Renal metabolism and urinary excretion of platelet-activating factor in the rat

The role of platelet-activating factor in mesangial pathophysiology

Platelet-activating factor (PAF) is a powerful proinflammatory mediator that displays an exceedingly diverse spectrum of biological effects. Importantly, PAF is shown to participate in a broad range of pathologic conditions. This review focuses on the role that PAF plays specifically in the pathophysiology of the kidney, the organ that is both a source and a target of PAF. Renal mesangial cells are responsible for glomerular PAF generation and, ultimately, are the victims of its excessive production. Mesangial pathology is widely acknowledged to reflect glomerular damage, which culminates in glomerulosclerosis and proteinuria. Therefore, modulation of mesangial cell responses would offer a pathophysiology-based therapeutic approach to prevent glomerular injury. However, the currently available therapeutic modalities do not allow for targeted intervention into these processes. A more profound understanding of the mechanisms that govern PAF metabolism and signaling in mesangial cells is important, because it could facilitate the quest for improved therapies for renal patients on the basis of PAF as a drug target.

Movement of monoglyceride derived from hydrolysis of fluorescence-labeled lyso platelet-activating factor by lysophospholipase C through plasma membranes of porcine kidney epithelial cell line LLC-PK1

To investigate the mechanisms of the release of lyso platelet-activating factor (PAF), an alkyl ether-linked lysophosphatidylcholine, from the kidney epithelial cell line LLC-PK1, the cell monolayer was incubated with a fluorescence-labeled lysoPAF analog, Bodipy-lysoPAF, on either the basolateral or apical side. The fluorescent lipids in the culture media mixed with or without bovine serum albumin at a final concentration of 2% were analyzed by thin layer chromatography. In both cases, two major bands, assignable to Bodipy-lysoPAF and Bodipy-monoglyceride (MG), were detected in the culture medium to which Bodipy-lysoPAF had been added, whereas the culture medium at the opposite side exhibited only the major band of Bodipy-MG. Our results suggest that lysoPAF was degraded by high ecto-lysophospholipase C activity. The possible physiological significance of this metabolic pathway is discussed.

Platelet-activating factor and solute transport processes in the kidney

We examined the hemodynamic and tubular transport mechanisms by which platelet-activating factor (PAF) regulates salt and water excretion. In anesthetized, renally denervated male Wistar rats, with raised systemic blood pressure and renal arterial blood pressure maintained at normal levels, intrarenal PAF infusion at 2.5 ng. min(-1) x kg(-1) resulted in a small fall in systemic blood pressure (no change in renal arterial blood pressure) and an increase in renal blood flow and urinary water, sodium, and potassium excretion rates. The PAF-induced changes in cardiovascular and renal hemodynamic function were abolished and renal excretory function greatly attenuated by treating rats with a nitric oxide synthase inhibitor. To determine whether a tubular site of action was involved in the natriuretic effect of PAF, cortical proximal tubules were enzymatically dissociated from male Wistar rat kidneys, and oxygen consumption rates (Qo(2)) were used as an integrated index of transcellular sodium transport. PAF at 1 nM maximally inhibited Qo(2) in both untreated and nystatin-stimulated (sodium entry into renal cell is not rate limiting) proximal tubules by approximately 20%. Blockade of PAF receptors or Na(+)-K(+)-ATPase pump activity with BN-52021 or ouabain, respectively, abolished the effect of PAF on nystatin-stimulated proximal tubule Qo(2). Inhibition of nitric oxide synthase or guanylate cyclase systems did not alter PAF-mediated inhibition of nystatin-stimulated proximal tubule Qo(2), whereas phospholipase A(2) or cytochrome-P-450 monooxygenase inhibition resulted in a 40-60% reduction. These findings suggest that stimulation of PAF receptors on the proximal tubule decreases transcellular sodium transport by activating phospholipase A(2) and the cytochrome-P-450 monooxygenase pathways that lead to the inhibition of an ouabain-sensitive component of the basolateral Na(+)-K(+)-ATPase pump. Thus PAF can activate both an arachidonate pathway-mediated suppression of proximal tubule sodium transport and a nitric oxide pathway-mediated dilatory action on renal hemodynamics that likely contributes to the natriuresis and diuresis observed in vivo.

Cyclosporin A (CsA) modulates the glomerular production of inflammatory mediators and proteoglycans in experimental nephrosis

Nephrosis is characterized by glomerular epithelial cell injury and a decrease in the glomerular basement membrane (GBM) proteoglycan content. Although CsA is a useful treatment for a group of patients with this disease, its mechanism of action is unclear. We have previously shown that in experimental nephrosis there is an increase in the glomerular production of tumour necrosis factor-alpha (TNF-alpha) and platelet-activating factor (PAF). Here we have studied the effect of CsA on kidney generation of TNF-alpha and PAF in puromycin aminonucleoside (PAN) nephrosis as well as on the synthesis of proteoglycans by cultured glomerular epithelial cells. Rats receiving CsA had, on day 8 of PAN injection, a significant reduction in proteinuria, blood cholesterol levels and in interstitial mononuclear cells. A diminution in glomerular production and urinary excretion of TNF-alpha and PAF was also noted. In in vitro studies, at 24 h of incubation PAF and TNF-alpha induced in glomerular epithelial cells a significant decrease in proteoglycan synthesis. Neither PAF nor TNF-alpha had any significant effect on glomerular epithelial cell proliferation. CsA alone induced a dose-response increase in proteoglycan synthesis and a slight decrease in cell proliferation. CsA also reversed the inhibitory effect of PAF and TNF-alpha on proteoglycan synthesis. However, CsA did not alter the pattern of proteoglycan production, remaining around 50% chondroitinase ABC-, 15% heparitinase-sensitive. Our results indicate that PAF and TNF-alpha could be implicated in the pathogenesis of nephrosis through the inhibition of proteoglycan synthesis by glomerular epithelial cells. The beneficial effect of CsA in nephrosis may be due to the recovery of the GBM charge selectivity caused by the normalization of glomerular PAF and TNF-alpha synthesis and the increase in proteoglycan synthesis by glomerular epithelial cells.

Urinary excretion of platelet activating factor in patients with immune-mediated glomerulonephritis

We have investigated whether human immune-mediated glomerulonephritis is associated with changes in platelet activating factor (PAF) biosynthesis. Urinary PAF, taken as a marker of its renal synthesis, was significantly higher in patients with membranous nephropathy (N = 9) than in healthy controls (N = 8). This was not due to a lower degradation of PAF since urinary acetylhydrolase activity was comparable in patients and controls. A significant positive correlation between urinary excretion of PAF and proteinuria was observed. PAF generation was comparable in polymorphonuclear cells isolated from patients with membranous nephropathy and controls. PAF levels in blood from patients with membranous nephropathy were significantly lower than in controls, suggesting that the excessive generation of PAF is confined to the kidney. The results document that signs of renal disease activity in human membranous nephropathy are associated with an excessive renal synthesis of PAF.

Urinary excretion of platelet-activating factor in haemolytic uraemic syndrome

Since some of the features of haemolytic uraemic syndrome (HUS), such as platelet activation and glomerular injury, could be brought about by platelet-activating factor (PAF), we have studied the urinary excretion of PAF in 10 children with HUS and in 10 healthy age-matched controls. Urinary PAF concentrations, measured by radioimmunoassay, were significantly higher in acute-phase HUS patients than in controls (mean 2.04 [SD 1.66] vs 0.72 [0.43] ng/mg creatinine, p less than 0.05) but were similar to those in controls in samples taken after recovery. High PAF concentrations during the acute phase of HUS may reflect platelet activation and glomerular injury; the lower values after recovery suggest that urinary PAF may be a marker of disease activity.

Increased urinary excretion of platelet activating factor in mice with lupus nephritis

Platelet activating factor (PAF) is present in urine from humans and experimental animals in normal conditions. Very little is known about changes in PAF urinary excretion under pathologic conditions and no data are available about the origin of PAF in the urine. In the present study we explored the possibility that immunologic renal disease is associated with an increase in PAF urinary excretion using gas chromatography-mass spectrometry technique. To clarify the renal or extrarenal origin of urinary PAF we evaluated whether exogenously administered PAF (1-[1', 2'-3H]alkyl) is filtered through the glomerulus and excreted in the urine. The results show that: 1) urine from mice with lupus nephritis in the early phase of the disease contained amounts of PAF comparable to those excreted in normal mouse urine, 2) PAF levels increased when animals started to develop high grade proteinuria, 3) after intravenous injection of [3H] PAF in nephritic mice, a negligible amount of [3H] ether lipid, corresponding to [3H]1-alkyl -2-acyl-3-phosphocholine (alkyl-2-acyl-GPC), was recovered from the 24 h urine extract.