-7-phenyl-2,2-dihydro-1H-pyrrolizine-5- ylacetic acid (ML 3000), and by treatment with the specific COX 2-inhibitor N-(2-cyclohexyloxy-4-nitrophenyl) methane-sulfonamide (NS-398). These results indicate that oxLDL, acLDL, and malBSA exhibit a COX 2-dependent and isotype specific effect on PKC in P388D1 cells following uptake via SR II/I and subsequent lysosomal degradation.

Nonenzymatic glycation of fibronectin impairs adhesive and proliferative properties of human vascular smooth muscle cells

Nonenzymatic glycation of proteins is involved in the pathogenesis of diabetes vascular complications. Extracellular matrix proteins are a prominent target for nonenzymatic glycation because of their slow turnover rates. The aim of this study was to investigate the influence of human fibronectin (F) nonenzymatic glycation on adhesion and proliferation of cultured human vascular smooth muscle cells (hVSMC). Incubation of human F with 500 mmol/L D-glucose at 37 degrees C induced a time-dependent increase in fluorescence detectable at 440 nm after excitation at 363 nm. Nonenzymatic glycation did not affect binding of F itself to the plates. Adhesion of hVSMC to F increased with the increase of incubation time of the cells on the protein from 30 minutes up to 120 minutes and remained stable thereafter. Adhesion to glycated fibronectin (GF) was reduced in comparison to control F at all the different adhesion times. Adhesion of hVSMC to GF was reduced when F was exposed to glucose for 4, 9, or 28 days (P=.0417 to .0025), but not when F was exposed for 1 day. Adhesion of hVSMC to GF was reduced compared with adhesion to nonglycated F at all coating concentrations from 0.2 to 10 micrograms/mL (P=.05 to .014). Thus, nonenzymatic glycation of F impairs adhesion of hVSMC in vitro. Proliferation of hVSMC on F increased with increasing concentrations of the protein as coating agent (ANOVA:P<.0001 for both nonglycated F and GF). Proliferation with F glycated for 4, 9, and 28 days was reduced at concentrations of 1, 3, and 10 micrograms/mL as compared with proliferation with nonglycated F (P=.0253 to .0001). Proliferation on F glycated for only 1 day was not significantly reduced. When the number of hVSMC plated on control F was reduced by 25% to take into account the reduced adhesion, the number of cells that proliferated on F was still reduced. In conclusion, nonenzymatic glycation of F impairs adhesive and proliferative properties of hVSMC.

Differentiated properties and proliferation of arterial smooth muscle cells in culture

The smooth muscle cell is the sole cell type normally found in the media of mammalian arteries. In the adult, it is a terminally differentiated cell that expresses cytoskeletal marker proteins like smooth muscle alpha-actin and smooth muscle myosin heavy chains, and contracts in response to chemical and mechanical stimuli. However, it is able to revert to a proliferative and secretory active state equivalent to that seen during vasculogenesis in the fetus, and this is a prerequisite for the involvement of the smooth muscle cell in the formation of atherosclerotic and restenotic lesions. A similar transition from a contractile to a synthetic phenotype occurs when smooth muscle cells are established in culture. Accordingly, an in vitro system has been used extensively to study the regulation of differentiated properties and proliferation of these cells. During the first few days after seeding, the cells are reorganized structurally with a loss of myofilaments and formation of a widespread endoplasmic reticulum and a prominent Golgi complex. In parallel, they lose their contractility and instead become competent to divide in response to a large variety of mitogens, including platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF). After entering the cell cycle, they start to produce these and other mitogens on their own, and continue to replicate in the absence of exogenous stimuli for a restricted number of generations. Furthermore, they start to secrete extracellular matrix components such as collagen, elastin, and proteoglycans. The mechanisms that control this change in morphology and function of the smooth muscle cells are still poorly understood. Adhesive proteins such as fibronectin and laminin apparently have an important role in determining the basic phenotypic state of the cells and exert their effects via integrin receptors. The proliferative and secretory activities of the cells are influenced by a multitude of growth factors, cytokines, and other molecules. Although much work remains before an integrated view of this regulatory machinery can be achieved, there is no doubt that the cell culture technique has contributed substantially to our knowledge of smooth muscle differentiation and growth. At the same time, it has been crucial in exploring the role of these cells in vascular disease and developing new therapeutic strategies to cope with major causes of human death and disability.

Sodium butyrate inhibits platelet-derived growth factor-induced proliferation of vascular smooth muscle cells

Sodium butyrate (SB), a naturally occurring short-chain fatty acid, was investigated for its therapeutic value as an antiproliferative agent for vascular smooth muscle cells (SMCs). At 5-mmol/L concentration, SB had no significant effect on rat SMC proliferation. However, at the same concentration, SB inhibited platelet-derived growth factor (PDGF)-AA-, -AB-, and -BB-induced proliferation of SMCs. Exposure of SMCs to PDGF-BB resulted in activation of receptor intrinsic tyrosine kinase activity and autophosphorylation of beta-PDGF-receptor (beta-PDGFR). The activated beta-PDGFR physically associated and phosphorylated signaling molecules such as ras-GTPase activating protein (GAP) and phospholipase C gamma (PLC gamma). SB, in the absence of PDGF-BB, caused neither beta-PDGFR tyrosine phosphorylation nor phosphorylation and association of GAP and PLC gamma with beta-PDGFR. PDGF-BB-enhanced activation of receptor intrinsic tyrosine kinase activity and autophosphorylation of tyrosine residues of beta-PDGFR were unaffected by SB irrespective of whether SMCs were preincubated with SB before exposure to PDGF-BB plus SB or incubated concomitantly with PDGF-BB plus SB. Likewise, phosphorylation and association of GAP and PLC gamma with PDGF-BB-activated beta-PDGFR were unaffected. In addition, SB did not block PDGF-BB-stimulated, PLC gamma-mediated production of inositol triphosphate. Similarly, PDGF-BB-induced beta-PDGFR degradation was unaffected when SMCs were exposed to PDGF-BB plus SB, and SB by itself had no influence on beta-PDGFR degradation. Unlike beta-PDGFR kinase activity, mitogen-activated protein kinase (MAP-kinase) activity was stimulated by SB by about 2.7-fold. Exposure of SMCs to PDGF-BB caused an approximately 11.4-fold increase in MAP-kinase activity and this increase in activity was not significantly affected when cells were coincubated with PDGF-BB and SB (10.3-fold). However, pretreatment of SMCs with SB for 30 minutes and subsequent incubation in PDGF-BB plus SB abolished most of the PDGF-BB-induced MAP-kinase activity (4.6-fold). Transcription of growth response genes such as c-fos, c-jun, and c-myc were induced by PDGF-BB, and their induction was suppressed, particularly c-myc, by incubating SMCs with PDGF-BB plus SB. Similarly, preincubation of cells with SB for 30 minutes and subsequent incubation in PDGF-BB plus SB diminished PDGF-BB-induced transcription of c-fos, c-jun, and c-myc. However, SB by itself had no significant effect on c-fos, c-jun, and c-myc transcription.(ABSTRACT TRUNCATED AT 400 WORDS)

Human Lp(a): regions in sequences of apoproteins similar to domains in signal transduction proteins

The major apoproteins of Lp(a)--apo(a) and apo B-100--are linked by only one intermolecular disulfide bond. This linkage has been suggested to be located between apo(a) Cys4057 and apo B-100 Cys3734. Several studies, however, have suggested other noncovalent interactions between different regions of apo(a) and apo B-100. One possible mechanism for these interactions may involve the apo(a) proline-rich interkringle regions that share sequence similarities with the proline-rich regions of Src homology 3 (SH3) domain-binding proteins such as 3BP-1. SH3 and SH2 domains, and their respective ligands, proline-rich regions, and phosphotyrosine motifs, are noncatalytic segments common to signal transduction proteins. Therefore, we used sequence comparison algorithms and molecular modeling programs to identify corresponding SH3 and SH2 candidate regions as well as potential phosphotyrosine sites in the apo B-100 sequence. Six SH2 and 16 SH3 candidate regions, along with 21 potential phosphotyrosine sites, are contained in the apo B-100 sequence. In Lp(a), these regions of apo B-100 may be involved in the noncovalent, protein-protein interactions between apo(a) and apo B-100. The presence of candidate SH3 and SH2 regions in apo B-100, and potential phosphotyrosine sites in apo B-100, apo(a), and apo A-I, suggests an alternative signaling pathway unrelated to the known B/E receptor.

Evidence that apoB-100 of low-density lipoproteins is a novel Src-related protein kinase

Protein-tyrosine kinases of signal transduction pathways occur and function intracellularly. In contrast, the low-density lipoprotein (LDL) particle circulates in plasma, where its function is to solubilize and transport lipid. Recently, several reports showed that LDL may have a role in signal transduction. We have identified a region in the apoB-100 primary structure which shows similarity to Src-homology-1 (SH1) domains, the kinase region of protein-tyrosine kinases. Results obtained in protein kinase assays of highly purified LDL showed that only the apoB-100 was phosphorylated, suggesting that apoB-100 has the capacity to undergo autophosphorylation like known protein-tyrosine kinases. Phosphorylation was not observed for any other apolipoprotein in LDL or for any component of high-density lipoprotein and lipoprotein [a]. Our results suggest that apoB-100 may be a novel and functional member of the src protein kinase family.

High-density lipoprotein and low-density lipoprotein-mediated signal transduction in cultured human skin fibroblasts

The signalling mechanisms whereby high-density lipoproteins (HDL) and low-density lipoproteins (LDL) affect a number of cellular functions in fibroblasts are unclear. This study has analyzed the influence of HDL3 and LDL on the phosphatidylinositol specific phospholipase C pathway in human skin fibroblasts. Exposure of myo-[2-3H]-inositol prelabelled fibroblasts to HDL3 or LDL elicited major increases in IP1 and minor increases in IP2 and IP3 within 30 s. In fura-2 loaded suspended fibroblasts, HDL3 and LDL increased intracellular Ca2+ concentrations ([Ca2+]i) with comparable rapid, transient kinetics. The dose-profiles for HDL3- and LDL-induced increases in [Ca2+]i were also comparable, with half-maximally and maximally effective concentrations being approximately 15 micrograms/mL and approximately 50 micrograms/mL, respectively. HDL3- and LDL-induced increases in [Ca2+]i were diminished by approximately 60% (vs. control fibroblasts) in thapsigargin-pretreated fibroblasts, indicating that release of Ca2+ from intracellular pools is the major contributor toward lipoprotein-induced increases in [Ca2+]i. Pertussis toxin-pretreatment of cells completely abolished lipoprotein induced Ca(2+)-transient, indicating the involvement of a guanine nucleotide-binding protein in the signalling process. In [3H]-palmitic acid-prelabelled fibroblasts, both HDL3 and LDL were observed to stimulate production of DAG. Activation of protein kinase C (PKC) was analysed by determining the cytosol-to-membrane translocation of both enzymatic activity and immunoreactivity of specific PKC isoforms (alpha, delta, epsilon, and zeta). Stimulation with HDL3 and LDL evoked a rapid (within 2.5 min) translocation of PKC activity, with PKC alpha and PKC epsilon being the isoforms translocated. It is concluded that HDL3 and LDL acutely stimulate a phosphoinositide-specific phospholipase C pathway in human skin fibroblasts. However, the specific cell membrane events mediating this signal transduction remain to be further elucidated.

Insulin increases cyclic nucleotide content in human vascular smooth muscle cells: a mechanism potentially involved in insulin-induced modulation of vascular tone

It has been suggested that insulin exerts a vasodilating effect, but the mechanisms involved are not completely understood. Since cyclic nucleotides mediate the vasodilation induced by endogenous substances, such as prostacyclin and nitric oxide, we aimed to investigate the influence of insulin (concentration range 240-960 pmol/l) on both cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) content in human vascular smooth muscle cells. Insulin dose-dependently increased both nucleotides (cAMP: from 0.7 +/- 0.1 to 2.6 +/- 0.4 pmol/10(6) cells, p = 0.0001; cGMP: from 1.3 +/- 0.2 to 3.4 +/- 0.7 pmol/10(6) cells, p = 0.033). This increase is receptor-mediated, since it was blunted when cells were preincubated with the tyrosine kinase inhibitor genistein. The effect of insulin remained significant (p = 0.0001) when preincubation with the phosphodiesterase inhibitor theophylline prevented cyclic nucleotide catabolism. The increase of cGMP was blunted when the cells were preincubated with the guanylate cyclase inhibitor methylene blue, and with the nitric oxide-synthase inhibitor NG-monomethyl-L-arginine. At all the concentrations tested, insulin potentiated the increase of cAMP induced by the stable prostacyclin analogue Iloprost (p = 0.0001), whereas only at 1920 pmol/l did it potentiate the cGMP increase induced by glyceryltrinitrate (p = 0.05). This study demonstrates that the vasodilating effects exerted by insulin may at least in part be attributable to an increase of both cGMP and cAMP via a receptor-mediated activation of adenylate and guanylate cyclases in human vascular smooth muscle cells and that the insulin effect on cGMP is mediated by nitric oxide.

Plasma lipids and lipoproteins and essential hypertension

In recent years there have been many studies demonstrating a correlation between increased arterial blood pressure and altered lipid profiles, and there has been an especially positive correlation between high cholesterol levels and blood pressure. There are differences between the various reports that are important. In our study the lipid distribution in 105 hypertensive patients with mild or moderate arterial hypertension according to WHO criteria without clinically or ultrasonographically apparent atherosclerosis was compared to the lipid distribution in 65 age-matched healthy persons. On the epidemiological level a significant, positive association was found between LDL serum levels (P < or = 0.001), Apo B serum levels (P < or = 0.001), serum triglyceride levels (P < or = 0.05) and VLDL serum levels (P < or = 0.01) and arterial hypertension. However, in contrast to recent reports, no significant difference was found between total serum cholesterol levels in normotensives and hypertensives, and there was no difference in HDL serum levels. No evidence could be found for a significant increase in lipoprotein (a) serum levels in hypertensives.

The role of lipids in nephrosclerosis and glomerulosclerosis

Hyperlipidemia and lipoprotein abnormalities are often encountered in patients with nephrotic syndrome or chronic renal disease and also in those undergoing haemodialysis and with renal transplant. Even though the significance of lipid deposition in renal tissue and the role of lipoproteins in the pathogenesis of renal disease in man is unclear, experimental and clinical data indicate a possible damaging effect of a disturbed lipid metabolism on the kidney. In humans, glomerular lipid deposition is observed in genetic diseases such as Fabry's disease, lecithin:cholesterol acyltransferase activity (LCAT) deficiency and arteriohepatic dysplasia, and in diseases with acquired disturbance of lipid metabolism such as nephrotic syndrome and cholestatic liver disease. Studies on animals with lupus nephritis, aminonucleoside nephrosis, reduced renal mass, diabetes mellitus or systemic hypertension have shown that cholesterol can increase the incidence of glomerulosclerosis. As most of these studies have been performed in the rat, which has a different lipoprotein profile to that of man, these results should be carefully interpreted with regard to their relevance for humans. In vitro cell culture studies on human glomerular cells have given some preliminary insights into the cellular mechanisms of lipid induced glomerular damage. Apo E-containing lipoproteins, which are pathologically elevated in many renal diseases, are avidly taken up by human mesangial cells. These cells seem to play a central role in the initiation of glomerulosclerosis by inducing proliferation and production of excess extracellular matrix. Lipoproteins are able to stimulate DNA synthesis in these cells, and increase the synthesis of mitogens and extracellular matrix protein. The pathogenic role of oxidized lipoproteins has not yet been defined. Human mesangial cells do not seem to take up these modified lipoproteins. However, macrophages infiltrate glomeruli and may constitute the stimulus for the generation of minimally modified lipoproteins and their cellular uptake. The data from animal experiments suggest that treatment that corrects hyperlipidemia may have an ameliorative effect on renal function. Thus, there are strong indications that lipoproteins may play a critical role in mediating the development of glomerulosclerosis.

The role of cholesterol accumulation in prosthetic vascular graft anastomotic intimal hyperplasia

PURPOSE

To demonstrate that modulation of plasma cholesterol concentrations affects prosthetic vascular graft anastomotic intimal hyperplasia (AIH), aortic grafts were examined histologically and biochemically in 41 rabbits.

METHODS

Twenty-seven rabbits were fed standard rabbit diet, whereas 14 were fed cholesterol-supplemented diet to induce hypercholesterolemia.

RESULTS

A smooth muscle cell proliferative response, similar to AIH in humans, was seen equally at the proximal and distal anastomoses. However, surface area and thickness of AIH were significantly greater in rabbits with hypercholesterolemia. Anastomotic tissue cholesterol concentrations were fifteenfold higher in rabbits with hypercholesterolemia than in rabbits with normal cholesterol concentrations and anastomotic cholesterol concentrations were fivefold higher than in the aorta away from the graft in rabbits with hypercholesterolemia. Preferential deposition of radioiodinated dilactitol tyramine coupled to low-density lipoproteins, but not albumin, was demonstrated in anastomotic areas and grafts of rabbits with normal cholesterol concentrations as well. Surface area and thickness of AIH correlated closely with plasma and tissue cholesterol concentrations.

CONCLUSIONS

Oxidized products of lipoproteins have been shown to stimulate production of growth factors that cause smooth muscle cell proliferation, migration, and synthetic function. It is likely they play an important part in prosthetic vascular graft AIH, similar to their role in atherogenesis.

Concerted effects of lipoproteins and angiotensin II on signal transduction processes in vascular smooth muscle cells

Low-density (LDL) and high-density (HDL3) lipoproteins dose-dependently activate phosphoinositide turnover and elevate cytosolic free Ca2+ concentrations ([Ca2+]i) in cultured vascular smooth muscle cells (VSMCs) from either human (microarterioles and aorta) or rat (aorta) sources. High-performance liquid chromatography analysis of cell extracts revealed comparable spectra of inositol phosphate isomers generated in response to either LDL, HDL3, or angiotensin II (Ang II). Thus, lipoproteins and Ang II may use similar, if not identical, signal transduction pathways for the generation and metabolism of inositol phosphates and intracellular Ca2+ mobilization in VSMCs. When Ang II was added in combination with either LDL or HDL3, the phosphoinositide and [Ca2+]i responses of VSMCs were either equal to or even greater than the sum of the effects elicited by the agonists individually. This additivity/synergy between Ang II and the lipoproteins was not accompanied by alteration in the half-maximally effective dose requirements of VSMCs for either Ang II (approximately 2 nmol/L, with or without lipoproteins) or lipoproteins (approximately 50 micrograms/mL for LDL and HDL3, with or without Ang II). Neither short-term (up to 10 minutes) nor long-term (48 hours) exposure of VSMCs to lipoproteins caused desensitization of phospholipase C and intracellular Ca2+ mobilization responses to either Ang II or lipoproteins. Since constant exposure of VSMCs to lipoproteins is a physiological circumstance, and because elevation of [Ca2+]i and activation of phosphoinositide turnover are pivotal events for VSMC contraction and growth, we suggest that the low concentrations of lipoproteins in the vessel intima may play an important role in regulating the response of the vasculature to Ang II.

Insulin influences immunoreactive endothelin release by human vascular smooth muscle cells

This study sought to investigate whether insulin influences immunoreactive endothelin release from cultured human vascular smooth muscle cells. For this purpose, we incubated cultured vascular smooth muscle cells (VSMC) obtained from human microvessels with 0, 80, and 320 microU/mL insulin with or without arginine vasopressin (10 nmol/L) and angiotensin II (10 nmol/L). After 6 hours, the culture supernatant was collected and immunoreactive endothelin was determined by radioimmunoassay. Insulin at a concentration of 320 microU/mL induced a significant increase of immunoreactive endothelin levels in medium (from 15.2 +/- 0.8 to 20.6 +/- 0.8 pg/200 microL, P < .01) and potentiated arginine vasopressin- and angiotensin II-induced immunoreactive endothelin release (P < .0001 and P < .04, respectively). Insulin at a concentration of 80 microU/mL did not induce a significant increase of spontaneous immunoreactive endothelin release, but significantly increased the effects of arginine vasopressin (P < .05). In conclusion, insulin influences immunoreactive endothelin release from human VSMC in culture.

Increased synthesis of plasminogen activator inhibitor-1 by cultured human endothelial cells exposed to native and modified LDLs. An LDL receptor-independent phenomenon

The effects of native and acetylated low density lipoproteins (LDLs and acetyl-LDLs, respectively) on the release of plasminogen activator inhibitor type 1 (PAI-1) by cultured human umbilical vein endothelial cells (ECs) were evaluated. LDL and acetyl-LDL incubated with ECs for 16-18 hours increased the PAI-1 antigen levels in conditioned medium. At a concentration of 100 micrograms/mL, LDL and acetyl-LDL increased PAI-1 by 10.8 and 12.0 ng/mL, respectively (p < 0.05 and p < 0.01 versus control). The increases in PAI-1 antigen levels exerted by the lipoproteins paralleled the changes in PAI-1 activity. The effect of LDL and acetyl-LDL was concentration dependent and specific for PAI-1 because tissue-type plasminogen activator and expression of procoagulant activity were not affected by either lipoprotein. In addition, total protein synthesis evaluated in [35S] methionine-labeled ECs was not affected, and studies with cycloheximide showed that the effect of LDL and acetyl-LDL on PAI-1 release was due to de novo protein synthesis. Experiments using the C7 monoclonal antibody against the LDL receptor and binding-defective LDL indicated that the effect of LDL on the synthesis of PAI-1 was not dependent on the interaction of the LDLs with their specific receptors. Finally, extensive oxidation of LDL prevented and even reversed the effect of LDL on PAI-1 release by ECs. It is concluded that LDL specifically increases the synthesis of PAI-1 by ECs with mechanisms that are not receptor mediated.

Phosphoinositide and calcium signalling responses in smooth muscle cells: comparison between lipoproteins, Ang II, and PDGF

The effects of low density lipoprotein (LDL) and high density lipoprotein (HDL3) on second messenger systems were investigated in cultured human vascular smooth muscle cells (VSMC) and compared with those of angiotensin II (Ang II) and platelet-derived growth factor (PDGF-BB). Phosphoinositide metabolism was studied in myo-[2-3H]-inositol prelabelled VSMC using high performance liquid anion-exchange chromatography. The spectra of inositol phosphate isomers increased after stimulation with either Ang II, LDL, HDL3 or PDGF-BB were qualitatively identical. Major increases occurred in 4-IP1, 1,4-IP2, 1,3,4-IP3 and 1,3,4,5-IP4. These are metabolic conversion products of 1,4,5-IP3 for which only a minor increase was found. Thus lipoproteins, like Ang II and PDGF-BB, activate polyphosphatidylinositol-specific phospholipase C. Intracellular Ca2+ concentrations ([Ca2+]i) were studied in fura-2 loaded VSMC. In monolayer cultures LDL and HDL3 increased [Ca2+]i with kinetics comparable to those for Ang II. Relative to the effects of these agonists, the PDGF-BB-induced increase in [Ca2+]i was slower in onset and the decay from peak [Ca2+]i levels more gradual. Fluorescence recordings from single cells exposed to LDL and HDL3 revealed a prolonged series of transient oscillations of [Ca2+]i, a phenomenon typical for calcium-mobilizing hormones. Additionally, as found for Ang II, preincubation of VSMC with either phorbol 12-myristate, 13-acetate, forskolin or 8-bromo-cyclic GMP inhibited LDL- and HDL-induced accumulation of [3H]-inositol monophosphate. We propose that LDL and HDL3 stimulate signal transduction in VSMC via mechanisms analogous to those of Ca(2+)-mobilizing hormones.

Platelet-derived growth factor suppresses and fibroblast growth factor enhances cytokine-induced production of nitric oxide by cultured smooth muscle cells. Effects on cell proliferation

Stimulation of thymidine incorporation by basic fibroblast growth factor or epidermal growth factor treatment of cultured quiescent smooth muscle cells (rat and human) was attenuated by the concomitant treatment with interleukin-1 beta in the presence of indomethacin. Platelet-derived growth factor-AB and -BB-induced thymidine incorporation was not inhibited by the presence of the cytokine under similar experimental conditions. Elevation of nitrite levels in the conditioned medium of cultures exposed to interleukin-1 beta correlated with the inhibition of thymidine incorporation. Platelet-derived growth factor-AB and -BB inhibited the production of nitric oxide (measured as nitrite levels in conditioned medium) by cells treated simultaneously with interleukin-1 beta and growth factor. However, platelet-derived growth factor-AA neither affected nitrite production nor thymidine incorporation by smooth muscle cells. Levels of cytokine-stimulated nitrite production by smooth muscle cells were increased synergistically by the presence of fibroblast growth factors or epidermal growth factor. The inhibition of thymidine incorporation and concomitant elevation of nitrite production was abolished in the presence of nitro-L-arginine. Cultures maintained in the presence of low levels of the cytokine for 9 days were growth-inhibited, and this was reversed when culture medium was supplemented with nitro-L-arginine. The treatment of smooth muscle cells, which were grown in coculture inserts with the cytokine to induce nitric oxide production, before their combination with other quiescent layers of cells resulted in the inhibition of thymidine incorporation by this second layer of cells regardless of the growth factor used for stimulation. Nitric oxide may act as an endogenous inhibitor of smooth muscle cell proliferation in the vessel wall, and impairment of its production may be one action of potent vascular mitogens such as platelet-derived growth factor.

Lipoprotein receptors and steroidogenesis in adrenocortical cells

Steroid-producing tissues require a continuous supply of cholesterol for hormone synthesis. In the majority of the steroidogenic tissues the cholesterol is imported via the receptor-mediated uptake of lipoproteins, and therefore the influence on the lipoprotein receptors provides an additional level for the regulation of hormone synthesis. Hormones regulating the adrenocortical activity exert both short- and long-term action, and thus they may control the interactions of the major cholesterol delivery particles--low- (LDLs) and high-density lipoproteins (HDLs)--and their receptors in short- and long-term action, possibly modulating the signal transduction in the former case and the number and distribution in the latter. The LDL and HDL pathway and the signal transduction mechanism is briefly reviewed. Data are discussed concerning short- and long-term action of hormones (alpha-MSH and ACTH, respectively) on the HDL3 receptors of isolated adrenocortical cells. Short-term treatment with alpha-MSH and long-term treatment with ACTH increased the binding of HDL3 to zona glomerulosa and fasciculata cells, respectively, while both treatments increased the hormone production in the presence of HDL. The lipoprotein receptors were frequently found on the microvilli of adrenocortical cell membranes.

Reversible desensitization of fibroblasts to cadmium receptor stimuli: evidence that growth in high zinc represses a xenobiotic receptor

The xenobiotic Cd2+ triggers the production of inositol trisphosphate and releases stored Ca2+ in certain cell types, apparently by binding to a zinc site in the external domain of an "orphan" receptor (no known endogenous stimulus). Cd2+ and bradykinin evoke similar spikes in cytosolic free Ca2+. Growth in high Zn2+ (100-200 microM) abolished the free Ca2+ spike evoked by Cd2+ without affecting the spike produced by bradykinin. Growth in high Zn2+ almost abolished Cd(2+)-evoked production of [3H]inositol mono-, bis-, and trisphosphate. Bradykinin-evoked [3H]inositol phosphate production was not affected by growth in high Zn2+. Growth in high Zn2+ nearly prevented the stimulation of 45Ca2+ efflux by Cd2+ without affecting the stimulation of 45Ca2+ efflux by bradykinin or histamine. Removing Zn2+ from the culture medium and incubating the cells for several hours fully restored responsiveness to Cd2+. Cycloheximide, actinomycin D, or tunicamycin prevented the restoration of Cd2+ responsiveness, indicating that resensitization requires macromolecular synthesis. Growth in high Zn2+ reversibly abolished Ca2+ mobilization evoked by two additional stimuli: a decrease in extracellular pH or Na+ concentration. These findings support the hypothesis that the three stimuli (Cd2+ or a decrease in external pH or Na+ concentration) activate the same orphan receptor. Growth in high Zn2+ apparently desensitizes the cells to the Cd2+ receptor stimuli by repressing receptor synthesis.

Oxidized low density lipoproteins stimulate phosphoinositide turnover in cultured vascular smooth muscle cells

Atherogenesis is associated with alterations in the properties of different cell types, including monocytes/macrophages (foam cell formation), platelets (increased aggregation), endothelial cells (injury), and smooth muscle cells (SMCs) (lipid accumulation or foam cell formation). Oxidized low density lipoproteins (ox-LDL) play a key role in this vascular pathology. This study investigated the ability of ox-LDL to elicit chemical signaling events in cultured human vascular smooth muscle cells (VSMCs). Ox-LDL was found to stimulate phospholipase C-mediated phosphoinositide turnover in human VSMCs. This response occurred rapidly (within 1 minute) and at low concentrations of ox-LDL (half-maximal effective concentration, approximately 5 micrograms/ml). Ox-LDL-stimulated inositol phosphate accumulation in human VSMCs was inhibited by pretreatment of cells with phorbol 12-myristate 13-acetate and with compounds that elevate cyclic AMP or cyclic GMP. Ca2+ antagonists also blocked the effects of ox-LDL on phosphoinositide turnover. Inhibitors of receptor-endocytotic processes (including receptor clustering, cross-linking, and cytoskeleton-dependent internalization) effectively prevented ox-LDL-induced inositol phosphate generation. The data suggest that ox-LDL promotes phospholipase C-mediated phosphoinositide turnover in a manner analogous to that for other Ca(2+)-mobilizing hormones. The results also support an association between phosphoinositide turnover and receptor-mediated endocytosis. Prevention of the direct effects of ox-LDL on SMCs could prove an interesting therapeutic avenue for the prevention of atherosclerosis.

Lysolecithin actions on vascular smooth muscle cells

Oxidation of low density lipoprotein increases its atherogenic potential. During oxidation there is an extensive conversion of lecithin to lysolecithin. In rat aortic smooth muscle cells, 2-25 micrograms/ml lysolecithin elevated cytosolic calcium concentration up to 560%. Lysolecithin (10-20 micrograms/ml) increased [3H]thymidine incorporation from 15 cpm/mg cell protein (controls) up to 189 cpm/mg cell protein. Lysolecithin (10 micrograms/ml) potentiated the PDGF-induced (50 ng/ml) [3H]thymidine incorporation up to 6.3 times. The results indicate that lysolecithin could induce mechanisms, by which oxidized low density lipoproteins could promote cell growth and thus contribute to atherosclerosis.

The effects of low density lipoprotein and high density lipoprotein on phosphoinositide hydrolysis in bovine aortic endothelial cells

Low density lipoprotein (LDL) and high density lipoprotein (HDL3) were tested for their ability to induce inositol phospholipid turnover and inositol phosphate production in bovine aortic endothelial cells (BAEC). The production of inositol phosphates following hydrolysis of the phosphoinositides was demonstrated by two methods; release of [3H]inositol phosphates after labelling with [3H]myo-inositol and by a direct binding assay for inositol 1,4,5-trisphosphate (InsP3). Acute exposure to LDL induced InsP3 release at low concentrations of the lipoprotein within the physiological range of LDL in tissues. HDL3 did not cause any release of the inositol phosphates. Pre-incubation of BAEC with HDL3 suppressed bradykinin- and LDL-induced inositol phosphate production in BAEC in a concentration-dependent manner. It is concluded that LDL acutely stimulates phosphoinositide breakdown and that pre-incubation of cells with HDL3 inhibits this effect. The mechanism responsible for these effects remains to be elucidated.

Modified low density lipoproteins activate human macrophages to secrete immunoreactive endothelin

This study attempted to determine if low density lipoproteins (LDL) induce the production of endothelins (ET) by human macrophages. Non-protected LDL from macrophage induced oxidation (n-LDL), copper-oxidized LDL (Ox-LDL), acetylated-LDL (Ac-LDL), butylated hydroxytoluene-LDL (BHT-LDL), BHT-Ac-LDL, polyinosinic acid (PiA, 1.5 micrograms/ml), phorbol myristate acetate (PMA; 0.5 microM) and BHT alone (20 microM) were studied. The different compounds had the following potency to stimulate the ET secretion: PMA greater than Ox-LDL greater than Ac-LDL greater than n-LDL greater than BHT-LDL greater than PiA greater than PiA + Ac-LDL greater than BHT. In conclusion, modified LDL stimulated ET secretion by human macrophages.

Release of nitric oxide from human vascular smooth muscle cells

It has recently been shown that nitric oxide (NO) or a labile NO-releasing compound is produced in endothelial cells. In the present study we measured the NO-release from human vascular smooth muscle cells in culture. The cells released an average 2.2 x 10(-9) moles nitric oxide per 10(8) cells in ten minutes with a large variation between different cell lines and passages without stimulators. The NO-release was markedly reduced by the inhibitor of NO-formation NG-momomethyl-L-arginine (L-NMMA, 10(-5) M) to 3% of the control levels (p less than 0.02, n = 4), but unaffected by acetylcholine, bradykinin or endothelin -1, -2 or -3. In a microperfusion system the compound released from human vascular smooth muscle cells caused profound relaxation of isolated rat mesenteric resistance arteries. Thus, human vascular smooth muscle cells in culture produce and release biologically active NO from L-arginine.

Modulation of gene expression by high and low density lipoproteins in human vascular smooth muscle cells

Low density lipoprotein and its oxidized form has been implicated in the process of arteriosclerosis which involves growth-related events in the smooth muscle cells of the arterial wall. The induction of so-called early-growth response genes e.g. c-myc and c-fos can serve as an indicator for these growth-related events. In cultured human vascular smooth muscle cells, both LDL and HDL3 were individually capable of stimulating c-myc and c-fos expression in a concentration dependent manner. However, when they were used in combination, depending on the proportion of HDL3 to LDL, c-fos but not c-myc expression was less pronounced than with the single components. In contrast to HDL3 and LDL alone, a combination of the two lipoproteins also blunted both the expression of autoinduced transforming growth factor beta transcripts and the transforming growth factor beta-induced increase of c-fos mRNA. It is concluded that a) the inhibition of transforming growth factor beta autostimulation by HDL3 plus LDL may involve reduced AP-1 activity via a reduction of c-fos expression by the lipoprotein combination and b) the ratio HDL3:LDL might influence the pathogenesis of arteriosclerosis via growth-related events in the arterial wall.

Stimulation of autocrine platelet--derived growth factor AA-homodimer and transforming growth factor beta in vascular smooth muscle cells

We have investigated the interrelationship between growth factors and vasoconstrictor peptides in terms of their possible paracrine/autocrine regulation of vascular smooth muscle cell differentiation/proliferation. Responses of quiescent cells from spontaneously hypertensive and Wistar-Kyoto rats to stimulation with a selected number of growth factors- and vasoconstrictor peptides were established (induction of mRNA as well as secretion of immunoreactive peptides). A single exposure of quiescent vascular smooth muscle cells to the vasoconstrictor peptides Angiotensin II and Endothelin-1 (10(-8) M each) resulted in a prolonged induction of platelet- derived growth factor A-chain and transforming growth factor beta transcripts (maximal at 5-6 hrs poststimulatory). The interrelationship between platelet- derived growth factor AA and transforming growth factor beta was investigated in experiments using the pure peptides individually for stimulation of mRNA and peptide secretion. Both growth factors enhanced their own and one anothers transcript expression. The results demonstrated that in spontaneously hypertensive rats, an established animal model of hypertension, the steady state balance of this set of growth factors may be disturbed. Defects involved may be attributable to alterations in the secretory machinery and/or amount of autocrine growth factor produced.

Native low-density lipoproteins stimulate leukotriene B4 production by human monocyte-derived macrophages

We have evaluated the effect of native low-density lipoproteins (LDL) on the production of leukotriene B4 (LTB4), a potent inflammatory and chemotactic factor, by human monocyte-derived macrophages. The capacity of LDL (d, 1.024-1.050 g/ml) to increase LTB4 secretion was dose-dependent with an optimal response at 100 micrograms LDL protein/ml, representing an approx. 7.5-fold stimulation over basal levels at 10 days of culture; the half-maximal response occurred at 20 micrograms/ml. The effect of LDL on LTB4 production was rapid (within 15 min) and was maintained for at least 21 h. The generation of LTB4 in response to LDL was partially inhibited (approx. 70% inhibition) by EDTA (5 mM) and by a monoclonal antibody (IgG-C7; 160 micrograms/ml) directed against the binding site of the cellular LDL receptor. In addition, the effects of native LDL and acetylated LDL were additive. These findings suggest that the specific interaction of LDL with its high affinity receptor represents a major component in the stimulation of the production of LTB4 by human monocyte-derived macrophages.

Low density lipoprotein activates monocytes to express tumor necrosis factor

We analyzed the effect of acetylated low density lipoprotein (aLDL) incubation on tumor necrosis factor (TNF) mRNA and protein expression in isolated resting human monocytes in serum free DMEM. TNF mRNA expression was about one third that of PMA and was dose dependent. The maximum stimulatory effect on TNF mRNA was at 250 micrograms/ml, while 500 micrograms/ml induced downregulation. The maximum stimulatory effect occurred at 6 hours, and by 24 hours TNF mRNA expression returned to the resting state. Acetyl LDL also induced the expression of immunoreactive TNF, reaching a sevenfold maximum above control at 12 hours following a 6 hour exposure period. The results suggest that aLDL is a potent stimulator of TNF expression in resting monocytes. This mechanism may be operational in atheroma evolution.

Novel cellular activities for low density lipoprotein in vascular smooth muscle cells

Hyperlipidemia and hypertension play important roles in the pathogenesis of atherosclerosis. To investigate the underlying intracellular mechanisms, we studied the effect of various concentrations of low density lipoprotein from normolipidemic subjects on concentrations of free intracellular calcium, intracellular pH, DNA synthesis, and vascular tone in vascular smooth muscle cells and rings from rat aortas. Low density lipoprotein in the range of 1-15 micrograms/ml induced a dose-dependent increase of concentration of free intracellular calcium and a biphasic change of the intracellular pH. Similar concentrations of low density lipoprotein led to an enhanced DNA synthesis. Furthermore, cumulative addition of 1-15 micrograms/ml low density lipoprotein produced a dose-dependent increase in contractile tension of thoracic aortic rings from rats. The maximal low density lipoprotein-induced contractile response was approximately 70% of that induced by 40 mM KCl. These findings indicate that low concentrations of low density lipoprotein occurring, for example, in the extravascular fluid might contribute to the pathogenesis of cardiovascular diseases by enhancing cell proliferation and vasoconstriction by changing intracellular calcium and intracellular pH.

Inducible endothelin mRNA expression and peptide secretion in cultured human vascular smooth muscle cells

This study demonstrates the induction of endothelin (ET) mRNA expression and synthesis of functional ET peptide in cultured human vascular smooth muscle cells (hVSMC). Compounds eliciting such responses in hVSMC include the vasoconstrictor hormones angiotensin II and arginine-vasopressin and the growth factors transforming growth factor beta, platelet derived growth factor AA and epidermal growth factor. Induction of ET mRNA expression in hVSMC exhibited transient kinetics (peak at 3-5 hrs. and return to basal within 7 hrs.) which differed from the more sustained ET transcript induction observed for porcine endothelial cells. ET peptide (determined by both radioimmuno- and radioreceptor assays) produced by stimulated hVSMC attained levels (approximately 120-160 pg/10(6) cells/4 hrs.; concentration approximately 3 x 10(-11) M) within the biologically effective concentration range of ET. Stimulated secretion of ET from hVSMC was abolished in the presence of the protein synthesis inhibitor cycloheximide. Sep-pak C18 extracts of medium from stimulated hVSMC elicited a concentration-dependent phosphoinositide catabolic response in myo-[2-3H]-inositol-prelabelled hVSMC. Our findings invoke a role for ET which extends beyond the paracrine regulation by peptide synthesized and secreted by endothelial cells. We propose that VSMC-synthesized ET may function in an autocrine manner to regulate both tone and structural modelling of vasculature.

Low-density lipoprotein elevates intracellular calcium and pH in vascular smooth muscle cells and fibroblasts without mediation of LDL receptor

Low-density lipoprotein (7 micrograms/ml) induced in the absence or in the presence of 7, 35, 70 micrograms/ml monoclonal antibodies against the specific Low-density lipoprotein receptor an elevation of intracellular Ca2+ from 105 to approximately 210 nM in vascular smooth muscle cells from rat aorta. Moreover, in both human cultured fibroblasts from normocholesterolemic individuals and from patients with familial hypercholesterolemia homozygote class 1, Low-density lipoprotein (7 micrograms/ml) induced a rise of free intracellular calcium and a biphasic change of intracellular pH. Low-density lipoprotein (1,7,15,30 micrograms/ml) had no significant influence on the phosphatidylinositol-turnover in vascular smooth muscle cells and fibroblasts. Since homozygote class 1 fibroblasts lack specific Low-density lipoprotein receptors, and as antibodies against this receptor did not attenuate the Low-density lipoprotein-induced elevation of cytosolic calcium and pH, we conclude that these intracellular changes are independent from the classical Low-density lipoprotein receptor.

Phorbol ester promotes a sustained down-regulation of endothelin receptors and cellular responses to endothelin in human vascular smooth muscle cells

The effect of phorbol ester pretreatment of human vascular smooth muscle cells (hVSMC) was studied with respect to regulation of endothelin (ET)-receptor binding and cellular responses to ET. The capacity of hVSMC to bind ET was decreased (by approximately 50% at maximum) after phorbol exposure, and this reductive effect was both rapid (t 1/2 approximately 10 min.) and sustained (for up to 24 hrs. of chronic phorbol exposure). Phorbol pretreatment inhibited both inositol phosphate and diacylclycerol production responses of hVSMC to ET in a manner that was time-dependent and sustained. Phorbol pretreatment also produced a persistent reduction in the ability of ET to release isotopically-labelled arachidonic and/or its metabolites from hVSMC, but importantly ionomycin-stimulated release was similarly negatively affected. Furthermore, ET-induced accumulation of the phospholipase A2/phospholipase B-derived inositol phospholipid metabolite, glycerophosphoinositol, was not different between control and phorbol-treated hVMSC. The mechanism whereby phorbol exerts differential, but notably sustained inhibitory effects on ET-promoted signal transduction pathways are thus complex and illustrative of the selectivity of protein kinase C in regulating cellular responses.