Extrahepatic production of acute phase serum amyloid A.

Amyloidosis is a group of diseases characterized by the extracellular deposition of protein that contains non-branching, straight fibrils on electron microscopy (amyloid fibrils) that have a high content of beta-pleated sheet conformation. Various biochemically distinct proteins can undergo transformation into amyloid fibrils. The precursor protein of amyloid protein A (AA) is the acute phase protein serum amyloid A (SAA). The concentration of SAA in plasma increases up to 1000-fold within 24 to 48 h after trauma, inflammation or infection. Individuals with chronically increased SAA levels may develop AA amyloidosis. SAA has been divided into two groups according to the encoding genes and the source of protein production. These two groups are acute phase SAA (A-SAA) and constitutive SAA (C-SAA). Although the liver is the primary site of the synthesis of A-SAA and C-SAA, extrahepatic production of both SAAs has been observed in animal models and cell culture experiments of several mammalian species and chicken. The functions of A-SAA are thought to involve lipid metabolism, lipid transport, chemotaxis and regulation of the inflammatory process. There is growing evidence that extrahepatic A-SAA formation may play a crucial role in amyloidogenesis and enhances amyloid formation at the site of SAA production.