Identification of Cells Expressing SAA3 mRNA by in Situ Hybridization

Abstract

Serum amyloid A (SAA) is a family of proteins found circulating mainly associated with high density lipoproteins.1–3 They behave as acute phase reactants: Only trace amounts are found normally but in response to inflammatory conditions or injury the levels are elevated several hundred fold.4,5 Amyloid A protein is the main protein constituent found in the amyloid fibrils of reactive amyloidosis and is a fragment of serum amyloid A.6–8 in the mouse, serum amyloid A is encoded by a family of three active genes,9 (SAA1, SAA2, SAA3) but only SAA2 is the precursor to amyloid A protein.10 The liver is the major site of SAA synthesis where, following LPS stimulation of mice, messenger RNA for each of the three genes is elevated approximately 1,000 fold.11 Serum amyloid A genes are also expressed in extrahepatic tissues, however this is almost exclusively limited to expression of the SAA3 mRNA.12,13 We wish to determine the source of SAA3 mRNA in extrahepatic sites: Is SAA3 message expressed by a single cell system dispersed throughout the extrahepatic tissues, or are the differentiated cells of the various tissues responsible for SAA3 expression?