LDL particles

As yet, no human diseases have been identified as a result of FATPl mutations. However, genetic polymorphisms in the human FATPl gene have been linked to dyslipidemia. An A/G exchange at position +48 in intron 8 of the FATPl gene has been shown to result in increased TG concentrations in female but not in male subjects. In a second study, the same polymorphism was linked to increased postprandial TG concentrations and smaller low density lipoprotein (LDL) particles. To date, it is still unknown if this polymorphism is associated with altered levels of FATPl expression and/or function. 

VLDL is the precursor of IDL, which is then converted to LDL. Only one molecule of apo B-lOO is present in each of these lipoprotein particles, and this is conserved during the transformations. Thus, each LDL particle is derived from only one VLDL particle (Figure 25-4). Two possible fates await IDL. It can be taken up by the liver directly via the LDL (apo B-lOO, E) receptor, or it is converted to LDL. In humans, a relatively large proportion forms LDL, accounting for the increased concentrations of LDL in humans compared with many other mammals. 

The antioxidant activities of carotenoids and other phytochemicals in the human body can be measured, or at least estimated, by a variety of techniques, in vitro, in vivo or ex vivo (Krinsky, 2001). Many studies describe the use of ex vivo methods to measure the oxidisability of low-density lipoprotein (LDL) particles after dietary intervention with carotene-rich foods. However, the difficulty with this approach is that complex plant foods usually also contain other carotenoids, ascorbate, flavonoids, and other compounds that have antioxidant activity, and it is difficult to attribute the results to any particular class of compounds. One study, in which subjects were given additional fruits and vegetables, demonstrated an increase in the resistance of LDL to oxidation (Hininger et al., 1997), but two other showed no effect (Chopra et al, 1996 van het Hof et al., 1999). These differing outcomes may have been due to systematic differences in the experimental protocols or in the populations studied (Krinsky, 2001), but the results do indicate the complexity of the problem, and the hazards of generalising too readily about the putative benefits of dietary antioxidants. 

In this reaction scheme, the steady-state concentration of peroxyl radicals will be a direa function of the concentration of the transition metal and lipid peroxide content of the LDL particle, and will increase as the reaction proceeds. Scheme 2.2 is a diagrammatic representation of the redox interactions between copper, lipid hydroperoxides and lipid in the presence of a chain-breaking antioxidant. For the sake of clarity, the reaction involving the regeneration of the oxidized form of copper (Reaction 2.9) has been omitted. The first step is the independent decomposition of the Upid hydroperoxide to form the peroxyl radical. This may be terminated by reaction with an antioxidant, AH, but the lipid peroxide formed will contribute to the peroxide pool. It is evident from this scheme that the efficacy of a chain-breaking antioxidant in this scheme will be highly dependent on the initial size of the peroxide pool. In the section describing the copper-dependent oxidation of LDL (Section 2.6.1), the implications of this idea will be pursued further. 

Oxidation of the fatty acids in an LDL particle shares many of the characteristics associated with lipid peroxidation in other biological or chemical systems. Once initiated peroxyl radicals are formed and this results in the oxidation of a-tocopherol to give the a-tocopheroyl radical (Kalyanaraman etal., 1990). This can be demonstrated by e.s.r. techniques that allow the direct observation of stable radicals such as the a-tocopheroyl radical. After the a-tocopheryl radical is consumed, lipid-derived peroxyl radicals can be detected after reaction with spin traps (Kalyanaraman etal., 1990, 1991). 

The potency of a chain-breaking antioxidant, which scavenges peroxyl radicals, will decrease as the concentration of lipid peroxides in the LDL particle increases (Scheme 2.2). This is illustrated in the experiment shown in Fig. 2.3 in which the antioxidant potency of a peroxyl radical scavenger (BHT) decreases as a function of added exogenous hpid hydroperoxide. If the endogenous lipid peroxide content of LDL were to vary between individuals, this could explain the observed diferences in the effectiveness of a-tocopherol in suppressing lipid peroxidation promoted by copper. 

In summary, in our view the principal fectors that contribute to the oxidizability of LDL assessed by the addition of a transition metal such as copper ate (1) the lipid hydroperoxide content of the LDL particle and (2) the a-tocopherol content. Other chain-breaking antioxidants such as ubiquinol and the carotenoids are present only at low concentrations in most individuals, and are unlikely to make a significant contribution. 

Esterbauer et cil. (1992) have studied the in vitro effects of copper on LDL oxidation and have shown that there are three distinct stages in this process. In the first part of the reaction, the rate of oxidation is low and this period is often referred to as the lag phase the lag phase is apparently dependent on the endogenous antioxidant content of the LDL, the lipid hydroperoxide content of the LDL particle and the fatty acid composition. In the second or propagation phase of the reaction, the rate of oxidation is much faster and independent of the initial antioxidant status of the LDL molecule. Ultimately, the termination reactions predominate and suppress the peroxidation process. The extensive studies of Esterbauer et al. have demonstrated the relative importance of the endogenous antioxidants within the LDL molecule in protecting it from oxidative modification. 

In contrast, Bowry and Stocker (1993) have recently proposed that a-tocopherol may act as a pro-oxidant within the LDL particle in vitro. Their studies have indicated that tocopherol-mediated LDL oxidation may take place when water-soluble alkyperoxyl radicals react with tocopheryl radicals in the absence of agents that regenerate the tocopheryl radical into a non-radical species (for example, ascorbate). Under these conditions. 

Lipid peroxidation is a radical-mediated chain reaction resulting in the degradation of polyunsaturated fatty acids (PUFAs) that contain more than two covalent carbon-carbon double bonds (reviewed by Esterbauer et al., 1992). One of the major carriers of plasma lipids is LDL, a spherical molecule with a molecular weight of 2.5x10 . A single LDL particle contains 1300 PUFA molecules (2700 total fatty-acid molecules) and is... 

The LDL particle, which has been oxidatively modified by the mechanisms described above, is no longer recognized by the classic LDL receptor and is taken up by the macrophage scavenger receptor. Importantly, ox-LDL also exhibits a variety of pro-inflammatory activities, as described below. 

After assessment and control of LDL cholesterol, patients with serum triglycerides of 200 to 499 mg/dL (2.26 to 5.64 mmol/L) should be assessed for atherogenic dyslipidemia (low HDL cholesterol and increased small-dense LDL particles) and metabolic syndrome. 

Patients with metabolic syndrome are twice as likely to develop type 2 diabetes and four times more likely to develop CHD.3,11 These individuals are usually insulin resistant, obese, have hypertension, are in a prothrombotic state, and have atherogenic dyslipidemia characterized by low HDL cholesterol and elevated triglycerides, and an increased proportion of their LDL particles are small and dense.3... 

Niacin (vitamin B3) has broad applications in the treatment of lipid disorders when used at higher doses than those used as a nutritional supplement. Niacin inhibits fatty acid release from adipose tissue and inhibits fatty acid and triglyceride production in liver cells. This results in an increased intracellular degradation of apolipoprotein B, and in turn, a reduction in the number of VLDL particles secreted (Fig. 9-4). The lower VLDL levels and the lower triglyceride content in these particles leads to an overall reduction in LDL cholesterol as well as a decrease in the number of small, dense LDL particles. Niacin also reduces the uptake of HDL-apolipoprotein A1 particles and increases uptake of cholesterol esters by the liver, thus improving the efficiency of reverse cholesterol transport between HDL particles and vascular tissue (Fig. 9-4). Niacin is indicated for patients with elevated triglycerides, low HDL cholesterol, and elevated LDL cholesterol.3... 

The predominant effects of fibrates are a decrease in triglyceride levels by 20% to 50% and an increase in HDL cholesterol levels by 9% to 30% (Table 9-8). The effect on LDL cholesterol is less predictable. In patients with high triglycerides, however, LDL cholesterol may increase. Fibrates increase the size and reduce the density of LDL particles much like niacin. 

Lipoprotein (a) is an independent risk factor for coronary artery disease [68]. It consists of two components an LDL particle and apolipoprotein (a) which are linked by a disulfide bridge. Apo(a) reveals a genetically determined size polymorphism, resulting from a variable number of plasminogen kringle IV-type repeats [69]. Statins either do not affect Lp(a) or may even increase Lp(a) [70, 71]. In a study of 51 FH patients, treated with 40 mgd 1 pravastatin, it has been shown that the increase in Lp(a) was greatest in patients with the low molecular-weight apo(a) phenotypes [70]. 

Low-density lipoproteins in plasma and arterial wall are susceptible to oxidation to form oxidized LDL, which are thought to promote the development of atherosclerosis. LDL particles have a density of about 1.05, a molecular weight of about 2.5 x 106, and a diameter of about 20 nm [119]. LDL composition from different donors varies widely an average LDL particle contains about 1200 molecules of unsaturated acids and antioxidants about six molecules of a-tocopherol, about 0.53 molecule of 7-tocopherol, about 0.33 molecule of (3-carotene, and about 0.18 molecule of lycopene [120], Rapid oxidation of LDL is started only after the depletion of tocopherols and carotenoids [121]. 

As mentioned earlier, ascorbate and ubihydroquinone regenerate a-tocopherol contained in a LDL particle and by this may enhance its antioxidant activity. Stocker and his coworkers [123] suggest that this role of ubihydroquinone is especially important. However, it is questionable because ubihydroquinone content in LDL is very small and only 50% to 60% of LDL particles contain a molecule of ubihydroquinone. Moreover, there is another apparently much more effective co-antioxidant of a-tocopherol in LDL particles, namely, nitric oxide [125], It has been already mentioned that nitric oxide exhibits both antioxidant and prooxidant effects depending on the 02 /NO ratio [42]. It is important that NO concentrates up to 25-fold in lipid membranes and LDL compartments due to the high lipid partition coefficient, charge neutrality, and small molecular radius [126,127]. Because of this, the value of 02 /N0 ratio should be very small, and the antioxidant effect of NO must exceed the prooxidant effect of peroxynitrite. As the rate constants for the recombination reaction of NO with peroxyl radicals are close to diffusion limit (about 109 1 mol 1 s 1 [125]), NO will inhibit both Reactions (7) and (8) and by that spare a-tocopherol in LDL oxidation. 

ATP III recognizes the metabolic syndrome as a secondary target of risk reduction after LDL-C has been addressed. This syndrome is characterized by abdominal obesity, atherogenic dyslipidemia (elevated triglycerides, small LDL particles, low HDL cholesterol), increased blood pressure, insulin resistance (with or without glucose intolerance), and prothrom-botic and proinflammatory states. If the metabolic syndrome is present, the patient is considered to have a CHD risk equivalent. 

More interest has been generated by the potential effects of estrogens as modulators of LDL oxidation, a mechanism considered to be the authentic mediator of the detrimental action of LDL particles in atherosclerosis. Oxidized LDL becomes trapped in an artery and is then internalized by macrophages (Steinberg 1997 Navab et al. 1996 Morel et al. 1983 Griendling and Alexander 1997). This internalization leads to the formation of lipid peroxides resulting... 

Because of the similarity, it is difficult to conclude whether the lipid changes induced by SERMs offer any advantage over the profile determined by HT. Triglyceride levels have been proposed as an independent risk factor for CVD in postmenopausal women (Miller 1998). Further, there are some indications that increases in triglycerides may favor the reduction in the size of LDL particles. Smaller LDL particles are more susceptible to oxidation and have been associated with a higher risk potential (Austin et al. 1988), but whether this observation confers any clinical prejudice to hypertriglyceridemia has not been proven at present. 

Fig. 9.4. One pure antiestrogen, ICI 182780, increased the resistance of LDL particles to oxidation. Isolated LDL particles were subjected to oxidation by copper, and the lag time to oxidation, as measured by changes in optical density, increased as a function of the concentration of ICI 182780 (upper panel). The increase in the lag time (min) determined by the different concentrations of ICI 182780 is shown in the lower panel... 

The relevance attributed to oxidized lipids, and particularly oxidized LDL, in atherogenesis has precipitated interest in the ability of SERMs to this regard. Ex vivo experiments have confirmed that both tamoxifen and raloxifene exert some protection against the oxidation of LDL particles (Arteaga et al. 2003 Zuckerman and Bryan 1996) and that, interestingly, raloxifene is a more powerful antioxidant than tamoxifen or estradiol. It seems that this antioxidant effect is not mediated by the activation of the ER since pure antiestrogens like ICI 182780 and other SERMs like EM 652 have proven to have similar protective effects on LDL (Hermenegildo et al. 2002) (Fig. 9.4). 

Hermenegildo C, Garcfa-Martinez MC, Tarfn JJ, Llcicer A, Cano A (2001) The effect of oral hormone replacement therapy on lipoprotein profile, resistance of LDL to oxidation and LDL particle size. Maturitas 38 287-295... 

Lp(a) can associate with LDL particles (Y3) and, as such, alter the intake of LDL by the apo-B E receptor pathway, thus indirectly influencing LDL and cholesterol metabolism. 

LDL particles are transported through the plasma and taken into peripheral cells by apoB receptor... 

E (a-tocopherol) Antioxidant in the hpid phase. Protects membrane lipids from peroxidation and helps prevent oxidation of LDL particles thought to be involved in atherosclerotic plaque formation... 

The HDL may subsequently be picked up by the liver throu the apoE receptor or deliver cholesterol throi the scavenger receptor SR-Bl (reverse cholesterol transport firom the periphery to the liver). The HDL may also transfer the cholesterol to an IDL reforming a normal, unoxidized LDL particle. 

Uncontrolled phagocytosis of oxidized LDL particles is a major stimulus for the development of foam cells and forty streaks in the vascular subendothelium. This process may be inhibited by increased dietary intake of... 

---