Angiotensin II causes formation of platelet activating factor in cultured rat mesangial cells

COVID-19, microthromboses, inflammation, and platelet activating factor

Recent articles report elevated markers of coagulation, endothelial injury, and microthromboses in lungs from deceased COVID-19 patients. However, there has been no discussion of what may induce intravascular coagulation. Platelets are critical in the formation of thrombi and their most potent trigger is platelet activating factor (PAF), first characterized by Demopoulos and colleagues in 1979. PAF is produced by cells involved in host defense and its biological actions bear similarities with COVID-19 disease manifestations. PAF can also stimulate perivascular mast cell activation, leading to inflammation implicated in severe acute respiratory syndrome (SARS). Mast cells are plentiful in the lungs and are a rich source of PAF and of inflammatory cytokines, such as IL-1β and IL-6, which may contribute to COVID-19 and especially SARS. The histamine-1 receptor antagonist rupatadine was developed to have anti-PAF activity, and also inhibits activation of human mast cells in response to PAF. Rupatadine could be repurposed for COVID-19 prophylaxis alone or together with other PAF-inhibitors of natural origin such as the flavonoids quercetin and luteolin, which have antiviral, anti-inflammatory, and anti-PAF actions.

The pathophysiological effects of Russell's viper (Daboia siamensis) venom and its fractions in the isolated perfused rabbit kidney model: A potential role for platelet activating factor

The pathophysiological effects of Russell's viper venom (RVV) and its fractions, including phospholipase A₂ (RvPLA₂), metalloprotease (RvMP), L-amino acid oxidase (RvLAAO), and phosphodiesterase (RvPDE) on renal functions were investigated using the isolated perfused rabbit kidney (IPK) model. Moreover, whether their effects on renal alterations were promoted by platelet activating factor (PAF) was tested using the PAF receptor antagonist, WEB 2086. There was a marked reduction in the perfusion pressure (PP) and renal vascular resistance (RVR) 10 min after RVV administration (1.0 mg/100 ml of perfusate), thereafter both PP and RVR gradually increased and approached the control level within 90 min. These effects were abolished by pretreatment with WEB2086 (2 μg/μl). Administration with RvPLA₂ (280 μg/ml), RvMP (280 μg/ml), or RvLAAO (135 μg/ml) alone increased both the PP and RVR, whereas RvPDE (100 μg/ml) reduced both the PP and RVR. Pretreatment with WEB 2086 completely abolished the effects induced by RvMP, but not the other fractions. The RVV also caused a marked decrease in the glomerular filtration rate (GFR), urinary flow rate (UF), and osmolar clearance (Cosm), and these effects were not inhibited by pretreatment with WEB2086. Each RVV fraction also increased, to varying extents, the GFR, UF, and Cosm, and these effects induced by RvPLA₂ or RvMP, but not the other fractions, were completely blocked by WEB 2086. Changes in percent filtered Na⁺ and K⁺ excreted in the IPK by RVV, RvPDE, and RvMP were abolished by pretreatment with WEB 2086. Histological evaluation profiled mainly tubulonephrosis in the treated kidney. These results reveal that the alterations in renal functions induced by RVV and its fractions are due to the synergistic action of the different components of snake venom, instead of the action of a single component. The effects of RVV and its fractions in rabbit IPK are mediated at least in part by PAF.

Hsa-let-7c controls the committed differentiation of IGF-1-treated mesenchymal stem cells derived from dental pulps by targeting IGF-1R via the MAPK pathways

The putative tumor suppressor microRNA let-7c is extensively associated with the biological properties of cancer cells. However, the potential involvement of let-7c in the differentiation of mesenchymal stem cells has not been fully explored. In this study, we investigated the influence of hsa-let-7c (let-7c) on the proliferation and differentiation of human dental pulp-derived mesenchymal stem cells (DPMSCs) treated with insulin-like growth factor 1 (IGF-1) via flow cytometry, CCK-8 assays, alizarin red staining, real-time RT-PCR, and western blotting. In general, the proliferative capabilities and cell viability of DPMSCs were not significantly affected by the overexpression or deletion of let-7c. However, overexpression of let-7c significantly inhibited the expression of IGF-1 receptor (IGF-1R) and downregulated the osteo/odontogenic differentiation of DPMSCs, as indicated by decreased levels of several osteo/odontogenic markers (osteocalcin, osterix, runt-related transcription factor 2, dentin sialophosphoprotein, dentin sialoprotein, alkaline phosphatase, type 1 collagen, and dentin matrix protein 1) in IGF-1-treated DPMSCs. Inversely, deletion of let-7c resulted in increased IGF-1R levels and enhanced osteo/odontogenic differentiation. Furthermore, the ERK, JNK, and P38 MAPK pathways were significantly inhibited following the overexpression of let-7c in DPMSCs. Deletion of let-7c promoted the activation of the JNK and P38 MAPK pathways. Our cumulative findings indicate that Let-7c can inhibit the osteo/odontogenic differentiation of IGF-1-treated DPMSCs by targeting IGF-1R via the JNK/P38 MAPK signaling pathways.

Angiotensin peptides attenuate platelet-activating factor-induced inflammatory activity in rats

Angiotensin (Ang)--a peptide that is part of the renin-angiotensin system-induces vasoconstriction and a subsequent increase in blood pressure; Ang peptides, especially AngII, can also act as potent pro-inflammatory mediators. Platelet-activating factor (PAF) is a potent phospholipid mediator that is implicated in many inflammatory diseases. In this study, we investigated the effects of Ang peptides (AngII, AngIII, and AngIV) on PAF-induced inflammatory activity. In experiments using a rat hind-paw oedema model, AngII markedly and dose-dependently attenuated the paw oedema induced by PAF. The inhibitory effects of AngIII and AngIV on PAF-induced paw oedema were lower than that of AngII. Two Ang receptors, the AT1 and AT2 receptors, did not affect the AngII-mediated attenuation of PAF-induced paw oedema. Moreover, intrinsic tyrosine fluorescence studies demonstrated that AngII, AngIII, and AngIV interact with PAF, and that their affinities were closely correlated with their inhibitory effects on PAF-induced rat paw oedema. Also, AngII interacted with metabolite/precursor of PAF (lyso-PAF), and an oxidized phospholipid, 1-palmitoyl-2-(5'-oxo-valeroyl)-sn-glycero-3-phosphocholine (POVPC), which bears a marked structural resemblance to PAF. Furthermore, POVPC dose-dependently inhibited AngII-mediated attenuation of PAF-induced paw oedema. These results suggest that Ang peptides can attenuate PAF-induced inflammatory activity through binding to PAF and lyso-PAF in rats. Therefore, Ang peptides may be closely involved in the regulation of many inflammatory diseases caused by PAF.

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.

A modern understanding of the traditional and nontraditional biological functions of angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) is a zinc-dependent peptidase responsible for converting angiotensin I into the vasoconstrictor angiotensin II. However, ACE is a relatively nonspecific peptidase that is capable of cleaving a wide range of substrates. Because of this, ACE and its peptide substrates and products affect many physiologic processes, including blood pressure control, hematopoiesis, reproduction, renal development, renal function, and the immune response. The defining feature of ACE is that it is composed of two homologous and independently catalytic domains, the result of an ancient gene duplication, and ACE-like genes are widely distributed in nature. The two ACE catalytic domains contribute to the wide substrate diversity of ACE and, by extension, the physiologic impact of the enzyme. Several studies suggest that the two catalytic domains have different biologic functions. Recently, the X-ray crystal structure of ACE has elucidated some of the structural differences between the two ACE domains. This is important now that ACE domain-specific inhibitors have been synthesized and characterized. Once widely available, these reagents will undoubtedly be powerful tools for probing the physiologic actions of each ACE domain. In turn, this knowledge should allow clinicians to envision new therapies for diseases not currently treated with ACE inhibitors.

Glomerular nephrotoxicity of aminoglycosides

Aminoglycoside antibiotics are the most commonly used antibiotics worldwide in the treatment of Gram-negative bacterial infections. However, aminoglycosides induce nephrotoxicity in 10-20% of therapeutic courses. Aminoglycoside-induced nephrotoxicity is characterized by slow rises in serum creatinine, tubular necrosis and marked decreases in glomerular filtration rate and in the ultrafiltration coefficient. Regulation of the ultrafiltration coefficient depends on the activity of intraglomerular mesangial cells. The mechanisms responsible for tubular nephrotoxicity of aminoglycosides have been intensively reviewed previously, but glomerular toxicity has received less attention. The purpose of this review is to critically assess the published literature regarding the toxic mechanisms of action of aminoglycosides on renal glomeruli and mesangial cells. The main goal of this review is to provide an actualized and mechanistic vision of pathways involved in glomerular toxic effects of aminoglycosides.

Coexpression of AT1 and AT2 receptors by human fibroblasts is associated with resistance to angiotensin II

Angiotensin II (AngII) is considered as a cytokine-like factor displaying a variety of proinflammatory and profibrotic cellular effects. Most of these effects seem mediated by AT1 signaling, whereas AT2 expression and function in adult human cells remain unclear. We have studied AT1 and AT2 expression in different human adult fibroblasts types and analyze their response to AngII. AngII did not induce thymidine incorporation, apoptosis nor collagen gene or protein expression in human fibroblasts. Specific AT1 or AT2 inhibitors did not modify this apparent resistance to AngII. We found abundant expression of both AT1 and AT2 receptors in all human fibroblasts studied, whereas vascular smooth muscle cells (VSMC) which only expressed AT1 receptor, displayed a clear AT1-dependent proliferative response to AngII. These data demonstrate that cultured human adult fibroblasts express both AT1 and AT2 receptor types and this phenomenon is associated with a lack of growth or collagen synthesis responses to AngII.

Developmental regulation of prostaglandin E2 synthase in porcine ductus arteriosus

The synthesis of PGE2, the major vasodilator prostanoid of the ductus arteriosus (DA), is catalyzed by PGE2 synthases (PGES). The factors implicated in increased PGE2 synthesis in the perinatal DA are not known. We studied the developmental changes of PGES along with that of cyclooxygenase (COX)-2 and cytosolic phospholipase A2 (cPLA2) in the DA of fetal (75-90% gestation) and immediately postnatal newborn (NB) piglets. Levels of microsomal PGES (mPGES), COX-2, and PGE2 in the DA of NB were ∼7-fold higher than in fetus; activities of cytosolic PGES (cPGES) and cPLA2 in DA of the fetus and NB did not differ. Because platelet-activating factor (PAF) could regulate COX-2 expression, the former was measured and found to be more abundant in the DA of the NB than of fetus. PAF elicited an increase in mPGES, COX-2, and PGE2 in fetal DA to levels approaching those of the NB; cPGES, cPLA2, and COX-1 were unaffected. In perinatal NB DA, PAF receptor antagonists BN-52021 and THG-315 reduced mPGES, COX-2, and PGE2 levels and were associated with increased DA tone. It is concluded that PAF contributes in regulating DA tone by governing mPGES, COX-2, and ensuing PGE2 levels in the perinate.

Interdependent effect of angiotensin-converting enzyme and platelet-activating factor acetylhydrolase gene polymorphisms on the progression of immunoglobulin A nephropathy

In order to investigate the interdependent action of the insertion/deletion polymorphism of the angiotensin-converting enzyme (ACE) gene and polymorphism in exon 11 (C1136-->T; Ala379Val) of the platelet-activating factor acetylhydrolase (PAF-AH) gene, which encodes a functional antagonist of PAF, on the progression of immunoglobulin A (IgA) nephropathy, we analysed both polymorphisms in patients with primary IgA nephropathy, who were followed-up for longer than 3 years. During the follow-up (87.3 +/- 50.0 months), the disease progressed in 38 of the 191 patients (19.9%). The D allele of the ACE gene in the absence of the T allele of the PAF-AH gene did not affect the prognosis [odds ratio (OR), 3.6; 95% confidence interval (CI), 0.8-16.4] and neither did the T allele in the absence of the D allele (OR, 3.0; 95% CI, 0.4-24.2). However, the presence of both was a significant prognostic factor (OR, 6.6; 95% CI, 1.4-31.3). After adjusting for other risk factors, the presence of both proved to be an independent risk factor (OR, 4.5; 95% CI, 1.6-12.7). These results suggest that the interdependent effects of ACE and PAF-AH polymorphisms on the progression of IgA nephropathy might be more important than the effect of the individual polymorphisms.

Type 1 angiotensin II receptor antagonism reduces antigen-induced airway reactions

Although the renin-angiotensin system is activated in patients with asthma during severe acute attacks and angiotensin II has been shown to cause bronchoconstriction in patients with asthma, the role of angiotensin II in patients with asthma is unclear. We investigated the effects of two specific antagonists at type 1 and type 2 angiotensin II receptors, candesartan cilexetil (TCV-116) and PD123319, on antigen-induced airway reactions in guinea pigs. Sixty minutes after intraperitoneal administration of candesartan cilexetil (0.1, 1.0, or 10 mg/kg) or PD123319 (30 mg/kg), animals received an antigen challenge. Airway responsiveness to inhaled methacholine was assessed as the dose of methacholine required to produce a 200% increase in the pressure at the airway opening (PC(200)). Differential cell counts in bronchoalveolar lavage fluids (BALF) were measured 24 h after antigen challenge. Candesartan cilexetil did not inhibit antigen-induced bronchoconstriction in sensitized guinea pigs or alter PC(200) in nonsensitized guinea pigs. Antigen inhalation significantly increased bronchoconstrictor responses to methacholine and increased airway accumulation of eosinophils; both responses showed dose-dependent prevention by candesartan but not by PD123319. These results indicate that endogenous angiotensin II promotes antigen-induced airway hyperresponsiveness and eosinophil accumulation by acting at type 1 receptors.

Secretion of platelet-activating factor is mediated by MDR1 P-glycoprotein in cultured human mesangial cells

MDR1 P-glycoprotein (Pgp), the product of the MDR1 gene involved in multidrug resistance in cancer cells, is also expressed in normal tissues. In the human kidney, it is localized in the mesangium, the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct. Pgp actively transports lipophilic xenobiotics, peptides, steroids, and lipids, and perhaps endogenous substrates. It has been shown previously that human mesangial cells in culture express active Pgp and that the expression of Pgp can be down-regulated by exposure to antisense oligonucleotides. Mesangial cells do not express multidrug resistance-related protein (MRP). Experiments were performed to determine whether 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (generically platelet-activating factor, PAF) is a substrate of Pgp in human mesangial cells in culture. This study found: (1) PAF C-16 and analogs inhibited Pgp-mediated efflux of rhodamine 123 by 59 to 88% in multidrug-resistant KBV-1 cells and by 85 to 97% in cultured human mesangial cells. (2) In mesangial cells stimulated with A23187, the secretion of endogenously produced PAF was inhibited by >80% by the Pgp blockers verapamil, cyclosporin A, PSC-833, vinblastine, and adriamycin. (3) Preincubation with MDR1 antisense oligonucleotides also blocked PAF secretion by human mesangial cells. PAF analogs do not modify the transport of MRP substrates in MCF-7/VP cells expressing MRP but not Pgp. These results indicate that PAF is an endogenous substrate of Pgp in human mesangial cells. Inhibition of Pgp transport may be useful in reducing glomerular damage occurring in pathologic conditions where PAF secretion is elevated.

Role of platelet-activating factor acetylhydrolase gene mutation in Japanese childhood IgA nephropathy

Platelet-activating factor (PAF) is a potent mediator of inflammatory injury in renal diseases. PAF is degraded to inactive products by PAF acetylhydrolase. Recently, a point mutation (G to T transversion) of the PAF acetylhydrolase gene was observed at position 994, and this mutation was found to contribute to the variability in plasma PAF levels, with undetectable plasma PAF acetylhydrolase activity occurring in homozygous patients (TT genotype) and reduced levels of activity in heterozygous patients (GT genotype). Therefore, we investigated the effect of the PAF acetylhydrolase gene mutation on the pathogenesis and progression of immunoglobulin A (IgA) nephropathy. Genomic DNA was obtained from 89 children with IgA nephropathy and 100 controls. We identified the PAF acetylhydrolase gene mutation (G994T) by polymerase chain reaction. There was no significant difference in genotypic frequency between patients and controls. However, urinary protein excretion at the time of biopsy was significantly greater in patients with the GT/TT genotypes than in those with the GG genotype. The percentage of glomeruli with mesangial cell proliferation was significantly greater in patients with the GT/TT genotypes than in those with the GG genotype. These results indicate the PAF acetylhydrolase gene mutation may influence the degree of proteinuria and the extent of mesangial proliferation in the early stage of childhood IgA nephropathy.

Plasma and urinary platelet activating factor concentrations and leukotriene releasing activity of leukocytes in steroid sensitive nephrotic syndrome of childhood

Platelet activating factor (PAF) is synthesized and secreted by glomerular mesangial and endothelial cells. It increases glomerular basement membrane permeability and induces proteinuria. Leukotrienes (LT) are mediators released by either leukocytes or glomerular cells under the PAF effect. The possible role of PAF in steroid sensitive nephrotic syndrome (SSNS) of childhood was studied in 8 children with SSNS in the acute stage, 5 children in remission and 8 healthy controls. The PAF concentrations in urine and plasma were determined. Leukocytes were stimulated in vitro and the LT release in response to stimulation was determined. The urinary and plasma concentrations of PAF were significantly higher in the acute phase than in remission and in control patients. Children with SSNS were found to have peripheral leukocytes with increased LT releasing activity in vitro. These results are in accordance with clinical and experimental observations indicating that PAF originates in the kidney and plays a role in normal kidney physiology. Urinary PAF concentrations may be related to proteinuria because they were strongly correlated in the present study. Elevated plasma PAF concentrations in the acute stage of SSNS could result from either its secretion from the circulating leukocytes or decreased acetyl hidrolase activity needed for its hydrolysis in plasma. The increased LT release in vitro suggests that these cells might have been activated by PAF secreted from glomeruli. It is proposed that PAF and different LT in systemic and glomerular circulation are important mediators in childhood SSNS.

Angiotensin as a renal growth promoting factor

The kidney has been traditionally considered to be one the pivotal organs involved in the systemic actions of the renin-angiotensin system (RAS) with renin produced in the juxtaglomerular apparatus and angiotensin II (ANG II) as a key player in the regulation of glomerular hemodynamics. However, many studies in the last decade, facilitated by a throughout molecular characterization of all elements of the RAS, have provided convincing evidence that the kidney exhibits a local RAS which may independently function from the systemic actions of the endocrine RAS. Moreover, even local distinct cell populations along the nephron possess all components of a functioning RAS. For example, proximal tubular cells express mRNA and protein for angiotensinogen, renin, and angiotensin converting enzyme (ACE). They bear different types of ANG II receptors with the appropriate signal transduction systems, and these cells also exhibit surface proteases like angiotensinase A which are required for the inactivation of ANG II. Moreover, recent studies in the isolated perfused kidney have clearly shown that proximal tubular cells produce considerable amounts of ANG II and these concentrations exceed approximately hundred times the systemic concentration of the peptide. Besides the well-known regulation of glomerular hemodynamics by contraction of the efferent glomerular arteriole and mesangium cells, ANG II influences transport and acidification processes in proximal and distal tubules. In addition, the octapeptide stimulates metabolic pathways like tubular gluconeogenesis and ammoniagenesis. Accumulating data over the last years derived from in vivo and in vitro studies have demonstrated that ANG II is also a growth factor for renal cells. For example, cell culture experiments have shown that the octapeptide stimulates proliferation or hypertrophy of mesangial cells. In contrast, proliferation of cultured proximal tubular cells is inhibited by ANG II and cellular hypertrophy of these cells is induced. Many studies have provided evidence that early mesangial proliferation/hypertrophy and tubular hypertrophy is a predecessor of the subsequent development of glomerulosclerosis and interstitial fibrosis, situations with irreversible morphological changes of the kidney's architecture leading finally to end-stage renal disease. Therefore, the identification of ANG II as a renal growth factor and a better understanding of its local intrarenal synthesis and growth stimulating effects on different cell types along the nephron may help to develop rational therapeutic interventions to prevent the progression of renal disease.

Effect of angiotensin converting enzyme inhibitor on collagen production by cultured mesangial cells

Cultured mesangial cells (MC) express renin mRNA and generate angiotensin I, supporting the action of local renin-angiotensin system. Also angiotensin II may act like a growth factor and was reported to increase collagen production (CP) in cultured MC. Angiotensin converting enzyme inhibitor is suggested to attenuate development and advancement of glomerulosclerosis, mainly with its hemodynamic effects. Therefore, we investigated the direct effects of enalapril (E) on CP by cultured MC. Rat MC were cultured in DMEM media alone, or containing high glucose (HG: 25 mM) or soluble immune complex (IC) prepared with bovine gamma globulin (BGG) and anti-BGG, with or without E (0.2 microgram/ml). CP was determined after 24 h by [3H] proline incorporation method. E significantly reduced CP by 43% in medium as compared with control (C) (C: 37,210 +/- 4,200 vs C + E: 21,350 +/- 5,080 cpm/well, p < 0.01). CP in medium increased in the presence of HG (123% of C) or IC (147% of C), which was, however, prevented with E (HG + E: 105% of C, IC + E: 116% of C). There were no differences of CP in cell layer between C (3,490 +/- 220 cpm/well) and C + E (3,340 +/- 190 cpm/well), and also no changes after addition of E in HG or IC groups. In conclusion, E directly attenuates CP by MC, even in the presence of HG or IC, independently of its hemodynamic effects.

Role of platelet activating factor in gentamicin and cisplatin nephrotoxicity

The present study was undertaken to evaluate the effects of platelet activating factor (PAF) antagonists on nephrotoxicity induced by gentamicin (GENTA) and cisplatin (DDP) in rats. PAF infusion provoked a 56% decline in single nephron (SN) GFR due to a decrease in glomerular plasma flow (QA, 55%), glomerular transcapillary hydraulic pressure (delta P, 13%), and glomerular ultrafiltration coefficient (Kf, 37%). Four days after a single dose of DDP (6 mg/kg, i.p.) we observed non-oliguric acute renal failure (ARF) with reduced SNGFR (45%), QA (46%) and delta P (10%) and unchanged Kf. GENTA administration for 10 days (40 mg/kg, i.p. daily) induced a decline in SNGFR (40%), QA (41%) and Kf (41%). Chronic treatment with a GENTA + PAF antagonist (BN 52021) partially prevented the decline in SNGFR (22%) by an amelioration in QA (25%) and Kf (13%). However, simultaneous treatment with DDP and BN 52063 completely prevented the ARF induced by DDP, normalizing all parameters of renal function. Thus, PAF may be a potential mediator involved in the nephrotoxicity induced by GENTA and DDP.

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.