Identification of Protein Ligands for Lipoprotein(a) - a risk factor for atherothrombotic disease

Lipoprotein(a) [=Lp(a)] is a complex in human plasma which is assembled from a low density lipoprotein (LDL) - which carries cholesterol - and a huge glycoprotein called apolipoprotein(a). The latter is a relative of plasminogen, a member of the fibronolytic cascade and contains so-called kringle domains. The homology to plasminogen prompted a number of studies which have implicated apo(a)/Lp(a) in fibrinolysis and blood clotting. Together the structural and functional properties of Lp(a) suggested that it bridges the lipid transport and clotting systems which may explain that high concentrations are a risk factor for atherothrombotic diseases (myocardial infarction, stroke). However the physiological role of Lp(a), the mechanisms by which it acts pro- atherothrombotic, and even basic properties e.g., the mechanism(s) and site(s) of removal from plasma are unclear or unknown. In order to get an unbiased insight into the role of Lp(a) in physiology and pathology we have used a proteomic approach. The so- called Yeast Two Hybrid System - a method that allows to "fish" binding partners of proteins among millions of clones containing the information for individual proteins was used to identify protein ligands which bind apo(a)/Lp(a). The Two Hybrid approach can also be used to identify binding domains in the respective proteins. Identified proteins and protein domains were further confirmed by biochemical analysis. The two results of our study considered most relevant were the following: i) A binding domain for apo(a) was identified in fibrinogen. This domain binds to kringle IV type 10 in apo(a). This finding is in line with the original suggestion that Lp(a) may be involved in wound healing delivering cholesterol to sites of injury. ii) An unexpected new protein ligand for Lp(a)/apo(a) was detected. This protein called DANCE (acronym for Development Arteries and Neural Crest Epidemiological Growth Factor Like) is also known as fibulin 5. This novel secreted RGD protein is an essential determinant of elastic fibre organisation in the skin, lung and vasculature. It links elastic fibres to cells and is expressed in developing, atherosclerotic, and balloon injured arteries. This may suggest that Lp(a) plays a role at sites of tissue damage and further provides still another link of Lp(a) to atherosclerosis.