Amino terminal region of acute phase, but not constitutive, serum amyloid A (apoSAA) specifically binds and transports cholesterol into aortic smooth muscle and HepG2 cells

The human apoSAA proteins comprise both acute phase (apoSAA1, apoSAA2) and constitutive (apoSAA4) iso- forms; all are expressed in human atherosclerotic lesions as well as in liver. Recombinant acute phase apoSAA binds cholesterol with an affinity of - 170 IIM and enhances choles- terol uptake by HepC2 cells v. Lipid Res. 1995.36 37-46). In the present study, we sought to define the region of acute phase apoSAA involved in cholesterol binding and to investi- gate the ability of constitutive apoS& to bind cholesterol. Binding of (3H)cholesterol to apoSAAp was inhibited by unla- beled cholesterol (1-100 nM), but not significantly by vitamin D and estradiol. Direct binding of acute phase, but not con- stitutive, apoSAA to the surfaces of polystyrene microtiter wells was strongly diminished in the presence of cholesterol. The ability of apoSAA, to bind cholesterol was inhibited by antibodies to human apoSAAl and to peptide 1-18 of apoSAA1. There was only slight inhibition of cholesterol bind- ing by antibodies to peptide 40-63, and no inhibition by antibodies to peptides spanning regions containing amino acid residues 14-44 and 59-104. (SH)cholesterol uptake by neonatal rabbit aortic smooth muscle and HepC2 cells was enhanced by a synthetic peptide corresponding to amino acids 1-18 of hSM1, but not by peptides corresponding to amino acids 1-18 of hS&. (SH)cholesterol uptake by HepC2 cells was slightly increased by a peptide corresponding to amino acids 40-63 of hSAA1. I These findings suggest that apoSAA