Arachidonic acid mediates angiotensin II effects on p21ras in renal proximal tubular cells via the tyrosine kinase-Shc-Grb2-Sos pathway

In kidney epithelial cells, an angiotensin II (Ang II) type 2 receptor subtype (AT2) is linked to a membrane-associated phospholipase A2 (PLA2) and the mitogen-activated protein kinase (MAPK) superfamily. However, the intervening steps in this linkage have not been determined. The aim of this study was to determine whether arachidonic acid mediates Ang II’s effect on p21ras and if so, to ascertain the signaling mechanism(s). We observed that Ang II activated p21ras and that mepacrine, a phospholipase A2 inhibitor, blocked this effect. This activation was also inhibited by PD123319, an AT2 receptor antagonist but not by losartan, an AT1 receptor antagonist. Furthermore, Ang II caused rapid tyrosine phosphorylation of Shc and its association with Grb2. Arachidonic acid and linoleic acid mimicked Ang II-induced tyrosine phosphorylation of Shc and activation of p21ras. Moreover, Ang II and arachidonic acid induced an association between p21ras and Shc. We demonstrate that arachidonic acid mediates linkage of a G protein-coupled receptor to p21ras via Shc tyrosine phosphorylation and association with Grb2/Sos. These observations have important implications for other G protein-coupled receptors linked to a variety of phospholipases.

CTLA4-Ig and anti-CD40 ligand prevent renal allograft rejection in primates

Selective inhibition of T cell costimulation using the B7-specific fusion protein CTLA4-Ig has been shown to induce long-term allograft survival in rodents. Antibodies preventing the interaction between CD40 and its T cell-based ligand CD154 (CD40L) have been shown in rodents to act synergistically with CTLA4-Ig. It has thus been hypothesized that these agents might be capable of inducing long-term acceptance of allografted tissues in primates. To test this hypothesis in a relevant preclinical model, CTLA4-Ig and the CD40L-specific monoclonal antibody 5C8 were tested in rhesus monkeys. Both agents effectively inhibited rhesus mixed lymphocyte reactions, but the combination was 100 times more effective than either drug alone. Renal allografts were transplanted into nephectomized rhesus monkeys shown to be disparate at major histocompatibility complex class I and class II loci. Control animals rejected in 5–8 days. Brief induction doses of CTLA4-Ig or 5C8 alone significantly prolonged rejection-free survival (20–98 days). Two of four animals treated with both agents experienced extended (>150 days) rejection-free allograft survival. Two animals treated with 5C8 alone and one animal treated with both 5C8 and CTLA4-Ig experienced late, biopsy-proven rejection, but a repeat course of their induction regimen successfully restored normal graft function. Neither drug affected peripheral T cell or B cell counts. There were no clinically evident side effects or rejections during treatment. We conclude that CTLA4-Ig and 5C8 can both prevent and reverse acute allograft rejection, significantly prolonging the survival of major histocompatibility complex-mismatched renal allografts in primates without the need for chronic immunosuppression.

Mutations in a NIMA-related kinase gene, Nek1, cause pleiotropic effects including a progressive polycystic kidney disease in mice

We previously have described a mouse model for polycystic kidney disease (PKD) caused by either of two mutations, kat or kat2J, that map to the same locus on chromosome 8. The homozygous mutant animals have a latent onset, slowly progressing form of PKD with renal pathology similar to the human autosomal-dominant PKD. In addition, the mutant animals show pleiotropic effects that include facial dysmorphism, dwarfing, male sterility, anemia, and cystic choroid plexus. We previously fine-mapped the kat2J mutation to a genetic distance of 0.28 ± 0.12 centimorgan between D8Mit128 and D8Mit129. To identify the underlying molecular defect in this locus, we constructed an integrated genetic and physical map of the critical region surrounding the kat2J mutation. Cloning and expression analysis of the transcribed sequences from this region identified Nek1, a NIMA (never in mitosis A)-related kinase as a candidate gene. Further analysis of the Nek1 gene from both kat/kat and kat2J/kat2J mutant animals identified a partial internal deletion and a single-base insertion as the molecular basis for these mutations. The complex pleiotropic phenotypes seen in the homozygous mutant animals suggest that the NEK1 protein participates in different signaling pathways to regulate diverse cellular processes. Our findings identify a previously unsuspected role for Nek1 in the kidney and open a new avenue for studying cystogenesis and identifying possible modes of therapy.

Factors affecting counteraction by methylamines of urea effects on aldose reductase

The concentration of urea in renal medullary cells is high enough to affect enzymes seriously by reducing Vmax or raising Km, yet the cells survive and function. The usual explanation is that the methylamines found in the renal medulla, namely glycerophosphocholine and betaine, have actions opposite to those of urea and thus counteract its effects. However, urea and methylamines have the similar (not counteracting) effects of reducing both the Km and Vmax of aldose reductase (EC 1.1.1.21), an enzyme whose function is important in renal medullas. Therefore, we examined factors that might determine whether counteraction occurs, namely different combinations of assay conditions (pH and salt concentration), methylamines (glycerophosphocholine, betaine, and trimethylamine N-oxide), substrates (dl-glyceraldehyde and d-xylose), and a mutation in recombinant aldose reductase protein (C298A). We find that Vmax of both wild-type and C298A mutant generally is reduced by urea and/or the methylamines. However, the effects on Km are much more complex, varying widely with the combination of conditions. At one extreme, we find a reduction of Km of wild-type enzyme by urea and/or methylamines that is partially additive, whereas at the other extreme we find that urea raises Km for d-xylose of the C298A mutant, betaine lowers the Km, and the two counteract in a classical fashion so that at a 2:1 molar ratio of betaine to urea there is no net effect. We conclude that counteraction of urea effects on enzymes by methylamines can depend on ion concentration, pH, the specific methylamine and substrate, and identity of even a single amino acid in the enzyme.

Human renal cell carcinoma expresses distinct binding sites for growth hormone-releasing hormone

Antagonists of growth hormone-releasing hormone (GHRH) inhibit the proliferation of various human cancers in vitro and in vivo by mechanisms that include apparent direct effects through specific binding sites expressed on tumors and that differ from pituitary human GHRH (hGHRH) receptors. In this study, GHRH antagonist JV-1–38 (20 μg/day per animal s.c.) inhibited the growth of orthotopic CAKI-1 human renal cell carcinoma (RCC) by 83% and inhibited the development of metastases to lung and lymph nodes. Using ligand competition assays with 125I-labeled GHRH antagonist JV-1–42, we demonstrated the presence of specific high-affinity (Kd = 0.25 ± 0.03 nM) binding sites for GHRH with a maximal binding capacity (Bmax) of 70.2 ± 4.1 fmol/mg of membrane protein in CAKI-1 tumors. These receptors bind GHRH antagonists preferentially and display a lower affinity for hGHRH. The binding of 125I-JV-1–42 is not inhibited by vasoactive intestinal peptide (VIP)-related peptides sharing structural homology with hGHRH. The receptors for GHRH antagonists on CAKI-1 tumors are distinct from binding sites detected with 125I-VIP (Kd = 0.89 ± 0.14 nM; Bmax = 183.5 ± 2.6 fmol/mg of protein) and also have different characteristics from GHRH receptors on rat pituitary as documented by the insignificant binding of [His1,125I-Tyr10,Nle27]hGHRH(1–32)NH2. Reverse transcription-PCR revealed the expression of splice variants of hGHRH receptor in CAKI-1 RCC. Biodistribution studies demonstrate an in vivo uptake of 125I-JV-1–42 by the RCC tumor tissue. The presence of specific receptor proteins that bind GHRH antagonists in CAKI-1 RCC supports the view that distinct binding sites that mediate the inhibitory effect of GHRH antagonists are present on various human cancers.

Direct toxicity of nonsteroidal antiinflammatory drugs for renal medullary cells

Antipyretic analgesics, taken in large doses over a prolonged period, cause a specific form of kidney disease, characterized by papillary necrosis and interstitial scarring. Epidemiological evidence incriminated mixtures of drugs including aspirin (ASA), phenacetin, and caffeine. The mechanism of toxicity is unclear. We tested the effects of ASA, acetaminophen (APAF, the active metabolite of phenacetin), caffeine, and other related drugs individually and in combination on mouse inner medullary collecting duct cells (mIMCD3). The number of rapidly proliferating cells was reduced by ≈50% by 0.5 mM ASA, salicylic acid, or APAF. The drugs had less effect on confluent cells, which proliferate slowly. Thus, the slow in vivo turnover of IMCD cells could explain why clinical toxicity requires very high doses of these drugs over a very long period. Caffeine greatly potentiated the effect of acetaminophen, pointing to a potential danger of the mixture. Cyclooxygenase (COX) inhibitors, indomethacin and NS-398, did not reduce cell number except at concentrations greatly in excess of those that inhibit COX. Therefore, COX inhibition alone is not toxic. APAF arrests most cells in late G1 and S and produces a mixed form of cell death with both oncosis (swollen cells and nuclei) and apoptosis. APAF is known to inhibit the synthesis of DNA and cause chromosomal aberrations due to inhibition of ribonucleotide reductase. Such effects of APAF might account for renal medullary cell death in vivo and development of uroepithelial tumors from surviving cells that have chromosomal aberrations.

Rapid activation of Na+/H+ exchange by aldosterone in renal epithelial cells requires Ca2+ and stimulation of a plasma membrane proton conductance.

There is increasing evidence for an additional acute, nongenomic action of the mineralocorticoid hormone aldosterone on renal epithelial cells, leading to a two-step model of mineralocorticoid action on electrolyte excretion. We investigated the acute effect of aldosterone on intracellular free Ca2+ and on intracellular pH in an aldosterone-sensitive Madin-Darby canine kidney cell clone. Within seconds of application of aldosterone, but not of the glucocorticoid hydrocortisone, there was a 3-fold sustained increase of intracellular Ca2+ at a half-maximal concentration of 10(-10) mol/liter. Omission of extracellular Ca2+ prevented this hormone response. In the presence of extracellular Ca2+ aldosterone led to intracellular alkalinization. The Na+/H+ exchange inhibitor ethyl-isopropanol-amiloride (EIPA) prevented the aldosterone-induced alkalinization but not the aldosterone-induced increase of intracellular Ca2+. Omission of extracellular Ca2+ also prevented aldosterone-induced alkalinization. Instead, aldosterone led to a Zn(2+)-dependent intracellular acidification in the presence of EIPA, indicative of an increase of plasma membrane proton conductance. Under control conditions, Zn2+ prevented the aldosterone-induced alkalinization completely. We conclude that aldosterone stimulated net-entry of Ca2+ from the extracellular compartment and a plasma membrane H+ conductance as prerequisites for the stimulation of plasma membrane Na+/H+ exchange which in turn modulates K+ channel acitivity. It is probable that the aldosterone-sensitive H+ conductance maintains Na+/H+ exchange activity by providing an acidic environment in the vicinity of the exchanger. Thus, genomic action of aldosterone determines cellular transport equipment, whereas the nongenomic action regulates transporter activity that requires responses within seconds or minutes, which explains the rapid effects on electrolyte excretion.

In vivo suppression of the renal Na+/Pi cotransporter by antisense oligonucleotides.

A 20-mer phosphorothioate oligonucleotide (AS1) was designed to hybridize to the message for the rat kidney sodium phosphate cotransporter NaPi-2 close to the translation initiation site. Single intravenous doses of this oligonucleotide were given to rats maintained on a low phosphorus diet to increase NaPi-2 expression. At 3 days after oligonucleotide infusion, rats receiving 2.5 micromol of AS1 exhibited a reduction in renal NaPi-2 to cyclophilin mRNA ratio by 40% +/- 17%, and rats receiving 7.5 micromol of AS1 exhibited a reduction in NaPi-2 to cyclophilin mRNA ratio by 46% +/- 21%. Reversed-sequence AS1 was without effect. The higher dose of 7.5 micromol of AS1 also reduced the rate of phosphate uptake into renal brush border membrane vesicles and the expression of NaPi-2 protein detected by Western blotting in these vesicles. Reversed sequence AS1 was again without effect on these parameters. These results suggest that systemically infused oligonucleotides can exert antisense effects in the renal proximal tubule.

Prevention of cardiovascular and renal pathology of aging by the advanced glycation inhibitor aminoguanidine.

Human aging is impacted severely by cardiovascular disease and significantly but less overtly by renal dysfunction. Advanced glycation endproducts (AGEs) have been linked to tissue damage in diabetes and aging, and the AGE inhibitor aminoguanidine (AG) has been shown to inhibit renal and vascular pathology in diabetic animals. In the present study, the effects of AG on aging-related renal and vascular changes and AGE accumulation were studied in nondiabetic female Sprague-Dawley (S-D) and Fischer 344 (F344) rats treated with AG (0.1% in drinking water) for 18 mo. Significant increases in the AGE content in aged cardiac (P < 0.05), aortic (P < 0.005), and renal (P < 0.05) tissues were prevented by AG treatment (P < 0.05 for each tissue). A marked age-linked vasodilatory impairment in response to acetylcholine and nitroglycerine was prevented by AG treatment (P < 0.005), as was an age-related cardiac hypertrophy evident in both strains (P < 0.05). While creatinine clearance was unaffected by aging in these studies, the AGE/ creatinine clearance ratio declined 3-fold in old rats vs. young rats (S-D, P < 0.05; F344, P < 0.01), while it declined significantly less in AG-treated old rats (P < 0.05). In S-D but not in F344 rats, a significant (P < 0.05) age-linked 24% nephron loss was completely prevented by AG treatment, and glomerular sclerosis was markedly suppressed (P < 0.01). Age-related albuminuria and proteinuria were markedly inhibited by AG in both strains (S-D, P < 0.01; F344, P < 0.01). These data suggest that early interference with AGE accumulation by AG treatment may impart significant protection against the progressive cardiovascular and renal decline afflicting the last decades of life.

Sphingosine: a mediator of acute renal tubular injury and subsequent cytoresistance.

The goal of this study was to determine whether sphingosine and ceramide, second messengers derived from sphingolipid breakdown, alter kidney proximal tubular cell viability and their adaptive responses to further damage. Adult human kidney proximal tubular (HK-2) cells were cultured for 0-20 hr in the presence or absence of sphingosine, sphingosine metabolites (sphingosine 1-phosphate, dimethylsphingosine), or C2, C8, or C16 ceramide. Acute cell injury was assessed by vital dye exclusion and tetrazolium dye transport. Their subsequent impact on superimposed ATP depletion/Ca2+ ionophore-induced damage was also assessed. Sphingosine (> or = 10 microM), sphingosine 1-phosphate, dimethylsphingosine, and selected ceramides (C2 and C8, but not C16) each induced rapid, dose-dependent cytotoxicity. This occurred in the absence of DNA laddering or morphologic changes of apoptosis, suggesting a necrotic form of cell death. Prolonged exposure (20 hr) to subtoxic sphingosine doses (< or = 7.5 microM) induced substantial cytoresistance to superimposed ATP depletion/Ca2+ ionophore-mediated damage. Conversely, neither short-term sphingosine treatment (< or = 8.5 hr) nor 20-hr exposures to any of the above sphingosine/ceramide derivatives/metabolites or various free fatty acids reproduced this effect. Sphingosine-induced cytoresistance was dissociated from the extent of cytosolic Ca2+ loading (indo-1 fluorescence), indicating a direct increase in cell resistance to attack. We conclude that sphingosine can exert dual effects on proximal renal tubular viability: in high concentrations it induces cell necrosis, whereas in low doses it initiates a cytoresistant state. These results could be reproduced in human foreskin fibroblasts, suggesting broad-based relevance to the area of acute cell injury and repair.

Basic fibroblast growth factor can mediate the early inductive events in renal development.

The earliest characterized events during induction of tubulogenesis in renal anlage include the condensation or compaction of metanephrogenic mesenchyme with the concurrent upregulation of WT1, the gene encoding the Wilms tumor transcriptional activator/suppressor. We report that basic fibroblast growth factor (FGF2) can mimic the early effects of an inductor tissue by promoting the condensation of mesenchyme and inhibiting the tissue degeneration associated with the absence of an inductor tissue. By in situ hybridization, FGF2 was also found to mediate the transcriptional activation of WT1 and of the hepatocyte growth factor receptor gene, c-met. Although FGF2 can induce these early events of renal tubulogenesis, it cannot promote the epithelial conversion associated with tubule formation in metanephrogenic mesenchyme. For this, an undefined factor(s) from pituitary extract in combination with FGF2 can cause tubule formation in uninduced mesenchyme. These findings support the concept that induction in kidney is a multiphasic process that is mediated by more than a single comprehensive inductive factor and that soluble molecules can mimic these inductive activities in isolated uninduced metanephrogenic mesenchyme.