Role of serum amyloid A during metabolism of acute-phase HDL by macrophages

The serum amyloid A (SAA) family of proteins is encoded by multiple genes t hat display allelic variation and a high degree of homology in mammals. Tri ggered by inflammation after stimulation of hepatocytes by lymphokine media ted processes, the concentrations of SAA may increase during the acute-phas e reaction to levels 1000-fold greater than those found in the noninflammat ory state. In addition to its role as an acute-phase reactant, SAA (104 ami no acids, 12 kDa) is considered to be the precursor protein of secondary re active amyloidosis, in which the N-terminal portion is incorporated into th e bulk of amyloid fibrils. However, the association with lipoproteins of th e high-density range and subsequent modulation of the metabolic properties of its physiological carrier appear to be the principal role of SAA. Becaus e SAA may displace apolipoprotein A-I, the major protein component of nativ e high density lipoprotein (HDL), during the acute-phase reaction, the pres ent study was aimed at (1) investigating binding properties of native and a cute-phase (SAA-enriched) HDL by J774 macrophages, (2) elucidating whether the presence of SAA on HDL particles affects selective uptake of HDL-associ ated cholesteryl esters, and (3) comparing cellular cholesterol efflux medi ated by native and acute-phase HDL. Both the total and the specific binding at 4 degrees C of rabbit acute-phase HDL were approximate to 2-fold higher than for native HDL. Nonlinear regression analysis revealed K-d values of 7.0X10(-7) moL/L (native HDL) and 3.1X10(-7) mol/L (acute-phase HDL), respe ctively. The corresponding B-max values were 203 ng of total lipoprotein pe r milligram of cell protein (native HDL) and 250 ng of total lipoprotein pe r milligram of cell protein (acute-phase HDL). At 37 degrees C, holoparticl e turnover was slightly enhanced for acute-phase HDL, a fact reflected by 2 -fold higher degradation rates. In contrast, the presence of SAA on HDL spe cifically increased (1.7-fold) the selective uptake of HDL cholesteryl este rs from acute-phase HDL by J774 macrophages, a widely used in vitro model t o study foam cell formation and cholesterol efflux properties. Although lig and blotting experiments with solubilized J774 membrane proteins failed to identify the scavenger receptor-BI as a binding protein for both native and acute-phase HDL, 2 binding proteins with molecular masses of 100 and 72 kD a, the latter comigrating with CD55 (also termed decay-accelerating factor) , were identified. During cholesterol efflux studies, it became apparent th at the ability of acute-phase HDL with regard to cellular cholesterol remov al was considerably lower than that for native HDL. This was reflected by a 1.7-fold increase in tau/2 values (22 versus 36 hours; native versus acute -phase HDL). Our observations of increased HDL cholesteryl ester uptake and reduced cellular cholesterol efflux (acute-phase versus native HDL) sugges t that displacement of apolipoprotein A-I by SAA results in considerable al tered metabolic properties of its main physiological carrier. These changes in the apolipoprotein moieties appear (at least in the in vitro system tes ted) to transform an originally antiatherogenic into a proatherogenic lipop rotein particle.