Altered cholesterol localization and caveolin expression during the evolution of acute renal failure

A molecular network-based pharmacological study on the protective effect of Panax notoginseng rhizomes against renal ischemia-reperfusion injury

Objective: We aimed to explore the protective effect of Panax notoginseng rhizomes (PNR) on renal ischemia and reperfusion injury (RIRI) and the underlying molecular network mechanism based on network pharmacology and combined systemic experimental validation. Methods: A bilateral RIRI model was established, and Cr, SCr, and BUN levels were detected. Then, the PNR was pretreated 1 week before the RIRI model was prepared. To determine the effects of the PNR in RIRI, histopathological damage and the effect of PNRs to the kidney was assessed, using TTC, HE, and TUNEL staining. Furthermore, the underlying network pharmacology mechanism was detected by screening drug-disease intersection targets from PPI protein interactions and GO and KEGG analysis, and the hub genes were screened for molecular docking based on the Degree value. Finally, the expression of hub genes in kidney tissues was verified by qPCR, and the protein expression of related genes was further detected by Western blot (WB). Results: PNR pretreatment could effectively increase Cr level, decrease SCr and BUN levels, reduce renal infarct areas and renal tubular cell injury areas, and inhibit renal cell apoptosis. By using network pharmacology combined with bioinformatics, we screened co-targets both Panax notoginseng (Sanchi) and RIRI, acquired ten hub genes, and successfully performed molecular docking. Of these, pretreatment with the PNR reduced the mRNA levels of IL6 and MMP9 at postoperative day 1 and TP53 at postoperative day 7, and the protein expression of MMP9 at postoperative day 1 in IRI rats. These results showed that the PNR could decrease kidney pathological injury in IRI rats and inhibit apoptotic reaction and cell inflammation so as to improve renal injury effectively, and the core network mechanism is involved in the inhibition of MMP9, TP53, and IL-6. Conclusion: The PNR has a marked protective effect for RIRI, and the underlying mechanism is involved in inhibiting the expression of MMP9, TP53, and IL-6. This striking discovery not only provides fruitful evidence for the protective effect of the PNR in RIRI rats but also provides a novel mechanic explanation.

Caveolin-1 Regulates Cellular Metabolism: A Potential Therapeutic Target in Kidney Disease

The kidney is an energy-consuming organ, and cellular metabolism plays an indispensable role in kidney-related diseases. Caveolin-1 (Cav-1), a multifunctional membrane protein, is the main component of caveolae on the plasma membrane. Caveolae are represented by tiny invaginations that are abundant on the plasma membrane and that serve as a platform to regulate cellular endocytosis, stress responses, and signal transduction. However, caveolae have received increasing attention as a metabolic platform that mediates the endocytosis of albumin, cholesterol, and glucose, participates in cellular metabolic reprogramming and is involved in the progression of kidney disease. It is worth noting that caveolae mainly depend on Cav-1 to perform the abovementioned cellular functions. Furthermore, the mechanism by which Cav-1 regulates cellular metabolism and participates in the pathophysiology of kidney diseases has not been completely elucidated. In this review, we introduce the structure and function of Cav-1 and its functions in regulating cellular metabolism, autophagy, and oxidative stress, focusing on the relationship between Cav-1 in cellular metabolism and kidney disease; in addition, Cav-1 that serves as a potential therapeutic target for treatment of kidney disease is also described.

[Significance of exosomes in cardiology: heralds of cardioprotection]

Los exosomas tienen un papel clave en la comunicación intercelular. Debido a sus múltiples interacciones, estas estructuras cumplen con el papel de «mensajeros» de forma dinámica, transportando su contenido a células blanco específicas y generando nuevas señales celulares. En este artículo se describen algunas de las proteínas, lípidos y ácidos nucleicos que son transportados por estas vesículas y que se han relacionado con cardioprotección, con la finalidad de proporcionar información y generar interés sobre la relevancia de los exosomas como posibles blancos diagnósticos y terapéuticos.

β-Cyclodextrin counteracts obesity in Western diet-fed mice but elicits a nephrotoxic effect

Obesity has become a worldwide health crisis and is associated with a plethora of comorbidities. The multi-organ effects of obesity have been linked to ectopic lipid accumulation. Thus, there is an urgent need to tackle the obesity crisis by developing effective lipid-lowering therapies. 2-hydroxypropyl-β-Cyclodextrin (2HP-β-CD) has been previously shown to reduce lysosomal cholesterol accumulation in a murine model of Niemann Pick Type C (NPC) disease. Using a murine model of Western diet-induced obesity (DIO), we report the effects of 2HP-β-CD in counteracting weight gain, expansion of adipose tissue mass and ectopic lipid accumulation. Interestingly, DIO caused intracellular storage of neutral lipids in hepatic tissues and of phospholipids in kidneys, both of which were prevented by 2HP-β-CD. Importantly, this report brings attention to the nephrotoxic effects of 2HP-β-CD: renal tubular damage, inflammation and fibrosis. These effects may be overlooked, as they are best appreciated upon assessment of renal histology.

Novel finding of caveolin-2 in apical membranes of proximal tubule and first detection of caveolin-2 in urine: A promising biomarker of renal disease

Caveolin-2 (Cav-2) is expressed in a variety of cell tissue, and it has also been found in renal tissue. The expression of Cav-2 in proximal tubules is still unclear. The aim of this study was to carry out a complete evaluation of the expression pattern of Cav-2 in rat renal cortex to clarify and deepen the knowledge about the localization of Cav-2 in the proximal tubules and also to evaluate its presence in urine. Male Wistar rats were used to assess Cav-2 expression by Western blot analysis in homogenates, apical, and basolateral membranes from kidney cortex, in lysates and total plasma membranes from renal cortical cell suspensions, in urine, and in urinary exosomes. Cav-2 was clearly expressed in renal cortex homogenates and in both apical and basolateral membranes isolated from kidney cortex, with a greater expression on the former membranes. It was also observed in lysates and in plasma membranes from cortical cell suspensions. Moreover, Cav-2 was found in urine and in its exosomal fraction. These results confirmed the presence of Cav-2 in proximal tubule cells in the kidney of healthy rats, and showed for the first time its expression at the apical membrane of these cells and in urine. Besides, urinary exosomal pathway could be involved in Cav-2 urinary excretion under normal conditions. We observed an increase in the urinary abundance of Cav-2 in two models of acute kidney injury, and thus we proposed the urinary excretion of Cav-2 as a potential biomarker of kidney injury.

Injury induced expression of caveolar proteins in human kidney tubules - role of megakaryoblastic leukemia 1

Background

Caveolae are membrane invaginations measuring 50–100 nm. These organelles, composed of caveolin and cavin proteins, are important for cellular signaling and survival. Caveolae play incompletely defined roles in human kidneys. Induction of caveolin-1/CAV1 in diseased tubules has been described previously, but the responsible mechanism remains to be defined.

Methods

Healthy and atrophying human kidneys were stained for caveolar proteins, (caveolin 1–3 and cavin 1–4) and examined by electron microscopy. Induction of caveolar proteins was studied in isolated proximal tubules and primary renal epithelial cells. These cells were challenged with hypoxia or H₂O₂. Primary tubular cells were also subjected to viral overexpression of megakaryoblastic leukemia 1 (MKL1) and MKL1 inhibition by the MKL1 inhibitor CCG-1423. Putative coregulators of MKL1 activity were investigated by Western blotting for suppressor of cancer cell invasion (SCAI) and filamin A (FLNA). Finally, correlative bioinformatic studies of mRNA expression of caveolar proteins and MKL1 were performed.

Results

In healthy kidneys, caveolar proteins were expressed by the parietal epithelial cells (PECs) of Bowman’s capsule, endothelial cells and vascular smooth muscle. Electron microscopy confirmed caveolae in the PECs. No expression was seen in proximal tubules. In contrast, caveolar proteins were expressed in proximal tubules undergoing atrophy. Caveolar proteins were also induced in cultures of primary epithelial tubular cells. Expression was not enhanced by hypoxia or free radical stress (H₂O₂), but proved sensitive to inhibition of MKL1. Viral overexpression of MKL1 induced caveolin-1/CAV1, caveolin-2/CAV2 and SDPR/CAVIN2. In kidney tissue, the mRNA level of MKL1 correlated with the mRNA levels for caveolin-1/CAV1, caveolin-2/CAV2 and the archetypal MKL1 target tenascin C (TNC), as did the MKL1 coactivator FLNA. Costaining for TNC as readout for MKL1 activity demonstrated overlap with caveolin-1/CAV1 expression in PECs as well as in atrophic segments of proximal tubules.

Conclusions

Our findings support the view that MKL1 contributes to the expression of caveolar proteins in healthy kidneys and orchestrates the induction of tubular caveolar proteins in renal injury.

Electronic supplementary material

The online version of this article (10.1186/s12882-017-0738-8) contains supplementary material, which is available to authorized users.

Plasma membrane wounding and repair in pulmonary diseases

Various pathophysiological conditions such as surfactant dysfunction, mechanical ventilation, inflammation, pathogen products, environmental exposures, and gastric acid aspiration stress lung cells, and the compromise of plasma membranes occurs as a result. The mechanisms necessary for cells to repair plasma membrane defects have been extensively investigated in the last two decades, and some of these key repair mechanisms are also shown to occur following lung cell injury. Because it was theorized that lung wounding and repair are involved in the pathogenesis of acute respiratory distress syndrome (ARDS) and idiopathic pulmonary fibrosis (IPF), in this review, we summarized the experimental evidence of lung cell injury in these two devastating syndromes and discuss relevant genetic, physical, and biological injury mechanisms, as well as mechanisms used by lung cells for cell survival and membrane repair. Finally, we discuss relevant signaling pathways that may be activated by chronic or repeated lung cell injury as an extension of our cell injury and repair focus in this review. We hope that a holistic view of injurious stimuli relevant for ARDS and IPF could lead to updated experimental models. In addition, parallel discussion of membrane repair mechanisms in lung cells and injury-activated signaling pathways would encourage research to bridge gaps in current knowledge. Indeed, deep understanding of lung cell wounding and repair, and discovery of relevant repair moieties for lung cells, should inspire the development of new therapies that are likely preventive and broadly effective for targeting injurious pulmonary diseases.

Sarcolemmal dependence of cardiac protection and stress-resistance: roles in aged or diseased hearts

Disruption of the sarcolemmal membrane is a defining feature of oncotic death in cardiac ischaemia-reperfusion (I-R), and its molecular makeup not only fundamentally governs this process but also affects multiple determinants of both myocardial I-R injury and responsiveness to cardioprotective stimuli. Beyond the influences of membrane lipids on the cytoprotective (and death) receptors intimately embedded within this bilayer, myocardial ionic homeostasis, substrate metabolism, intercellular communication and electrical conduction are all sensitive to sarcolemmal makeup, and critical to outcomes from I-R. As will be outlined in this review, these crucial sarcolemmal dependencies may underlie not only the negative effects of age and common co-morbidities on myocardial ischaemic tolerance but also the on-going challenge of implementing efficacious cardioprotection in patients suffering accidental or surgically induced I-R. We review evidence for the involvement of sarcolemmal makeup changes in the impairment of stress-resistance and cardioprotection observed with ageing and highly prevalent co-morbid conditions including diabetes and hypercholesterolaemia. A greater understanding of membrane changes with age/disease, and the inter-dependences of ischaemic tolerance and cardioprotection on sarcolemmal makeup, can facilitate the development of strategies to preserve membrane integrity and cell viability, and advance the challenging goal of implementing efficacious 'cardioprotection' in clinically relevant patient cohorts. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

Role of caveolin 1 in AT1a receptor-mediated uptake of angiotensin II in the proximal tubule of the kidney

Caveolin 1 (CAV-1) functions not only as a constitutive scaffolding protein of caveolae but also as a vesicular transporter and signaling regulator. In the present study, we tested the hypothesis that CAV-1 knockout (CAV-1 KO) inhibits ANG II type 1 [AT1 (AT1a)] receptor-mediated uptake of ANG II in the proximal tubule and attenuates blood pressure responses in ANG II-induced hypertension. To determine the role of CAV-1 in mediating the uptake of FITC-labeled ANG II, wild-type (WT) mouse proximal convoluted tubule cells were transfected with CAV-1 small interfering (si)RNA for 48 h before AT1 receptor-mediated uptake of FITC-labeled ANG II was studied. CAV-1 siRNA knocked down CAV-1 expression by >90% (P < 0.01) and inhibited FITC-labeled ANG II uptake by >50% (P < 0.01). Moreover, CAV-1 siRNA attenuated ANG II-induced activation of MAPK ERK1/2 and Na(+)/H(+) exchanger 3 expression, respectively (P < 0.01). To determine whether CAV-1 regulates ANG II uptake in the proximal tubule, Alexa 488-labeled ANG II was infused into anesthetized WT and CAV-1 KO mice for 60 min (20 ng/min iv). Imaging analysis revealed that Alexa 488-labeled ANG II uptake was decreased by >50% in CAV-1 KO mice (P < 0.01). Furthermore, Val(5)-ANG II was infused into WT and CAV-1 KO mice for 2 wk (1.5 mg·kg(-1)·day(-1) ip). Basal systolic pressure was higher, whereas blood pressure and renal excretory and signaling responses to ANG II were attenuated, in CAV-1 KO mice (P < 0.01). We concluded that CAV-1 plays an important role in AT1 receptor-mediated uptake of ANG II in the proximal tubule and modulates blood pressure and renal responses to ANG II.

Impaired expression of caveolin-1 contributes to hepatic ischemia and reperfusion injury

Caveolae are membrane structures enriched in glycosphingolipids and cholesterol, and caveolin-1 (Cav-1) has been recognized to be pivotal in ischemic tolerance. Sphingosine-1-phosphate (S1P), one of the sphingolipid metabolites, is well known for its anti-apoptotic properties, counteracting ischemia and reperfusion (IR) injury. Here, we investigated the cytoprotective mechanism of Cav-1 against IR injury. Male C57BL/6 mice underwent 70% hepatic ischemia for 60 min, followed by reperfusion. Mice were pretreated with methyl-beta-cyclodextrin (MβCD, 10, 25 and 50mg/kg, i.p.), a caveolae disruptor, or saline 48 and 24h before ischemia. Serum and liver tissues were collected at the end of ischemia, at 0, 1, 4 and 24h of reperfusion. Decreases in the expression of Cav-1 protein and in the number of caveolae of the liver ultrastructure were observed during IR, which were augmented by pretreatment with MβCD. MβCD also augmented the IR-induced increases in serum alanine aminotransferase and tumor necrosis factor-α levels. IR decreased the levels of sphingosine kinase 2 (SK2) and S1P receptor 2 (S1P2) mRNA expressions, while MβCD also augmented these decreases. Moreover, IR resulted in increases of mitochondrial cytochrome c release, caspase 3, 8 activities and Bax/Bcl-xL ratio, and MβCD augmented all of these apoptotic parameters. MβCD also increased p38 MAPK and JNK phosphorylation, but did not affect ERK and PI3K/Akt. Our findings demonstrate that downregulation of Cav-1 mediates IR-induced liver damage by inhibiting SK2/S1P2 signaling and enhancing the apoptotic pathway.

Epigallocatechin-3-gallate-mediated cardioprotection by Akt/GSK-3β/caveolin signalling in H9c2 rat cardiomyoblasts

Background

Epigallocatechin-3-gallate (EGCg) with its potent anti-oxidative capabilities is known for its beneficial effects ameliorating oxidative injury to cardiac cells. Although studies have provided convincing evidence to support the cardioprotective effects of EGCg, it remains unclear whether EGCg affect trans-membrane signalling in cardiac cells. Here, we have demonstrated the potential mechanism for cardioprotection of EGCg against H₂O₂-induced oxidative stress in H9c2 cardiomyoblasts.

Results

Exposing H9c2 cells to H₂O₂ suppressed cell viability and altered the expression of adherens and gap junction proteins with increased levels of intracellular reactive oxygen species and cytosolic Ca²⁺. These detrimental effects were attenuated by pre-treating cells with EGCg for 30 min. EGCg also attenuated H₂O₂-mediated cell cycle arrest at the G1-S phase through the glycogen synthase kinase-3β (GSK-3β)/β-catenin/cyclin D1 signalling pathway. To determine how EGCg targets H9c2 cells, enhanced green fluorescence protein (EGFP) was ectopically expressed in these cells. EGFP-emission fluorescence spectroscopy revealed that EGCg induced dose-dependent fluorescence changes in EGFP expressing cells, suggesting that EGCg signalling events might trigger proximity changes of EGFP expressed in these cells.

Proteomics studies showed that EGFP formed complexes with the 67 kD laminin receptor, caveolin-1 and -3, β-actin, myosin 9, vimentin in EGFP expressing cells. Using in vitro oxidative stress and in vivo myocardial ischemia models, we also demonstrated the involvement of caveolin in EGCg-mediated cardioprotection. In addition, EGCg-mediated caveolin-1 activation was found to be modulated by Akt/GSK-3β signalling in H₂O₂-induced H9c2 cell injury.

Conclusions

Our data suggest that caveolin serves as a membrane raft that may help mediate cardioprotective EGCg transmembrane signalling.

Caveolin and caveolae in age associated cardiovascular disease

It is estimated that the elderly (> 65 years of age) will increase from 13%−14% to 25% by 2035. If this trend continues, > 50% of the United States population and more than two billion people worldwide will be “aged” in the next 50 years. Aged individuals face formidable challenges to their health, as aging is associated with a myriad of diseases. Cardiovascular disease is the leading cause of morbidity and mortality in the United States with > 50% of mortality attributed to coronary artery disease and > 80% of these deaths occurring in those age 65 and older. Therefore, age is an important predictor of cardiovascular disease. The efficiency of youth is built upon cellular signaling scaffolds that provide tight and coordinated signaling. Lipid rafts are one such scaffold of which caveolae are a subset. In this review, we consider the importance of caveolae in common cardiovascular diseases of the aged and as potential therapeutic targets. We specifically address the role of caveolin in heart failure, myocardial ischemia, and pulmonary hypertension.

The significance of caveolae in the glomeruli in glomerular disease

AIMS

The aim of this study was to demonstrate expression of cell membrane invagination 'caveolae' in glomeruli and to correlate this with functional and structural characteristics of the human glomerular diseases.

METHODS

The expression of caveolin-1 (Cav-1), which is the main component of caveolae, was examined in the glomeruli, and the relationship between Cav-1 expression and pathological and clinical findings was determined in 99 patients with glomerular disease and in 50 renal transplantation donors as controls.

RESULTS

Cav-1 was expressed very weakly in the controls, and the area of Cav-1 expression relative to the total glomerular area was 0.57±0.65%. However, the area of Cav-1 expression was significantly larger in each glomerular disease (IgA nephropathy, 1.05±1.36%, p<0.05; crescent glomerulonephritis, 1.86±1.19%, p<0.001; minimal change disease, 2.38±1.24%, p<0.001; focal segmental glomerulosclerosis, 2.88±2.05%, p<0.01; membranous nephritis, 4.27±2.95%, p<0.001; membranoproliferative glomerulonephritis, 4.49±3.15%, p<0.001; and diabetic nephropathy, 2.45±1.52%, p<0.001; compared with the controls. Cav-1 expression was significantly decreased in glomerular disease treated with steroids. Co-localisation of Cav-1 and the endothelial marker 'pathologische anatomie leiden-endothelium' was prominent in an immunofluorescence study, and caveolae on the glomerular endothelial cells were observed in electron microscopy.

CONCLUSIONS

The expression of Cav-1 was significantly increased in the glomeruli of patients with glomerular disease, and it was related to urinary albumin excretion. Cav-1 expression and caveolae were observed in glomerular endothelial cells. It is hypothesised that they play a role in the recovery phase of capillary injury or endocytosis of albumin into endothelial cells. Basic research should be performed to elucidate the role played by Cav-1 and caveolae.

Inhibition of renal caveolin-1 reduces natriuresis and produces hypertension in sodium-loaded rats

Renal dopamine receptor function and ion transport inhibition are impaired in essential hypertension. We recently reported that caveolin-1 (CAV1) and lipid rafts are necessary for normal D(1)-like receptor-dependent internalization of Na-K-ATPase in human proximal tubule cells. We now hypothesize that CAV1 is necessary for the regulation of urine sodium (Na(+)) excretion (U(Na)V) and mean arterial blood pressure (MAP) in vivo. Acute renal interstitial (RI) infusion into Sprague-Dawley rats of 1 μg·kg⁻¹·min⁻¹ fenoldopam (FEN; D(1)-like receptor agonist) caused a 0.46 ± 0.15-μmol/min increase in U(Na)V (over baseline of 0.29 ± 0.04 μmol/min; P < 0.01). This increase was seen in Na(+)-loaded rats, but not in those under a normal-sodium load. Coinfusion with β-methyl cyclodextrin (βMCD; lipid raft disrupter, 200 μg·kg⁻¹·min⁻¹) completely blocked this FEN-induced natriuresis (P < 0.001). Long-term (3 day) lipid raft disruption via continuous RI infusion of 80 μg·kg⁻¹·min⁻¹ βMCD decreased renal cortical CAV1 expression (47.3 ± 6.4%; P < 0.01) and increased MAP (32.4 ± 6.6 mmHg; P < 0.001) compared with vehicle-infused animals. To determine whether the MAP rise was due to a CAV1-dependent lipid raft-mediated disruption, Na(+)-loaded rats were given a bolus RI infusion of CAV1 siRNA. Two days postinfusion, cortical CAV1 expression was decreased by 73.6 ± 8.2% (P < 0.001) and the animals showed an increase in MAP by 17.4 ± 2.9 mmHg (P < 0.01) compared with animals receiving scrambled control siRNA. In summary, acute kidney-specific lipid raft disruption decreases CAV1 expression and blocks D(1)-like receptor-induced natriuresis. Furthermore, chronic disruption of lipid rafts or CAV1 protein expression in the kidney induces hypertension.

Is caveolin involved in normal proximal tubule function? Presence in model PT systems but absence in situ

Kidney proximal tubule (PT) cells are specialized for the uptake and transport of ions, solutes, peptides, and proteins. These functions are often regulated by hormones that signal at the cell surface and are internalized by clathrin-mediated endocytosis. However, the caveolin/caveolae pathway has also been implicated in normal PT function, often based on data from isolated PTs or PT cells in culture. Although we reported previously that caveolae and caveolin 1 are not detectable in PTs in vivo, reports of caveolin expression and function in PT cells appear periodically in the literature. Therefore, we reexamined caveolin expression in PTs in vivo, in isolated "purified" PTs following collagenase digestion, and in cultured PT cells. Caveolin 1 and 2 protein, mRNA, or immunofluorescence was undetectable in PTs in vivo, but PT cell cultures expressed caveolin 1 and/or 2. Furthermore, caveolin 1 and 2 mRNAs were detected in isolated PTs along with the endothelial markers CD31 and ICAM1. In contrast, no caveolin or endothelial marker mRNAs were detectable in samples isolated from snap-frozen kidneys by laser cut microdissection, which eliminates contamination by other cell types. We conclude 1) caveolin 1 and 2 are not normally expressed by PT cells in situ, 2) caveolin expression is "activated" in cultured PT cells, 3) contamination with non-PT, caveolin-expressing cells is a potential source of caveolin 1 and 2 that must be taken into account when isolated PTs are used in studies to correlate expression of these proteins with PT function.

Organic extract of tire debris causes localized damage in the plasma membrane of human lung epithelial cells

The potential toxicity of tire debris organic extracts on human alveolar epithelial cells (A549) was investigated. We analysed time- and dose dependent modifications produced on plasma membrane molecular composition and on lipid microdomains expression (caveolae and lipid rafts) that represent specific signalling platforms. Cells were exposed to increasing organic extract concentrations (10, 60 and 75mug/ml) for 24, 48 and 72h. An up to three fold dose and time dependent increase in specific protein markers of lipid microdomains was found, suggesting a corresponding increase in signalling platforms. Since the total pool of these plasma membrane markers was unchanged, we supposed that these proteins were translocated within the plasma membrane as to assemble the newly formed lipid microdomains. Despite no major modifications in lipid bilayer composition, a time- and dose dependent toxic effect was documented at 48h of exposure by an increase of cells positive to Trypan Blue assay. After 48h a dose dependent increase in the cell medium of the cytosolic enzyme lactate dehydrogenase was also observed, indicating greater damage of the plasma membrane as prenecrotic sign. The overall ultrastructural morphology of the plasma membrane of treated cells was not greatly modified, suggesting that organic extracts from tire debris cause focalized discontinuities on cell surfaces.

Immunohistochemical study on caveolin-1alpha in regenerating process of tubular cells in gentamicin-induced acute tubular injury in rats

Caveolin-1, a principal component of caveolae, modulates growth signaling, endocytosis, and intracellular transport. We examined the expression of caveolin-1alpha and its relation to cell cycle and caveolin-interacting growth factor receptors in regenerating proximal tubules (PTs) after gentamicin-induced acute renal failure in rats. Caveolin-1alpha appeared in regenerating PTs as early as day 4 after last gentamicin, peaked at days 6 to 8, and showed cytoplasmic pattern after day 8. Immunoelectron microscopy revealed caveolin-1alpha-positive caveolae on the cell membrane and in cytoplasms in regenerating PTs at days 4 to 8 and caveolin-positivity confined to cytoplasms after day 10. The number of PT cells with proliferation markers peaked at day 6 and decreased afterwards as expression of cyclin-dependent kinase inhibitors increased. Platelet-derived growth factor receptor-beta (PDGFR-beta) and epidermal growth factor receptor (EGFR) were colocalized with caveolin-1alpha in proliferating PTs as early as day 4. Phosphorylated EGFR increased at day 8 and afterwards when caveolins dissociated from EGFR or decreased. In case of PDGFR-beta, phosphorylation seemed to be associated with the increase and association of caveolins to the receptors. Our results suggest that transient expression of caveolin-1alpha in early regenerating PTs might contribute to the regenerating process of PTs through modulating growth factor receptors.

‘Endotoxin tolerance’: TNF-α hyper-reactivity and tubular cytoresistance in a renal cholesterol loading state

The term 'endotoxin tolerance' defines a state in which prior endotoxin (lipopolysaccharide (LPS)) exposure induces resistance to subsequent LPS attack. However, its characteristics within kidney have not been well defined. Hence, this study tested the impact of LPS 'preconditioning' (LPS-PC; 18 or 72 h earlier) on: (i) selected renal inflammatory mediators (tumor necrosis factor (TNF)-alpha, interleukin-10 (IL-10), monocyte chemotactic protein-1 (MCP-1), inducible nitric oxide synthase (iNOS), Toll-like receptor 4 (TLR4); protein or mRNA); (ii) cholesterol homeostasis (a stress reactant); and (iii) isolated proximal tubule (PT) vulnerability to hypoxia or membrane cholesterol (cholesterol oxidase/esterase) attack. Two hours post LPS injection, LPS-PC mice manifested reduced plasma TNF-alpha levels, consistent with systemic LPS tolerance. However, in kidney, paradoxical TNF-alpha hyper-reactivity (protein/mRNA) to LPS existed, despite normal TLR4 protein levels. PT TNF-alpha levels paralleled renal cortical results, implying that PTs were involved. LPS-PC also induced: (i) renal cortical iNOS, IL-10 (but not MCP-1) mRNA hyper-reactivity; (ii), PT cholesterol loading, and (iii) cytoresistance to hypoxia and plasma membrane cholesterol attack. A link between cholesterol homeostasis and cell LPS responsiveness was suggested by observations that cholesterol reductions in HK-2 cells (methylcyclodextrin), or reductions in HK-2 membrane fluidity (A2C), blunted LPS-mediated TNF-alpha/MCP-1 mRNA increases. In sum: (i) systemic LPS tolerance can be associated with renal hyper-responsiveness of selected components within the LPS signaling cascade (e.g., TNF-alpha, iNOS, IL-10); (ii) PT cytoresistance against hypoxic/membrane injury coexists; and (iii) LPS-induced renal/PT cholesterol accumulation may mechanistically contribute to each of these results.

Renal caveolin-1 expression in children with unilateral ureteropelvic junction obstruction

Caveolae are plasma membrane invaginations that contain a variety of signal transduction molecules and receptors for growth factors and cytokines. This study was performed to examine the in vivo expression and localization of caveolin-1 in kidneys from 19 children who underwent surgery release of ureteropelvic junction obstruction (UPJO) in relation to renal function and degree of tubulointerstitial fibrosis. Renal biopsies were carried out at the time of surgery for obstruction release. Kidney tissue from children of similar age removed because of carcinoma was used as control. Expression of caveolin-1 at the protein level in renal tissue and urine was demonstrated in patients with technetium 99 m labeled diethylene triamine pentaacetate ((99)Tc DTPA) renal scan 28.8+/-2% and increased tubular interstitial fibrosis in seven patients at the time of obstruction release. Colocalization staining of AT(1) angiotensin II receptor with caveolin-1 in basolateral membrane of epithelial tubule cells, enhanced AT(1) messenger ribonucleic acid (mRNA) and decreased endothelial nitric oxide synthase (eNOS), were shown in these patients. In contrast, absence of association of caveolin-1 with AT(1) receptor expression in proximal and collecting tubule membranes with AT(1) receptor mRNA and eNOS mRNA expression near control were demonstrated in 12 patients, with (99)Tc DTPA renal scan 39.7+2.1% and no evidence of tubulointerstitial fibrosis. From our results, the role of caveolin-1 as a factor contributing to the severity of the tubulointerstitial process resulting from obstructive nephropathy could be suggested.

Regional myocardial ischemia-induced activation of MAPKs is associated with subcellular redistribution of caveolin and cholesterol

Ischemia-reperfusion activates ERK and p38 MAPK in cardiac membranes, but the role of caveolae in MAPK signaling during this stress has not been studied. The purpose of this study was to determine the effect of in vivo myocardial ischemia-reperfusion on the level and distribution of caveolin-1 and -3 and cholesterol as well as MAPK activation in caveolin-enriched fractions. Adult male rats were subjected to in vivo regional myocardial ischemia induced by 25 min of coronary artery occlusion and 10 min ( n = 5) or 2 h ( n = 4) of reperfusion. Another group of rats served as appropriate nonischemic time controls ( n = 4). A discontinuous sucrose density gradient was used to isolate caveolae/lipid rafts from ischemic and nonischemic heart tissue. Caveolin-1 and -3, as well as cholesterol, were enriched in the light fractions. A redistribution of caveolin-3 and a reduction in caveolin-1 and cholesterol levels in the light fractions occurred after 10 min of reperfusion. The ERKs were activated in ischemic zone light and heavy fractions by 10 min of reperfusion. p44 ERK was activated after 2 h of reperfusion only in the light fractions, whereas p42 ERK phosphorylation was increased in the light and heavy fractions. Although no p38 MAPK activation occurred after 10 min of reperfusion, 2 h of reperfusion caused significant activation of p38 MAPK in nonischemic zone light and heavy fractions. These results show the importance of caveolar membrane/lipid rafts in MAPK signaling and suggest that subcellular compartmentation of p44/p42 ERKs and p38 MAPK may play distinct roles in the response to myocardial ischemia-reperfusion.

MCP-1 is up-regulated in unstressed and stressed HO-1 knockout mice: Pathophysiologic correlates

BACKGROUND

Up-regulation of heme oxygenase-1 (HO-1) occurs in, and often confers protection to, the injured kidney. Up-regulation of monocyte chemoattractant protein-1 (MCP-1) promotes not only acute and chronic nephritides but also acute ischemic and nephrotoxic injury. The present study was stimulated by the hypothesis that expression of MCP-1 is suppressed by HO-1, and analyzed the effect of HO-1 on the expression of MCP-1 in stressed and unstressed conditions.

METHODS

Expression of MCP-1 and pathophysiologic correlates were examined in HO-1 knockout (HO-1-/-) and wild-type (HO-1+/+) mice in the unstressed state in young and aged mice, and following nephrotoxic and ischemic insults.

RESULTS

In unstressed HO-1-/- mice, plasma levels of MCP-1 protein were elevated, and MCP-1 mRNA expression was increased in circulating leukocytes and in the kidney. Such early and heightened up-regulation of MCP-1 was eventually accompanied by phenotypic changes in the aged kidney consistent with MCP-1, namely, proliferative changes in glomeruli, tubulointerstitial disease, and up-regulation of transforming growth factor-beta1 (TGF-beta1) and collagens I, III, and IV. In response to a nephrotoxic insult such as hemoglobin, MCP-1 mRNA was up-regulated in a markedly sustained manner in HO-1-/- mice. In response to a duration of ischemia that exerted little effect in HO-1+/+ mice, HO-1-/- mice exhibited higher expression of MCP-1 mRNA, enhanced activation of nuclear factor-kappaB (NF-kappaB) (the transcription factor that regulates MCP-1), markedly greater functional and structural renal injury, increased caspase-3 expression, and increased mortality.

CONCLUSION

In the absence of HO-1, expression of MCP-1 is significantly and consistently enhanced in unstressed and stressed conditions. We speculate that the protective effects of HO-1 in injured tissue may involve, at least in part, the capacity of HO-1 to restrain up-regulation of MCP-1.

Transgenic sickle mice are markedly sensitive to renal ischemia-reperfusion injury

Ischemic injury is invoked as a mechanism contributing to end-organ damage and other complications of sickle cell disease (SCD). However, the intrinsic sensitivity of tissues in SCD to ischemic insults has never been addressed. We examined the effect of renal ischemia in a transgenic mouse expressing human sickle hemoglobin. Twenty-four hours after bilateral, total renal artery occlusion for 15 minutes, transgenic sickle mice exhibited worse renal function and more marked histological injury. With bilateral renal ischemia of greater duration (22.5 minutes), and after 6 hours, transgenic sickle mice exhibited massive vascular congestion, sickling of red blood cells, more marked histological injury in the kidney, and more prominent congestion in the capillary beds in the lungs and heart. Additionally, serum amyloid P-component, the murine homologue of C-reactive protein, was markedly increased in transgenic sickle mice as compared to wild-type mice. Twenty-four hours after bilateral renal ischemia for 22.5 minutes, transgenic sickle mice exhibited 28% mortality, with no mortality observed in any other group. With bilateral renal ischemia of short or long duration, renal expression of caspase-3 was most prominent in transgenic sickle mice subjected to ischemia. Thus, renal ischemia in this murine model induces more severe renal injury and extrarenal complications. We conclude that tissues in SCD exhibit heightened vascular congestion and sensitivity to ischemia and that clinically apparent or silent episodes of ischemia may contribute to the complications of SCD.

Caveolins in the repair phase of acute renal failure after oxidative stress

Ischaemia-reperfusion and toxic injury are leading causes of acute renal failure (ARF). Both of these injury initiators use secondary mediators of damage in oxygen-derived free radicals. Several recent publications about ischaemia-reperfusion and toxin-induced ARF have indicated that plasma membrane structures called caveolae, and their proteins, the caveolins, are potential participants in protecting or repairing renal tissues. Caveolae and caveolins have previously been ascribed many functions, a number of which may mediate cell death or survival of injured renal cells. This review proposes possible pathophysiological mechanisms by which altered caveolin-1 expression and localization may affect renal cell survival following oxidative stress.

Recovery of Na-glucose cotransport activity after renal ischemia is impaired in mice lacking vimentin

Vimentin, an intermediate filament protein mainly expressed in mesenchyma-derived cells, is reexpressed in renal tubular epithelial cells under many pathological conditions, characterized by intense cell proliferation. Whether vimentin reexpression is only a marker of cell dedifferentiation or is instrumental in the maintenance of cell structure and/or function is still unknown. Here, we used vimentin knockout mice ( Vim−/−) and an experimental model of acute renal injury (30-min bilateral renal ischemia) to explore the role of vimentin. Bilateral renal ischemia induced an initial phase of acute tubular necrosis that did not require vimentin and was similar, in terms of morphological and functional changes, in Vim+/+and Vim−/−mice. However, vimentin was essential to favor Na-glucose cotransporter 1 localization to brush-border membranes and to restore Na-glucose cotransport activity in regenerating tubular cells. We show that the effect of vimentin inactivation is specific and results in persistent glucosuria. We propose that vimentin is part of a structural network that favors carrier localization to plasma membranes to restore transport activity in injured kidneys.

Kidney ischemia–reperfusion regulates expression and distribution of tubulin subunits, β-actin and rho GTPases in proximal tubules

Ischemic injury is characterized by a loss of cell polarity and a release of proximal tubule epithelial cells resulting from cytoskeletal reorganization. This study used a reversible unilateral renal ischemia-reperfusion model to investigate the expression and distribution of cytoskeletal components and Rho GTPases at protein and mRNA levels in proximal tubule fractions. Ischemia strongly increased beta-actin and alpha-tubulin expressions that were predominantly found in nuclear fractions. Rho GTPases and caveolin-1 expression were upregulated by ischemia and were enriched mainly in Triton-soluble membranes. Rac1 expression was stimulated in the soluble fractions during reperfusion. Rho GTPases mRNA levels were similarly regulated by ischemia-reperfusion suggesting that changes in their expressions could occur at gene or mRNA levels. ERM protein expression and distribution were unaffected by ischemia-reperfusion. Together, these data show that renal ischemia-reperfusion induced expression and redistribution of actin and microtubule cytoskeleton components in addition to Rho GTPases in proximal tubules, suggesting that they participate in an adaptive response to cellular lesions.

The plasma membrane Ca2+pump from proximal kidney tubules is exclusively localized and active in caveolae

Plasma membrane Ca2+-ATPase is involved in the fine-tuned regulation of intracellular Ca2+. In this study, the presence of Ca2+-ATPase in caveolae from kidney basolateral membranes was investigated. With the use of a discontinuous sucrose gradient, we show that Ca2+-ATPase is exclusively located and fully active in caveolin-containing microdomains. Treatment with methyl-beta-cyclodextrin--a cholesterol chelator--leads to a spreading of both caveolin and completely inactive Ca2+-ATPase toward high-density fractions. These data support the view that Ca2+ fluxes mediated by Ca2+-ATPase in kidney epithelial cells occur only in caveolae, being strictly dependent on the integrity of these microdomains.

Ouabain assembles signaling cascades through the caveolar Na+/K+-ATPase

Based on the observation that the Na(+)/K(+)-ATPase alpha subunit contains two conserved caveolin-binding motifs, we hypothesized that clustering of the Na(+)/K(+)-ATPase and its partners in caveolae facilitates ouabain-activated signal transduction. Glutathione S-transferase pull-down assay showed that the Na(+)/K(+)-ATPase bound to the N terminus of caveolin-1. Significantly, ouabain regulated the interaction in a time- and dose-dependent manner and stimulated tyrosine phosphorylation of caveolin-1 in LLC-PK1 cells. When added to the isolated membrane fractions, ouabain increased tyrosine phosphorylation of proteins from the isolated caveolae but not other membrane fractions. Consistently, ouabain induced the formation of a Na(+)/K(+)-ATPase-Src-caveolin complex in the isolated caveolae preparations as it did in live cells. Finally, depletion of either cholesterol by methyl beta-cyclodextrin or caveolin-1 by siRNA significantly reduced the caveolar Na(+)/K(+)-ATPase and Src. Concomitantly, cholesterol depletion abolished ouabain-induced recruitment of Src to the Na(+)/K(+)-ATPase signaling complex. Like depletion of caveolin-1, it also blocked the effect of ouabain on ERKs, which was restored after cholesterol repletion. Clearly, the caveolar Na(+)/K(+)-ATPase represents the signaling pool of the pump that interacts with Src and transmits the ouabain signals.

Proximal tubular cholesterol loading after mitochondrial, but not glycolytic, blockade

Diverse forms of injury cause proximal tubular cholesterol accumulation. However, underlying mechanisms in general, and those involved with ATP depletion injury in particular, remain poorly defined. To help elucidate this issue, cholesterol homeostasis and its determinants were assessed after partial ATP depletion states. Serum-exposed HK-2 cells were subjected to mild ATP depletion, induced by mitochondrial inhibition (antimycin A; AA) or glycolytic blockade (2-deoxyglucose; DG). Four or 18 h later, cell cholesterol levels, hydroxymethylglutaryl (HMG)-CoA reductase (HMGCR), the LDL receptor (LDL-R), and ABCA1/SR-B1 cholesterol transporters were assessed. AA and DG each induced mild, largely sublethal ATP depletion injury. Each also caused significant HMGCR increments and SR-B1 decrements and left ABCA1 intact. In contrast, only AA increased the LDL-R, and only AA evoked a cholesterol-loading state (approximately 25% up). One-half of this increase was statin inhibitable, and one-half could be blocked by serum deletion, implying that both synthetic and nonsynthetic (e.g., LDL-R transport) pathways were involved. The AA-induced HMGCR and LDL-R protein changes were paralleled by their mRNAs, suggesting the presence of altered transcriptional events. We conclude that 1) sublethal ATP depletion, whether induced by mitochondrial or glycolytic blockade, can upregulate HMGCR and decrease SR-B1, and these changes represent a previously unrecognized ATP depletion "phenotype"; 2) mitochondrial blockade can also upregulate the LDL-R and evoke a cholesterol-loading state; 3) the latter likely occurs via synthetic and transport pathways; and 4) the mitochondrion may be a critical, and previously unrecognized, determinant of postinjury cell cholesterol homeostasis, potentially by impacting the LDL-R.

Caveolae, caveolin and cav-p60 in smooth muscle and renin-producing cells in the rat kidney

AIMS

In vascular smooth muscle cells caveolae are important for signalling mechanisms regulating vascular contraction. In smooth muscle layer of the renal afferent arteriole juxtaglomerular cells (JG cells) are non-contractile renin producing cells that have the capacity to change their phenotype into smooth muscle cells and back again by metaplastic transformation. Signalling mechanisms in JG cells are not fully understood and we therefore investigated if caveolae were present, and thereby could be involved as integrators of cellular signalling in both of these phenotypes of smooth muscle cells.

METHODS

Using electron microscopy we compared the number of caveolae in JG cells and smooth muscle cells in the afferent arteriole of the rat kidney. The expression of caveolin and cav-p60 was examined using a combination of immunogold electron microscopy and immunofluorescence microscopy.

RESULTS

We found that JG cells have sixfold less caveolae per cell surface sectional length than smooth muscle cells. The expression of cavolin-1 and cav-p60 correlated with the number of caveolae. An examination of the general distribution of caveolae, cav-p60 and caveolins in the rat kidney showed that cav-p60, like caveolin-1, is a specific maker of caveolae.

CONCLUSION

The number of caveolae in JG cells is very low, and this makes it unlikely that caveolae are of major importance for the renin secretion specific for JG cells.

Acute tubular injury causes dysregulation of cellular cholesterol transport proteins

Acute renal injury causes accumulation of free and esterified cholesterol (FC, CE) in proximal tubules, mediated, at least in part, by increased cholesterol synthesis. Normally, this would trigger compensatory mechanisms such as increased efflux and decreased influx to limit or reverse the cholesterol overload state. This study sought to determine the integrity of these compensatory pathways following acute renal damage. Rhabdomyolysis-induced acute renal failure was induced in mice by glycerol injection. Normal mice served as controls. After 18 hours, BUN levels and renal cortical FC/CE content were determined. Expression of ABCA-1 and SR-B1 (cholesterol efflux proteins) were assessed by Western blot. Renal cortical LDL receptor (LDL-R; a cholesterol importer) regulation was gauged by quantifying its mRNA. To obtain proximal tubule cell-specific data, the impact of oxidant (Fe) stress on cultured HK-2 cell LDL-R, SR-B1, and ABCA-1 proteins and their mRNAs (versus controls) was assessed. Glycerol evoked marked azotemia and striking FC/CE increments (44%, 384%, respectively). Paradoxically, renal cortical SR-B1 and ABCA-1 protein reductions and LDL-R mRNA increments resulted. Fe-induced injury suppressed HK-2 cell SR-B1, ABCA-1, and their mRNAs. LDL-R protein rose with the in vitro Fe challenge. Renal tubular cell injury causes dysregulation of SR-B1, ABCA-1, and LDL-R protein expression, changes which should contribute to a cholesterol overload state. Reductions in HK-2 cell SR-B1 and ABCA-1 mRNAs and increases in renal cortical LDL-R mRNA imply that this dysregulation reflects, at least in part, altered genomic/transcriptional events.

In vivo and in vitro models demonstrate a role for caveolin-1 in the pathogenesis of ischaemic acute renal failure

Caveolae and their proteins, the caveolins, transport macromolecules; compartmentalize signalling molecules; and are involved in various repair processes. There is little information regarding their role in the pathogenesis of significant renal syndromes such as acute renal failure (ARF). In this study, an in vivo rat model of 30 min bilateral renal ischaemia followed by reperfusion times from 4 h to 1 week was used to map the temporal and spatial association between caveolin-1 and tubular epithelial damage (desquamation, apoptosis, necrosis). An in vitro model of ischaemic ARF was also studied, where cultured renal tubular epithelial cells or arterial endothelial cells were subjected to injury initiators modelled on ischaemia-reperfusion (hypoxia, serum deprivation, free radical damage or hypoxia-hyperoxia). Expression of caveolin proteins was investigated using immunohistochemistry, immunoelectron microscopy, and immunoblots of whole cell, membrane or cytosol protein extracts. In vivo, healthy kidney had abundant caveolin-1 in vascular endothelial cells and also some expression in membrane surfaces of distal tubular epithelium. In the kidneys of ARF animals, punctate cytoplasmic localization of caveolin-1 was identified, with high intensity expression in injured proximal tubules that were losing basement membrane adhesion or were apoptotic, 24 h to 4 days after ischaemia-reperfusion. Western immunoblots indicated a marked increase in caveolin-1 expression in the cortex where some proximal tubular injury was located. In vitro, the main treatment-induced change in both cell types was translocation of caveolin-1 from the original plasma membrane site into membrane-associated sites in the cytoplasm. Overall, expression levels did not alter for whole cell extracts and the protein remained membrane-bound, as indicated by cell fractionation analyses. Caveolin-1 was also found to localize intensely within apoptotic cells. The results are indicative of a role for caveolin-1 in ARF-induced renal injury. Whether it functions for cell repair or death remains to be elucidated.

Radiographic contrast media-induced tubular injury: evaluation of oxidant stress and plasma membrane integrity

BACKGROUND

Experimental and clinical investigations suggest that oxidant stress is a critical determinant of radiocontrast nephropathy (RCN), and that N acetyl cysteine (NAC) can prevent this damage. This study addresses these issues directly at the tubular cell level. Potential alternative mechanisms for RCN have also been sought.

METHODS

Isolated mouse proximal tubule segments (PTS), or cultured proximal tubule (HK-2) cells, were subjected to radiocontrast media (RCM) (Ioversol, Optiray 320) exposure, followed by assessments of cellular viability [% lactate dehydrogenase (LDH) release, tetrazolium dye (MTT), uptake] and lipid peroxidation. These experiments were conducted in the absence or presence of a variety of antioxidants [NAC, glutathione (GSH), superoxide dismutase, catalase] or pro-oxidant (GSH depletion, heme oxygenase inhibition) strategies. RCM effects on mitochondrial and plasma membrane integrity were also assessed.

RESULTS

RCM exposure did not induce PTS lipid peroxidation. Neither antioxidant nor pro-oxidant interventions mitigated or exacerbated RCM-induced tubular cell injury, respectively. RCM impaired mitochondrial integrity, as assessed by ouabain-resistant ATP reductions, and by cytochrome c release (before cell death). RCM also induced plasma membrane damage, as indicated by loss of key resident proteins (NaK-ATPase, caveolin) and by increased susceptibility to phospholipase A2 (PLA2) attack (increase of >/=2 times in free fatty acid and NaK-ATPase release). Hyperosmolality could not account for RCM's toxic effects.

CONCLUSION

RCM toxicity can be dissociated from tubular cell oxidant stress. Alternative mechanisms may include mitochondrial injury/cytochrome c release and plasma membrane damage. The latter results in critical protein loss, as well as a marked increase in plasma membrane susceptibility to exogenous/endogenous PLA2 attack.

Caveolin interacts with the angiotensin II type 1 receptor during exocytic transport but not at the plasma membrane

The mechanisms involved in angiotensin II type 1 receptor (AT1-R) trafficking and membrane localization are largely unknown. In this study, we examined the role of caveolin in these processes. Electron microscopy of plasma membrane sheets shows that the AT1-R is not concentrated in caveolae but is clustered in cholesterol-independent microdomains; upon activation, it partially redistributes to lipid rafts. Despite the lack of AT1-R in caveolae, AT1-R.caveolin complexes are readily detectable in cells co-expressing both proteins. This interaction requires an intact caveolin scaffolding domain because mutant caveolins that lack a functional caveolin scaffolding domain do not interact with AT1-R. Expression of an N-terminally truncated caveolin-3, CavDGV, that localizes to lipid bodies, or a point mutant, Cav3-P104L, that accumulates in the Golgi mislocalizes AT1-R to lipid bodies and Golgi, respectively. Mislocalization results in aberrant maturation and surface expression of AT1-R, effects that are not reversed by supplementing cells with cholesterol. Similarly mutation of aromatic residues in the caveolin-binding site abrogates AT1-R cell surface expression. In cells lacking caveolin-1 or caveolin-3, AT1-R does not traffic to the cell surface unless caveolin is ectopically expressed. This observation is recapitulated in caveolin-1 null mice that have a 55% reduction in renal AT1-R levels compared with controls. Taken together our results indicate that a direct interaction with caveolin is required to traffic the AT1-R through the exocytic pathway, but this does not result in AT1-R sequestration in caveolae. Caveolin therefore acts as a molecular chaperone rather than a plasma membrane scaffold for AT1-R.

Experimental glomerulopathy alters renal cortical cholesterol, SR-B1, ABCA1, and HMG CoA reductase expression

Previous studies indicate that acute tubular injury causes free cholesterol (FC) and cholesteryl ester (CE) accumulation within renal cortex/proximal tubules. This study assessed whether similar changes occur with glomerulopathy/nephrotic syndrome, in which high-circulating/filtered lipoprotein levels increase renal cholesterol supply. Potential adaptive changes in cholesterol synthetic/transport proteins were also assessed. Nephrotoxic serum (NTS) or passive Heymann nephritis (PHN) was induced in Sprague-Dawley rats. Renal injury (blood urea nitrogen, proteinuria) was assessed 2 and 7 days (NTS), or 10 and 30 days (PHN) later. FC and CE levels in renal cortex, isolated glomeruli, and proximal tubule segments were determined. SR-B1 (a CE influx protein), ABCA1 (a FC exporter), and HMG CoA reductase protein/mRNA levels were also assessed. FC was minimally elevated in renal cortex (0 to 15%), the majority apparently localizing to proximal tubules. More dramatic CE elevations were found ( approximately 5 to 15x), correlating with the severity of proteinuria at any single time point (r >/= 0.85). Cholesterol increments were associated with decreased SR-B1, increased ABCA1, and increased HMG CoA reductase (HMGCR) protein and its mRNA. Tubule (HK-2) cell culture data indicated that SR-B1 and ABCA1 levels are responsive to cholesterol supply. Experimental nephropathy can increase renal FC, and particularly CE, levels, most notably in proximal tubules. These changes are associated with adaptations in SR-B1 and ABCA1 expression, which are physiologically appropriate changes for a cholesterol overload state. However, HMGCR protein/mRNA increments can also result. These seem to reflect a maladaptive response, potentially contributing to a cell cholesterol overload state.

Sepsis syndrome stimulates proximal tubule cholesterol synthesis and suppresses the SR-B1 cholesterol transporter

BACKGROUND

Previous studies demonstrate that renal cortical/proximal tubule cholesterol accumulation is part of the renal "stress response." The present study was performed to help define underlying mechanisms, using experimental sepsis as a test model.

METHODS

Male CD-1 mice and female low-density lipoprotein receptor (LDLR) knockout mice were injected with a heat-killed Escherichia coli suspension. Renal cortex and serum were obtained from these and control mice either 4, 6, or 18 hours later. Tissues samples were assayed for free cholesterol (FC), cholesteryl esters (CE), HMG CoA reductase (HMGCR) mRNA, and SR-B1 [the high-density lipoprotein (HDL) receptor/cholesterol transporter]. Statin effects on renal cortical HMGCR mRNA and FC/CE levels also were assessed. Finally, the impact of serum from septic versus normal mice on cultured proximal tubule (HK-2) cell cholesterol levels was assessed.

RESULTS

Sepsis induced approximately 30% and 300 to 500% increases in renal FC and CE content, respectively. Cholesterol accumulation was not blunted in LDLR-/- mice versus their controls. Statin therapy also did not alter sepsis-induced renal FC/CE accumulation. However, statin treatment exerted no discernible intra-renal activity (for example, no rise in renal HMGCR mRNA), despite significant extra-renal activity (25% reduction in serum cholesterol; 400% increase in hepatic HMGCR mRNA). HK-2 cells exposed to septic serum sustained a 40% cholesterol increase, compared to cells exposed to control serum. This response was completely statin inhibited, proving that de novo synthesis was involved. Sepsis markedly suppressed renal levels of SR-B1 (an FC efflux protein). Renal HMGCR mRNA did not fall despite sepsis triggered cholesterol loading, indicating a failure of negative feedback activity.

CONCLUSIONS

Sepsis-induced renal cholesterol accumulation is not simply an intrinsic renal response, since it can be enhanced by circulating "stress factors" that drive HMGCR activity. Sepsis also down-regulates SR-B1. Thus, decreased cell FC efflux, coupled with increased synthesis, may synergistically induce the post-sepsis cholesterol overload state.

The mevalonate pathway during acute tubular injury: selected determinants and consequences

Renal injury evokes tubular cholesterol accumulation, mediated in part by increased HMG CoA reductase (HMGCR) levels. The present study was undertaken to define potential molecular determinants of these changes and to ascertain the relative importance of increased cholesterol production versus mevalonate pathway-driven protein prenylation, on the emergence of the so-called postrenal injury "cytoresistant state." Cultured proximal tubule (HK-2) cells were subjected to Fe or ATP depletion injury, followed 1 to 24 hours later by assessments of: 1) sterol transcription factor expression (SREBP)-1 and -2); 2) HMGCR mRNA levels; and 3) Ras/Rho prenylation. HMGCR mRNA and Ras/Rho prenylation were also assessed after in vivo ischemic and Fe-mediated renal damage. Using specific inhibitors, the relative importance of protein prenylation versus terminal cholesterol synthesis on HK-2 cell susceptibility to injury was also assessed. Acute injury induced HK-2 cell SREBP disruption and reductions in HMGCR mRNA. Renal cortical HMGCR mRNA also fell in response to either in vivo ischemic or Fe-mediated oxidant damage. At 24 hours after in vitro/in vivo injury, a time of cholesterol buildup, no increase in Ras/Rho prenylation was observed. Prenylation inhibitors did not sensitize HK-2 cells to injury. Conversely, squalene synthase (terminal cholesterol synthesis) blockade sensitized HK-2 cells to both Fe and ATP depletion attack. We concluded that: 1) acute tubular cell injury can destroy SREBPs and lower HMGCR mRNA. This suggests that posttranscriptional/translational events are responsible for HMGCR enzyme and cholesterol accumulation after renal damage. 2) Injury-induced cholesterol accumulation appears dissociated from increased protein prenylation. 3) Cholesterol accumulation, per se, seems to be the dominant mechanism by which the mevalonate pathway contributes to the postrenal injury cytoresistant state.