Lactate boosts TLR4 signaling and NF-kappaB pathway-mediated gene transcription in macrophages via monocarboxylate transporters and MD-2 up-regulation.

It has been shown that lactate induces insulin resistance. However, the underlying mechanisms have not been well understood. Based on our observation that lactate augments LPS-stimulated inflammatory gene expression, we proposed that lactate may enhance TLR4 signaling in macrophages, which has been shown to play an important role in insulin resistance in adipocytes. In this study, we demonstrated that lactate stimulated MD-2, a coreceptor for TLR4 signaling activation, NF-kappaB transcriptional activity, and the expression of inflammatory genes in human U937 histiocytes (resident macrophages). Similar enhancement of the inflammatory gene expression by lactate was also observed in human monocyte-derived macrophages. The essential role of MD-2 in lactate-augmented TLR4 signaling was confirmed by observation that the suppression of MD-2 expression by small interfering RNA led to significant inhibition of inflammatory gene expression. To further elucidate how lactate treatment enhances TLR4 activation, we showed that the augmentation of inflammatory gene expression by lactate was abrogated by antioxidant treatment, suggesting a critical role of reactive oxygen species in the enhancement of TLR4 activation by lactate. Finally, we showed that alpha-cyano-4-hydroxycinnamic acid, a classic inhibitor for monocarboxylate transporters, blocked lactate-augmented inflammatory gene expression and nuclear NF-kappaB activity, indicating that lactate transport through monocarboxylate transporters is required for lactate-enhanced TLR4 activation. Collectively, this study documents that lactate boosts TLR4 activation and NF-kappaB-dependent inflammatory gene expression via monocarboxylate transporters and MD-2 up-regulation.

Simvastatin suppresses LPS-induced MMP-1 expression in U937 mononuclear cells by inhibiting protein isoprenylation-mediated ERK activation.

Matrix metalloproteinase (MMP) plays a crucial role in periodontal disease and is up-regulated by oral Gram-negative, pathogen-derived LPS. In this study, we reported that simvastatin, a 3-hydroxyl-3-methylglutaryl-CoA reductase inhibitor, effectively inhibited LPS-stimulated MMP-1 as well as MMP-8 and MMP-9 expression by U937 mononuclear cells. Our studies showed that the geranylgeranyl transferase inhibitor inhibited LPS-stimulated MMP-1 expression, and addition of isoprenoid intermediate geranylgeranyl pyrophosphate (GGPP) reduced the inhibitory effect of simvastatin on LPS-stimulated MMP-1 expression. We also demonstrated that simvastatin inhibited the activation of Ras and Rac, and the inhibition was abolished by addition of GGPP. The above results indicate that protein isoprenylation is involved in the regulation of MMP-1 expression by LPS and simvastatin. Moreover, we showed that simvastatin inhibited LPS-stimulated nuclear AP-1, but not NF-kappaB activity, and the inhibition was reversed by addition of GGPP. Simvastatin also inhibited LPS-stimulated ERK but not p38 MAPK and JNK. Finally, we showed that the inhibition of LPS-stimulated ERK activation by simvastatin was reversed by GGPP. Taken together, this study showed that simvastatin suppresses LPS-induced MMP-1 expression in U937 mononuclear cells by targeting protein isoprenylation-mediated ERK activation.

Pioglitazone inhibits connective tissue growth factor expression in advanced atherosclerotic plaques in low-density lipoprotein receptor-deficient mice.

Connective tissue growth factor (CTGF) is expressed in atherosclerotic plaques. It is generally recognized that CTGF contributes to atherosclerosis by stimulating vascular smooth muscle cell (VSMC) proliferation and extracellular matrix production during the development of atherosclerosis. Recent studies indicate that CTGF may also contribute to plaque destabilization as it induces apoptosis and stimulates MMP-2 expression in VSMCs. Thiazolidinediones (TZDs), a new class of insulin sensitizing drugs for type 2 diabetes, inhibit atherosclerosis. However, their effect on CTGF expression in atherosclerotic plaques remains unknown. In this study, male LDL receptor-deficient mice were fed high-fat diet for 4 months to induce the formation of atherosclerotic plaques and then given the high-fat diet with or without pioglitazone for the next 3 months. At the end of the 7-month study, CTGF expression in aortic atherosclerotic lesions was examined. Results showed that CTGF expression was increased in mice fed the high-fat diet by seven-fold as compared to that in mice fed normal chow, but the treatment with pioglitazone significantly inhibited the high-fat diet-induced CTGF expression. To verify these in vivo observations, in vitro studies using human aortic SMC were conducted. Quantitative real-time PCR and Western blot showed that pioglitazone inhibited TGF-beta-stimulated CTGF expression. In conclusion, the present study has demonstrated that pioglitazone inhibits CTGF expression in mouse advanced atherosclerotic plaques and in cultured human SMCs, and hence unveiled a possible mechanism potentially involved in the inhibition of atherosclerosis by TZD.

Oxidized LDL immune complexes induce release of sphingosine kinase in human U937 monocytic cells.

The transformation of macrophages into foam cells is a critical event in the development of atherosclerosis. The most studied aspect of this process is the uptake of modified LDL through the scavenger receptors. Another salient aspect is the effect of modified LDL immune complexes on macrophages activation and foam cell formation. Macrophages internalize oxidized LDL immune complexes (oxLDL-IC) via the Fc-gamma receptor and transform into activated foam cells. In this study we examined the effect of oxLDL-IC on sphingosine kinase 1 (SK1), an enzyme implicated in mediating pro-survival and inflammatory responses through the generation of the signaling molecule sphingosine-1-phosphate (S1P). Intriguingly, oxLDL-IC, but not oxLDL alone, induced an immediate translocation and release of SK1 into the conditioned medium as evidenced by fluorescence confocal microscopy. Immunoblot analysis of cell lysates and conditioned medium revealed a decrease in intracellular SK1 protein levels accompanied by a concomitant increase in extracellular SK1 levels. Furthermore, measurement of S1P formation showed that the activity of cell-associated SK decreased in response to oxLDL-IC compared to oxLDL alone, whereas the activity of SK increased extracellularly. Blocking oxLDL-IC binding to Fc-gamma receptors resulted in decreased levels of extracellular S1P. The data also show that cell survival of human U937 cells exposed to oxLDL-IC increased compared to oxLDL alone. Exogenously added S1P further increased cell survival induced by oxLDL-IC. Taken together, these findings indicate that S1P may be generated extracellularly in response to modified LDL immune complexes and may therefore promote cell survival and prolong cytokine release by activated macrophages.

Iron loading increases cholesterol accumulation and macrophage scavenger receptor I expression in THP-1 mononuclear phagocytes.

Epidemiological studies have established that a high level of iron body stores is associated with increased risk of acute coronary heart disease. To explain this association, it has been proposed that iron catalyzes the production of highly reactive forms of free oxygen species, and thus, promotes low-density lipoprotein (LDL) oxidation, a lipoprotein that plays a critical role in atherogenesis. However, few studies have provided evidence to support this hypothesis. In the present study, we determined the effect of iron loading of THP-1 mononuclear phagocytes on LDL metabolism. We demonstrated that iron loading of THP-1 cells stimulated conjugated diene formation in LDL in the culture medium. In addition, iron loading of THP-1 cells significantly increased cholesteryl ester accumulation in cells exposed to native LDL, suggesting that during the incubation of the cells with native LDL, the LDL became oxidized and was taken up by the cells. We further demonstrated that the degradation of 125I-oxidized LDL was significantly increased in iron-loaded THP-1 cells. Lastly, we demonstrated that iron loading of THP-1 cells stimulated scavenger receptor expression in these cells. In conclusion, this study demonstrates that loading of mononuclear phagocytes with iron leads to oxidization of LDL, increased cellular cholesterol accumulation and scavenger receptor expression, and supports the hypothesis that increased macrophage iron levels promote atherogenesis.

High glucose and interferon gamma synergistically stimulate MMP-1 expression in U937 macrophages by increasing transcription factor STAT1 activity.

Recent diabetes control and complications trial and epidemiology of diabetes interventions and complications (DCCT/EDIC) and other clinical studies have reported that glucose control in patients with diabetes leads to a significant reduction of cardiovascular events and atherosclerosis, indicating that hyperglycemia plays an essential role in cardiovascular disease in diabetic patients. Although several mechanisms by which hyperglycemia promotes atherosclerosis have been proposed, it remains unclear how hyperglycemia promotes atherosclerosis by interaction with inflammatory cytokines. To test our hypothesis that hyperglycemia interplays with interferon gamma (IFN gamma), a key factor involved in atherosclerosis, to up-regulate the expression of genes such as matrix metalloproteinases (MMPs) and cytokines that are involved in plaque destabilization, U937 macrophages cultured in medium containing either normal or high glucose were challenged with IFN gamma and the expression of MMPs and cytokines were then quantified by real-time polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA). Results showed that high glucose and IFN gamma had a synergistic effect on the expression of MMP-1, MMP-9 and IL-1 beta. High glucose also enhanced IFN gamma-induced priming effect on lipopolysaccharide (LPS)-stimulated MMP-1 secretion. Furthermore, high glucose and IFN gamma exert the synergistic effect on MMP-1 expression by enhancing STAT1 phosphorylation and STAT1 transcriptional activity. In summary, this study revealed a novel mechanism potentially involved in diabetes-promoted cardiovascular disease.

High glucose enhances lipopolysaccharide-stimulated CD14 expression in U937 mononuclear cells by increasing nuclear factor kappaB and AP-1 activities.

We have demonstrated recently that high glucose augments lipopolysaccharide (LPS)-stimulated matrix metalloproteinase (MMP) and cytokine expression by U937 mononuclear cells and human monocyte-derived macrophages. Since CD14 is a receptor for LPS, one potential underlying mechanism is that high glucose enhances CD14 expression. In the present study, we determined the effect of high glucose on CD14 expression by U937 mononuclear cells. After being chronically exposed to normal or high glucose for 2 weeks or longer, cells were treated with LPS for 24 h. Real-time PCR showed that although high glucose by itself did not increase CD14 expression significantly, it augmented LPS-stimulated CD14 expression by 15-fold. Immunoassay showed a marked enhancement of both membrane-associated and soluble CD14 protein levels by high glucose. Further investigations using transcription factor activity assays and gel shift assays revealed that high glucose augmented LPS-stimulated CD14 expression by enhancing transcription factor nuclear factor kappaB (NFkappaB) and activator protein-1 (AP-1) activities. Finally, studies using anti-CD14 neutralizing antibody showed that CD14 expression is essential for the enhancement of LPS-stimulated MMP-1 expression by high glucose. Taken together, this study has demonstrated a robust augmentation by high glucose of LPS-stimulated CD14 expression through AP-1 and NFkappaB transcriptional activity enhancement, elucidating a new mechanism by which hyperglycemia boosts LPS-elicited gene expression involved in inflammation and tissue destruction.

Administration of pioglitazone in low-density lipoprotein receptor-deficient mice inhibits lesion progression and matrix metalloproteinase expression in advanced atherosclerotic plaques.

Recent clinical trials have provided evidence that pioglitazone reduces cardiovascular events in patients with type 2 diabetes. However, the underlying mechanisms are not well understood. Because it has been well established that disruption of atherosclerotic plaques is a key event involved in acute myocardial infarction, we hypothesized that pioglitazone reduces cardiovascular events by stabilizing atherosclerotic lesions. In this study, we used an animal model to test our hypothesis. Low-density lipoprotein receptor-deficient (LDLR-/-) male mice were first fed a high-fat diet for 4 months to induce the formation of aortic atherosclerotic plaques and then treated with pioglitazone for the next 3 months. Analysis of atherosclerotic plaques at the end of the study showed that treatment with pioglitazone at 20 mg/kg/day reduced the progression of atherosclerotic plaques as compared to untreated mice. Furthermore, gene array analysis, quantitative real-time polymerase chain reaction, and immunohistochemical analysis showed that pioglitazone inhibited high-fat diet-induced upregulation of matrix metalloproteinase (MMP) expression. Finally, Sirius red staining showed that atherosclerotic lesions in mice receiving pioglitazone had higher collagen contents than those in untreated mice. This study demonstrated for the first time that administration of pioglitazone in LDLR-/- mice inhibited lesion progression and MMP expression in established atherosclerotic plaques and thus delineated a potential mechanism by which pioglitazone reduces cardiovascular events in patients with type 2 diabetes.

Sodium lactate increases LPS-stimulated MMP and cytokine expression in U937 histiocytes by enhancing AP-1 and NF-kappaB transcriptional activities.

The plasma lactate concentration in patients with obesity and type 2 diabetes is often higher than that in nondiabetic individuals. Although it is known that increased lactate concentration is an independent risk factor for developing type 2 diabetes, the underlying mechanisms are not well understood. Because inflammation plays an important role in the development of type 2 diabetes, we postulated that increased lactate level might contribute to the pathogenesis of type 2 diabetes by enhancing inflammation. In the present study, we demonstrated that preexposure of U937 macrophage-like cells to sodium lactate increased LPS-stimulated matrix metalloproteinase (MMP)-1, IL-1beta, and IL-6 secretion. Augmentation of LPS-stimulated MMP-1 secretion was diminished when sodium lactate was replaced by lactic acid that reduced pH in the culture medium. Furthermore, quantitative real-time PCR indicated that the increased secretion of MMP-1, IL-1beta, and IL-6 was due to increased mRNA expression. To explore the underlying signaling mechanism, blocking studies using specific inhibitors for NF-kappaB and MAPK cascades were performed. Results showed that blocking of either NF-kappaB or MAPK pathways led to the inhibition of MMP-1, IL-1beta, and IL-6 expression stimulated by sodium lactate, LPS, or both. Finally, electrophoretic mobility shift assays showed a synergy between sodium lactate and LPS on AP-1 and NF-kappaB transcriptional activities. In conclusion, this study has demonstrated for the first time that sodium lactate and LPS exert synergistic effect on MMP and cytokine expression through NF-kappaB and MAPK pathways and revealed a novel mechanism potentially involved in the development of type 2 diabetes and its complications.

Pre-exposure to high glucose augments lipopolysaccharide-stimulated matrix metalloproteinase-1 expression by human U937 histiocytes.

BACKGROUND AND OBJECTIVES: It has been well established that patients with diabetes have increased prevalence and severity of periodontal diseases. However, the underlying mechanisms are not well understood. Given that bacterial infection is the primary cause of periodontal disease, we postulated that hyperglycemia may interplay with bacterial virulence factors such as lipopolysaccharide to up-regulate matrix metalloproteinase (MMP), leading to increased periodontal tissue destruction. METHODS AND RESULTS: We showed that prolonged pre-exposure of U937 histiocytes to high glucose markedly increased lipopolysaccharide-stimulated MMP-1 secretion and mRNA expression. Our results also showed that the effect of high glucose on lipopolysaccharide-induced MMP-1 expression is cell type-specific because no similar response was observed in human gingival fibroblasts. In addition to MMP-1, high glucose also augments lipopolysaccharide-stimulated MMP-7, -8, and -9 mRNA expression. In the investigation of the signaling pathways involved in the enhancement of lipopolysaccharide-induced MMP-1 expression by high glucose, we found that both high glucose and lipopolysaccharide regulate MMP-1 expression through the nuclear factor kappaB (NFkappaB) and mitogen-activated protein kinase (MAPK) cascades. CONCLUSIONS: The present study has shown that pre-exposure to high glucose and subsequent lipopolysaccharide treatment synergistically stimulates MMP-1 expression by mononuclear phagocytes through the NFkappaB and MAPK signaling pathways. This study has thus delineated a pathogenic mechanism that may be involved in the exacerbated periodontal disease in diabetic patients.