The path of murine serum amyloid a through peritoneal macrophages

Abstract

Serum amyloid A (SAA) is a family of proteins encoded by four related genes. Of the four, isoforms 1.1 and 2.1 are acute phase proteins synthesized by the liver. They become major components of the HDL plasma fraction during acute tissue injury and the HDL/SAA complex is readily taken up by macrophages. Herein we investigated the path SAA follows when presented to macrophages as HDL/SAA or in liposomes. Using antibodies specific to SAA and confocal microscopy, or EM autoradiography where only SAA is radio-labeled, we show that HDL/SAA is taken up rapidly by macrophages and within 30 min SAA, or fragments thereof, proceeds through the cytoplasm to the peri-nuclear region and then the nucleus. Within 45–60 min SAA, or fragments thereof, is found back in the cytoplasm and at the plasma membrane where it is subsequently extruded. The observation that SAA, or fragments thereof, traverse the nucleus is a novel finding and may implicate SAA in macrophage gene regulation. It also raises questions by what mechanism SAA enters and leaves the nucleus. We further investigated if both SAA isoforms traffic through the macrophage in a similar manner. Isoform differences were observed. Both isoforms bind well to the plasma membrane of macrophages at 4°C, but at 37°C only SAA2.1 is taken up by the cell in significant quantity, and is observed in the nucleus, suggesting that the two isoforms are handled differently and that they may have discrete physiological roles.