New evidence suggests a role for HDL in inflammatory and coagulation processes

A study reported in the Journal of Clinical Investigation shows that HDL is enriched in proteins involved in complement activation, as well as protease inhibitors, supporting the concept that HDL plays a key role in the regulation of inflammation, coagulation and tissue remodelling. The protein composition of HDL is also altered in patients with coronary artery disease (CAD).1

In this study, researchers used shotgun proteomics to analyse the protein composition of HDL from healthy subjects (n=26) and subjects with CAD (n=7). In this process, the enzyme trypsin was used to cleave HDL proteins into a peptide mixture that could then be analysed using mass spectrometry.

Although the researchers found proteins involved in lipid metabolism, they also identified a number of protein families not previously known to reside in HDL. These included complement pathway and regulatory proteins and endopeptidase inhibitors. Notably, 23 of the 48 HDL-associated proteins were acute-phase-response proteins. Levels of these proteins are modified in acute infection and chronic inflammation. Taken together, these observations suggest that HDL carries proteins that play key roles in regulating complement activation, in tissue damage during acute myocardial infarction2, and in inhibiting proteolysis; this latter finding suggests that HDL may attenuate intraplaque hydrolysis, thereby protecting against plaque rupture. These functions have not been previously recognised for HDL

Compared with unmodified apoA-I and HDL, lipid-free apoA-I that had undergone AGE modification and AGE-modified HDL significantly reduced (by 40-70%) ABCA1- and ABCG1- mediated cholesterol effluxfrom the various cells,. This effect was reversed by partly inhibiting AGE formation with aminoguanidine or by reversing AGE modification using alagebrium chloride. The AGE-modified apoA-I was also less effective in stabilizing ABCA1 in THP-1 cells, as well as inhibiting expression of CD11B, an integrin that is expressed on the surface of monocytes.

Furthermore, the study showed that HDL isolated from healthy subjects and subjects with established CAD display distinct protein composition. When the analysis was extended to HDL3, this lipoprotein was shown to be significantly enriched in 5 proteins in men with CAD: apolipoprotein C-IV (apoC-IV), paraoxonase-1, complement C3, apoA-IV and apoE. These proteins play critical roles in macrophage biology, lipid metabolism and the inflammatory response. ApoE, a protein involved in lipoprotein transport, was the most significantly enriched protein in HDL3 from CAD patients. This finding was confirmed in subsequent analysis of samples from a second set of 64 female subjects, 32 with CAD and 32 age-matched controls. Immunochemical analysis showed that levels of apoE were significantly higher in HDL3 from subjects with CAD than from control subjects, although both groups had similar levels of HDL cholesterol, apoA-I, apoA-II and plasma triglycerides. These results suggest that apoE may play a role in modulating atherogenesis, by redistribution of apoE from HDL3 to HDL2, which may subsequently lead to impairment of cholesterol efflux from macrophages.

Although the findings of this report require confirmation in larger samples, they may help to elucidate the role of HDL in atherosclerosis, and could facilitate development of biomarkers that predict the outcome of interventions that modify HDL levels and function.3

References

1. Vaisar T, Pennathur S, Green PS et al. Shotgun proteomics implicates protease inhibition and complement activation in the anti-inflammatory properties of HDL. J Clin Invest 2007;117:746-56.

2. Niculescu F, Rus H. The role of complement activation in atherosclerosis. Immunol Res 2004;30:73-80.

3. Reilly MP, Tall AR. HDL proteomics: pot of gold or Pandora’s box? J Clin Invest 2007;117:595-8.

Commentary

Over the last decade, several studies have suggested that the protein content of HDL particles extends well beyond the major AI and AII apolipoproteins together with minor amounts of apolipoproteins CI, CII, CIII and E. Taking a shotgun to blast apart the entire complement of HDL protein components (“shotgun proteomics”) with subsequent identification of their constituent fragments by tandem LC-MS/MS has revealed an entirely new dimension of the functionality and potential atheroprotectivity of HDL particles.

Most strikingly, proteins implicated in processes independent of cholesterol transport, and more specifically in the regulation of diverse proteolytic processes, coagulation, the acute phase response and the complement system were especially prominent. Data are already available to suggest that HDL particles can inhibit complement activation and thus may modulate innate immunity; serine-type endopeptidase (SERPIN) activity, consistent with the HDL-mediated regulation of inflammatory processes; and platelet activation and thrombosis.

The implications of these findings are wide reaching and open new horizons in the following areas. (i) Development of functional assays to evaluate the status of HDL anti-atherogenic activity across a wide range of human pathologies involving elevated risk of cardiovascular disease. (ii) Identification of simple HDL biomarkers not only of clinical diagnostic value, but equally of value in assessing the impact of specific therapeutic approaches targeted to improve HDL functionality and atheroprotective potential. The relevance of such biomarkers should also be evaluated in terms of their congruency with respect to the assessment of atherosclerosis progression by imaging techniques, and equally in terms of their predictive power relative to cardiovascular morbidity and mortality. (iii) Identification of distinct HDL particle subspecies, with potent inhibitory activity towards specific and potentially deleterious biological processes, and which might ultimately represent privileged targets for pharmacological modulation in the quest to halt atherosclerosis progression or even induce plaque regression.

Considered together, these exciting and innovative findings may be of special relevance to the future development of novel agents that not only raise HDL levels but also concomitantly induce formation of HDL subspecies with potent anti- atherogenic activity reflecting defined protein composition. Some caution is, however, warranted in the interpretation of these innovative findings, as protein activity and function are frequently intimately linked to conformation. Clearly then, direct evidence of the functionality of the individual proteins transported by HDL will provide us with additional information on their relevance to atheroprotection mediated by these particles in vivo.