No abstract

Animal cells can convert 20-carbon polyunsaturated fatty acids into prostaglandins (PGs) and leukotrienes. These locally produced mediators of inflammatory and immunological reactions act in an autocrine or paracrine fashion. Arachidonic acid (AA), the precursor of most PGs and leukotrienes, is present in the form of lipid esters within plasma lipoproteins and cannot be synthesised de novo by animal cells. Therefore, AA or its plant-derived precursor, linoleic acid, must be provided to cells if PGs or leukotrienes are to be formed. Because several classes of lipoproteins, including low-density lipoproteins (LDL), very-low-density lipoproteins, and chylomicron remnants, are taken up by means of the LDL receptor, and because LDL and very-low-density lipoproteins, but not high-density lipoproteins, stimulate PG synthesis, we have suggested previously that PG formation is directly linked to the LDL pathway. Using fibroblasts with the receptor-negative phenotype of familial hypercholesterolaemia and anti-LDL receptor antibodies, we show here that LDL deliver AA for PG production and that an LDL receptor-dependent feedback mechanism inhibits the activity of PGH synthase, the rate-limiting enzyme of PG synthesis. These results indicate that the LDL pathway has a regulatory role in PG synthesis, in addition to its well-known role in the maintenance of cellular cholesterol homeostasis.

We studied the regulation of thromboxane (TX) synthesis in promyelocytic leukemia cells during macrophage differentiation.Cells treated with 12-O-tetradecanoylphorbol-13-acetate (TPA) showed rates of TXB2 synthesis from exogenous arachidonic acid that exceeded that of control cells by a factor of up to 81. Cells treated with sn-1,2-dioctanoylglycerol (diC8) showed similarly high TXB2 synthesis rates when diC8 was added concomitantly with a subthreshold concentration of TPA or when given in multiple doses. These activities depended on de novo synthesis of prostaglandin H (PGH) synthase because: (a) microsomal PGH synthase activity showed large increases in V.ax values, and (b) mass measurements of PGH synthase revealed the presence of PGH synthase in differentiating cells whereas the enzyme was undetectable in control cells. These results indicate that macrophage differentiation is associated with stimulation of TXB2 synthesis that requires both activation of protein kinase C and de novo synthesis of PGH synthase.

Arterial and hepatovenous concentrations of circulating prostaglandin E2 and prostaglandin F2 alpha, the stable metabolites of prostacyclin and thromboxane A2 were measured in patients with chronic liver disease and compared with those in control patients with coronary artery disease but without hepatic dysfunction. Specific radioimmunoassays were used after extraction on octadecyl C 18-silica gel columns and thin-layer chromatography. While low levels of all cyclooxygenase products were found in hepatic arterial blood in patients with proven cirrhosis (n = 10) and fibrosis (n = 8), significantly higher concentrations were detected in the hepatic vein. A similar concentration profile was observed in controls (n = 4). Thus, there is a marked but comparable release of prostanoids from the normal as well as the diseased liver. Hepatovenous prostaglandin E2 was 11.6-fold, prostaglandin F2 alpha was 7.5-fold, prostacyclin was 12.2.-fold and thromboxane B2 was 3.9-fold above the level in the artery in both groups of patients. The hepatovenous concentrations of all arachinodate metabolites were unrelated to changes of liver morphology, biochemical abnormalities or the presence of ascites. No correlation could be demonstrated between hepatic venous pressure gradient and the concentration of prostanoids in the hepatic vein with the exception of thromboxane B2 (r = 0.55, p less than 0.05). The occurrence of esophageal varices was not associated with a specific pattern of circulating prostanoids in the posthepatic vasculature. Moreover, the portal-venous concentrations of all prostanoids (five patients: two with fibrosis, three with cirrhosis) exceeded the level in the hepatic vein substantially.(ABSTRACT TRUNCATED AT 250 WORDS)