HDL in humans: a complex system of proteins, each with its own unique size distribution, metabolism, and diet regulation

Abstract

Despite the fact that epidemiological studies have established that high levels of HDL- cholesterol are associated with a decreased risk of cardiovascular disease, therapies that raise HDL-cholesterol have failed to show beneficial effect in cardiovascular disease prevention. This led us to step away from the HDL-cholesterol-centric vision of HDL, and instead investigate the role of HDL proteins in regulating HDL function in humans in vivo and how these functions may be altered by diet. Participants with low HDL-cholesterol and who were overweight or obese were placed on a controlled diet high in monounsaturated fat or carbohydrate. After each diet period, participants were infused with a stable isotope tracer. Blood was collected and tracer was followed for 70 hours. We isolated HDL by its primary protein apolipoprotein (Apo) A1, and separated HDL into 5 sizes. We determined the distribution of the HDL proteome across the 5 HDL sizes, and developed a targeted mass spectrometry method to quantify tracer in multiple HDL proteins simultaneously. The proteins monitored included ApoA1, ApoA2, ApoA4, ApoC3, ApoE, ApoJ, ApoL1, ApoM, and LCAT. Compartmental modeling was used to determine protein metabolic rates. We identified 97 HDL proteins that were shared by >50% of participants. These proteins each had a unique distribution across HDL sizes, and proteins with similar functions tended to localize on the same HDL size. Compartmental analysis showed that the 9 HDL proteins were directly secreted into plasma, and circulate within their specific size until removed from circulation. Dietary fat and carbohydrate did not alter the HDL proteome’s size-specific architecture or metabolic structure. Nonetheless, carbohydrate, when replacing fat, increased the metabolic rates of several proteins on specific HDL sizes. This study demonstrates that HDL in humans is composed of a complex system of proteins, each with its own unique size distribution, metabolism, and diet regulation. The metabolism of HDL proteins within a given size range, taken together with the unique proteome composition, suggest that HDL’s multiple biological functions may be orchestrated by several size and protein-based subspecies. The carbohydrate-induced hypermetabolic state of HDL proteins may indicate a mechanism by which carbohydrate decreases the cardioprotective properties of HDL.