The effect of reduction of lipoprotein (a) on cellular cholesterol synthesis in non-diabetic and type 2 diabetic subjects

Lipoprotein(a): an emerging cardiovascular risk factor

Lipoprotein(a) is a cholesterol-enriched lipoprotein, consisting of a covalent linkage joining the unique and highly polymorphic apolipoprotein(a) to apolipoprotein B100, the main protein moiety of low-density lipoproteins. Although the concentration of lipoprotein(a) in humans is mostly genetically determined, acquired disorders might influence synthesis and catabolism of the particle. Raised concentration of lipoprotein(a) has been acknowledged as a leading inherited risk factor for both premature and advanced atherosclerosis at different vascular sites. The strong structural homologies with plasminogen and low-density lipoproteins suggest that lipoprotein(a) might represent the ideal bridge between the fields of atherosclerosis and thrombosis in the pathogenesis of vascular occlusive disorders. Unfortunately, the exact mechanisms by which lipoprotein(a) promotes, accelerates, and complicates atherosclerosis are only partially understood. In some clinical settings, such as in patients at exceptionally low risk for cardiovascular disease, the potential regenerative and antineoplastic properties of lipoprotein(a) might paradoxically counterbalance its athero-thrombogenicity, as attested by the compatibility between raised plasma lipoprotein(a) levels and longevity.

Enzymatic and chemical modifications of lipoprotein(a) selectively alter its lysine-binding functions

The pathogenicity of lipoprotein(a) [Lp(a)] as a risk factor for cardiovascular disease may depend upon its lysine binding sites (LBS) which impart unique functions to Lp(a) not shared with low density lipoprotein. Biologically relevant modifications of Lp(a) were tested for alterations of LBS activity using two previously described functional assays, a LBS-Lp(a) immunoassay and a lysine-Sepharose bead assay. In the LBS-Lp(a) immunoassay, minimal changes in the LBS activity of Lp(a) were observed after modification with lipoprotein lipase, sphingomyelinase, or phospholipase C. In contrast, a significant (p<0.003) increase in the LBS activity of Lp(a) occurred after phospholipase A2 (PLA2) treatment, and this increase was confirmed using the lysine-Sepharose bead assay. The increase depended upon the release of fatty acids from Lp(a) by PLA2. A decrease in the LBS activity of Lp(a) occurred after oxidation of Lp(a) with 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) (44% decrease), but CuSO4 oxidation increased LBS activity (210%). N-acetylcysteine (NAC) treatment of Lp(a) decreased (48%) LBS activity while homocysteine treatment had no (89%) effect. Thus, modification of phospholipids and protein moieties can alter the LBS-activity of Lp(a). Such enzymatic and chemical modifications may contribute to the variability in LBS function of Lp(a) seen within the population.