High-density lipoprotein and inflammation - new insights

Atherosclerosis is a chronic inflammatory disease, which is characterised by the accumulation of macrophages and T lymphocytes in the intima of the artery together with an increase in plasma levels of a number of inflammatory markers.  The anti-inflammatory properties of HDL and their main apolipoprotein, apoA-I, is a topic that is currently attracting considerable interest, as discussed by Forum Editor Professor Kerry-Anne Rye, Heart Research Institute, Sydney, Australia.

A key initial step in this process involves the recruitment and binding of monocytes to the endothelium of the artery. Activated endothelial cells express a number of adhesion molecules, including vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1), which bind the monocytes to the surface of endothelial cells. The monocytes subsequently migrate into the subendothelial space in a process that is mediated by the chemokine monocyte chemoattractant protein-1 (MCP-1), a key regulator of monocyte recruitment and hence an early component of the inflammatory response in atherosclerosis (Fig. 1).

Fig. 1. Recruitment and binding of monocytes to the endothelium

  

Monocytes that have entered the artery wall differentiate into macrophages and take up modified low-density lipoproteins (LDL) via scavenger receptors to form foam cells, the hallmark of atherosclerosis (Fig. 2).

Fig. 2. Once inside the artery wall, the monocytes differentiate to foam cells

  

High-density lipoproteins (HDL) have anti-inflammatory effects, which are implicated in its anti-atherogenic properties. Effects on endothelial cells are generally well described. In vitro studies have shown that spherical HDL from human plasma, as well as discoidal reconstituted HDL containing apolipoprotein A-I (apoA-I), inhibit expression of VCAM‑1 and ICAM-1 in endothelial cells and reduce the binding of monocytes to the endothelial surface (Fig. 3).

Fig. 3. Anti-inflammatory effects of HDL

  

However, there is no information about the effects of HDL on monocyte activation.

Recent new data have increased knowledge about the anti-inflammatory effects of HDL and apoA-I by showing that they inhibit monocyte activation as well as endothelial expression of MCP-1.

Murphy AJ, Woollard KJ, Hoang A et al. High-density lipoprotein reduces the human monocyte inflammatory response. Arterioscler Thromb Vasc Biol 2008 [available on-line].

Tölle M, Pawlak A, Schuchardt M et al. HDL-associated lysosphingolipids inhibit NAD(P)H oxidase-dependent monocyte chemoattractant protein-1 production. Arterioscler Thromb Vasc Biol 2008;28:1542-8.

In the first of these studies, researchers investigated the mechanism by which HDL and apoA-I prevent monocyte activation using human monocytes isolated from whole blood of healthy volunteers.

Stimulation with PMA led to activation of monocytes by increasing expression of the integrin CD11b, and this was inhibited by HDL in a dose-dependent manner. Notably, reconstituted HDL and apoA-I both inhibited CD11b expression to a similar extent as HDL, whereas incubation with phospholipid liposomes or albumin had no effect.

The mechanism of this effect differed from the anti-inflammatory actions of HDL on endothelial cells, in that cholesterol efflux appeared to be a requirement. Cholesterol acceptors studied in the experiments included HDL (isolated from plasma by sequential ultracentrifugation), reconstituted HDL and phospholipid liposomes, apoA-I and the apoA-I mimetic peptide, L37pA. The researchers also evaluated the role of the HDL receptor – SR-B1 (scavenger receptor class B1) and the ATP-binding cassette transporter A1 (ABCA1) in this mechanism. SR-B1 has been shown to support cholesterol efflux to HDL. In contrast, ABCA1 facilitates the delivery of cholesterol from cells to lipid-poor apoA-I in the extracellular space.

While blocking SR-B1 led to reduction although not elimination of the anti-inflammatory effects of HDL, this did not influence the effects of apoA-I. However, blocking ABCA1 (with an anti-ABCA1 antibody) completely abolished the inhibitory effect of apoA-I but had no effect on HDL. Furthermore, when the experiments were repeated using monocytes derived from patients with Tangier disease (a genetic disease in which ABCA1 is dysfunctional and unable to support cholesterol efflux to apoA-I), apo-AI did not decrease CD11b activation. However, HDL was still able to produce some inhibition in monocyte activation. Together, these findings indicate that the effects of apoA-I on monocyte activation are mediated by ABCA1, whereas in the case of HDL, several pathways may be implicated.

The authors highlighted potential clinical implications of their findings. Firstly, apoA-I mimetics such as L37pA, which was also shown to mimic the anti-inflammatory effects of apoA-I in the study, may have therapeutic potential for the treatment of inflammatory diseases, including atherosclerosis. Secondly, the observation that HDL was still able to affect inflammatory status in monocytes from Tangier disease patients, suggests potential for the application of therapeutic HDL strategies in patients at risk of cardiovascular disease. Finally, the effects of HDL in preventing or reversing monocyte activation may have application in the management of inflammatory disease.

In the second study, researchers from Germany (Medizinische Klinik, Charite, Berlin, University of Münster, University of Essen and Assmann-Stiftung für Prävention, Münster), the Netherlands (University Medical Center Groningen) and the USA (Scripps Research Institute, La Jolla California), investigated the influence of HDL on MCP-1, a key regulator of monocyte recruitment to sites of vascular inflammation. In particular, the study investigated the involvement of the HDL‑associated lysosphingolipids (S1P, sphingosine-1-phosphate and SPC, sphingosylphosphorylcholine) in this process.

In the first of these in vitro experiments, vascular smooth muscle cells were stimulated with thrombin in the absence and presence of HDL. In the presence of HDL, there was a concentration-dependent reduction in MCP-1 expression. The inhibitory effects of HDL were also seen in endothelial cells and macrophages, as well as in isolated mouse aortas at concentrations of HDL close to those observed in physiological settings. The reduction in MCP-1 production with HDL was associated with suppression of reactive oxygen species. The researchers showed that HDL directly inhibited NAD(P)H-oxidase, which is the main source of reactive oxygen species in the vasculature.

The researchers then investigated the mechanism by which HDL influences reactive oxygen species formation, by testing the effects of apoA-I and the lysosphingolipids on MCP-1 levels and superoxide generation, in both experimental models.  Both S1P and SPC had similar effects to HDL in inhibiting MCP-1 production, reactive oxygen species generation and NAD(P)H activation; however, apoA-I did not emulate the effects of HDL.

Using antagonists of the lysosphingolipid receptors (S1P1, S1P2 and S1P3), the researchers were able to show that the effects of HDL-associated lysosphingolipids were mediated via S1P3, as well as SR-B1. 

The authors concluded that inhibition of NAD(P)H oxidase-dependent MCP-1 production by HDL-associated lysosphingolipids may be an important mechanism by which HDL exerts anti-atherogenic effects.

The anti-inflammatory properties of HDL and their main apolipoprotein, apoA-I, is a topic that is currently attracting considerable interest. While the ability of HDL to inhibit inflammation in cultured cells in vitro has been known for some time,1-3 more recent studies have established that they also inhibit acute vascular inflammation in animals.4-5 This inhibition is associated with a reduction in endothelial expression of VCAM-1, ICAM-1 and MCP-1. The recent paper of Murphy et al. expands the focus of the anti-inflammatory properties of HDL by showing that they, as well as apoA-I, also inhibit the expression of CD11b, a key member of the CD11/CD18 integrin family, on the monocyte surface. Murphy and co-workers used a number of approaches to investigate the mechanism of this inhibition and established that it is dependent on the removal of cholesterol from the monocytes via ABCA1. In addition to having implications for inhibiting the events that occur during the initiation of atherosclerotic lesions, the inhibition of monocyte activation by HDL and apoA-I has the capacity to be potentially important in a number of inflammatory pathologies as well as acute coronary syndromes.

In another more mechanistically oriented study focusing on the intracellular signalling events that underlie the anti-inflammatory effects of HDL, Tölle et al. found that two HDL constituents - S1P and SPC - inhibit MCP-1 expression and the formation of reactive oxygen species by NAD(P)H oxidase in a range of cell types including vascular smooth muscle cells, macrophages, endothelial cells, as well as in aortic segments. S1P and SPC are structurally similar sphingolipid metabolites that are derived from sphingomyelin. They act as second messengers and signal via the G-coupled protein receptor S1P3. The fact that apoA-I did not mediate these beneficial events is consistent with what has been observed previously in vitro for the inhibition of ICAM-1 and VCAM-1 expression in cultured endothelial cells1 and highlights the functional importance of the lipids that are transported by HDL. While the lack of an effect of apoA-I in this study is at odds with what Murphy et al. reported for the ability of apoA-I to inhibit monocyte activation, this simply serves to highlight the diverse biological effects of this lipoprotein class.

In summary, while both of these studies give new mechanistic insights into the anti-inflammatory properties of HDL, they emphasise the fact that there are still many gaps in our knowledge. For example, we do not know if specific HDL subpopulations inhibit monocyte activation, or if this is an effect that is mediated by all types of HDL. Similarly, we do not know if S1P and SPC are constituents of all HDL subpopulations or whether the beneficial effects they confer are confined to specific subpopulations of particles.

References

1. Cockerill GW, Rye KA, Gamble JR, et al. High-density lipoproteins inhibit cytokine-induced expression of endothelial cell adhesion molecules. Arterioscler Thromb Vasc Biol 1995;15:1987-94

2. Park SH, Park JH, Kang JS, Kang YH. Involvement of transcription factors in plasma HDL protection against TNF-alpha-induced vascular cell adhesion molecule-1 expression. Int J Biochem Cell Biol 2003;35:168-82.

3. Calabresi L, Franceschini G, Sirtori CR et al. Inhibition of VCAM-1 expression in endothelial cells by reconstituted high density lipoproteins. Arterioscler Thromb Vasc Biol 1997;238:61-5.

4. Nicholls SJ, Dusting GJ, Cutri B et al. Reconstituted high-density lipoproteins inhibit the acute pro-oxidant and proinflammatory vascular changes induced by a periarterial collar in normocholesterolemic rabbits. Circulation 2005;111:1543-50.

5. Puranik R, Bao S, Nobécourt E et al. Low dose apolipoprotein A-I rescues carotid arteries from inflammation in vivo. Atherosclerosis 2008:196:240-7.


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