Aggregated IgG bind to glomerular epithelial cells to stimulate urokinase release through an endocytosis-independent process.

BACKGROUND: In membranous nephropathy, the development of glomerular lesions is related to the formation of immune complexes at subepithelial sites. These deposits are associated with modifications in the fibrinolytic activity of glomerular cells leading to the appearance of fibrin degradation products in the deposits and the urine. A previous study has shown that immune complexes interact with glomerular epithelial cells (GEC) through the neonatal Fc receptor (FcRn). We therefore determined whether this binding could be responsible for a modification in the fibrinolytic activity of GEC. METHODS: Endocytosis of heat-aggregated immunoglobulins (AgIgG) in cultured human GEC was studied by immunofluorescence and confocal microscopy. The release of urokinase-type plasminogen activator (uPA) and plasminogen activator inhibitor 1 (PAI) by GEC or whole glomeruli was assessed by ELISA, fibrin zymography and Northern blot. RESULTS: Human GEC in culture bound AgIgG that possess characteristics similar to soluble immune complexes and internalized them by 10 min. This process was mediated by FcRn since chicken aggregated IgG (AgIgY), that do not bind FcRn, did not colocalize with AgIgG in GEC. AgIgG but not AgIgY induced a decrease of FcRn expression at the membrane and within the cells. The binding of AgIgG to GEC elicited a dose- and time-dependent increase in the release of uPA activity, as in the uPA protein and mRNA expression without modification in the release of PAI. This process was not abrogated by agents inhibiting endocytosis and/or transcytosis such as cytochalasin B, suggesting an endocytosis-independent uPA regulation. CONCLUSION: GEC response to AgIgG overload comprises at least two sequential steps: (1) a FcRn-mediated endocytosis; (2) an endocytosis-independent fibrinolytic imbalance leading to plasmin generation which could favor in vivo AgIgG clearance and matrix remodeling.

Renin/prorenin-receptor biochemistry and functional significance.

The renin-angiotensin system (RAS) has become increasingly complex. New components have been identified, and additional roles for angiotensin peptides and their receptors are being uncovered. A functional (pro)renin receptor has been cloned that acts as (pro)renin cofactor on cell surface, enhancing the efficiency of angiotensinogen cleavage by (pro)renin and unmasking prorenin catalytic activity. Binding of (pro)renin to the receptor mediates (pro)renin cellular effects by activating mitogen-activating protein (MAP) kinases, extracellular signal-regulated kinases (ERK)1/2. Immunofluorescence studies have localized the receptor on mesangial and vascular smooth muscle cells in human heart and kidney. This suggests that the renin receptor might represent a means to capture (pro)renin from the circulation and to concentrate (pro)renin at the interface between smooth muscle and endothelial cells. In this article, we review the biochemical characteristics of this receptor and of other renin-binding proteins, and discuss their physiologic significance.

Regulation of growth-hormone-receptor gene expression by growth hormone and pegvisomant in human mesangial cells.

BACKGROUND: Mice transgenic for growth hormone develop mesangial proliferation, glomerular hypertrophy, and progressive glomerular sclerosis suggesting that the growth hormone-insulin-like growth factor I (IGF-I) pathway plays an important role. Therefore, we studied the impact of variable concentrations of 22 kD, 20 kD growth hormone, as well as of the growth hormone receptor antagonist pegvisomant (B2036-PEG), on both the growth hormone receptor (GHR/GHBP) gene expression and growth hormone binding protein (GHBP) formation in a human glomerular mesangial cell line. Further, the impact on collagen, IGF-I and IGF binding protein-1 (IGFBP-1) formation was studied. METHODS: In order to assess transcription, quantitative reverse transcription-polymerase chain reaction (RT-PCR) was used. RESULTS: Physiologic doses of 22 kD or 20 kD growth hormone caused a dose-dependent and significant (P < 0.01) up-regulation of GHR/GHBP gene transcription, whereas supraphysiologic doses (50 and 500 ng/mL) resulted in down-regulation (P < 0.001). Whenever pegvisomant was used, there was no increase in GHR/GHBP expression. These data were confirmed using run-on experiments. Further, the assessment of GHBP presented a constant, dose-dependent increase, which was completely abolished in the experiments where pegvisomant was used. CONCLUSION: We present data showing that growth hormone has a direct impact on GHR/GHPB gene transcription and that pegvisomant is a potent growth hormone receptor antagonist in human mesangial cells. In addition, although the GHR/GHBP gene transcription is down-regulated by supraphysiologic growth hormone concentrations, this effect was not found when GHBP levels were measured. This finding may reflect a self-inhibitory effect of growth hormone on the level of GHR/GHBP gene transcription, which does not involve the regulation of the shedding of GHBP and may, therefore, be of physiologic interest.

The renin receptor: the facts, the promise and the hope.

PURPOSE OF REVIEW: The renin-angiotensin system plays a major role in the control of blood pressure and of salt balance, but it is also involved in physiological and pathological processes, development, inflammation and cardiac hypertrophy. A concept has emerged suggesting that these effects are due to a local activation of the renin-angiotensin system. The search for a receptor of renin was based on the idea that tissue (pro)renin is taken up from the circulation and on data suggesting that renin has cellular effects independent of angiotensin II. RECENT FINDINGS: Endothelial cells and cardiac myocytes bind (pro)renin via the mannose-6-phosphate receptor, mainly a clearance receptor as no cellular effect has been specifically attributed to prorenin binding. A functional receptor was cloned recently. It mediates intracellular signalling by activating the mitogen activated protein kinases, extracellular signal regulated kinases 1 and 2, and acts as a co-factor by increasing the efficiency of angiotensinogen cleavage by receptor-bound (pro)renin. The receptor is abundantly expressed in heart, brain, placenta and eye, compared with a lower expression in liver and kidney. In normal human kidney and heart, it is localized in the mesangium and in the coronary and kidney artery, associated with smooth-muscle cells and co-localized with renin. SUMMARY: This receptor provides a functional role for prorenin and may help to understand the physiological and pathological role of elevated levels of prorenin and of local activation of the renin-angiotensin system. From a practical point of view, it questions the need for a pharmacological compound blocking (pro)renin binding and activity as an alternative to the classical inhibitors of the renin-angiotensin system.

Adult haemolytic and uraemic syndrome: causes and prognostic factors in the last decade.

BACKGROUND: Haemolytic uraemic syndrome (HUS) is a rare and severe disease of various aetiologies in adults. The effect of fresh frozen plasma (FFP) infusion in adults suffering from HUS is not well defined. The aim of this retrospective study was to analyse the causes of HUS in adults admitted in a single renal intensive care unit (ICU) and to determine the life and renal prognosis factors, while most patients (78%) received FFP infusion. METHODS: We recorded clinical, biological, and histological data of 55 adults admitted in our renal ICU for HUS between 1990 and 1998, 49 of them having had a renal biopsy. By stepwise logistic regression analysis, we examined the parameters that were associated with the in-hospital mortality and renal function at discharge. RESULTS: HUS complicated different diseases in 40 patients (HIV infection n=18, nephropathies n=10, allotransplantation n=7, malignant diseases n=5) and appeared as a primary in 15 patients. Factors influencing the in-hospital mortality were positive HIV serology (odds ratio (OR) >20, P=0.0002) and requirement for haemodialysis (OR >35, P=0.004). A pre-existing nephropathy was a bad prognosis factor for renal function (OR >99, P=0.02), while fever was associated with better renal prognosis (OR=1/10, P=0.033). CONCLUSIONS: HUS in adults remains a severe disease, with a high mortality rate in HIV patients and in those who required haemodialysis. However, as compared with previous studies, we observed an improvement in renal outcome, particularly in patients with primary HUS, suggesting a beneficial effect of FFP infusion, at least in these forms.

Pivotal role of the renin/prorenin receptor in angiotensin II production and cellular responses to renin.

Renin is an aspartyl protease essential for the control of blood pressure and was long suspected to have cellular receptors. We report the expression cloning of the human renin receptor complementary DNA encoding a 350-amino acid protein with a single transmembrane domain and no homology with any known membrane protein. Transfected cells stably expressing the receptor showed renin- and prorenin-specific binding. The binding of renin induced a fourfold increase of the catalytic efficiency of angiotensinogen conversion to angiotensin I and induced an intracellular signal with phosphorylation of serine and tyrosine residues associated to an activation of MAP kinases ERK1 and ERK2. High levels of the receptor mRNA are detected in the heart, brain, placenta, and lower levels in the kidney and liver. By confocal microscopy the receptor is localized in the mesangium of glomeruli and in the subendothelium of coronary and kidney artery, associated to smooth muscle cells and colocalized with renin. The renin receptor is the first described for an aspartyl protease. This discovery emphasizes the role of the cell surface in angiotensin II generation and opens new perspectives on the tissue renin-angiotensin system and on renin effects independent of angiotensin II.

11beta-hydroxysteroid dehydrogenase type 2 is expressed in the human kidney glomerulus.

Our previous study demonstrated that the GR is expressed in the human kidney glomerulus. The function of the GR of glomerular cells might be affected by the concentration of intracellular glucocorticoids, which is modulated by 11beta-hydroxysteroid dehydrogenase type 2 (11betaHSD2). Because the expression of 11betaHSD2 in the glomerular cells remains unclear, we used competitive RT-PCR and immunoblotting to detect the expression of 11betaHSD2 mRNA and protein in isolated human glomeruli, in whole kidney cortex as a positive control, and in a human glomerular visceral epithelial cell line. 11betaHSD2 mRNA was detected in all samples. Specific antihuman 11betaHSD2 antibody recognized a single band at 41 kDa, consistent with the molecular mass of human 11betaHSD2, in the samples of the isolated glomeruli and whole kidney cortex. Furthermore, definite 11betaHSD2 enzymatic activity was also determined with the sample of isolated glomeruli. We also performed immunohistochemistry by light and electron microscopy to determine the cellular and subcellular localization of 11betaHSD2 in the human glomeruli. Immunoreactivity of the enzyme was clearly observed in the glomerular visceral epithelial cells and endothelial cells as well as in the distal convoluted tubules and collecting ducts. The subcellular localization of 11betaHSD2 was shown to be endoplasmic reticulum. These results suggest that 11betaHSD2 might play a crucial role in modulating the intracellular concentration of glucocorticoids in human glomerular cells.

[Renal sclerosis: a public health problem]. / La sclérose rénale: un problème de Santé Publique.

In France, 45,000 patients are treated by hemodialysis and/or transplantation for chronic renal failure. Every year 7,000 new patients need such a therapeutic approach. The estimated cost of this pathology is about 1% of the total amount of the budget of social security, even though the number of patients is limited. Some treatments were shown to be effective in improving the progression of chronic renal failure, as for example the anti-hypertensive therapy and the adequate treatment of diabetes mellitus. Collagen deposition in each segment of the kidney, mainly in the interstitium, plays a pivotal role in the progression of renal deficiency in chronic renal failure. In order to ameliorate the progression renal failure it would be essential to know: 1) the different types of collagen deposit; 2) the proteinase/antiproteinase systems involved in the remodelling of the extracellular matrix (serine-protease, metallo-protease and their inhibitors); 3) the autocrine/paracrine effects of proteases and of growth factors on collagen synthesis. The more precise the knowledge of these factors, the more useful will be new pharmacological-, gene- or cellular therapies for limiting the progression of chronic renal failure.

[Proteases and antiproteases in the progression of chronic renal insufficiency lesions. The role of the tissue renin-angiotensin system and the renin receptor]. / Protéases et antiprotéases dans la progression des lésions de l'insuffisance rénale chronique. Rôle du système rénine-angiotensine tissulaire et du récepteur de la rénine?

The role of proteases and of antiproteases in the progression of renal disease is well established. Most studies have focused on the serine-proteases of the plasmin/plasminogen activator system and on matrix metalloproteases. Recently, renin, an aspartyl-protease, has attracted much attention because of the role of angiotensin II in the progression of renal lesions and because of the discovery of a functional renin receptor. This receptor is a 45 kDa membrane-protein that binds specifically renin and prorenin. The binding of renin induces an increase of the catalytic efficiency of angiotensinogen conversion into angiotensin I by receptor-bound renin compared to renin in soluble phase, and a rapid phosphorylation of the receptor on serine and tyrosine residues associated with an activation of MAP kinases ERK1/2. Immunofluorescence and confocal analyses on normal human kidney and cardiac biopsies show that the receptor is localized within the mesangial area of glomeruli and in the sub-endothelium of kidney and coronary arteries, associated to smooth-muscle cells. In summary, this receptor exerts dual effects, mediating renin cellular response and increasing the efficiency of angiotensinogen cleavage by membrane-bound renin. These observations emphasizes the importance of angiotensin II generation at the cell surface and the cellular effects of renin add new dimensions (and complexity) to the classical dogma that angiotensin II is the only effector of the RAS.

The decorin high glucose response element and mechanism of its activation in human mesangial cells.

The decorin gene encodes a proteoglycan with putative structural and regulatory functions whose expression is markedly increased in human mesangial cells (HMC) exposed to high concentrations of glucose (15 to 30 mM). The gene has two promoters (P1 and P2) upstream of two alternative first exons. Transcripts driven by both promoters are present in HMC maintained in 4 mM D-glucose medium. After exposure to 30 mM D-glucose for 7 to 21 d, transcripts driven by P1 are markedly increased, whereas those driven by P2 decrease. Culture in 4 mM D-glucose medium containing transforming growth factor-beta1 (TGF-beta1) (1.25 ng/ml) has the same effect. However, addition of an excess of TGF-beta neutralizing antibody to the 30 mM D-glucose cultures only partly suppressed increased decorin transcription from P1. In transformed HMC transfected with a reporter (p-SAEP) driven by P1 or P2, P1 activity increased twofold on treatment with either 30 mM D-glucose or TGF-beta1 in 4 mM medium. P2 had little activity under any conditions. 5' deletion of P1 showed that basal transcriptional activity lies within the proximal 378 bp, while the major high glucose and TGF-beta response element is located in the -683 to -583-bp region. A putative cAMP response-like sequence (TGACGTTT) lies within this region. Electrophoretic mobility shift assays revealed the same pattern of multiple complexes between oligonucleotides containing this sequence and nuclear proteins extracted from HMC maintained in either 4 or 30 mM D-glucose conditions, but the latter were more prominent. cAMP response element binding protein (CREB) was identified as one transcription factor forming these complexes but other factors remain unidentified. Increased levels of phospho-(Ser 133) CREB were found in HMC exposed to 30 mM D-glucose. High glucose also activated and led to nuclear translocation of p42/44 mitogen-activated protein kinase and p38 mitogen-activated protein kinase, both of which can activate CREB by phosphorylation of serine 133.

Characterization and localization of the neonatal Fc receptor in adult human kidney.

The binding of Fc fragments of Ig on glomerular epithelial cells (GEC) was observed previously, but the receptor could not be identified. In immunofluorescence and immunohistochemical studies using normal adult human kidney sections, the presence of the so-called neonatal Fc receptor (FcRn) was demonstrated on GEC as well as in the brush border of proximal tubular cells. FcRn transcripts were also detected on isolated glomeruli by reverse transcription-PCR. Using an immortalized GEC line, the presence of the FcRn was confirmed by flow cytometry, reverse transcription-PCR, Western blotting, and by the pH dependence of the binding of heat-aggregated IgG. Because it is well established that the FcRn is involved in IgG transcytosis, it is hypothesized that the FcRn in the kidney may play a role in the reabsorption of IgG. Ongoing studies should clarify the role of the FcRn as a potential target for immune complexes on GEC and should assess its relevance in physiology and pathology.