New Clues to HDL’s Benefits RevealedNew Clues to HDL’s Benefits Revealed

While “good” high-density lipoprotein (HDL) cholesterol is thought to ward off heart attacks, researchers suspect that there may be both beneficial and dysfunctional types of HDL. This theory was bolstered in December when Pfizer Inc halted a phase 3 clinical trial of torcetrapib, an HDL-boosting agent, after the drug was found to cause an increase in deaths and heart problems in patients.

“HDL is the next big therapeutic frontier [for cardiovascular disease], but the torcetrapib trial is telling us that it may not be enough to just raise HDL cholesterol,” said Jay Heinecke, MD, from the University of Washington in Seattle.

To determine which properties of HDL are cardioprotective, Heinecke and colleagues have broken down the lipoprotein into pieces. A close examination of the resulting peptides has produced some surprising findings that may lead to the development of new indicators of cardiovascular risk and better treatments for heart-related conditions (Vaisar T et al. J Clin Invest. 2007;117:746-756).

Researchers have suspected that, in addition to its ability to promote the efflux of cholesterol from cells, HDL may have anti-inflammatory and antioxidant properties that contribute to its cardioprotective effects (Barter PJ et al. Circ Res. 2004;95:764-772). To tease out these and other potential characteristics of HDL, Heinecke's laboratory group used a technique called shotgun proteomics, a direct analysis of the complex makeup of proteins. Specifically, the investigators used the enzyme trypsin to break HDL into a peptide mixture that they analyzed with mass spectrometry.

Despite HDL’s usefulness as a predictor of cardiovascular risk, scientists didn't really know its precise composition, said Heinecke. “But with this technology, we can see what it's made of,” he said.

As expected, Heinecke and his team found proteins involved in lipid metabolism in the mixture, but they found other components as well. “The big surprise was that we found things that no one had anticipated,” said Heinecke. These included protease inhibitors, which block enzymes that degrade proteins. The protease inhibitors within HDL may be involved in keeping atherosclerotic plaque from breaking apart and causing myocardial infarctions.

“One of the leading hypotheses for what triggers plaque rupture is proteolytic activation,” explained Heinecke. “Breaking down the structural elements of the plaque weakens it and makes it prone to rupture.”

The investigators also found that HDL contains complement regulatory proteins, which are important for innate immunity. Because complement activation is implicated in tissue damage during acute myocardial infarction, HDL's complement regulatory properties may help limit injury to cardiac cells.

Differences in HDL between healthy individuals and patients with coronary artery disease were also revealed in this study. Compared with the HDL found in healthy controls, HDL from patients contained higher levels of apoE, a protein involved in lipoprotein transport. “That was also unanticipated,” said Heinecke, but the finding was confirmed when the researchers looked at additional individuals with coronary artery disease matched for age, sex, and triglyceride and cholesterol levels. What role apoE may play in heart health is currently unknown, but Heinecke and others are conducting animal experiments to find the answer.

While cholesterol measurements are used to predict who is at risk for heart disease, they provide an incomplete picture, said Heinecke. And because the first manifestation of heart disease in one third of individuals is sudden death, better predictive tools could be life saving. A clearer understanding of which components of HDL impart health benefits may help researchers develop better therapies for patients with cardiovascular disease as well as design tests to determine who is at risk of developing the disease. If researchers succeed in identifying which components of HDL are significant, using mass spectrometry may be an effective way to analyze HDL samples and help predict which patients are at increased risk, said Heinecke.

“HDL proteomics is in its infancy, and preliminary findings will need to be confirmed using standardized approaches in larger clinical samples,” said experts in an accompanying commentary (Reilly MP and Tall AR. J Clin Invest. 2007;117:595-598). “However, this approach promises to better elucidate the relationship of HDL to atherosclerosis and its complications and could eventually help in the development of biomarkers to predict the outcome of interventions that alter HDL levels and functions,” they wrote.