Acute inflammation, acute phase serum amyloid A and cholesterol metabolism in the mouse

Acute inflammation results in a profound change in the apolipoprotein composition of high density lipoprotein (HDL). Several isoforms of the serum amyloid A (SAA) family, SAA1 and SAA2, become major components of HDL. This structural relationship has suggested that acute phase SAA plays some as yet unidentified role in HDL function, possibly related to cholesterol transport, during the course of acute inflammation. Using subcutaneous AgNO3 to induce a sterile abscess changes in plasma cholesterol and SAA were monitored over the subsequent 144 h. Total plasma cholesterol began to increase within 12 h of the induction of inflammation and reached a peak in 24 h. Thereafter its plasma levels fell returning to normal values by 96-120 h. The bulk of the increase in plasma cholesterol was found in the free cholesterol fraction of HDL. This pattern of cholesterol increase corresponds to the established temporal changes for acute phase SAA (AP-SAA). AP-SAA levels increased within 8 h of the induction of inflammation and reached a peak at 24 h. They began to decrease by 48 h with small quantites still present 120 h later. In concert, but inversely, with the changes in AP-SAA the apoA-I, apoA-II, and apo-E, content of HDL decreased during the AP-SAA increases and increased as AP-SAA levels fell. The plasma appearance of cholesterol from the periphery, and central parts of the inflammatory site was assessed by the use of radiolabelled cholesterol. The peripherally placed cholesterol rapidly reached a peak plasma concentration within 24 h of injection. Cholesterol placed in the central part of the sterile abscess, a site relatively inaccessible to the vasculature required 48 h to reach its peak and was 5-times lower than that placed peripherally. The influence of AP-SAA on neutral cholesterol ester hydrolase (nCEH) activity in mouse liver homogenates, mouse peritoneal macrophage homogenates, and a purified porcine pancreatic enzyme with nCEH activity was also assessed. Following optimization with regard to pH, bile salt concentration, protein concentration and incubation time, mouse peritoneal macrophages had a significantly higher nCEH specific activity than that found in liver (7-8 fold). Purified AP-SAA, assessed over a concentration range of 0-10 microg/ml, enhanced nCEH activity at concentrations above 2 microg/ml. The nCEH activity, regardless of its source, increased by 3-7 fold in the presence of AP-SAA. Equivalent concentrations of apolipoprotein A-I (apo A-I) and bovine serum albumin (BSA) failed to alter the activity of nCEH. The effect of AP-SAA on a purified form of nCEH suggests that AP-SAA may have a direct effect on the activity of this enzyme. The temporal correlation of circulating AP-SAA and plasma cholesterol and the significant stimulation of nCEH by AP-SAA (but not apoA-I or BSA) provides further evidence that AP-SAA plays a role in cholesterol metabolism during the course of acute inflammation.