CHARACTERIZATION AND ACUTE PHASE MODULATION OF CANINE APOLIPOPROTEIN-H (BETA-2-GLYCOPROTEIN-I)

Apolipoprotein H (ApoH), also known as β2-glycoprotein I (β2I), is a serum glycoprotein of approximately 50 Kda (I) with poorly defined physiological function. While some ApoH is associated with the lipoprotein fraction (30%), the majority is found free in serum (70%) (2). ApoH binds to a number of negatively charged substrates including anionic phospholipids (3) and platelets (4). Consequently, a number of possible roles for ApoH including inhibition of (i) the blood coagulation pathway via binding to phospholipids on the surface of damaged cells (5,6), (ii) ADP-mediated platelet aggregation (7), and (iii) platelet prothrombinase activity (6) have been proposed. A recent report also suggests that ApoH participates in the pathogenesis of autoimmune disease as it has been shown to be a component of the antigen to which anti-cardiolipin autoantibodies are directed (8). Northern blot analysis has established that the expression of ApoH mRNA in a cultured human hepatoma cell line HepG2 is down-regulated in response to the pro- inflammatory cytokines interleukin Iβ (IL-iβ) and interleukin 6 (IL-6) (9); if ApoH is also a negative acute phase reactant (APR) in vivo, the reduction in the inhibitory functions listed above may have significant clinical consequences. © 1993 Academic Press.