Serum amyloid A, the major vertebrate acute‐phase reactant

Abstract

The serum amyloid A (SAA) family comprises a number of differentially expressed apolipoproteins, acute‐phase SAAs (A‐SAAs) and constitutive SAAs (C‐SAAs). A‐SAAs are major acute‐phase reactants, the in vivo concentrations of which increase by as much as 1000‐fold during inflammation. A‐SAA mRNAs or proteins have been identified in all vertebrates investigated to date and are highly conserved. In contrast, C‐SAAs are induced minimally, if at all, during the acute‐phase response and have only been found in human and mouse. Although the liver is the primary site of synthesis of both A‐SAA and C‐SAA, extrahepatic production has been reported for most family members in most of the mammalian species studied. In vitro, the dramatic induction of A‐SAA mRNA in response to pro‐inflammatory stimuli is due largely to the synergistic effects of cytokine signaling pathways, principally those of the interleukin‐1 and interleukin‐6 type cytokines. This induction can be enhanced by glucocorticoids. Studies of the A‐SAA promoters in several mammalian species have identified a range of transcription factors that are variously involved in defining both cytokine responsiveness and cell specificity. These include NF‐κB, C/EBP, YY1, AP‐2, SAF and Sp1. A‐SAA is also post‐transcriptionally regulated. Although the precise role of A‐SAA in host defense during inflammation has not been defined, many potential clinically important functions have been proposed for individual SAA family members. These include involvement in lipid metabolism/transport, induction of extracellular‐matrix‐degrading enzymes, and chemotactic recruitment of inflammatory cells to sites of inflammation. A‐SAA is potentially involved in the pathogenesis of several chronic inflammatory diseases: it is the precursor of the amyloid A protein deposited in amyloid A amyloidosis, and it has also been implicated in the pathogenesis of atheroscelerosis and rheumatoid arthritis.