Solubilization and partial purification of prostaglandin endoperoxide synthetase of rabbit kidney medulla

Prostaglandin D2 and E2 syntheses in rat Kupffer cells are antagonistically regulated by lipopolysaccharide and phorbol ester

Prostaglandin-synthesizing activities were demonstrated in cell-free extracts of rat Kupffer cells and characterized. The enzymatic properties of PGH2 synthase were found to be similar to those of synthases present in other organs or cell types. The specific activity of the enzyme was not changed by substances that stimulate prostanoid release by intact Kupffer cells; however, it was reduced by pretreatment of the cells with glucocorticoid hormones. On the other hand, the activities of PGD2 and PGE2 synthase were influenced differently by the kind of cell stimulation. While pretreatment of the intact cells with endotoxin and/or inhibition of protein kinase C led to an enhanced PGE2 formation in cell-free extracts, exposure to agents that enhance protein kinase C-dependent signalling pathways, e.g. phagocytotic stimuli or phorbol ester, suppressed PGE2 synthase activity and, therefore, led to enhanced PGD2 synthesis. It is in line with this observation that in vitro activation of protein kinase C of Kupffer cells resulted in a reduced PGE2 and an enhanced PGD2 synthase activity.

Prostaglandin biosynthesis in rabbit kidney: mepacrine inhibits renomedullary cyclooxygenase

Mepacrine was found to exert a dose-dependent inhibition of prostaglandin E2 synthesis in rabbit kidney medulla slices and in medullary microsomes. Mepacrine at 0.5 mM produced 90% inhibition of microsomal prostaglandin E2 biosynthesis from added arachidonic acid. This effect results from inhibition of medullary cyclooxygenase; the activities of the prostaglandin G2 hydroperoxidase and the prostaglandin H2 isomerases are unaffected. In experiments with medulla slices prelabelled with [14C]arachidonate, the effect of mepacrine on the inhibition of [14C]prostaglandin generation was significantly higher (2.5 to 3.5-fold) than its inhibition of [14C]arachidonate release. Hence, although mepacrine reduces prostaglandin production by decreasing the lipolytic release of arachidonate from medullary lipids, its inhibitory effect on prostaglandin cyclooxygenase activity is substantial and appears to contribute significantly to its overall inhibition of prostaglandin generation in kidney medulla. Mepacrine is thus not only a non-selective antilipolytic agent but also a potent cyclooxygenase inhibitor.

Effect of essential polyunsaturated fatty acid modifications on prostaglandin production by MDCK canine kidney cells

Supplementation of growing MDCK canine kidney tubular epithelial cultures with linoleic acid produced a 3.6- to 4.9-fold increase in bradykinin-stimulated PGE2 release as measured by radioimmunoassay. Under these conditions the cell phospholipids contained 3.9- times more linoleic acid and 5.6-times more arachidonic acid, with the inositol, ethanolamine and choline phosphoglyceride fractions becoming enriched in arachidonic acid. By contrast, supplementation with arachidonic acid did not enhance bradykinin-stimulated PGE2 release even though the arachidonic acid content of the cell phospholipids was increased 8.8-fold. The distribution of radioactive prostaglandin products was unchanged by these fatty acid enrichments, with PGE2 accounting for 55 to 68% of the total output from [1-14C]arachidonic acid. Linoleic acid supplementation also produced a 2.5-fold increase in PGE2 formation stimulated by extracellular arachidonic acid, whereas supplementation during culture with arachidonic acid caused a 55 to 80% inhibition. This difference cannot be accounted for by changes in the ability of the cells to incorporate extracellular arachidonic acid. It is suggested that at least some of the effects of linoleate supplementation on prostaglandin production are due to the resulting enrichment of the intracellular phospholipid substate pools with arachidonic acid. In addition, it appears that prolonged exposure to arachidonic acid during culture has an overriding inhibitory effect on prostaglandin production even though the total cell lipids become highly enriched in arachidonate.