Lymphatic transport of high-density lipoproteins and chylomicrons

Abstract

The life cycles of VLDLs and most LDLs occur within plasma. By contrast, the role of HDLs in cholesterol transport from cells requires that they readily gain access to and function within interstitial fluid. Studies of lymph derived from skin, connective tissue, and adipose tissue have demonstrated that particles as large as HDLs require transport through lymphatics to return to the bloodstream during reverse cholesterol transport. Targeting HDL for therapeutic purposes will require understanding its biology in the extravascular compartment, within the interstitium and lymph, in health and disease, and we herein review the processes that mediate the transport of HDLs and chylomicrons through the lymphatic vasculature.

Figure 1. Organization of the lymphatic vasculature.

Lymphatic capillaries form blind-ended vessels in all organs (referred to as lacteals in the intestine). These vessels converge and transition into collecting lymphatic vessels that are surrounded by muscle (red lines overlaying lymphatic vessels). Collecting vessels are interrupted by LNs. Collecting vessels and the LNs are surrounded by adipose tissue. The largest collecting vessel, the thoracic duct, drains lymph collected from all organs into the bloodstream at the subclavian vein. Insets detail the structure of lymphatic capillaries with respect to lipoprotein absorption. In tissues such as skin and lung, button-like junctions separate endothelial cells and create flaps, which allow receptor-independent entry of molecules into lymph. In intestine, large pores may form at the tips. The precise artery-capillary organization is unknown. Entry of discoidal HDL into skin lymph may be mediated by receptors through binding of SR-B1 (left inset), but more work is needed to confirm this. The right inset depicts the fenestrated transition from arterial capillary to venous blood capillary around each intestinal lacteal. Smaller nutrients not packaged in chylomicrons enter the blood vasculature that leads to the portal vein, but chylomicrons are too large. They enter the lacteal vessels by means of size exclusion, which also plays a key role in routing of HDL into lymph in other organs. Macrophages and DCs endocytose at least some molecules in intestinal villi, reducing their transit into the lacteal vessels, even if they bypass entry into the blood vasculature.

Figure 2. The intravascular/extravascular cycle of HDL remodeling that maintains reverse cholesterol transport.

(i) Transfer of HDL across vascular endothelium. (ii) Production of small, lipid-poor apoA1-containing preβ-HDLs in interstitial fluid through the remodeling of spheroidal CE-rich α-HDLs. (iii) Conversion of preβ-HDLs to discoidal HDLs through uptake of unesterified cholesterol (chol) and phospholipid (PL) via the ABCA1 transporters of peripheral cells. (iv) Transport of the discs via the lymphatic system to the blood via the thoracic duct. (v) Conversion of the discs to spheroidal CE-rich α-HDLs in plasma through the action of LCAT. (vi) Transfer of CE from α-HDLs to liver cells, directly via SR-B1 receptors and indirectly via CETP and apoB-containing lipoproteins (VLDLs and LDLs) that are endocytosed by apoB100 receptors. The principal function of LCAT is to generate CEs for delivery to the liver. The net production of preβ-HDLs in interstitial fluid appears to be maintained by a high ratio of active to inactive PLTP in the presence of a near-zero cholesterol esterification rate, in contrast to a high esterification rate and lower active/inactive PLTP ratio in plasma. Black arrows represent the path of apoA1 as a component of different HDLs as they move between the intravascular and extravascular compartments. Red arrows represent the flow of cholesterol, initially as unesterified cholesterol in interstitial fluid and lymph, and then as CE in blood.