Hepatic triglyceride synthesis

Wolfe has presented an excellent description of the systematic application of stable and radioactive isotope tracers in determining the kinetics of intestinal fat absorption, hepatic triglyceride synthesis, lipid mobilization, triglyceride-fatty acid recycling, and cholesterol turnover. 

Nicotinic acid exerts a variety of effects on lipoprotein metabolism (7,16,49). One of its most important actions is the inhibition of lipolysis in adipose tissue. This initial inhibition, like those of previously discussed antihyperlipidemic agents, produces a sequence of events that ultimately result in the lowering of plasma triglycerides and cholesterol. Impaired lipolysis decreases the mobilization of free fatty acids, thus reducing their plasma levels and their delivery to the liver. In turn, this decreases hepatic triglyceride synthesis and results in a decreased production of VLDL. Enhanced clearance of VLDL through stimulation of lipoprotein lipase also has been proposed to contribute to the reduction of plasma VLDL levels. Because LDL is derived from VLDL (Fig. 30.5), the decreased production of VLDL ultimately leads to a decrease in LDL levels. The sequential nature of this process has been clinically demonstrated. The reduction in triglyceride levels occurs within several hours after ... 

It seems well established that fatty livers produced by starvation, diabetes or the administration of anterior pituitary hormones are primarily due to an increased mobilization of fatty acids from adipose tissue (see chapter III 2) (Barrett et al. 1938 Stetten and Salcedo 1944). The rate of hepatic triglyceride synthesis is directly proportional to the level of free fatty acids in the blood (Feigelson et al. 1961), and increased mobilization will therefore residt in accelerated triglyceride synthesis. 

FIGURE 5 Sources of fatty acid for hepatic triglyceride synthesis. (1) Adipose tissue lipolysis, catalyzed by hormone-sensitive lipase, provides fatty acids that travel through the bloodstream to the liver. (2) Chylomiaon remnants still carry some triglyceride and are cleared from the circulation by the liver. (3) Carbohydrate is converted to fatty acids when glycogen stores are maintained. 

The hypertriglyceridemia of Type II diabetes, unlike that which is found with Type I diabetes, is not due to excessive adipocyte lipolysis. This is because only a small level of insulin action is required to suppress excessive adipose tissue hormone-sensitive lipase activity. In Type II diabetes, there is insufficient adipose tissue lipoprotein lipase and excessive hepatic triglyceride synthesis. Thus, inefficient VLDL triglyceride catabolism and excessive VLDL triglyceride secretion both contribute to the excess VLDL in Type II diabetes. 

That portion of acetyl CoA which has been already been converted into citrate may be exported into the cytoplasm where it will act as a substrate for fatty add synthesis. As mentioned above, this may be followed by increased hepatic triglyceride synthesis and lead to hypertriglycerid-aemia. Alternatively, some of the excess acetyl CoA may be incorporated into 3-hydroxy-3-methylglutaryl CoA which is a starting point for cholesterol synthesis or the formation of ketone bodies that may increase addosis. This addosis can inhibit urea excretion by the kidney which in turn may exacerbate symptoms of gout. A summary diagram of the main effects of ethanol on metabolism is shown in Figure 29.3. 

ACTIN ASSEMBLY KINETICS MICROTUBULE ASSEMBLY KINETICS Triglyceride synthesis kinetics (hepatic), LIRID TRACER KINETICS 2,4,5-Trihydroxyphenylalanine quinone, REDOX-ACTIVE AMINO ACIDS TRIMOLECULAR MOLECUARITY ORDER... 

Mechanism of Action Afibricacid derivative that inhibits lipolysis of fat in adipose tissue decreases liver uptake of free fatty acids and reduces hepatic triglyceride production. Inhibits synthesis of VLDL carrier apolipoprotein B. Therapeutic Effect Lowers serum cholesterol and triglycerides (decreases VLDL, LDL increases HDL). Pharmacokinetics Well absorbed from the GI tract. Protein binding 99%. Metabolized in liver. Primarily excreted in urine. Not removed by hemodialysis. Half-life 1.5 hr. 

LCAT), which catalyzes the synthesis of cholesterol esters (F14, S46, S59) apoA-II, which activates hepatic triglyceride lipase (J2) and apoC-II, which activates lipoprotein lipase, responsible for the hydrolysis of triglycerides in chylomicrons and VLDL (H20, L5). Their mode of action is considered in Section 4 when the individual apolipoproteins are discussed. 

Nicotinic acid inhibits hepatic triglyceride production and VLDL secretion, which lowers the plasma level of LDL and increases HDL. Nicotinic acid is mostly used to treat elevated LDL and VLDL by decreasing VLDL synthesis. 

The HMG-CoA reductase inhibitors (Statins like simvastatin, lovastatin, pravastatin, fluvastatin, etc.) inhibit the enzyme and thereby decrease the hepatic cholesterol synthesis and increase the synthesis of LDL receptors causing increased clearance of LDL and a reduced concentration of LDL cholesterol in plasma. HMG-CoA reductase inhibitors are used to treat elevated LDL which also causes a small reduction in plasma triglycerides and an increase in HDL cholesterol. 

The liver synthesizes two enzymes involved in intra-plasmic lipid metabolism hepatic triglyceride lipase (HTL) and lecithin-cholesterol-acyltransferase (LCAT). The liver is further involved in the modification of circulatory lipoproteins as the site of synthesis for cholesterol-ester transfer protein (CETP). Free fatty acids are in general potentially toxic to the liver cell. Therefore they are immobilized by being bound to the intrinsic hepatic fatty acid-binding protein (hFABP) in the cytosol. The activity of this protein is stimulated by oestrogens and inhibited by testosterone. Peripheral lipoprotein lipase (LPL), which is required for the regulation of lipid metabolism, is synthesized in the endothelial cells (mainly in the fatty tissue and musculature). 

Intolerance to intravenous lipid emulsion (IVFE), evidenced by increased serum triglyceride concentrations, is common in ARE Hypertriglyceridemia is thought to be caused by decreased catabolism of triglycerides and increased synthesis from free fatty acids (FFAs). Hepatic triglyceride lipase and peripheral lipoprotein lipase activ-... 

The release and utilization of hepatic triglycerides are blocked, although the hepatic synthesis of triglycerides is normal. 

Without the enzyme microsomal triglyceride transfer protein (MTP), hepatic triglycerides cannot be transferred to apoB-100. Patients with dysfunctional MTP fail to make any of the apoB-containing lipoproteins (VLDL, IDL, or LDL). MTP also plays a key role in the synthesis of chylomicrons in the intestine, and mutations of MTP that result in the inability of triglycerides to he transferred to either apoB-100 in the liver or apoB-48 in the intestine prevent VLDL and chylomicron production and cause the genetic disorder abetalipoproteinemia. 

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