Serum amyloid A facilitates the binding of high-density lipoprotein from mice injected with lipopolysaccharide to vascular proteoglycans

HDL dysfunction in diabetes: causes and possible treatments

HDL is known to be inversely correlated with cardiovascular disease due to its diverse antiatherogenic functions. These functions include cholesterol efflux and reverse cholesterol transport, antioxidative and anti-inflammatory activities. However, HDL has been shown to undergo a loss of function in several pathophysiological states, as in the acute phase response, obesity and chronic inflammatory diseases. Some of these diseases were also shown to be associated with increased risk for cardiovascular disease. One such disease that is associated with HDL dysfunction and accelerated atherosclerosis is diabetes mellitus, a disease in which the HDL particle undergoes diverse structural modifications that result in significant changes in its function. This review will summarize the changes that occur in HDL in diabetes mellitus and how these changes lead to HDL dysfunction. Possible treatments for HDL dysfunction are also briefly described.

Molecular mechanisms of vascular effects of High-density lipoprotein: alterations in cardiovascular disease

Low high-density lipoprotein (HDL)-cholesterol levels are associated with an increased risk of coronary artery disease (CAD) and myocardial infarction, which has triggered the hypothesis that HDL, in contrast to low-density lipoprotein (LDL), acts as an anti-atherogenic lipoprotein. Moreover, experimental studies have identified potential anti-atherogenic properties of HDL, including promotion of macrophage cholesterol efflux and direct endothelial-protective effects of HDL, such as stimulation of endothelial nitric oxide production and repair, anti-apoptotic, anti-inflammatory and anti-thrombotic properties. Studies in gene-targeted mice, however, have also indicated that increasing HDL-cholesterol plasma levels can either limit (e.g. apolipoprotein A-I) or accelerate (e.g. Scavenger receptor class B type I) atherosclerosis. Moreover, vascular effects of HDL have been observed to be heterogenous and are altered in patients with CAD or diabetes, a condition that has been termed 'HDL dysfunction'. These alterations in biological functions of HDL may need to be taken into account for HDL-targeted therapies and considering raising of HDL-cholesterol levels alone is likely not sufficient in this respect. It will therefore be important to further determine, which biological functions of HDL are critical for its anti-atherosclerotic properties, as well as how these can be measured and targeted.

High-density lipoprotein loses its anti-inflammatory capacity by accumulation of pro-inflammatory-serum amyloid A

AIMS

High-density lipoprotein (HDL) is known to have potent anti-inflammatory properties. Monocyte chemoattractant protein-1 is an important pro-inflammatory cytokine in early atherogenesis. There is evidence that HDL can lose its protective function during inflammatory disease. In patients with end-stage renal disease (ESRD), epidemiological studies have documented that the inverse correlation between HDL-cholesterol and cardiovascular risk is lost. Many structural modifications leading to reduced HDL function have been characterized, but the functional consequences are not fully understood.

METHODS AND RESULTS

We showed that HDL from patients with ESRD has a lower anti-inflammatory potential by reduced inhibition of monocyte chemoattractant protein-1 formation in vascular smooth muscle cells. Via a proteomic approach, we identified proteins in HDL from ESRD patients exerting pro-inflammatory actions. By chromatographic separation of proteins and mass-spectrometric analysis, we found serum amyloid A (SAA) to be one molecule acting as a potent pro-inflammatory protein. SAA is enriched in HDL from ESRD patients, correlating with reduced anti-inflammatory capacity. In SAA signal transduction, activation of formyl-peptide receptor 2 is involved. SAA enrichment in HDL of healthy subjects reduced the anti-inflammatory capacity of HDL and correlated with its decreased function.

CONCLUSION

These results suggest that SAA enrichment of HDL during disease conditions contributes to the decreased protective function. It is a novel finding that SAA acts as a pro-inflammatory molecule to reduce the anti-inflammatory properties of HDL.

Emerging role of high density lipoproteins as a player in the immune system

High density lipoproteins (HDL) possess a number of physiological activities. The most studied and, perhaps, better understood is the ability of HDL to promote excess cholesterol efflux from peripheral tissues and transport to the liver for excretion, a mechanism believed to confer protection against atherosclerotic cardiovascular disease. The ability of HDL to modulate cholesterol bioavailability in the lipid rafts, membrane microdomains enriched in glycosphingolipids and cholesterol, is evolutionary conserved and affects the properties of cells involved in the innate and adaptive immune response, tuning inflammatory response and antigen presentation functions in macrophages as well as B and T cell activation. Also sphingosine-1 phosphate (S1P), a major active sphingolipid carried by HDL, is of relevance in the pathogenesis of several immuno-inflammatory disorders through the modulation of macrophage and lymphocyte functions. Furthermore, HDL influence the humoral innate immunity by modulating the activation of the complement system and the expression of pentraxin 3 (PTX3). Finally, in humans, HDL levels and functions are altered in several immune-mediated disorders, such as rheumatoid arthritis, systemic lupus eritematosus, Crohn's disease and multiple sclerosis as well as during inflammatory responses. Altogether these observations suggest that the effects of HDL in immunity could be related, to either the ability of HDL to modulate cholesterol content in immune cell lipid rafts and to their role as reservoir for several biologically active substances that may impact the immune system.

High-density lipoprotein proteome dynamics in human endotoxemia

Background

A large variety of proteins involved in inflammation, coagulation, lipid-oxidation and lipid metabolism have been associated with high-density lipoprotein (HDL) and it is anticipated that changes in the HDL proteome have implications for the multiple functions of HDL. Here, SELDI-TOF mass spectrometry (MS) was used to study the dynamic changes of HDL protein composition in a human experimental low-dose endotoxemia model. Ten healthy men with low HDL cholesterol (0.7+/-0.1 mmol/L) and 10 men with high HDL cholesterol levels (1.9+/-0.4 mmol/L) were challenged with endotoxin (LPS) intravenously (1 ng/kg bodyweight). We previously showed that subjects with low HDL cholesterol are more susceptible to an inflammatory challenge. The current study tested the hypothesis that this discrepancy may be related to differences in the HDL proteome.

Results

Plasma drawn at 7 time-points over a 24 hour time period after LPS challenge was used for direct capture of HDL using antibodies against apolipoprotein A-I followed by subsequent SELDI-TOF MS profiling. Upon LPS administration, profound changes in 21 markers (adjusted p-value < 0.05) were observed in the proteome in both study groups. These changes were observed 1 hour after LPS infusion and sustained up to 24 hours, but unexpectedly were not different between the 2 study groups. Hierarchical clustering of the protein spectra at all time points of all individuals revealed 3 distinct clusters, which were largely independent of baseline HDL cholesterol levels but correlated with paraoxonase 1 activity. The acute phase protein serum amyloid A-1/2 (SAA-1/2) was clearly upregulated after LPS infusion in both groups and comprised both native and N-terminal truncated variants that were identified by two-dimensional gel electrophoresis and mass spectrometry. Individuals of one of the clusters were distinguished by a lower SAA-1/2 response after LPS challenge and a delayed time-response of the truncated variants.

Conclusions

This study shows that the semi-quantitative differences in the HDL proteome as assessed by SELDI-TOF MS cannot explain why subjects with low HDL cholesterol are more susceptible to a challenge with LPS than those with high HDL cholesterol. Instead the results indicate that hierarchical clustering could be useful to predict HDL functionality in acute phase responses towards LPS.