Lipid Metabolism

Hyperlipidemia or hyperlipemia refers to an increased cloudiness in serum due to an excess of circulating lipids. The term lipemia, the presence of lipids in serum, is often incorrectly used to describe excess circulating lipids. Hyperlipidemia and hyperlipoproteinemia are often used interchangeably, but hyperlipoproteinemia more correctly refers to an excess of circulating lipoproteins. Hypercholesterolemia and hypertriglyceridemia refer respectively to an excess of circulating cholesterol or triglyceride. Hypercholesterolemia or hypertriglyceridemia may occur alone or in combination with hyperlipoproteinemia. Normally hyperlipidemia occurs after ingesting a meal, but fasting hyperlipidemia is indicative of abnormal lipid metabolism.

Patricia SCHENCK
DVM, PhD

Dr. Schenck received her Masters degree in Animal Science and her DVM degree from the University of Illinois in Champaign-Urbana. After owning her own small animal practice, she returned to the University of Florida where she completed her PhD in lipid biochemistry. After completing a post-doc at the USDA in Peoria Illinois, she joined the Ohio State University, where she became interested in research in calcium regulation. After working in the pet food industry for a number of years, she joined the Endocrinology section in the Diagnostic Center for Population and Animal Health at Michigan State University in 2001. Her current research interests include developing new tests for increasing diagnostic utility in calcium and lipid disorders, hyperlipidemias in the dog, idiopathic hypercalcemia in the cat, and the relationships between lipids and parathyroid hormone.

1. Lipid Metabolism

Perturbations in any aspect of lipid metabolism may result in abnormal hyperlipidemia. Abnormalities may occur in lipid absorption, synthesis, esterification, lipoprotein synthesis, receptor-mediated uptake, bile formation and circulation, or reverse cholesterol transport.

Lipid Absorption

Cholesterol and triglycerides are absorbed in the small intestine. Cholesterol may be ingested in the diet (exogenous), or is derived from biliary secretion and desquamation of intestinal epithelial cells (endogenous) which may account for up to 50% of the total cholesterol present in the small intestinal lumen (Holt, 1972). Absorption requires bile acids and micelle formation (Figure 1). Bile salts are secreted by the liver and enter the small intestine via the bile, and most salts exist as conjugates with glycine or taurine. When the concentration of bile salts reaches a high enough level, bile salts form aggregates or micelles (Feldman et al., 1983), and allow approximately 30 to 60% of available cholesterol to be absorbed. Within the lumen of the intestine, cholesteryl esters from micelles are hydrolyzed by pancreatic cholesterol esterase. Free cholesterol passively diffuses across the intestinal mucosal cell wall (Westergaard et al., 1976). Within the intestinal cell, free cholesterol is re-esterified with fatty acids, a process mediated by the enzyme acyl CoA:cholesterylacyltransferase (ACAT). A combination of free cholesterol and cholesteryl esters are then secreted into chylomicron particles.

Figure 1. Digestion and absorption of lipids (from Gogny, 1994).

Within the intestinal lumen, triglycerides are hydrolyzed by pancreatic lipase to mono-glycerides, diglycerides, and free fatty acids. In combination with cholesterol, phospholipid, and bile salts, these monoglycerides, diglycerides, and free fatty acids form mixed micelles. These micelles release monoglycerides, diglycerides, and free fatty acids at the intestinal cell wall where they are absorbed (Figure 1). Within the intestinal cell, monoglycerides and diglycerides are re-esterified to form triglycerides. Triglycerides along with cholesteryl esters, free cholesterol, phospholipid, and proteins will be incorporated into chylomicron particles for release into the circulation via the lymphatic system and the thoracic duct.

Cholesterol Synthesis

Endogenous cholesterol synthesis contributes to the total body cholesterol concentration. Cholesterol can be synthesized by almost all cells, with the highest rate of synthesis in the liver and intestine (Turley et al., 1981). Approximately 1 g cholesterol per day is synthesized within the body from acetyl CoA. The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCoA reductase) is the rate-limiting enzyme in cholesterol synthesis (Alberts, 1988).

Lipoprotein Production

Lipoproteins are the main carriers of triglycerides and cholesterol in the blood and are important in the delivery of cholesterol to all tissues. Circulating lipoproteins are classified by their size, density, and electrophoretic behavior (Mahley et al., 1974a). Lipoproteins in humans have been well characterized (Alaupovic et al., 1968; Assmann, 1982; Shepherd et al., 1989), but direct correlations cannot be made to the dog due to many differences in lipoprotein characteristics (Mahley et al., 1974a; Mahley et al., 1974b).

Lipoproteins are micellar particles with a hydrophobic core containing triglycerides and cholesteryl esters, and an amphipathic outer surface containing phospholipid, unesterified cholesterol, and proteins (Assmann, 1982). Proteins within a lipoprotein tend to be specific for that lipoprotein class. Lipoprotein particles are not static, but are in a dynamic state of equilibrium, with transfer of components occurring between lipoproteins.

Five major classes of lipoproteins have been characterized, including:

• Chylomicrons
• Very low density lipoproteins (VLDL)
• Intermediate density lipoproteins (IDL)
• Low density lipoproteins (LDL)
• And high density lipoproteins (HDL).

Some mammals (such as humans and most monkeys) have a predominance of LDL and are classified as "LDL mammals" (Chapman, 1986). LDL mammals are more sensitive to elevations in LDL cholesterol and the development of atherosclerosis. Dogs and most other mammals are considered "HDL mammals" due to the predominance of circulating HDL. HDL mammals are less sensitive to elevated LDL cholesterol concentrations, and are more resistant to the development of atherosclerosis (Table 1).

In general, the larger lipoproteins are less dense, contain less protein, and more lipid. Chylomicrons are the largest of the lipoproteins with the lowest density. HDL are the smallest and heaviest of the lipoproteins. Characteristics of the individual lipoproteins are summarized in Table 2.

In the peripheral circulation, chylomicrons gain apoprotein C and apoprotein E from HDL (Figure 2), increasing their protein content (Capurso, 1987). Lipoprotein lipase activated by apoprotein C-II of chylomicrons hydrolyzes the triglyceride present in chylomicrons, creating a phospholipid-rich particle. Lipoprotein lipase is associated with endothelial cell surfaces, interacting with membrane associated heparan sulfate (Nilsson-Ehle et al., 1980). Apoprotein A is transferred to HDL, and a chylomicron remnant is formed.

Figure 2. Chylomicron metabolism. Chylomicron particles containing a high concentration of triglyceride are released from the intestinal mucosal cell into the lymphatics and to the circulation. Lipoprotein lipase hydrolysis of triglycerides within chylomicrons releases fatty acids and decreases the triglyceride content of chylomicrons, creating a chylomicron remnant. In addition, there is an exchange of apoproteins between HDL and chylomicrons. Chylomicrons contribute apoprotein A to HDL in exchange for apoproteins C and E. The chylomicron remnant formed is recognized by an apoprotein E receptor on hepatocytes and is removed from the circulation. A deficiency of lipoprotein lipase activity can result in decreased metabolism of chylomicrons to chylomicron remnants and thus a prolonged appearance of chylomicrons in the circulation.  

Chylomicron remnant formation is necessary for hepatic clearance (Cooper, 1977). Once chylomicron remnants are formed, they are rapidly removed from the circulation by the apoprotein E receptor in liver cells (Mahley et al., 1989).

VLDL are synthesized by hepatocytes (Figure 3), and are a major transporter of triglyceride (Mills et al., 1971). VLDL binds to lipoprotein lipase, and lipoprotein lipase hydrolyzes the triglyceride present in VLDL. This process may create VLDL remnants which can be removed by the liver via receptor or non-receptor-mediated uptake (Havel, 1984). HDL transfers apoprotein E to VLDL, creating an IDL particle. With further loss of triglyceride, phospholipid, and apoprotein, LDL is formed. Removal of LDL from the circulation is via the LDL receptor which binds both apoprotein B and apoprotein E (Goldstein et al., 1984).

Figure 3. Chylomicron, VLDL, LDL, and liver cholesterol metabolism. Chylomicron particles containing lipids are released from the intestine into the circulation. Cholesterol-rich chylomicron remnants form and are recognized by the apoprotein E receptor on hepatocytes. Once in the hepatocyte, cholesterol can be stored as cholesteryl ester (via the action of ACAT), can be excreted into bile as cholesterol or bile acids, or secreted into VLDL particles. Synthesis of cholesterol in the hepatocyte (via HMGCoA reductase) contributes to the available cholesterol pool. Lipoprotein lipase hydrolysis of triglyceride within secreted VLDL and exchange of apoproteins create a triglyceride-depleted IDL which forms the triglyceride-poor, cholesterol-enriched LDL particle. The LDL receptor recognizes apoproteins B and E and mediates uptake and removal of LDL from the circulation. A deficiency of lipoprotein lipase activity can result in decreased metabolism of VLDL to LDL and thus a prolonged appearance of VLDL in the circulation.  

Nascent HDL is secreted by the liver (Figure 4), and contains very little free cholesterol and cholesteryl ester. Free cholesterol is transferred from peripheral cells to nascent HDL, and these cholesterol- rich particles serve as substrate for lecithin:cholesterol acyltransferase (LCAT), converting free cholesterol to cholesteryl esters. With the increased concentration of cholesteryl esters, the core of HDL enlarges and becomes more spherical. Hepatic lipase may also play a role in the interconversion of HDL subfractions (Groot et al., 1981). The conversion of free cholesterol to cholesteryl esters and its subsequent transfer to other lipoproteins allows additional free cholesterol to transfer from the surface of cells and other lipoproteins to HDL (Kostner et al., 1987). Thus LCAT plays a key role in the transfer of free cholesterol from peripheral tissues to the liver (Albers et al., 1986).

Figure 4. Reverse cholesterol transport. Discoidal HDL (nascent HDL) is secreted by the liver and obtains unesterified cholesterol from peripheral cells. LCAT in the circulation esterifies this cholesterol, resulting in a more spherical cholesteryl ester-rich particle. If cholesteryl ester transfer protein (CETP) is present, cholesteryl ester is transferred from HDL to LDL, with exchange of triglyceride from LDL to HDL. LDL carrying cholesteryl ester derived from peripheral cells returns to the liver completing reverse cholesterol transport. In dogs with little CETP, other mechanisms exist to return cholesterol to the liver via HDL directly.  

In humans, cholesteryl ester transfer protein (CETP) is responsible for cholesteryl ester and triglyceride exchange between HDL and LDL or VLDL. Cholesteryl ester derived from free cholesterol in peripheral cells is transferred to LDL, which can then return to the liver via receptormediated uptake (reverse cholesterol transport) (Noel et al., 1984). Dogs however have low levels of CETP (Mahley et al., 1983); thus there is little transfer of cholesteryl ester to LDL. Without cholesteryl ester transfer, HDL remains enriched with cholesteryl esters, and is designated HDL1, or HDLc. In the dog, reverse cholesterol transport is completed via HDL uptake by the liver. The dog is a "HDL mammal" since most of the circulating cholesterol is carried by HDL and cannot be transferred to LDL as in humans (an "LDL mammal").