Glomeruli cooperate with macrophages in converting arachidonic acid to prostaglandins and hydroxyeicosatetraenoic acids

The interaction of proliferating macrophages with the glomerulus may play an important role in certain forms of glomerulonephritis. This interaction could involve metabolites of arachidonic acid (C20:4) such as prostaglandins (PG) and lipoxygenase products. We therefore investigated the conversion of exogenous [3H] C20:4 into hydroxyeicosatetraenoic acids (HETE) and PG by isolated glomeruli and macrophages from rats, alone and in combination. As demonstrated by HPLC, glomeruli converted C20:4 predominantly to lipoxygenase products -mainly 12-HETE- and, to a lesser extent, to PG. Resident macrophages converted C20:4 to equivalent amounts of HETE and PG, mainly 12-HETE and 6 keto-PGF1 alpha. When macrophages and glomeruli were studied in combination, a striking interaction was detected in both pathways of C20:4 metabolism. Production of 6 keto-PGF1 alpha was stimulated and considerable amounts of TXB2, PGD2 and hydroxyheptadecatrienoic acid (HHT) were also produced. Total 12-HETE production was unchanged. When a lipid extract of glomeruli, containing oxygenated metabolites of C20:4, was added to macrophages, stimulation of 12-HETE occurred without any change in HHT or PG formation. When, on the contrary, a lipid extract from macrophages was added to glomeruli, 12-HETE production by the glomeruli was nearly completely abolished. Thus the unchanged total 12-HETE production by coincubated glomeruli and macrophages resulted from its increased production by macrophages and its decreased production by glomeruli. These data suggest that interaction between glomeruli and macrophages results in activation of C20:4 metabolism by macrophages followed by inhibition of C20:4 metabolism by glomeruli. Such a regulatory process could play a role in the inflammatory response to immunological injuries during macrophage-dependent human and experimental glomerulonephritis.

PGE2 binding sites and PG-stimulated cyclic AMP accumulation in rat isolated glomeruli and glomerular cultured cells

[3H]PGE2 specifically bound to isolated glomeruli. The KD value and the number of sites were 80 nM and 528 fmoles/mg respectively. PGE1 and PGE2 resulted in equipotent inhibition of binding whereas PGI2 was markedly less active. It was not possible to demonstrate specific receptors for PGE2 in glomerular mesangial and epithelial cultured cells. PGE1, PGE2 and PGI2 (0.1-100 microM) stimulated cyclic AMP concentration both in isolated glomeruli and glomerular cultured cells. Basal cyclic AMP in epithelial cells was greater than in mesangial cells or glomeruli. The cyclic AMP accumulation in the presence of PGs was greatest in mesangial cells. Maximum stimulation was in the range 300-1400%. For the three preparations, PGE2 and PGE1 produced a greater effect than PGI2. ED50 values were identical for PGE1 and PGE2 (5 microM for epithelial cells and glomeruli, 20 microM for mesangial cells). ED50 value for PGI2 were lower than those for PGE1 or PGE2 (0.2, 2 and 5 microM for glomeruli, epithelial cells and mesangial cells, respectively). The effects of the three PGs were not additive when tested at maximally effective concentrations. These results demonstrate that PGE1, PGE2 and PGI2 stimulate glomerular and cellular cyclic AMP. A relationship between [3H]PGE2 binding sites and this biological effect has not been established. The physiological events secondary to the increase in glomerular cyclic AMP are also yet to be determined.

Metabolism of arachidonic acid via the lipoxygenase pathway in human and murine glomeruli

Glomeruli isolated from murine and human renal cortex metabolize arachidonic acid to prostaglandins via the cyclooxygenase pathway but whether such glomeruli can also metabolize arachidonic acid via the lipoxygenase pathway is controversial. [1-14C]Arachidonic acid was incubated with glomeruli or glomerular fractions isolated from rat and human renal cortex. The products were extracted, purified by high performance liquid chromatography, and identified by comparison of their retention times with those of authentic hydroxyeicosatetraenoic acid (HETE) standards and by gas chromatography-mass spectrometry. At low substrate concentrations, human glomeruli synthesized in equivalent amounts 12- and 15-HETE, whereas rat glomeruli synthesized only 12-HETE and in larger quantities than in man. At higher substrate concentrations, both species synthesized 12- and 15-HETE and the rate of synthesis for both products was higher in human glomeruli. No other HETE was detected in either species. The lipoxygenase products were stored within the glomeruli and recovered almost equally in the 10,000 x g pellet and in the 100,000 X g supernatant of the homogenized glomeruli. The properties of the lipoxygenase system were the following: the enzyme was distributed equally in the membranes and the cytosol; 12-HETE accumulation was linear with time over 15 min; and 12-HETE production correlated linearly with the amount of glomerular protein. 12-Lipoxygenase activity was maximum at pH 7.5 (rat) or 9.0 (human) and at 40-42 degrees C (both species). Km values calculated at low concentrations of substrate (10-200 microM) were for 15-HETE, 125 and 667 microM with murine and human glomeruli, respectively, and for 12-HETE, 44 microM with the glomeruli of both species. This study demonstrates lipoxygenase activity in murine and, for the first time, in human glomeruli. The products of such enzymatic activity, 12- and 15-HETE, may mediate the glomerular inflammatory response in various experimental or spontaneous glomerular diseases.