Cis-acting DNA elements of mouse granulocyte/macrophage colony-stimulating factor gene responsive to oxidized low density lipoprotein

Pioglitazone suppresses macrophage proliferation in apolipoprotein-E deficient mice by activating PPARγ

BACKGROUND AND AIMS

Local macrophage proliferation is linked to enhanced atherosclerosis progression. Our previous study found that troglitazone, a thiazolidinedione (TZD), suppressed oxidized low-density lipoprotein (Ox-LDL)-induced macrophage proliferation. However, its effects and mechanisms are unclear. Therefore, we investigated the effects of pioglitazone, another TZD, on macrophage proliferation.

METHODS

Normal chow (NC)- or high-fat diet (HFD)-fed apolipoprotein E-deficient (Apoe^-/-) mice were treated orally with pioglitazone (10 mg/kg/day) or vehicle (water) as a control. Mouse peritoneal macrophages were used in in vitro assays.

RESULTS

Atherosclerosis progression was suppressed in aortic sinuses of pioglitazone-treated Apoe^-/- mice, which showed fewer proliferating macrophages in plaques. Pioglitazone suppressed Ox-LDL-induced macrophage proliferation in a dose-dependent manner. However, treatment with peroxisome proliferator-activated receptor-γ (PPARγ) siRNA ameliorated pioglitazone-induced suppression of macrophage proliferation. Low concentrations (less than 100 μmol/L) of pioglitazone, which can suppress macrophage proliferation, activated PPARγ in macrophages, but did not induce macrophage apoptosis. Pioglitazone treatment did not induce TUNEL-positive cells in atherosclerotic plaques of aortic sinuses in Apoe^-/- mice.

CONCLUSIONS

Pioglitazone suppressed macrophage proliferation through PPARγ without inducing macrophage apoptosis. These findings imply that pioglitazone could prevent macrovascular complications in diabetic individuals.

T cell-intrinsic function of the noncanonical NF-κB pathway in the regulation of GM-CSF expression and experimental autoimmune encephalomyelitis pathogenesis

The noncanonical NF-κB pathway induces processing of the NF-κB2 precursor protein p100, and thereby mediates activation of p52-containing NF-κB complexes. This pathway is crucial for B cell maturation and humoral immunity, but its role in regulating T cell function is less clear. Using mutant mice that express a nonprocessible p100, NF-κB2(lym1), we show that the noncanonical NF-κB pathway has a T cell-intrinsic role in regulating the pathogenesis of a T cell-mediated autoimmunity, experimental autoimmune encephalomyelitis (EAE). Although the lym1 mutation does not interfere with naive T cell activation, it renders the Th17 cells defective in the production of inflammatory effector molecules, particularly the cytokine GM-CSF. We provide evidence that p52 binds to the promoter of the GM-CSF-encoding gene (Csf2) and cooperates with c-Rel in the transactivation of this target gene. Introduction of exogenous p52 or GM-CSF to the NF-κB2(lym1) mutant T cells partially restores their ability to induce EAE. These results suggest that the noncanonical NF-κB pathway mediates induction of EAE by regulating the effector function of inflammatory T cells.

Apocynin suppresses the progression of atherosclerosis in apoE-deficient mice by inactivation of macrophages

Production of reactive oxygen species (ROS) and other proinflammatory substances by macrophages plays an important role in atherogenesis. Apocynin (4-hydroxy-3-methoxy-acetophenone), which is well known as a NADPH oxidase inhibitor, has anti-inflammatory effects including suppression of the generation of ROS. However, the suppressive effects of apocynin on the progression of atherosclerosis are not clearly understood. Thus, we investigated anti-atherosclerotic effects of apocynin using apolipoprotein E-deficient (apoE(-/-)) mice in vivo and in mouse peritoneal macrophages in vitro. In atherosclerosis-prone apoE(-/-) mice, apocynin suppressed the progression of atherosclerosis, decreased 4-hydroxynonenal-positive area in atherosclerotic lesions, and mRNA expression of monocyte chemoattractant protein-1 (MCP-1) and interleukin-6 (IL-6) in aorta. In mouse peritoneal macrophages, apocynin suppressed the Ox-LDL-induced ROS generation, mRNA expression of MCP-1, IL-6 and granulocyte/macrophage colony-stimulating factor, and cell proliferation. Moreover, immunohistochemical studies revealed that apocynin decreased the number of proliferating cell nuclear antigen-positive macrophages in atherosclerotic lesions of apoE(-/-) mice. These results suggested that apocynin suppressed the formation of atherosclerotic lesions, at least in part, by inactivation of macrophages. Therefore, apocynin may be a potential therapeutic material to prevent the progression of atherosclerosis.

Telmisartan Exerts Antiatherosclerotic Effects by Activating Peroxisome Proliferator-Activated Receptor-γ in Macrophages

Objective—

Telmisartan, an angiotensin type I receptor blocker (ARB), protects against the progression of atherosclerosis. Here, we investigated the molecular basis of the antiatherosclerotic effects of telmisartan in macrophages and apolipoprotein E–deficient mice.

Methods and Results—

In macrophages, telmisartan increased peroxisome proliferator-activated receptor-γ (PPARγ) activity and PPAR ligand-binding activity. In contrast, 3 other ARBs, losartan, valsartan, and olmesartan, did not affect PPARγ activity. Interestingly, high doses of telmisartan activated PPARα in macrophages. Telmisartan induced the mRNA expression of CD36 and ATP-binding cassette transporters A1 and G1 (ABCA1/G1), and these effects were abrogated by PPARγ small interfering RNA. Telmisartan, but not other ARBs, inhibited lipopolysaccharide-induced mRNA expression of monocyte chemoattractant protein-1 (MCP-1) and tumor necrosis factor-α, and these effects were abrogated by PPARγ small interfering RNA. Moreover, telmisartan suppressed oxidized low-density lipoprotein-induced macrophage proliferation through PPARγ activation. In apolipoprotein E −/− mice, telmisartan increased the mRNA expression of ABCA1 and ABCG1, decreased atherosclerotic lesion size, decreased the number of proliferative macrophages in the lesion, and suppressed MCP-1 and tumor necrosis factor-α mRNA expression in the aorta.

Conclusion—

Telmisartan induced ABCA1/ABCG1 expression and suppressed MCP-1 expression and macrophage proliferation by activating PPARγ. These effects may induce antiatherogenic effects in hypertensive patients.

Nifedipine Induces Peroxisome Proliferator-Activated Receptor-γ Activation in Macrophages and Suppresses the Progression of Atherosclerosis in Apolipoprotein E-Deficient Mice

Objective— Nifedipine, an L-type calcium channel blocker, protects against the progression of atherosclerosis. We investigated the molecular basis of the antiatherosclerotic effect of nifedipine in macrophages and apolipoprotein E-deficient mice.

Methods and Results— In macrophages, nifedipine increased peroxisome proliferator-activated receptor-γ (PPARγ) activity without increasing PPARγ-binding activity. Amlodipine, another L-type calcium channel blocker, and 1,2-bis-(o-aminophenoxy)-ethane-N,N,-N′,N′-tetraacetic acid tetraacetoxy-methyl ester (BAPTA-AM), a calcium chelator, decreased PPARγ activity, suggesting that nifedipine does not activate PPARγ via calcium channel blocker activity. Inactivation of extracellular signal-regulated kinase 1/2 suppressed PPARγ2-Ser112 phosphorylation and induced PPARγ activation. Nifedipine suppressed extracellular signal-regulated kinase 1/2 activation and PPARγ2-Ser112 phosphorylation, and mutating PPARγ2-Ser112 to Ala abrogated nifedipine-mediated PPARγ activation. These results suggested that nifedipine inhibited extracellular signal-regulated kinase 1/2 activity and PPARγ2-Ser112 phosphorylation, leading to PPARγ activation. Nifedipine inhibited lipopolysaccharide-induced monocyte chemoattractant protein-1 expression and induced ATP-binding cassette transporter A1 mRNA expression, and these effects were abrogated by small interfering RNA for PPARγ. Furthermore, in apolipoprotein E-deficient mice, nifedipine treatment decreased atherosclerotic lesion size, phosphorylation of PPARγ2-Ser112 and extracellular signal-regulated kinase 1/2, and monocyte chemoattractant protein-1 mRNA expression and increased ATP-binding cassette transporter A1 expression in the aorta.

Conclusion— Nifedipine unlike amlodipine inhibits PPARγ-Ser phosphorylation and activates PPARγ to suppress monocyte chemoattractant protein-1 expression and induce ATP-binding cassette transporter A1 expression in macrophages. These effects may induce antiatherogenic effects in hypertensive patients.

Activation of AMP-activated protein kinase suppresses oxidized low-density lipoprotein-induced macrophage proliferation

Macrophage-derived foam cells play important roles in the progression of atherosclerosis. We reported previously that ERK1/2-dependent granulocyte/macrophage colony-stimulating factor (GM-CSF) expression, leading to p38 MAPK/ Akt signaling, is important for oxidized low density lipoprotein (Ox-LDL)-induced macrophage proliferation. Here, we investigated whether activation of AMP-activated protein kinase (AMPK) could suppress macrophage proliferation. Ox-LDL-induced proliferation of mouse peritoneal macrophages was assessed by [(3)H]thymidine incorporation and cell counting assays. The proliferation was significantly inhibited by the AMPK activator 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) and restored by dominant-negative AMPKalpha1, suggesting that AMPK activation suppressed macrophage proliferation. AICAR partially suppressed Ox-LDL-induced ERK1/2 phosphorylation and GM-CSF expression, suggesting that another mechanism is also involved in the AICAR-mediated suppression of macrophage proliferation. AICAR suppressed GM-CSF-induced macrophage proliferation without suppressing p38 MAPK/Akt signaling. GM-CSF suppressed p53 phosphorylation and expression and induced Rb phosphorylation. Overexpression of p53 or p27(kip) suppressed GM-CSF-induced macrophage proliferation. AICAR induced cell cycle arrest, increased p53 phosphorylation and expression, and suppressed GM-CSF-induced Rb phosphorylation via AMPK activation. Moreover, AICAR induced p21(cip) and p27(kip) expression via AMPK activation, and small interfering RNA (siRNA) of p21(cip) and p27(kip) restored AICAR-mediated suppression of macrophage proliferation. In conclusion, AMPK activation suppressed Ox-LDL-induced macrophage proliferation by suppressing GM-CSF expression and inducing cell cycle arrest. These effects of AMPK activation may represent therapeutic targets for atherosclerosis.

The regulated in development and DNA damage response 2 (REDD2) gene mediates human monocyte cell death through a reduction in thioredoxin-1 expression

In a previous study, we identified the regulated in development and DNA damage response 2 (REDD2) gene as a highly expressed gene in human atherosclerotic lesions in comparison to normal artery, as well as in cultured human macrophages, and showed its implication in oxidized low-density lipoprotein (LDL)-induced macrophage death sensitivity. In this article, we attempt to identify the mechanism by which REDD2 induces such a phenomenon. Transient transfection of U-937 monocytic cells with a pCI.CMV.REDD2 expression vector increased by approximately twofold the mRNA levels of REDD2 in comparison to control cells transfected with pCI.CMV.GFP. Reactive oxygen species (ROS) production was significantly induced in REDD2-transfected cells compared with control cells (157+/-48 and 100+/-8 arbitrary units/mg cell protein, respectively; p<0.05). Moreover, a significant increase in parameters known to reflect the oxidative modifications of LDL was observed. Among enzymes involved in ROS production or degradation, we found a specific reduction in thioredoxin-1 (Trx-1) mRNA ( approximately 52+/-7% decrease, p<0.01 vs control cells) and protein ( approximately 60+/-4% decrease, p<0.001 vs control cells) levels in cells overexpressing REDD2 in comparison to control cells. In contrast, transfection of U-937 cells with siRNA against REDD2 decreased the mRNA levels of REDD2 by approximately 60% and increased Trx-1 mRNA and protein levels. Moreover, we observed no or a moderate increase in Bax (proapoptotic) and a significant decrease in Bcl2 (antiapoptotic) gene expression in cells that overexpress REDD2 compared to control cells. In addition, we showed that Trx-1 mRNA and protein levels were increased at low H(2)O(2) doses and decreased at higher doses. Interestingly, macrophages isolated from human atherosclerotic lesions differentially express REDD2 and Trx-1. Indeed, in certain patients, levels of REDD2 mRNA were low and those of Trx-1 mRNA were high. In contrast, in other patients, levels of REDD2 were high and levels of Trx-1 mRNA were low.

Troglitazone inhibits oxidized low-density lipoprotein-induced macrophage proliferation: Impact of the suppression of nuclear translocation of ERK1/2

Thiazolidinediones (TZDs), which were known as novel insulin-sensitizing antidiabetic agents, have been reported to inhibit the acceleration of atherosclerotic lesions. Macrophages play important roles in the development of atherosclerosis. We previously reported that oxidized low-density lipoprotein (Ox-LDL) induces macrophage proliferation through ERK1/2-dependent GM-CSF production. In the present study, we investigated the effects of two TZDs, troglitazone and ciglitazone on Ox-LDL-induced macrophage proliferation. Troglitazone significantly inhibited Ox-LDL-induced increases in [(3)H]thymidine incorporation into and proliferation of mouse peritoneal macrophages, whereas ciglitazone had no effects. Troglitazone and ciglitazone both significantly induced PPARgamma activity, suggesting that the inhibitory effect of troglitazone was not mediated by PPARgamma. Ox-LDL-induced production of GM-CSF was significantly inhibited by troglitazone, but not by ciglitazone. Troglitazone inhibited Ox-LDL-induced production of intracellular reactive oxygen species, whereas ciglitazone had no effect. The antioxidant reagents NAC and NMPG each inhibited phosphorylation of ERK1/2, whereas troglitazone and ciglitazone had no effects. However, troglitazone, NAC and NMPG all inhibited nuclear translocation of ERK1/2. In conclusion, troglitazone inhibited Ox-LDL-induced GM-CSF production by suppressing nuclear translocation of ERK1/2, thereby inhibiting macrophage proliferation. This suppression of macrophage proliferation by troglitazone may, at least in part, explain its antiatherogenic effects.

Extracellular signal-regulated kinase and p38 mitogen-activated protein kinase mediate macrophage proliferation induced by oxidized low-density lipoprotein

We previously reported that oxidized low-density lipoprotein (Ox-LDL)-induced expression of granulocyte/macrophage colony-stimulating factor (GM-CSF) via PKC, leading to activation of phosphatidylinositol-3 kinase (PI-3K), was important for macrophage proliferation [J Biol Chem 275 (2000) 5810]. The aim of the present study was to elucidate the role of extracellular-signal regulated kinase 1/2 (ERK1/2) and of p38 MAPK in Ox-LDL-induced macrophage proliferation. Ox-LDL-induced proliferation of mouse peritoneal macrophages assessed by [3H]thymidine incorporation and cell counting assays was significantly inhibited by MEK1/2 inhibitors, PD98059 or U0126, and p38 MAPK inhibitors, SB203580 or SB202190, respectively. Ox-LDL-induced GM-CSF production was inhibited by MEK1/2 inhibitors but not by p38 MAPK inhibitors in mRNA and protein levels, whereas recombinant GM-CSF-induced macrophage proliferation was inhibited by p38 MAPK inhibitors but enhanced by MEK1/2 inhibitors. Recombinant GM-CSF-induced PI-3K activation and Akt phosphorylation were significantly inhibited by SB203580 but enhanced by PD98059. Our results suggest that ERK1/2 is involved in Ox-LDL-induced macrophage proliferation in the signaling pathway before GM-CSF production, whereas p38 MAPK is involved after GM-CSF release. Thus, the importance of MAPKs in Ox-LDL-induced macrophage proliferation was confirmed and the control of MAPK cascade could be targeted as a potential treatment of atherosclerosis.

REDD2 gene is upregulated by modified LDL or hypoxia and mediates human macrophage cell death

OBJECTIVE

Cholesterol accumulation in macrophages is known to alter macrophage biology. In this article we studied the impact of macrophage cholesterol loading on gene expression and identified a novel gene implicated in cell death.

METHODS AND RESULTS

The regulated in development and DNA damage response 2 (REDD2) gene was strongly upregulated as THP-1 macrophages are converted to foam cells. These results were confirmed by Northern blot of RNA from human monocyte-derived macrophages (HMDM) treated with oxidized LDL (oxLDL). Human REDD2 shares 86% amino acid sequence identity with murine RTP801-like protein, which is 33% identical to RTP801, a hypoxia-inducible factor 1-responsive gene involved in apoptosis. Treatment of HMDM with desferrioxamine, a molecule that mimics the effect of hypoxia, increased expression of REDD2 in a concentration-dependent fashion. Transfection of U-937 and HMEC cells with a REDD2 expression vector increased the sensitivity of the cells for oxLDL-induced cytotoxicity, by inducing a shift from apoptosis toward necrosis. In contrast, suppression of mRNA expression using siRNA approach resulted in increased resistance to oxLDL treatment.

CONCLUSIONS

We showed that stimulation of REDD2 expression in macrophages increases oxLDL-induced cell death, suggesting that REDD2 gene might play an important role in arterial pathology.

Thioredoxin reductase 1 is upregulated in atherosclerotic plaques: specific induction of the promoter in human macrophages by oxidized low-density lipoproteins

Uptake of modified low-density lipoproteins (LDLs) by macrophages in the arterial wall is an important event in atherogenesis. Indeed, oxidatively modified LDLs (oxLDLs) are known to affect various cellular processes by modulating oxidation-sensitive signaling pathways. Here we found that the ubiquitous 55 kDa selenoprotein thioredoxin reductase 1 (TrxR1), which is a key enzyme for cellular redox control and antioxidant defense, was upregulated in human atherosclerotic plaques and expressed in foam cells. Using reverse transcription polymerase chain reaction analysis, we also found that oxLDLs, but not native LDLs (nLDLs), dose-dependently increased TrxR1 mRNA in human monocyte-derived macrophages (HMDMs). This stimulating effect was specific for oxLDLs, as pro-inflammatory factors, such as lipopolysaccharides (LPSs), interleukin-1beta (IL-1beta), interleukin-6 (Il-6), and tumor necrosis factor alpha (TNFalpha), under the same conditions, failed to induce TrxR1 mRNA levels to the same extent. Moreover, phorbol ester-differentiated THP-1 cells or HMDMs transiently transfected with TrxR1 promoter fragments linked to a luciferase reporter gene allowed identification of a defined promoter region as specifically responding to the phospholipid component of oxLDLs (p <.05 vs. phospholipid component of nLDLs). Gel mobility shift analyses identified a short 40-nucleotide stretch of the promoter carrying AP-1 and HoxA5 consensus motifs that responded with an altered shift pattern in THP-1 cells treated with oxLDLs, however, without evident involvement of either the Fos, Jun, Nrf2 or HoxA5 transcription factors.

15d-PGJ2 inhibits oxidized LDL-induced macrophage proliferation by inhibition of GM-CSF production via inactivation of NF-κB

Macrophage-derived foam cells play an important role in atherosclerotic lesions. Oxidized low-density lipoprotein (Ox-LDL) induces macrophage proliferation via production of GM-CSF in vitro. This study investigated the effects of 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)), a natural ligand for peroxisome proliferator-activated receptor gamma, on macrophage proliferation. Mouse peritoneal macrophages and RAW264.7 cells were used for proliferation study and reporter gene assay, respectively. Twenty microgram per milliliter of Ox-LDL induced [3H]thymidine incorporation in mouse peritoneal macrophages, and 15d-PGJ(2) inhibited Ox-LDL-induced [3H]thymidine incorporation in a dose-dependent manner. Ox-LDL increased GM-CSF release and GM-CSF mRNA expression, and activated GM-CSF gene promoter, all of which were prevented by 15d-PGJ(2) or 2-cyclopenten-1-one, a cyclopentenone ring of 15d-PGJ(2). The suppression of GM-CSF promoter activity by 15d-PGJ(2) and 2-cyclopenten-1-one was mediated through reduction of NF-kappaB binding to GM-CSF promoter. These results suggest that 15d-PGJ(2) inhibits Ox-LDL-induced macrophage proliferation through suppression of GM-CSF production via NF-kappaB inactivation.

Role of monocytes in atherogenesis

This review focuses on the role of monocytes in the early phase of atherogenesis, before foam cell formation. An emerging consensus underscores the importance of the cellular inflammatory system in atherogenesis. Initiation of the process apparently hinges on accumulating low-density lipoproteins (LDL) undergoing oxidation and glycation, providing stimuli for the release of monocyte attracting chemokines and for the upregulation of endothelial adhesive molecules. These conditions favor monocyte transmigration to the intima, where chemically modified, aggregated, or proteoglycan- or antibody-complexed LDL may be endocytotically internalized via scavenger receptors present on the emergent macrophage surface. The differentiating monocytes in concert with T lymphocytes exert a modulating effect on lipoproteins. These events propagate a series of reactions entailing generation of lipid peroxides and expression of chemokines, adhesion molecules, cytokines, and growth factors, thereby sustaining an ongoing inflammatory process leading ultimately to lesion formation. New data emerging from studies using transgenic animals, notably mice, have provided novel insights into many of the cellular interactions and signaling mechanisms involving monocytes/macrophages in the atherogenic processes. A number of these studies, focusing on mechanisms for monocyte activation and the roles of adhesive molecules, chemokines, cytokines and growth factors, are addressed in this review.

Nondisposable materials, chronic inflammation, and adjuvant action

Why inflammatory responses become chronic and how adjuvants work remain unanswered. Macrophage-lineage cells are key components of chronic inflammatory reactions and in the actions of immunologic adjuvants. One explanation for the increased numbers of macrophages long term at sites of chronic inflammation could be enhanced cell survival or even local proliferation. The evidence supporting a unifying hypothesis for one way in which this macrophage survival and proliferation may be promoted is presented. Many materials, often particulate, of which macrophages have difficulty disposing, can promote monocyte/macrophage survival and even proliferation. Materials active in this regard and which can initiate chronic inflammatory reactions include oxidized low-density lipoprotein, inflammatory microcrystals (calcium phosphate, monosodium urate, talc, calcium pyrophosphate), amyloidogenic peptides (amyloid beta and prion protein), and joint implant biomaterials. Additional, similar materials, which have been shown to have adjuvant activity (alum, oil-in-water emulsions, heat-killed bacteria, CpG oligonucleotides, methylated bovine serum albumin, silica), induce similar responses. Cell proliferation can be striking, following uptake of some of the materials, when macrophage-colony stimulating factor is included at low concentrations, which normally promote mainly survival. It is proposed that if such responses were occurring in vivo, there would be a shift in the normal balance between cell survival and cell death, which maintains steady-state, macrophage-lineage numbers in tissues. Thus, there would be more cells in an inflammatory lesion or at a site of adjuvant action with the potential, following activation and/or differentiation, to perpetuate inflammatory or antigen-specific, immune responses, respectively.

Granulocyte/macrophage colony-stimulating factor plays an essential role in oxidized low density lipoprotein-induced macrophage proliferation

We and other groups have recently demonstrated that oxidized low density lipoprotein (Ox-LDL) induces proliferation of macrophages in vitro. Since previous immunohistochemical studies demonstrated that macrophages and macrophage derived foam cells proliferated in situ in atherosclerotic lesions, it seems reasonable to expect that the Ox-LDL-induced macrophage proliferation might be linked to the development of atherosclerotic lesions. Thus, clarification of the molecular cascades of Ox-LDL-induced macrophage proliferation is expected to enhance our knowledge of the pathogenesis of atherosclerosis. Recently, we demonstrated that the activation of PKC leads to release into the culture medium of granulocyte/macrophage colony-stimulating factor (GM-CSF) which plays an important role in Ox-LDL-induced macrophage proliferation. In this review article, we mainly show the role of GM-CSF in the Ox-LDL-induced macrophage proliferation. Moreover, based on our recent findings, we summarize the Ox-LDL-induced signaling pathway for macrophage proliferation.

Comparison of macrophage responses to oxidized low-density lipoprotein and macrophage colony-stimulating factor (M-CSF or CSF-1)

Modification of low-density lipoprotein (LDL), for example by oxidation, could be involved in foam cell formation and proliferation observed in atherosclerotic lesions. Macrophage colony-stimulating factor (CSF-1 or M-CSF) has been implicated in foam cell development. It has been reported previously that oxidized LDL (ox.LDL) and CSF-1 synergistically stimulate DNA synthesis in murine bone-marrow-derived macrophages (BMM). The critical signal-transduction cascades responsible for the proliferative response to ox.LDL, as well as their relationship to those mediating CSF-1 action, are unknown. We report here that ox.LDL stimulated extracellular signal-regulated protein kinase (ERK)-1, ERK-2 and phosphoinositide 3-kinase activities in BMM but to a weaker extent than optimal CSF-1 concentrations at the time points examined. Inhibitor studies suggested at least a partial role for these kinases, as well as p70 S6-kinase, in ox.LDL-induced macrophage survival and DNA synthesis. For the DNA synthesis response to CSF-1, the degree of inhibition by PD98059, wortmannin and rapamycin was significant at low CSF-1 concentrations but was reduced as the CSF-1 dose increased. Using BMM from CSF-1-deficient mice (op/op) and a neutralizing antibody approach, we found no evidence for an essential role for endogenous CSF-1 in ox.LDL-mediated survival or DNA synthesis; likewise, with the same approaches, no evidence was obtained for an essential role for endogenous granulocyte/macrophage-CSF in ox.LDL-mediated macrophage survival and, in contrast with the literature, ox.LDL-induced macrophage DNA synthesis.

Metabolism of oxidized LDL by macrophages

Oxidation products of lipids and proteins are found in atherosclerotic plaque and in macrophage foam cells. Macrophages avidly endocytose in-vitro oxidized LDL and accumulate sterols. What is the evidence that such a process is involved in in-vivo foam cell formation? The present review surveys current knowledge on the metabolism of oxidized LDL by macrophages, and the types, amounts and location of oxidation products that accumulate in these cells. Comparable studies of lesion lipoproteins and foam cells indicate that limited extracellular lipoprotein oxidation, perhaps followed by more extensive intracellular oxidation subsequent to uptake by macrophages, is a more likely scenario in vivo.

Macrophage proliferation in atherosclerosis

Oxidized LDL can induce an increase in intracellular calcium concentration and the activation of protein kinase C in mouse peritoneal macrophages. The activation of protein kinase C leads to the release into the culture medium of granulocyte-macrophage colony-stimulating factor, which plays a priming role in oxidized LDL-induced macrophage proliferation. The expression of granulocyte-macrophage colony-stimulating factor in macrophages by oxidized LDL is positively regulated in the 5'-flanking region of granulocyte-macrophage colony-stimulating factor gene from sequence -169 to -160, but negatively regulated from -91 to -82. Granulocyte-macrophage colony-stimulating factor released by oxidized LDL from macrophages induces proliferation in autocrine or paracrine fashion via the activation of phosphatidylinositol 3-kinase. The capacity of oxidized LDL to induce macrophage proliferation in vitro may be involved in the enhanced progression of atherosclerosis in vivo.

Sites of action of protein kinase C and phosphatidylinositol 3-kinase are distinct in oxidized low density lipoprotein-induced macrophage proliferation

Oxidized low density lipoprotein (Ox-LDL) can induce macrophage proliferation in vitro. To explore the mechanisms involved in this process, we reported that activation of protein kinase C (PKC) is involved in its signaling pathway (Matsumura, T., Sakai, M., Kobori, S., Biwa, T., Takemura, T., Matsuda, H., Hakamata, H., Horiuchi, S., and Shichiri, M. (1997) Arterioscler. Thromb. Vasc. Biol. 17, 3013-3020) and that expression of granulocyte/macrophage colony-stimulating factor (GM-CSF) and its subsequent release in the culture medium are important (Biwa, T., Hakamata, H., Sakai, M., Miyazaki, A., Suzuki, H., Kodama, T., Shichiri, M., and Horiuchi, S. (1998) J. Biol. Chem. 273, 28305-28313). However, a recent study also demonstrated the involvement of phosphatidylinositol 3-kinase (PI3K) in this process. In the present study, we investigated the role of PKC and PI3K in Ox-LDL-induced macrophage proliferation. Ox-LDL-induced macrophage proliferation was inhibited by 90% by a PKC inhibitor, calphostin C, and 50% by a PI3K inhibitor, wortmannin. Ox-LDL-induced expression of GM-CSF and its subsequent release were inhibited by calphostin C but not by wortmannin, whereas recombinant GM-CSF-induced macrophage proliferation was inhibited by wortmannin by 50% but not by calphostin C. Ox-LDL activated PI3K at two time points (10 min and 4 h), and the activation at the second but not first point was significantly inhibited by calphostin C and anti-GM-CSF antibody. Our results suggest that PKC plays a role upstream in the signaling pathway to GM-CSF induction, whereas PI3K is involved, at least in part, downstream in the signaling pathway after GM-CSF induction.