Statin inhibition of Fc receptor-mediated phagocytosis by macrophages is modulated by cell activation and cholesterol

OBJECTIVES

An inflammatory response to altered lipoproteins that accumulate in the arterial wall is a major component of the pathogenesis of atherosclerosis. Statins reduce plasma levels of low-density lipoprotein (LDL) and are effective treatments for atherosclerosis. It is hypothesized that they also modulate inflammation. The aim of this study was to examine whether lovastatin inhibits macrophage inflammatory processes and clarify its mechanism of action.

METHODS AND RESULTS

We examined the effects of statins on phagocytosis of antibody-coated red blood cells by cultured human monocytes and mouse peritoneal macrophages. Lovastatin, simvastatin, and zaragozic acid, a squalene synthase inhibitor, blocked Fc receptor-mediated phagocytosis by cultured human monocytes and mouse peritoneal macrophages. The inhibitory effect of lovastatin on Fc receptor-mediated phagocytosis was prevented completely by addition of mevalonate, farnesyl pyrophosphate, LDL, or cholesterol to the culture medium. The inhibitory effect of zaragozic acid was reversed by addition of LDL, but not by the addition of geranylgeranyl pyrophosphate, to the medium. In addition, the effect of lovastatin on phagocytosis is a function of cell activation because treatment of cells with tumor necrosis factor-alpha or lipopolysaccharide prevented inhibition of phagocytosis by lovastatin.

CONCLUSIONS

The inhibition of Fc receptor-mediated phagocytosis of lovastatin is related to its effect on cholesterol biosynthesis rather than its effect on the formation of isoprenoids.

Blockade of alpha 5 beta 1 integrins reverses the inhibitory effect of tenascin on chemotaxis of human monocytes and polymorphonuclear leukocytes through three-dimensional gels of extracellular matrix proteins

Tenascin is an extracellular matrix protein found in adults in T cell-dependent areas of lymphoid tissues, sites of inflammation, and tumors. We report here that it inhibited chemotaxis of chemoattractant-stimulated human monocytes and chemoattractant-stimulated polymorphonuclear leukocytes (PMN) through three-dimensional gels composed of collagen I or Matrigel, and chemotaxis of leukotriene B4-stimulated PMN through fibrin gels. The inhibitory effect of tenascin on monocyte or PMN chemotaxis through these matrices was reversed by Abs directed against alpha5beta1 integrins or by a peptide (GRGDSP) that binds to beta1 integrins. Tenascin did not affect leukotriene B4- or fMLP-stimulated expression of beta1 or beta2 integrins, but did exert a small inhibitory effect on PMN adhesion and closeness of apposition to fibrin(ogen)-containing surfaces. Thus, alpha5beta1 integrins mediate the inhibitory effect of tenascin on monocyte and PMN chemotaxis, without promoting close apposition between these leukocytes and surfaces coated with tenascin alone or with tenascin bound to other matrix proteins. This contrasts with the role played by alpha5beta1 integrins in promoting close apposition between fMLP-stimulated PMN and fibrin containing surfaces, thereby inhibiting chemotaxis of fMLP-stimulated PMN through fibrin gels. Thus, chemoattractants and matrix proteins regulate chemotaxis of phagocytic leukocytes by at least two different mechanisms: one in which specific chemoattractants promote very tight adhesion of leukocytes to specific matrix proteins and another in which specific matrix proteins signal cessation of migration without markedly affecting strength of leukocyte adhesion.

Scavenger receptor class B type I (SR-BI) mediates adhesion of neonatal murine microglia to fibrillar beta-amyloid

Class A scavenger receptors (SR-A) mediate microglial interaction with fibrillar beta-amyloid (fAbeta). We report here that neonatal microglia from SR-A knockout mice (SR-A-/-) adhere to surface-bound fAbeta, and produce reactive oxygen species (ROS) as efficiently as wildtype microglia; that both wildtype and SR-A-/- microglia express SR-BI; that antibodies against SR-BI do not affect adhesion or ROS production by wildtype microglia, but inhibit adhesion and ROS production of SR-A-/- microglia to immobilized fAbeta by approximately 40%. Adhesion to fAbeta-coated surfaces, and uptake of fAbeta by both wildtype and SR-A-/- microglia was almost completely inhibited by incubation with fucoidan. Thus SR-BI and SR-A mediate similar effector functions in neonatal microglia, which suggests that SR-BI plays as important a role as SR-A, and can maintain the wildtype phenotype in SR-A-/- microglia.

Mitochondrial DNA genotypes in nuclear transfer-derived cloned sheep

Eukaryotic cells contain two distinct genomes. One is located in the nucleus (nDNA) and is transmitted in a mendelian fashion, whereas the other is located in mitochondria (mtDNA) and is transmitted by maternal inheritance. Cloning of mammals typically has been achieved via nuclear transfer, in which a donor somatic cell is fused by electoporation with a recipient enucleated oocyte. During this whole-cell electrofusion, nDNA as well as mtDNA ought to be transferred to the oocyte. Thus, the cloned progeny should harbour mtDNAs from both the donor and recipient cytoplasms, resulting in heteroplasmy. Although the confirmation of nuclear transfer has been established using somatic cell-specific nDNA markers, no similar analysis of the mtDNA genotype has been reported. We report here the origin of the mtDNA in Dolly, the first animal cloned from an established adult somatic cell line, and in nine other nuclear transfer-derived sheep generated from fetal cells. The mtDNA of each of the ten nuclear-transfer sheep was derived exclusively from recipient enucleated oocytes, with no detectable contribution from the respective somatic donor cells. Thus, although these ten sheep are authentic nuclear clones, they are in fact genetic chimaeras, containing somatic cell-derived nuclear DNA but oocyte-derived mtDNA.

Differential regulation of beta1 integrins by chemoattractants regulates neutrophil migration through fibrin

Chemoattractants differ in their capacity to stimulate neutrophils to adhere to and to migrate through matrices containing fibrin. Formyl methionyl leucyl phenylalanine (fMLP) stimulates neutrophils to adhere closely to, but not to migrate into, fibrin gels. Leukotriene B4 (LTB4) stimulates neutrophils to adhere loosely to and to migrate through fibrin gels. We report that alpha5beta1 integrins regulate the different migratory behaviors on fibrin gels of neutrophils in response to these chemoattractants. fMLP, but not LTB4, activated neutrophil beta1 integrins, as measured by binding of mAb 15/7 to an activation epitope on the beta1 integrins. Antibodies or peptides that block alpha5beta1 integrins prevented fMLP-stimulated neutrophils from forming zones of close apposition on fibrin and reversed fMLP's inhibitory effect on neutrophil chemotaxis through fibrin. In contrast, neither peptides nor antibodies that block beta1 integrins affected the capacity of LTB4-stimulated neutrophils to form zones of loose apposition or to migrate through fibrin gels. These results suggest that chemoattractants generate at least two different messages that direct neutrophils, and perhaps other leukocytes, to accumulate at specific anatomic sites: a general message that induces neutrophils to crawl and a specific message that prepares neutrophils to stop when they contact appropriate matrix proteins for activated beta1 integrins.

Complementary roles for scavenger receptor A and CD36 of human monocyte-derived macrophages in adhesion to surfaces coated with oxidized low-density lipoproteins and in secretion of H2O2

Oxidized low-density lipoprotein (oxLDL) is considered one of the principal effectors of atherogenesis. To explore mechanisms by which oxLDL affects human mononuclear phagocytes, we incubated these cells in medium containing oxLDL, acetylated LDL (acLDL), or native LDL, or on surfaces coated with these native and modified lipoproteins. The presence of soluble oxLDL, acLDL, or native LDL in the medium did not stimulate H2O2 secretion by macrophages. In contrast, macrophages adherent to surfaces coated with oxLDL secreted three- to fourfold more H2O2 than macrophages adherent to surfaces coated with acLDL or native LDL. Freshly isolated blood monocytes secreted little H2O2 regardless of the substrate on which they were plated. H2O2 secretion was maximal in cells maintained for 4-6 d in culture before plating on oxLDL-coated surfaces. Fucoidan, a known ligand of class A macrophage scavenger receptors (MSR-A), significantly reduced macrophage adhesion to surfaces coated with oxLDL or acLDL. Monoclonal antibody SMO, which blocks oxLDL binding to CD36, did not inhibit adhesion of macrophages to oxLDL-coated surfaces but markedly reduced H2O2 secretion by these cells. These studies show that MSR-A is primarily responsible for adhesion of macrophages to oxLDL-coated surfaces, that CD36 signals H2O2 secretion by macrophages adherent to these surfaces, and that substrate-bound, but not soluble, oxLDL stimulates H2O2 secretion by macrophages.

Microglia, scavenger receptors, and the pathogenesis of Alzheimer's disease

The senile plaque is the pathological hallmark of Alzheimer's disease. Senile plaques are composed of beta amyloid fibrils, associated with activated microglia, astrocytes, and dystrophic neurons. We have recently identified class A scavenger receptors as the main receptors mediating the interaction of microglia with beta amyloid fibrils. Adhesion of microglia to beta amyloid fibrils leads to immobilization of these cells on the fibrils, and induces them to produce reactive oxygen species. We propose that interactions of microglial scavenger receptors with fibrillar beta amyloid may stimulate the microglia to secrete apolipoprotein E and complement proteins, which may further contribute to neurotoxicity and neuronal degeneration. Therefore, microglial scavenger receptors may be novel targets for therapeutic interventions in Alzheimer's disease.

Lipoprotein lipase regulates Fc receptor-mediated phagocytosis by macrophages maintained in glucose-deficient medium

During periods of intense activity such as phagocytosis, macrophages are thought to derive most of their energy from glucose metabolism under both aerobic and anaerobic conditions. To determine whether fatty acids released from lipoproteins by macrophage lipoprotein lipase (LPL) could substitute for glucose as a source of energy for phagocytosis, we cultured peritoneal macrophages from normal and LPL knockout (LPL-KO) mice that had been rescued from neonatal demise by expression of human LPL via the muscle creatine kinase promoter. Normal and LPL-KO macrophages were cultured in medium containing normal (5 mM) or low (1 mM) glucose, and were tested for their capacity to phagocytose IgG-opsonized sheep erythrocytes. LPL-KO macrophages maintained in 1 and 5 mM glucose phagocytosed 67 and 79% fewer IgG-opsonized erythrocytes, respectively, than macrophages from normal mice. Addition of VLDL to LPL-expressing macrophages maintained in 1 mM glucose enhanced the macrophages' phagocytosis of IgG-opsonized erythrocytes, but did not stimulate phagocytosis by LPL-KO macrophages. Inhibition of secreted LPL with a monoclonal anti-LPL antibody or with tetrahydrolipstatin blocked the ability of VLDL to enhance phagocytosis by LPL-expressing macrophages maintained in 1 mM glucose. Addition of oleic acid significantly enhanced phagocytosis by both LPL-expressing and LPL-KO macrophages maintained in 1 mM glucose. Moreover, oleic acid stimulated phagocytosis in cells cultured in non-glucose-containing medium, and increased the intracellular stores of creatine phosphate. Inhibition of oxidative phosphorylation, but not of glycolysis, blocked the capacity of oleic acid to stimulate phagocytosis. Receptor-mediated endocytosis of acetyl LDL by macrophages from LPL-expressing and LPL-KO mice was similar whether the cells were maintained in 5 or 1 mM glucose, and was not augmented by VLDL. We postulate that fatty acids derived from macrophage LPL-catalyzed hydrolysis of triglycerides and phospholipids provide energy for macrophages in areas that have limited amounts of ambient glucose, and during periods of intense metabolic activity.

Mac-1 (CD11b/CD18) is an oligodeoxynucleotide-binding protein

We have studied the interactions of phosphodiester and phosphorothioate oligodeoxynucleotides with Mac-1 (CD11b/CD18; alpha M beta 2), a heparin-binding integrin found predominantly on the surface of polymorphonuclear leukocytes (PMNs), macrophages and natural killer cells. Binding of a homopolymer of thymidine occurred on both the alpha M and beta 2 subunits. Soluble fibrinogen, a natural ligand for Mac-1, was an excellent competitor of the binding of a phosphorothioate oligodeoxynucleotide to both TNF-alpha-activated and nonactivated PMNs. Upregulation of cell-surface Mac-1 expression increased cell-surface binding of oligodeoxynucleotides. Binding was inhibited by anti-Mac-1 monoclonal antibodies, and the increase in cell-surface binding was correlated with a three- to fourfold increase in internalization by PMNs. An oligodeoxynucleotide inhibited beta 2-dependent migration through Matrigel, but the production of reactive oxygen species in PMNs adherent to fibrinogen dramatically increased. Thus, our data demonstrate that Mac-1 is a cell-surface receptor for oligodeoxynucleotides that can mediate their internalization and that this binding may have important functional consequences.

Sodium-glucose cotransporters display sodium- and phlorizin-dependent water permeability

Expression of Na(+)-glucose cotransporters of the SGLT-1 type by Xenopus laevis oocytes increased the osmotic water permeability (Pf) of oocytes by a factor of 1.9-2.8, in the presence and in the absence of 5 mM extracellular glucose. The Pf increase was correlated with the amount of SGLT-1 cRNA injected. In oocytes expressing SGLT-1, either addition of phlorizin to the medium or the replacement of Na+ by choline inhibited the uptake of methyl-alpha-D-glucopyranoside, a specific substrate for SGLT-1, and returned oocyte Pf to its level in uninjected oocytes. Phlorizin inhibited the SGLT-1-attributable increase in Pf with an inhibition constant (Ki) of 6.1 microM, a value analogous to the Ki for phlorizin inhibition of sugar uptake. However, neither the presence of phlorizin nor the absence of extracellular Na+ significantly affected the increase in Pf elicited in oocytes expressing GLUT-1, a facilitative glucose transporter. These findings suggest that SGLT-1 forms a pore that allows the transmembrane passage of water and that water and glucose traverse the protein through this pore. The finding that removal of extracellular Na+ abolishes the increase in Pf attributable to SGLT-1 suggests that extracellular Na+ is required to maintain patency of this transporter's water-permeable transmembrane pore.

Scavenger receptor-mediated adhesion of microglia to beta-amyloid fibrils

A pathological hallmark of Alzheimer's disease is the senile plaque, containing beta-amyloid fibrils, microglia and astrocytes. Beta-amyloid fibrils exert a cytotoxic effect on neurons, and stimulate microglia to produce neurotoxins, such as reactive oxygen species. Mononuclear phagocytes, including microglia, express scavenger receptors that mediate endocytosis of oxidized low-density lipoproteins, and adhesion to glucose-modified extra-cellular matrix proteins. Here we report that class A scavenger receptors mediate adhesion of rodent microglia and human monocytes to beta-amyloid fibril-coated surfaces leading to secretion of reactive oxygen species and cell immobilization. Thus, class A scavenger receptors are potential therapeutic targets in Alzheimer's disease.

Fibrin regulates neutrophil migration in response to interleukin 8, leukotriene B4, tumor necrosis factor, and formyl-methionyl-leucyl-phenylalanine

We have examined the capacity of four different chemoattractants/cytokines to promote directed migration of polymorphonuclear leukocytes (PMN) through three-dimensional gels composed of extracellular matrix proteins. About 20% of PMN migrated through fibrin gels and plasma clots in response to a gradient of interleukin 8 (IL-8) or leukotriene B4 (LTB4). In contrast, < 0.3% of PMN migrated through fibrin gels in response to a gradient of tumor necrosis factor alpha (TNF) or formyl-methionyl-leucyl-phenylalanine (FMLP). All four chemoattractants stimulated PMN to migrate through gels composed of collagen IV or of basement membrane proteins (Matrigel), or through filters to which fibronectin or fibrinogen had been adsorbed. PMN stimulated with TNF or FMLP adhered and formed zones of close apposition to fibrin, as measured by the exclusion of a 10-kD rhodamine-polyethylene glycol probe from the contact zones between PMN and the underlying fibrin gel. By this measure, IL-8- or LTB4-treated PMN adhered loosely to fibrin, since 10 kD rhodamine-polyethylene glycol permeated into the contact zones between these cells and the underlying fibrin gel. PMN stimulated with FMLP and IL-8, or FMLP and LTB4, exhibited very little migration through fibrin gels, and three times as many of these cells excluded 10 kD rhodamine-polyethylene glycol from their zones of contact with fibrin as PMN stimulated with IL-8 or LTB4 alone. These results show that PMN chemotaxis is regulated by both the nature of the chemoattractant and the composition of the extracellular matrix; they suggest that certain combinations of chemoattractants and matrix proteins may limit leukocyte movements and promote their localization in specific tissues in vivo.

Macrophages adhere to glucose-modified basement membrane collagen IV via their scavenger receptors

Scavenger receptors have been reported to mediate macrophage adhesion to serum-coated plastic surfaces. We report here that scavenger receptors promote the divalent cation independent adhesion of human monocytes and macrophages to surfaces coated with non-enzymatically glycated collagen IV but not to surfaces coated with native collagen IV. Ligands for scavenger receptor types I and II blocked adhesion of monocytes and macrophages to non-enzymatically glycated collagen IV but had no effect on adhesion of these cells to albumin-coated surfaces. U937 human promonocyte-like cells transfected with cDNA encoding bovine scavenger receptor I or II adhered to surfaces coated with glycated-collagen IV but not to surfaces coated with native collagen IV. A synthetic peptide homologous to the domain of bovine scavenger receptor that binds modified low density lipoproteins (residues 327-343) inhibited the adhesion of U937 cells transfected with cDNA encoding bovine scavenger receptor II to glycated collagen IV, whereas a control peptide from the alpha helical domain of scavenger receptor II (residues 121-137) had no effect on adhesion of these cells. Macrophages plated on surfaces coated with glycated collagen IV were unable to endocytose acetylated low density lipoproteins from the medium, suggesting that their scavenger receptors were occupied in binding these cells to the substrate. These findings suggest new roles for scavenger receptors in the accelerated development of vascular lesions observed in diabetics.

Functional characterization of antiadhesion molecules

Cytotactin, cytotactin binding (CTB) proteoglycan, and several other extracellular matrix (ECM) proteins and proteoglycans are described as antiadhesion molecules because they inhibit cell spreading and attachment to normally permissive ECM proteins. For cytotactin and CTB proteoglycan, this effect appears to be due to the binding of these proteins to their cell-surface receptors, which initiates a transmembrane signal that inhibits cell spreading. In contrast, the binding of fibronectin or laminin to its cell-surface receptors promotes cell spreading. Cell behavior may be regulated in a variety of systems by the interplay between these two opposing signals. For example, eosinophils on laminin spreading, form numerous foci containing filamentous actin, and remain viable in culture. In contrast, eosinophils on a mixture of laminin and cytotactin do not spread or form foci containing filamentous actin and the maintenance of viability is inhibited. In another system designed to model the immune surveillance of tumors, monocytes migrate in response to tumor necrosis factor through a gel comprised of a mixture of basement membrane proteins (Matrigel). Migration is blocked if the gel is coated with cytotactin. This result is of particular significance because cytotactin is expressed at high levels in the stroma of adult breast tissue surrounding tumors but only at low levels in normal breast tissue. These observations suggest that inflammation and the immune surveillance of tumors are among the processes regulated by cytotactin.

Role of facilitative glucose transporters in diffusional water permeability through J774 cells

We have reported previously that in the presence of an osmotic gradient, facilitative glucose transporters (GLUTs) act as a transmembrane pathway for water flow. Here, we find evidence that they also allow water passage in the absence of an osmotic gradient. We applied the linear diffusion technique to measure the diffusional permeability (Pd) of tritiated water (3H-H2O) through plasma membranes of J774 murine macrophage-like cells. Untreated cells had a Pd of 30.9 +/- 1.8 microns/s; the inhibitors of facilitative glucose transport cytochalasin B (10 microM) and phloretin (20 microM) reduced that value to 15.3 +/- 1.8 (50%) and 11.0 +/- 0.7 (62%) microns/s, respectively. In contrast, no significant effect on Pd was observed in cells treated with dihydrocytochalasin B (Pd = 28.4 +/- 1.5 microns/s). PCMBS (3 mM) inhibited glucose uptake by greater than 95%, and 3H-H2O diffusion by approximately 30% (Pd = 22.9 +/- 1.5 microns/s). The combination of cytochalasin B plus pCMBS reduced Pd by about 87% (Pd = 3.9 +/- 0.3 microns/s). Moreover, 1 mM pCMBS did not affect the osmotic water permeability in Xenopus laevis oocytes expressing the brain/erythroid form of facilitative glucose transporters (GLUT1). These results indicate for the first time that about half of the total Pd of J774 cells may be accounted for by water passage across GLUTs. Hence, they highlight the multifunctional properties of these transporters serving as conduits for both water and glucose. Our results also suggest for the first time that pCMBS blocks glucose transport without affecting water permeation through GLUTs. Lastly, because pCMBS decreases the Pd of J774 cells, this suggests the presence in their plasma membranes of another protein(s) exhibiting water channel properties.

Activated platelets form protected zones of adhesion on fibrinogen and fibronectin-coated surfaces

Leukocytes form zones of close apposition when they adhere to ligand-coated surfaces. Because plasma proteins are excluded from these contact zones, we have termed them protected zones of adhesion. To determine whether platelets form similar protected zones of adhesion, gel-filtered platelets stimulated with thrombin or ADP were allowed to adhere to fibrinogen- or fibronectin-coated surfaces. The protein-coated surfaces with platelets attached were stained with either fluorochrome-conjugated goat anti-human fibrinogen or anti-human fibronectin antibodies, or with rhodamine-conjugated polyethylene glycol polymers. Fluorescence microscopy revealed that F(ab')2 anti-fibrinogen (100 kD) did not penetrate into the contact zones between stimulated platelets and the underlying fibrinogen-coated surface, while Fab antifibrinogen (50 kD) and 10 kD polyethylene glycol readily penetrated and stained the substrate beneath the platelets. Thrombin- or ADP-stimulated platelets also formed protected zones of adhesion on fibronectin-coated surfaces. F(ab')2 anti-fibronectin and 10 kD polyethylene glycol were excluded from these adhesion zones, indicating that they are much less permeable than those formed by platelets on fibrinogen-coated surfaces. The permeability properties of protected zones of adhesion formed by stimulated platelets on surfaces coated with both fibrinogen and fibronectin were similar to the zones of adhesion formed on fibronectin alone. mAb 7E3, directed against the alpha IIb beta 3 integrin blocked the formation of protected adhesion zones between thrombin-stimulated platelets and fibrinogen or fibronectin coated surfaces. mAb C13 is directed against the alpha 5 beta 1 integrin on platelets. Stimulated platelets treated with this mAb formed protected zones of adhesion on surfaces coated with fibronectin. These protected zones were impermeable to F(ab')2 antifibronectin but were permeable to 10 kD polyethylene glycol. These results show that activated platelets form protected zones of adhesion and that the size of molecules excluded from these zones depends upon the composition of the matrix proteins to which the platelets adhere. They also show that formation of protected zones of adhesion by platelets requires alpha IIb beta 3 integrins while the permeability properties of these zones of adhesion are regulated by both alpha IIb beta 3 and alpha 5 beta 1 integrins.

Lactate transport in macrophages

Macrophages perform phagocytic and effector activities in a number of different tissues. The environment of the inflammatory foci in which they function is often acidic and contains an abundance of lactate. We characterized the ability of thioglycollate-elicited mouse peritoneal macrophages to accumulate lactate from the medium and to use this lactate to maintain intracellular energy stores. Lactate uptake was stereospecific for L-lactate and was inhibited by the organic anion transport blocker probenecid but not by concentrations of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid that block anion exchangers. L-[14C]Lactate uptake was not affected by variation of the extracellular Na+ concentration but was enhanced by acidification of the extracellular medium, suggesting that lactate uptake was mediated by a proton cotransport system. The enhanced accumulation of [14C]-lactate seen in medium at pH 6.0 to 6.5 was inhibited by probenecid or by an excess of unlabeled L-lactate. When macrophages were incubated in PBS without glucose for 6 h, intracellular stores of phosphocreatine were 13 nmol/mg of protein, compared with 44 nmol/mg of protein in cells incubated in medium containing glucose. When lactate was substituted for glucose, phosphocreatine stores were 32 nmol/mg of protein. These studies reveal that macrophages take up L-lactate in a pH-dependent manner and that lactate uptake occurs via a probenecid-inhibitable monocarboxylate transporter; they suggest that macrophages can utilize this lactate as an energy source.

Lactate transport in macrophages

Macrophages perform phagocytic and effector activities in a number of different tissues. The environment of the inflammatory foci in which they function is often acidic and contains an abundance of lactate. We characterized the ability of thioglycollate-elicited mouse peritoneal macrophages to accumulate lactate from the medium and to use this lactate to maintain intracellular energy stores. Lactate uptake was stereospecific for L-lactate and was inhibited by the organic anion transport blocker probenecid but not by concentrations of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid that block anion exchangers. L-[14C]Lactate uptake was not affected by variation of the extracellular Na+ concentration but was enhanced by acidification of the extracellular medium, suggesting that lactate uptake was mediated by a proton cotransport system. The enhanced accumulation of [14C]-lactate seen in medium at pH 6.0 to 6.5 was inhibited by probenecid or by an excess of unlabeled L-lactate. When macrophages were incubated in PBS without glucose for 6 h, intracellular stores of phosphocreatine were 13 nmol/mg of protein, compared with 44 nmol/mg of protein in cells incubated in medium containing glucose. When lactate was substituted for glucose, phosphocreatine stores were 32 nmol/mg of protein. These studies reveal that macrophages take up L-lactate in a pH-dependent manner and that lactate uptake occurs via a probenecid-inhibitable monocarboxylate transporter; they suggest that macrophages can utilize this lactate as an energy source.

The role of protected extracellular compartments in interactions between leukocytes, and platelets, and fibrin/fibrinogen matrices

Polymorphonuclear leukocytes express multiple surface receptors that mediate their adhesion to extracellular matrices and to other cells. These receptors also play roles in cell migration and phagocytosis. We have studied the role of one class of polymorphonuclear leukocytes surface receptors, the beta 2 integrins, in the interactions of these cells with fibrinogen. We have found that the beta 2 integrins, CD11b/CD18 (complement receptor three) and CD11c/CD18 mediate attachment of polymorphonuclear leukocytes to fibrinogen-coated surfaces. Polymorphonuclear leukocytes must be stimulated with chemoattractants, such as fMLP, or with cytokines, such as tumor necrosis factor to bind to fibrinogen via these integrins. Moreover, each of these integrins interacts with a different segment of the fibrinogen molecule. PMN adherence to fibrinogen via CD11b/CD18 depends on the carboxy terminus of fibrinogen, whereas adherence via CD11c/CD18 depends on the amino terminus of fibrinogen's alpha-chain. One of the physiological consequences of these interactions is that polymorphonuclear leukocytes stimulated with either chemoattractants or TNF form protected compartments at their interface with fibrinogen-coated surfaces and that elastase released into these compartments is inaccessible to protease inhibitors present in the plasma. These protected compartments exclude plasma proteins of > 40,000 daltons such as alpha 1 anti-proteinase, thereby allowing polymorphonuclear leukocyte proteases to degrade matrix proteins within this compartment without interference by plasma anti-proteinases.

Hypoxia induces glucose transporter expression in endothelial cells

Endothelial cells in various tissues of the body are often exposed to hypoxic conditions. To examine the effects of sustained hypoxia on energy metabolism in endothelial cells, we have maintained bovine aortic and human umbilical vein endothelial cells in an atmosphere containing low oxygen concentrations (14 mmHg) for up to 96 h. We report here that endothelial cells maintained under these conditions upregulate their glucose transport activity, consume more glucose, and produce greater amounts of lactic acid than normoxic cells. Upregulation of glucose transport activity by hypoxic endothelial cells required several hours to occur, was associated with increased expression of mRNA and protein for the erythroid/brain form of the facilitative glucose transporter, and was not due to depletion of glucose from the medium. Prolonged treatment of endothelial cells with inhibitors or uncouplers of oxidative phosphorylation (antimycin, azide, dinitrophenol) under normoxic conditions also upregulated glucose transporter expression. These results suggest that reduced rates of oxidative metabolism may represent an important signal for cells to adapt metabolically to hypoxia. Furthermore, in our examination of endothelial cell energy metabolism, we discovered that endothelial cells contain phosphocreatine and express both the brain and muscle isozymes of creatine kinase.

CD11c/CD18 on neutrophils recognizes a domain at the N terminus of the A alpha chain of fibrinogen

Fibrinogen and fibrin serve as adhesive substrates for a variety of cells including platelets, endothelial cells, and leukocytes. Previously, we identified the C terminus of the gamma chain of fibrinogen as the region of the fibrinogen molecule that contains a ligand for CD11b/CD18 (complement receptor 3) on phorbol ester-stimulated polymorphonuclear leukocytes. In contrast, we report here that neutrophils stimulated with tumor necrosis factor adhere to fibrinogen-coated surfaces, but not to human serum albumin-coated surfaces, via the integrin CD11c/CD18 (p150/95). Monoclonal antibodies LeuM5 and 3.9, which are directed against the alpha subunit of CD11c/CD18, but not monoclonal antibodies OKM10 and OKM1, which are directed against the alpha subunit of CD11b/CD18, inhibit the adhesion of tumor necrosis factor-stimulated neutrophils to fibrinogen-coated surfaces. To identify the site on fibrinogen recognized by CD11c/CD18, we have examined the adhesion of tumor necrosis factor-stimulated neutrophils to surfaces coated with various fibrinogen fragments. Stimulated neutrophils adhere to surfaces coated with the N-terminal disulfide knot fragment of fibrinogen or fibrinogen fragment E. Moreover, peptides containing the sequence Gly-Pro-Arg (which corresponds to amino acids 17-19 of the N-terminal region of the A alpha chain of fibrinogen), and monoclonal antibody LeuM5, block tumor necrosis factor-stimulated neutrophil adhesion to fibrinogen and to the N-terminal disulfide knot fragment of fibrinogen. Thus, CD11c/CD18 on tumor necrosis factor-stimulated neutrophils functions as a fibrinogen receptor that recognizes the sequence Gly-Pro-Arg in the N-terminal domain of the A alpha chain of fibrinogen.

Characterization of dystrophin in muscle-biopsy specimens from patients with Duchenne's or Becker's muscular dystrophy

A deficiency of the protein dystrophin has recently been shown to be the probable cause of Duchenne's muscular dystrophy. We sought to determine the relation between the clinical phenotype and the status of dystrophin in muscle-biopsy specimens from 103 patients with various neuromuscular disorders. We found very low levels (less than 3 percent of normal levels) or no dystrophin in the severe Duchenne phenotype (35 of 38 patients), low concentrations of dystrophin in the intermediate (outlier) phenotype (4 of 7), and dystrophin of abnormal molecular weight in the mild Becker phenotype (12 of 18). Normal levels of dystrophin of normal molecular weight were found in nearly all the patients (38 of 40) with 20 other neuromuscular disorders we studied. These data show the clinical consequences of both quantitative alterations (in Duchenne's and intermediate dystrophy) in a single protein. The biochemical assay for dystrophin should prove helpful in delineating myopathies that overlap clinically with Duchenne's and Becker's dystrophies, and it shows promise as an accurate diagnostic tool.

Extracellular creatine regulates creatine transport in rat and human muscle cells

Muscle cells do not synthesize creatine; they take up exogenous creatine by specific Na+-dependent plasma membrane transporters. We found that extracellular creatine regulates the level of expression of these creatine transporters in L6 rat muscle cells. L6 myoblasts maintained for 24 hr in medium containing 1 mM creatine exhibited 1/3rd of the creatine transport activity of cells maintained for 24 hr in medium without creatine. Down-regulation of creatine transport was partially reversed when creatine-fed L6 cells were incubated for 24 hr in medium lacking creatine. Down-regulation of creatine transport occurred independently of amino acid and glucose transport. Furthermore, the down-regulation of creatine transporters by extracellular creatine was slowed by inhibitors of protein synthesis. These results suggest that creatine induces the expression of a protein that functionally inactivates the creatine transporters. Regulation of creatine transport by extracellular creatine also was observed in L6 myotubes and in cultures of human myoblasts and myotubes. Hence, the activity of creatine transport represents another site for the regulation of creatine homeostasis.

Creatine uptake, metabolism, and efflux in human monocytes and macrophages

The uptake of creatine in human monocytes and monocyte-derived macrophages can be divided into two components. One component of uptake is saturable (Km = 30 microM), specific for creatine, requires Na+ in the extracellular medium, and enables cells to take up creatine against a concentration gradient of creatine. The second component of uptake is nonsaturable and accounts for less than 10% of creatine uptake at physiological creatine concentrations (20-60 microM). Both monocytes and macrophages have similar Km values for creatine transport. However, macrophages exhibit a fivefold greater rate of creatine uptake than monocytes (3.6 vs. 0.7 pmol X micrograms protein-1 X h-1), indicating that monocyte differentiation into macrophages is associated with an increase in the number of creatine transporters in the membrane. Monocytes do not contain creatine kinase and do not phosphorylate creatine; monocyte-derived macrophages express creatine kinase and phosphorylate approximately 25% of intracellular creatine. These results and the finding that 2-deoxyglucose inhibits creatine phosphorylation by macrophages without affecting the transport of creatine indicate that phosphorylation of creatine is not required for net creatine accumulation.

Creatine kinase expression and creatine phosphate accumulation are developmentally regulated during differentiation of mouse and human monocytes

We have studied the expression of creatine kinase (CK) and the accumulation of creatine phosphate during the differentiation of human and mouse peripheral blood monocytes. Mouse monocytes cultured for 24 h do not contain detectable levels of CK and creatine phosphate. However, resident tissue macrophages and inflammatory elicited macrophages obtained from the peritoneal cavities of mice have 70 and 300 mU per mg protein of CK activity and contain 3 and 6 mol of creatine phosphate per mol of ATP, respectively. The major isozyme of CK in these cells has been identified as the brain form. These findings suggest that the differentiation of monocytes into macrophages is associated with the expression of CK and the accumulation of creatine phosphate. We have found a similar pattern in human monocytes. Human blood monocytes, maintained in culture for 24 or 48 h, do not contain detectable levels of CK or creatine phosphate. Monocyte-derived macrophages (monocytes maintained in tissue cultures for 1 to 2 wk) have up to 100 mU per mg protein of CK activity and contain 0.5 mol of creatine phosphate per mol of ATP. Human macrophages express multiple isozymes of CK including the brain (BB) and possibly the mitochondrial forms of this enzyme. Thus, the expression of CK and the accumulation of creatine phosphate in human monocytes is induced by their in vitro cultivation. The induction of CK during in vitro cultivation occurs independently of the concentration of creatine in the medium. However, the size of the creatine phosphate pool varies with respect to extracellular creatine concentration. Creatine phosphate and CK are not detectable in freshly isolated human lymphocytes, polymorphonuclear leukocytes or erythrocytes, but are found in freshly isolated human platelets.

A fluorescence quenching technique using trypan blue to differentiate between attached and ingested glutaraldehyde-fixed red blood cells in phagocytosing murine macrophages

A fluorescence quenching method, using trypan blue, is described for quantifying the ingestion of either glutaraldehyde-fixed sheep red blood cells or antibody coated glutaraldehyde-fixed sheep red blood cells by murine peritoneal macrophages. This method is based on the observations that glutaraldehyde-fixed red blood cells fluoresce at about 585 nm when excited at 490 nm and that when trypan blue is in intimate contact with fixed red blood cells, the fluorescence is converted from a chartreuse to a red color. Thus, the ingestion of fixed erythrocytes by murine macrophages can be monitored by fluorescence microscopy after the addition of 1 mg/ml of trypan blue at the end of the assay. Extracellular glutaraldehyde-fixed red blood cells fluoresce a red color whereas the intracellular particles continue to fluoresce a chartreuse color. This method offers a simple and convenient technique for rapidly distinguishing between intracellular and extracellular glutaraldehyde-fixed red blood cells in macrophages.

VP16-213 and podophyllotoxin. A study on the relationship between chemical structure and biological activity

VP16-213, a semi-synthetic derivative of podophyllotoxin, is an effective antitumor agent in the treatment of a variety of leukemias and solid tumors. A comparison of the mechanism of action of VP16-213 and podophyllotoxin has revealed that although both drugs inhibit the uptake of nucleosides into HeLa cells, they exhibit other biological properties which are quite distinct. Podophyllotoxin is a potent inhibitor of microtubule assembly in vitro, while VP16-213 has no effect in this system. VP16-213 induces single stranded breaks in HeLa cells DNA, an effect which may be related to its antitumor activity. In contrast to VP16-213 treated cells, podophyllotoxin-treated cells maintain DNA integrity. Structure-activity relationship studies have identified some of chemical sites of VP16-213 and podophyllotoxin responsible for each of their biological properties. These studies illustrate that chemical modification of podophyllotoxin can generate derivatives which possess new and unique biological properties.

Application of differential scanning microcalorimetry to the study of cellular processes: heat production and glucose oxidation of murine macrophages

Differential scanning microcalorimetry provides a noninvasive method for studying heat evolution in living cells. We used this technique to measure the heat evolved by thioglycollate broth-elicited mouse macrophages, and the effects of NaF, KCN, cycloheximide, and cytochalasins B and D on this parameter. The total heat evolved in the interval 10--37 degrees C scanned at 1 degree C min-1 ranged from 300 to 2500 X 10(-12) cal (1 Cal = 4.184 J) per cell, depending on cell density, glucose concentration, and the presence or absence of various drugs.

Increased ATP and creatine phosphate turnover in phagocytosing mouse peritoneal macrophages

Resident and thioglycollate-elicited macrophages maintained in culture for 24 h contain approximately 5 x 10(-16) and 12 x 10(-16) mol of ATP per cell, respectively. During particle ingestion, the levels of ATP in these cells did not change. However, the specific activity of ATP extracted from macrophages labeled with [32P]Pi during phagocytosis was 40% lower than ATP extracted from control cells. These results suggested that macrophages contain a high energy phosphate reservoir, in addition to the ATP pool(s). A search for such a reservoir led to the identification of creatine phosphate in both resident and thioglycollate-elicited macrophages at concentrations that are in 3- to 5-fold-molar excess over ATP. Creatine phosphate levels in phagocytosing resident macrophages decreased by 45%, while creatine phosphate levels in phagocytosing thioglycollate-elicited macrophages did not change. Creatine phosphate turnover was measured in macrophages prelabeled with [14C]creatine. Over 90% of the intracellular label was in the form of creatine phosphate. During phagocytosis, there was a 40% decrease in intracellular [14C]creatine phosphate in both resident and thioglycollate-elicited macrophages. These results indicate that creatine phosphate turns over more rapidly during phagocytosis and replenishes the ATP consumed.

Structure-activity study of the inhibition of microtubule assembly in vitro by podophyllotoxin and its congeners

This study investigates the inhibition of microtubule assembly in vitro by podophyllotoxin and its derivatives, which include in part the antitumor compounds 4'-demethylepipodophyllotoxin ethylidene beta-D-glucoside (VP-16-213) and 4'-demethylepipodophyllotoxin thenylidene beta-D-glucoside (VM-26); the cyclic ethers, cyclic sulfides, and cyclic sulfones of podophyllotoxin and deoxypodophyllotoxin; epipodophyllotoxin; picropodophyllotoxin; and several 4'-demethyl compounds. The inhibitory activity of these derivatives is sensitive to the configuration and size of substituents at position 4 in ring C and to steric features of substituents at position 12 in ring D. Decreasing activity correlates with the increasing size of the substituent at position 12, as indexed by their van der Waals radii. These results suggest that rings C and D of these drugs are involved in their interaction with the podophyllotoxin-binding site in tubulin.

A comparison of the mechanisms of action of VP-16-213 and podophyllotoxin

A comparison of the mechanism of action of VP 16-213 and podophyllotoxin has revealed that although both drugs inhibit nucleoside uptake in HeLa cells, they exhibit other biological properties which are quite distinct. Podophyllotoxin inhibits microtubule assembly in vitro while VP-16-213 has no effect in this system. VP-16-213 induces single-stranded breaks in DNA in HeLa cells, an effect which may explain the cytoxicity of VP-16-213. In contrast to VP-16-213-treated cells, cells treated with podophyllotoxin maintain DNA integrity and show no evidence of single-stranded breaks.

Effect of VP-16-213 on the intracellular degradation of DNA in HeLa cells

The effect of VP-16-213 on cellular DNA was studied by following the sedimentation profiles of radioactive DNA in HeLa cells on alkaline sucrose gradients. In VP-16-213 treated cells, high-molecular-weight DNA is converted to a lower molecular-weight form in a dose-dependent, temperature-dependent reaction. The effect of VP-16-213 on cellular DNA is reversed after the drug has been removed from the growth medium for 150 min. These results suggest that VP-16-213 induces single-stranded breaks in DNA in HeLa cells and that HeLa cells can repair these breaks within 150 min. The nonglucoside derivative of VP-16-213, 4'-demethylepipodophyllotoxin, also induces the cleavage of cellular DNA but podophyllotoxin has no effect on DNA. A structure-activity relationship study, in which the effects of various VP-16-213 and podophyllotoxin congeners were tested for their ability to cleave cellular DNA,revealed that an hydroxyl group at the C-4' position is required for activity and that the configuration of the C-4 carbon influences the activity of a congener. These results may offer insights into the mechanism of action of VP-16-213 as an antitumor agent.

Effects of podophyllotoxin and VP-16-213 on microtubule assembly in vitro and nucleoside transport in HeLa cells

VP-16-213, a semisynthetic derivative of podophyllotoxin, is an active antitumor agent. In this paper, the effects of VP-16-213 and podophyllotoxin on microtubule assembly in vitro and nucleoside transport in HeLa cells are compared. At 100 muM, VP-16-213 does not inhibit microtubule assembly in vitro, while 5 muM podophyllotoxin completely prevents the formation of microtubules. The presence of the glucoside moiety in VP-16-213 is responsible for the inactivity of VP-16-213 in this system because 4'-demethylepipodo-phyllotoxin, the nonglucoside congener of VP-16-213, inhibits microtubule assembly. In HeLa cells, VP-16-213 and podophyllotoxin share a common biological property; both agents inhibit the uptake of thymidine and uridine into cells by inhibiting the facilitated diffusional component of nucleoside transport. The conncentrations of drug necessary to inhibit thymidine and uridine uptake into HeLa cells by 50% are 10 and 5 muM, respectively, for podophyllotoxin, and 25 and 20 muM for VP-16-213. The action of podophyllotoxin on nucleoside transport appears unrelated to its effect on microtubule assembly, since VP-16-213, which does not inhibit microtubule assembly, inhibits nucleoside transport.