ApoB containing lipoproteins can interact with proteoglycans such as heparan sulphate and chondroitin sulphate found on the arterial wall. There are seven regions o f basic amino acids in apoB that can bind heparin (Section 1.1.1, W eisgraber & Rail, 1987) and at least three that can interact with chondroitin sulphate-rich aortic proteoglycans (Camejo et al. 1988). The primary receptor binding domain o f apoB coincides with regions identified in both the above studies, which may account for the fact that heparin is capable o f displacing LDL from the LDL-receptor (Goldstein et al. 1976). A physiological purpose o f this binding could be to enable triglyceride rich lipoproteins to bind to the endothelial surface prior to hydrolysis by lipoprotein lipase (Zilversmit, 1973) which is also anchored to the surface of endothelial cells by heparan sulphate (Cheng et al. 1981). However, the fact that triglyceride-rich VLDL have a lower affinity for arterial proteoglycans than LDL (Camejo et al. 1988) does not support this hypothesis, and may indicate that these regions only become exposed after hydrolysis of the triglyceride from the particle. The interaction is believed to be mediated by electrostatic forces between positively charged residues (Arg and Lys) on apoB and the negative charges on heparin. This interaction, which binds LD L to the endothelial surface o f arteries and capillaries, may be a contributing factor in atherogenesis (Hollander, 1976).
1 .2.5 The Lp(a) particle
The ability o f apoB to bind to the receptor can also be reduced by its covalent attachment to apo(a) to form Lp(a) particles. First identified by Berg (1963), high serum Lp(a) levels are an independent risk factor for the development o f CAD (Kostner et al. 1976, Armstrong et al. 1986). Lp(a) particles are larger and more dense than LD L since apo(a) has a molecular weight o f between 400 and TOOkDa. The amino acid sequence o f apo(a) was deduced by McLean et al. (1987), and shows striking homology with plasminogen. Plasminogen is the zymogen o f plasmin, a protease which digests fibrin clots, it has five homologous structures called kringles (due to their resemblance in shape to a Danish biscuit), and a C-terminal protease domain. The gene for apo(a) has been derived from the plasminogen gene by duplication, and the two genes lie within a 50kb region o f chromosome 6 (Malgaretti et al. 1992). The apo(a) gene has its own promoter (Wade et al. 1993), it has lost the N-terminal domain and kringles 1 to 3, but does encode many repeats o f kringle 4, followed by one kringle 5 and an inactive protease domain. Lp(a) particles vary markedly between individuals in both size and plasma levels. The size variation is due to the different numbers o f repeats of kringle 4, which can range from 2 to more than 20 (Lackner et al. 1991). The serum levels o f apo(a) are inversely related to the size (Utermann et al. 1987). M ore than 90% o f variation in Lp(a) levels are due to differences in the apo(a) gene (Boerwinkle et al. 1992), although the genes for apoE and the LD L-receptor also have an effect on Lp(a) levels (DeKnijff et al. 1991, Leitersdorf et al. 1991). Apo(a), like apoB, is synthesised in the hepatocytes but it is not yet clear where or when during production the two become covalently linked. Studies in baboons suggest that linkage could occur in the medium surrounding the hepatocytes or just before secretion from the cells (W hite et al. 1993) and studies with transgenic mice expressing apo(a) and infused with human LD L indicate that linkage can occur in the plasma (Chiesa et al. 1992). The disulphide bridge that forms this covalent link is believed to involve Cys3 7 3 4 or Cys4,9oin apoB (Coleman et al. 1990) and a Cys residue in the penultimate repeat o f
lipoprotein produced is not VLD L but Lp(a). Lp(a) has low affinity for the LDL-receptor - individuals with homozygous and heterozygous LDL-receptor defects catabolise Lp(a) at the same rate as normal individuals (Krempler et al. 1982, Knight et al. 1991). Furtherm ore, individuals with the apoB Glujjoo mutation (Section 1.4.3) accumulate Gln^goo-LDL, but not Arg3 5oo-Lp(a), in
their plasma (Perombelon et al. 1992). Armstrong et al. (1986) showed that skin fibroblasts in vitro degrade Lp(a) slower than LD L. However, the reason why attachment o f apo(a) to apoB affects the receptor-binding domain o f apoB is not understood. The alternative mechanism, through which most Lp(a) particles are catabolised, is not elucidated either. Native Lp(a) are not significantly taken up by the macrophage scavenger receptor (Snyder et al. 1992). However, Lp(a) particles are more susceptible to oxidation than LDL (Naruszewicz et al. 1992a & 1992b), and once oxidised they are cleared by the scavenger receptor (Haberland et al. 1992). Williams et al. (1992a) have also demonstrated that fibroblasts take up native Lp(a) in the presence o f LPL, this does not involve the lipolytic function o f LPL but may use the LPL associated heparan sulphate proteoglycans. If demonstrated to have a physiological role this mechanism could also lead to atherogenesis.
In addition to poor clearance of Lp(a) leading to high serum cholesterol levels and other lipid-related atherogenic factors, Lp(a) may also have an adverse effect on the clotting cascade. Lerch et al. (1980) have demonstrated that kringle 4 o f Lp(a) has weak binding to fibrin but no protease catalytic activity. Harpel et al. (1989) showed that Lp(a) competes with plasminogen for binding to fibrin and fibrinogen, and thus may inhibit the normal degradation o f blood clots. In addition, if Lp(a) binds to the fibrin which is present at the site o f arterial wall injury, it may supply the regenerating artery with a supply o f cholesterol. In moderation, this would be important for the synthesis o f new cell membranes, but in excess it may initiate atherogenesis. Rath et al. (1989) and Cushing et al. (1989) have demonstrated the presence o f apo(a) in human atherosclerotic plaques but not in normal aorta. Furtherm ore, transgenic mice expressing human
apo(a), on a high fat diet, develop atherosclerotic lesions which also contain apo(a) (Lawn et al. 1992), indicating that Lp(a) does play a direct role in atherogenesis.