Decay-accelerating factor regulates complement-mediated damage in the human atherosclerotic wall

Decay-accelerating factor suppresses complement C3 activation and retards atherosclerosis in low-density lipoprotein receptor-deficient mice

Decay-accelerating factor (DAF; CD55) is a membrane protein that regulates complement pathway activity at the level of C3. To test the hypothesis that DAF plays an essential role in limiting complement activation in the arterial wall and protecting from atherosclerosis, we crossed DAF gene targeted mice (daf-1(-/-)) with low-density lipoprotein-receptor deficient mice (Ldlr(-/-)). Daf-1(-/-)Ldlr(-/-) mice had more extensive en face Sudan IV staining of the thoracoabdominal aorta than Ldlr(-/-) mice, both following a 12-week period of low-fat diet or a high-fat diet. Aortic root lesions in daf-1(-/-)Ldlr(-/-) mice on a low-fat diet showed increased size and complexity. DAF deficiency increased deposition of C3d and C5b-9, indicating the importance of DAF for downstream complement regulation in the arterial wall. The acceleration of lesion development in the absence of DAF provides confirmation of the proinflammatory and proatherosclerotic potential of complement activation in the Ldlr(-/-) mouse model. Because upstream complement activation is potentially protective, this study underlines the importance of DAF in shielding the arterial wall from the atherogenic effects of complement.

CD59 but not DAF deficiency accelerates atherosclerosis in female ApoE knockout mice

Although the complement system has been implicated in atherosclerosis, the influence of membrane-bound complement regulators in this process has not been well understood. We studied the role of two membrane complement regulators, decay-accelerating factor (DAF) and CD59, in a murine model of atherosclerosis. DAF(-/-) and CD59(-/-) mice were crossed with apolipoprotein E (ApoE)-deficient mice to generate DAF(-/-)ApoE(-/-) and CD59(-/-)ApoE(-/-) mice. Mice were fed a high fat diet (HFD) for 8 or 16 weeks. En face analysis showed that CD59 deficiency led to more extensive lesions in female ApoE(-/-) mice both at 8 weeks (2.07+/-0.27% vs.1.34+/-0.21%, P=0.06) and 16 weeks (17.13+/-1.14% vs. 9.72+/-1.14%, P<0.001). Similarly, lesions measured by aortic root sectioning were larger in female CD59(-/-)ApoE(-/-) mice than in controls at 8 weeks of HFD feeding (20.74+/-1.33% vs. 13.12+/-1.46%, P<0.005). On the other hand, DAF deficiency did not significantly influence atherosclerosis in ApoE(-/-) mice. Immunohistochemistry revealed more abundant membrane attack complex (MAC) deposition and more collagen staining in the aortic roots of CD59(-/-)ApoE(-/-) mice. Unexpectedly, total plasma cholesterol levels in female CD59(-/-)ApoE(-/-) mice were found to be elevated compared with CD59(+/+)ApoE(-/-) mice. We conclude that CD59 but not DAF offered protection in atherosclerosis in the context of ApoE deficiency. The protective role of CD59 was gender-biased and most likely involved prevention of MAC-mediated vascular injury, with possible contribution from an undefined effect on plasma cholesterol homeostasis.

Statin-induced expression of decay-accelerating factor protects vascular endothelium against complement-mediated injury

Complement-mediated vascular injury is important in the pathophysiology of atherosclerosis and myocardial infarction. Because recent evidence shows that statins have beneficial effects on endothelial cell (EC) function independent of lipid lowering, we explored the hypothesis that statins modulate vascular EC resistance to complement through the upregulation of complement-inhibitory proteins. Human umbilical vein and aortic ECs were treated with atorvastatin or simvastatin, and decay-accelerating factor (DAF), membrane cofactor protein, and CD59 expression was measured by flow cytometry. A dose-dependent increase in DAF expression of up to 4-fold was seen 24 to 48 hours after treatment. Statin-induced upregulation of DAF required increased steady-state mRNA and de novo protein synthesis. L-Mevalonate and geranylgeranyl pyrophosphate reversed the effect, confirming the role of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibition and suggesting that constitutive DAF expression is negatively regulated by geranylgeranylation. Neither farnesyl pyrophosphate nor squalene inhibited statin-induced DAF expression, suggesting that the effect is independent of cholesterol lowering. Statin-induced DAF upregulation was mediated by the activation of protein kinase Calpha and inhibition of RhoA and was independent of phosphatidylinositol-3 kinase and NO activity. The increased DAF expression was functionally effective, resulting in significant reduction of C3 deposition and complement-mediated lysis of antibody-coated ECs. These observations provide evidence for a novel cytoprotective action of statins on vascular endothelium that is independent of the effect on lipids and results in enhanced protection against complement-mediated injury. Modulation of complement regulatory protein expression may contribute to the early beneficial effects of statins in reducing the morbidity and mortality associated with atherosclerosis.

bFGF and VEGF synergistically enhance endothelial cytoprotection via decay-accelerating factor induction

The complement-regulatory protein decay-accelerating factor (DAF) can be upregulated on endothelial cells (EC) by protein kinase C (PKC)-dependent and -independent pathways. We hypothesized that basic fibroblast growth factor (bFGF) might induce EC DAF expression, providing a cytoprotective mechanism for angiogenic neovessels against complement-mediated injury. Incubation of umbilical vein, aortic, and dermal EC with bFGF or vascular endothelial growth factor (VEGF) significantly increased DAF expression. Growth factor-induced EC proliferation was inhibited by PKC antagonists. In contrast, although PKC antagonists inhibited VEGF-induced DAF expression, bFGF-induced DAF was unaffected. Investigation of mitogen-activated kinase (MAPK) pathways also revealed differences, with bFGF-induced DAF dependent on p44/42 and p38 MAPK and VEGF requiring activation of p38 MAPK alone. Upregulation of DAF by bFGF was functionally relevant, reducing C3 deposition on EC after complement activation by 60% and resulting in marked reduction in complement-mediated EC lysis. bFGF and VEGF were synergistic in terms of DAF expression, resulting in enhanced cytoprotection. These observations reveal parallel PKC-dependent and -independent pathways regulating complement activation during angiogenesis. Further elucidation of these pathways may provide important insights into innate cytoprotective mechanisms in endothelium.

Mechanisms of signal transduction activated by sublytic assembly of terminal complement complexes on nucleated cells

The sublytic assembly of C5b-7, C5b-8, and C5b-9, activates membrane phospholipases through heterotrimeric G proteins and stimulates a variety of cellular activities including prostanoids, leukotrienes, and cytokines synthesis. Activation of mitotic signaling through Ras, Raf-1, ERK1, and phosphatidylinositol-3 kinase (PI3-K) was induced in B lymphocytes, endothelial, and smooth muscle cells. PI3-K activation by C5b-9 induced STAT3 phosphorylation and translocation from cytoplasm to nucleus. This complex signaling mechanism is directly involved in many biological functions such as endo- and exocytosis, cell cycle progression, activation of transcription, and protein synthesis. The key role of this signaling pathway is reflected on cell survival and proliferation in acute and chronic inflammation where complement activation is an ubiquitous event.

Complement activation and atherosclerosis

Atherosclerosis is an inflammatory disease mediated through the action of monocyte/macrophages, complement and T-lymphocytes. C5a and monocyte chemotactic factor released during complement activation in the arterial wall may participate in the initial monocyte recruitment. Assembly of C5b-9 on cells of the arterial wall may also induce cell lysis. On the other hand, sublytic assembly of C5b-9 on smooth muscle cells (SMC) and endothelial cells (EC) induces cell activation and proliferation. Analysis of mitogen activated protein kinases (MAPK) pathways induced by C5b-9 in aortic SMC revealed that extracellular signal regulated kinase (ERK) 1, c-jun NH2-terminal kinase (JNK) 1, and p38 MAPK are all activated by C5b-9. ERK1 activity was inhibited by wortmannin suggesting that ERK1 pathway is activated through phosphatidyl inositol -3 (PI 3-) kinase. Sublytic C5b-9 assembly on the plasma membrane was also able to activate Janus kinase (JAK) 1, signal transducer and activator (STAT) 3 and STAT4 in EC. JAK1 but not STAT3 activation induced by C5b-9 is dependent on Gi protein activation. New evidence accumulated during the last decade support the role of complement activation in both initiation and progression of the atherosclerotic lesions. Complement system activation is a major component of the chronic inflammatory process associated with atherosclerosis.

Interleukin-6 and interleukin-8 protein and gene expression in human arterial atherosclerotic wall

Interleukin 6 (IL-6) and interleukin 8 (IL-8) are present in the human arterial atherosclerotic wall as cellular and extracellular deposits in the connective tissue matrix. Quantitative determinations of IL-6 by ELISA showed mean values of 27.6 +/- 3.3 ng/100 mg protein in normal intima, 37.3 +/- 2.1 ng/100 mg protein in fibrous plaque and 25.7 +/- 4.3 ng/100 mg total extracted protein in media. IL-8 levels were 3.5 +/- 0.6 ng/100 mg protein in normal intima, 11.3 +/- 2.1 ng/100 mg protein in fibrous plaque and 8.5 +/- 1.4 ng/100 mg total extracted protein in media. Fibrous plaques presented statistically significant higher levels of both IL-6 and IL-8. IL-6 and IL-8 gene transcripts were present in human iliac fibrous plaque and media prelevated at surgery indicating that a local production by the cells of the arterial wall participate to their accumulation. We also tested the role of complement activation in induction of IL-6 and IL-8 protein synthesis as well as the subsequent activation of endothelial cells. Only IL-8 was induced by complement activation and this may contribute to increased IL-8 levels found in the atherosclerotic wall. When exposed to terminal complement complexes, endothelial cells in culture also showed an increase of both DNA-synthesis and p70 S6 kinase activity indicating that complement is able to induce not only IL-8 synthesis but also cell activation. The presence of IL-6 and IL-8 in the arterial wall where complement activation also occurred, clearly show the involvement of inflammatory events in initiation and progression of atherosclerosis.

Tumor necrosis factor-alpha in human arterial wall with atherosclerosis

Tumor necrosis factor-alpha (TNF-alpha) was demonstrated in human normal and atherosclerotic aorta, iliac and femoral arteries by immunohistochemistry using monoclonal and polyclonal antibodies. TNF was present in the cells of the arterial wall and as granular and diffuse extracellular deposits in the connective tissue matrix. Quantitative determinations of TNF by ELISA showed mean values of 21.7 +/- 0.7 ng/100 mg total extracted protein in normal intima, 38.2 +/- 0.5 in intimal thickenings, 25.5 +/- 1.1 in fibrous plaques and 16.8 +/- 0.2 ng/100 mg total extracted protein in media. Intimal thickenings presented the highest amounts of TNF with a statistically significant difference when compared to normal intima (P less than 0.05) and media (P less than 0.01). TNF-alpha concentrations in arterial eluates were about 200 times higher than in the corresponding serum samples. Western blotting analysis confirmed TNF-alpha eluted from the arterial wall to be about 17 kDa similar to human recombinant TNF-alpha. TNF-alpha in human atherosclerotic wall could be actively involved in the inflammatory events associated with atherosclerosis.