Altered activation of endothelial anti- and proapoptotic pathways by high-density lipoprotein from patients with coronary artery disease: role of high-density lipoprotein-proteome remodeling.

Riwanto M, Rohrer L, Roschitzki B, et al.
Circulation. 2013 Feb 26;127(8):891-904. doi: 10.1161/CIRCULATIONAHA.112.108753.

Background

Reduced levels of high-density lipoprotein (HDL) have long been associated with increased risk of coronary artery disease (CAD)[1]. HDL not only promotes reverse cholesterol transport [2,3], it has also been demonstrated to have anti-atherosclerotic effects, including anti-inflammatory characteristics and stimulation of endothelial nitric oxide (NO) production [4-8], although heterogeneous effects have been observed in CAD patients.
Endothelial dysfunction is thought to contribute to CAD progression [9-11]. More specifically, increased endothelial apoptosis appears to be associated with atherosclerotic lesion propensity, thus contributing to CAD [12-14].
To gain more insight in the variable vascular effects of HDL, this study compared the effects of HDL obtained from patients with stable CAD or acute coronary syndrome (ACS) with HDL from healthy controls, on endothelial antiapoptotic and proapoptotic signalling pathways.

Main results

• Both in a cell-based assay and in an in vivo study in apoE-deficient mice healthy HDL reduced endothelial apoptosis, whereas CAD-HDL and ACS-HDL did not.
• When fractionating healthy HDL, delipidated HDL was more potent at exerting an endothelial antiapoptotic activity than did reconstituted HDL or purified apolipoprotein A-1. This suggests an important role for the HDL proteome and its remodelling in endothelial apoptosis.
• Indeed, HDL from healthy and CAD individuals showed differential abundance of HDL-associated proteins.
• Healthy HDL and CAD-HDL differentially regulate downstream targets of HDL:  healthy HDL activates an antiapoptotic protein, whereas CAD-HDL activates a proapoptotic protein.

Conclusion

Healthy or CAD-HDL exert different effects on the endothelial cell wall, such that the healthy antiapoptotic effect of HDL is absent in HDL obtained from individuals with CAD or ACS. Instead, proapoptotic signalling pathways are activated in CAD-HDL. These differential downstream effects seem the result of the exact protein composition of HDL. These altered vascular effects are interesting in light of recent disappointing results from trials in which HDL was raised in patients with coronary disease. Remodelling of the HDL proteome in patients with CAD has important functional implications with respect to endothelial cell survival.

Editorial comment [15]

The observations of Riwanto et al support the proposal that inflammation can generate dysfunctional forms of HDL that lack the normal cardioprotective properties or even become detrimental. Some HDL proteins appear to increase the risk for CVD whereas others decrease it, and those effects seem independent of HDL-C and perhaps apoA-L. It is essential to develop new metrics for assessing HDL’s cardioprotective effects.

References

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BACKGROUND:
Endothelial dysfunction and injury are thought to play an important role in the progression of coronary artery disease (CAD). High-density lipoprotein from healthy subjects (HDL(Healthy)) has been proposed to exert endothelial antiapoptotic effects that may represent an important antiatherogenic property of the lipoprotein. The present study therefore aimed to compare effects of HDL(CAD) and HDL(Healthy) on the activation of endothelial anti- and proapoptotic pathways and to determine which changes of the lipoprotein are relevant for these processes.

METHODS AND RESULTS:
HDL was isolated from patients with stable CAD (HDL(sCAD)), an acute coronary syndrome (HDL(ACS)), and healthy subjects. HDL(Healthy) induced expression of the endothelial antiapoptotic Bcl-2 protein Bcl-xL and reduced endothelial cell apoptosis in vitro and in apolipoprotein E-deficient mice in vivo. In contrast, HDL(sCAD) and HDL(ACS) did not inhibit endothelial apoptosis, failed to activate endothelial Bcl-xL, and stimulated endothelial proapoptotic pathways, in particular, p38-mitogen-activated protein kinase-mediated activation of the proapoptotic Bcl-2 protein tBid. Endothelial antiapoptotic effects of HDL(Healthy) were observed after inhibition of endothelial nitric oxide synthase and after delipidation, but not completely mimicked by apolipoprotein A-I or reconstituted HDL, suggesting an important role of the HDL proteome. HDL proteomics analyses and subsequent validations and functional characterizations suggested a reduced clusterin and increased apolipoprotein C-III content of HDL(sCAD) and HDL(ACS) as mechanisms leading to altered effects on endothelial apoptosis.

CONCLUSIONS:
The present study demonstrates for the first time that HDL(CAD) does not activate endothelial antiapoptotic pathways, but rather stimulates potential endothelial proapoptotic pathways. HDL-proteome remodeling plays an important role for these altered functional properties of HDL. These findings provide novel insights into mechanisms leading to altered vascular effects of HDL in coronary disease