Caveolae in mesangial cells and caveolin expression in mesangial proliferative glomerulonephritis

Diabetic Nephropathy and Gaseous Modulators

Diabetic nephropathy (DN) remains the leading cause of vascular morbidity and mortality in diabetes patients. Despite the progress in understanding the diabetic disease process and advanced management of nephropathy, a number of patients still progress to end-stage renal disease (ESRD). The underlying mechanism still needs to be clarified. Gaseous signaling molecules, so-called gasotransmitters, such as nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S), have been shown to play an essential role in the development, progression, and ramification of DN depending on their availability and physiological actions. Although the studies on gasotransmitter regulations of DN are still emerging, the evidence revealed an aberrant level of gasotransmitters in patients with diabetes. In studies, different gasotransmitter donors have been implicated in ameliorating diabetic renal dysfunction. In this perspective, we summarized an overview of the recent advances in the physiological relevance of the gaseous molecules and their multifaceted interaction with other potential factors, such as extracellular matrix (ECM), in the severity modulation of DN. Moreover, the perspective of the present review highlights the possible therapeutic interventions of gasotransmitters in ameliorating this dreaded disease.

Overexpression of Plasmalemmal Vesicle-Associated Protein-1 Reflects Glomerular Endothelial Injury in Cases of Proliferative Glomerulonephritis with Monoclonal IgG Deposits

Introduction

Proliferative glomerulonephritis with monoclonal IgG deposits (PGNMID) occasionally presents refractory nephrotic syndrome resulting in poor renal prognosis, but its etiology is not fully elucidated. Given that glomerular endothelial cell (GEC) stress or damage may lead to podocytopathy and subsequent proteinuria, as in thrombotic microangiopathy (TMA), diabetic kidney disease, and focal segmental glomerulosclerosis, we investigated the evidence of glomerular endothelial injury by evaluating the expression of plasmalemmal vesicle-associated protein-1 (PV-1), a component of caveolae in the cases of PGNMID.

Methods

We measured the immunofluorescent PV-1 intensities of 23 PGNMID cases and compared with those of primary membranoproliferative glomerulonephritis (MPGN) (n = 5) and IgA nephropathy (IgAN) (n = 54) cases. PV-1 localization was evaluated with Caveolin-1, and CD31 staining, and the ultrastructural analysis was performed using a low-vacuum scanning electron microscope (LVSEM). To check the association of podocyte injury, we also conducted 8-oxoguanine and Wilms tumor 1 (WT1) double stain. We then evaluated PV-1 expression in other glomerulitis and glomerulopathy such as lupus nephritis and minimal change disease.

Results

The intensity of glomerular PV-1 expression in PGNMID is significantly higher than that in the other glomerular diseases, although the intensity is not associated with clinical outcomes such as urinary protein levels or renal prognosis. Immunostaining and LVSEM analysis revealed that glomerular PV-1 expression is localized in GECs in PGNMID. 8-oxoguanine accumulation was detected in WT1-positive podocytes but not in PV-1-expressing GECs, suggesting GEC-derived podocyte injury in PGNMID.

Conclusion

PV-1 overexpression reflects glomerular endothelial injury, which could be associated with podocyte oxidative stress in PGNMID cases.

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.

Glomerulopathic Light Chain-Mesangial Cell Interactions: Sortilin-Related Receptor (SORL1) and Signaling

INTRODUCTION

Deciphering the intricacies of the interactions of glomerulopathic Ig light chains with mesangial cells is key to delineate signaling events responsible for the mesangial pathologic alterations that ensue.

METHODS

Human mesangial cells, caveolin 1 (CAV1), wild type (WT) ,and knockout (KO), were incubated with glomerulopathic light chains purified from the urine of patients with light chain-associated (AL) amyloidosis or light chain deposition disease. Associated signaling events induced by surface interactions of glomerulopathic light chains with caveolins and other membrane proteins, as well as the effect of epigallocatechin-3-gallate (EGCG) on the capacity of mesangial cells to intracellularly process AL light chains were investigated using a variety of techniques, including chemical crosslinking with mass spectroscopy, immunofluorescence, and ultrastructural immunolabeling.

RESULTS

Crosslinking experiments provide evidence suggesting that sortilin-related receptor (SORL1), a transmembrane sorting receptor that regulates cellular trafficking of proteins, is a component of the receptor on mesangial cells for glomerulopathic light chains. Colocalization of glomerulopathic light chains with SORL1 in caveolae and also in lysosomes when light chain internalization occurred, was documented using double immunofluorescence and immunogold labeling ultrastructural techniques. It was found that EGCG directly blocks c-Fos cytoplasmic to nuclei signal translocation after interactions of AL light chains with mesangial cells, resulting in a decrease in amyloid formation.

CONCLUSION

Our findings document for the first time a role for SORL1 linked to glomerular pathology and signaling events that take place when certain monoclonal light chains interact with mesangial cells. This finding may lead to novel therapies for treating renal injury caused by glomerulopathic light chains.

Upregulation of MUC5AC by VEGF in human primary bronchial epithelial cells: implications for asthma

Background

Airway mucus hypersecretion is an important pathophysiological feature in asthma. Mucins are glycoproteins that are mainly responsible for the viscoelastic property of mucus, and MUC5AC is a major mucin glycoprotein that is overproduced in asthma. Vascular endothelial growth factor (VEGF) has been implicated in inflammatory and airway blood vessel remodeling in asthmatics. Therefore, we sought to investigate the effect of VEGF on MUC5AC expression and study the underlying mechanisms.

Methods

In order to elucidate the precise mechanism underlying the effect of VEGF on MUC5AC expression, we tested the effects of VEGF on RhoA activation and the association of caveolin-1 and VEGFR2 in Primary Bronchial Epithelial Cells.

Results

VEGF up-regulated MUC5AC mRNA and protein levels in a dose- and time-dependent manner, and activated RhoA. Additionally, VEGF-induced MUC5AC expression and RhoA activation were enhanced by disrupting caveolae with cholesterol depletion and reversed by cholesterol repletion, and inhibited by a selective VEGF receptor 2 (VEGFR2) inhibitor SU1498. Furthermore, phospho-VEGFR2 expression was decreased via overexpression of caveolin-1. VEGF treatment reduced the association of caveolin-1 and VEGFR2.

Conclusion

Collectively, our findings suggest that VEGF up-regulates MUC5AC expression and RhoA activation by interaction with VEGFR2, and this phenomenon was related with the association of caveolin-1 and VEGFR2. Further studies on these mechanisms are needed to facilitate the development of treatments for asthma.

Caveolin-1 regulation of Sp1 controls production of the antifibrotic protein follistatin in kidney mesangial cells

BACKGROUND

We previously showed that caveolin-1 (cav-1), an integral membrane protein, is required for the synthesis of matrix proteins by glomerular mesangial cells (MC). In a previous study to understand how cav-1 is involved in regulating matrix production, we had identified significant upregulation of the antifibrotic protein follistatin in cav-1 knockout MC. Follistatin inhibits the profibrotic effects of several members of the transforming growth factor beta superfamily, in particular the activins. Here, we characterize the molecular mechanism through which cav-1 regulates the expression of follistatin.

METHODS

Kidneys from cav-1 wild type and knockout (KO) mice were analyzed and primary cultures of MC from cav-1 wild-type and KO mice were utilized. FST promoter deletion constructs were generated to determine the region of the promoter important for mediating FST upregulation in cav-1 KO MC. siRNA-mediated down-regulation and overexpression of Sp1 in conjunction with luciferase activity assays, immunoprecipitation, western blotting and ChiP was used to assess the role of Sp1 in transcriptionally regulating FST expression. Pharmacologic kinase inhibitors and specific siRNA were used to determine the post-translational mechanism through which cav-1 affects Sp1 activity.

RESULTS

Our results establish that follistatin upregulation occurs at the transcript level. We identified Sp1 as the critical transcription factor regulating activation of the FST promoter in cav-1 KO MC through binding to a region within 123 bp of the transcription start site. We further determined that the lack of cav-1 increases Sp1 nuclear levels and transcriptional activity. This occurred through increased phosphoinositide 3-kinase (PI3K) activity and downstream protein kinase C (PKC) zeta-mediated phosphorylation and activation of Sp1.

CONCLUSIONS

These findings shed light on the transcriptional mechanism by which cav-1 represses the expression of a major antifibrotic protein, and can inform the development of novel antifibrotic treatment strategies.

Caveolin-1 in the Pathogenesis of Diabetic Nephropathy: Potential Therapeutic Target?

PURPOSE OF REVIEW

Diabetic nephropathy, a major microvascular complication of diabetes and the most common cause of end-stage renal disease, is characterized by prominent accumulation of extracellular matrix. The membrane microdomains caveolae, and their integral protein caveolin-1, play critical roles in the regulation of signal transduction. In this review we discuss current knowledge of the contribution of caveolin-1/caveolae to profibrotic signaling and the pathogenesis of diabetic kidney disease, and assess its potential as a therapeutic target.

RECENT FINDINGS

Caveolin (cav)-1 is key to facilitating profibrotic signal transduction induced by several stimuli known to be pathogenic in diabetic nephropathy, including the most prominent factors hyperglycemia and angiotensin II. Phosphorylation of cav-1 on Y14 is an important regulator of these responses. In vivo studies support a pathogenic role for caveolae in the progression of diabetic nephropathy. Targeting caveolin-1/caveolae would enable inhibition of multiple profibrotic pathways, representing a novel and potentially potent therapeutic option for diabetic nephropathy.

Role of licochalcone A in VEGF-induced proliferation of human airway smooth muscle cells: implications for asthma

Asthma is a chronic respiratory disease characterized by reversible airway obstruction with persistent airway inflammation and airway remodeling, which is associated with increased airway smooth muscle (ASM) mass. Licochalcone A is the predominant characteristic chalcone in licorice root. We found that licochalcone A inhibited vascular endothelial growth factor (VEGF)-induced ASM cell proliferation and induced cell cycle arrest. Additionally, VEGF-induced ASM cell proliferation was suppressed via inhibition of extracellular signal-regulated kinase 1/2 (ERK1/2) activity, but not that of Akt. Furthermore, licochalcone A treatment inhibited VEGF-induced activation of VEGF receptor 2 (VEGFR2) and ERK and blocked the downregulation of caveolin-1 in a concentration-dependent manner. Collectively, our findings suggested that licochalcone A inhibited VEGF-induced ASM cell proliferation by suppressing VEGFR2 and ERK1/2 activation and downregulating caveolin-1. Further studies of these mechanisms are needed to facilitate the development of treatments for smooth muscle hyperplasia-associated diseases of the airway, such as asthma.

Roxithromycin inhibits VEGF-induced human airway smooth muscle cell proliferation: Opportunities for the treatment of asthma

Asthma is a chronic respiratory disease characterized by reversible airway obstruction with persistent airway inflammation and airway remodelling, which is associated with increased airway smooth muscle (ASM) mass. Roxithromycin (RXM) has been widely used in asthma treatment; however, its mechanism of action is poorly understood. Vascular endothelial growth factor (VEGF) has been implicated in inflammatory and airway blood vessel remodelling in patients with asthma, and shown to promote ASM cell proliferation. Here, we investigated the effect of RXM on VEGF-induced ASM cell proliferation and attempted to elucidate the underlying mechanisms of action. We tested the effect of RXM on proliferation and cell cycle progression, as well as on the expression of phospho-VEGF receptor 2 (VEGFR2), phospho-extracellular signal-regulated kinase 1/2 (ERK1/2), phospho-Akt, and caveolin-1 in VEGF-stimulated ASM cells. RXM inhibited VEGF-induced ASM cell proliferation and induced cell cycle arrest. Additionally, VEGF-induced ASM cell proliferation was suppressed by inhibiting the activity of ERK1/2, but not that of Akt. Furthermore, RXM treatment inhibits VEGF-induced activation of VEGFR2 and ERK and downregulation of caveolin-1 in a dose-dependent manner. RXM also inhibited TGF-β-induced VEGF secretion by ASM cells and BEAS-2B cells. Collectively, our findings suggest that RXM inhibits VEGF-induced ASM cell proliferation by suppression of VEGFR2 and ERK1/2 activation and caveolin-1 down-regulation, which may be involved in airway remodelling. Further elucidation of the mechanisms underlying these observations should enable the development of treatments for smooth muscle hyperplasia-associated diseases of the airway such as asthma.

Roxithromycin treatment inhibits TGF-β1-induced activation of ERK and AKT and down-regulation of caveolin-1 in rat airway smooth muscle cells

BACKGROUND

Roxithromycin (RXM) has been widely used in asthma treatment; however, the mechanism has not been fully understood. The aim of our study was to investigate the underlying mechanism of RXM treatment in mediating the effect of transforming growth factor (TGF)-β1 on airway smooth muscle cells (ASMCs) proliferation and caveolinn-1 expression.

METHODS

Firstly, the rat ovalbumin (OVA) model was built according to the previous papers. Rat ASMCs were prepared and cultured, and then TGF-β1 production in ASMCs was measured by enzyme-linked immunosorbent assay (ELISA). Moreover, the proliferation of ASMCs was determined using cell counting kit (CCK-8) assay. Additionally, the expressions of caveolin-1, phosphorylated-ERK1/2 (p-ERK1/2) and phosphorylated-AKT (p-AKT) in ASMCs treated with or without PD98059 (an ERK1/2 inhibitor), wortannin (a PI3K inhibitor), β-cyclodextrin (β-CD) and RXM were measured by Western blot. Finally, data were evaluated using t-test or one-way ANOVA, and then a P value < 0.05 was set as a threshold.

RESULTS

Compared with normal control, TGF-β1 secretion was significantly increased in asthmatic ASMCs; meanwhile, TGF-β1 promoted ASMCs proliferation (P < 0.05). However, ASMCs proliferation was remarkably inhibited by RXM, β-CD, PD98059 and wortmannin (P < 0.05). Moreover, the expressions of p-ERK1/2 and p-AKT were increased and peaked at 20 min after TGF-β1 stimulation, and then suppressed by RXM. Further, caveolin-1 level was down-regulated by TGF-β1 and up-regulated by inhibitors and RXM.

CONCLUSION

Our findings demonstrate that RXM treatment inhibits TGF-β1-induced activation of ERK and AKT and down-regulation of caveolin-1, which may be the potential mechanism of RXM protection from chronic inflammatory diseases, including bronchial asthma.

Akt and RhoA activation in response to high glucose require caveolin-1 phosphorylation in mesangial cells

Glomerular matrix accumulation is a hallmark of diabetic renal disease. Serine/threonine kinase PKC-β1 mediates glucose-induced Akt S473 phosphorylation, RhoA activation, and transforming growth factor (TGF)-β1 upregulation and finally leads to matrix upregulation in mesangial cells (MCs). It has been reported that glucose-induced PKC-β1 activation is dependent on caveolin-1 and the presence of intact caveolae in MCs; however, whether activated PKC-β1 regulates caveolin-1 expression and phosphorylation are unknown. Here, we showed that, although the caveolin-1 protein level had no significant change, the PKC-β-specific inhibitor LY-333531 blocked caveolin-1 Y14 phosphorylation in high glucose (HG)-treated MCs and in the renal cortex of diabetic rats. The Src-specific inhibitor SU-6656 prevented the HG-induced association between PKC-β1 and caveolin-1 and PKC-β1 membrane translocation, whereas PKC-β1 small interfering RNA failed to block Src activation, indicating that Src kinase is upstream of PKC-β1 activation. Although LY-333531 blocked PKC-β1 membrane translocation, it had no effect on the PKC-β1/caveolin-1 association, suggesting that PKC-β1 activation requires the interaction of caveolin-1 and PKC-β1. PKC-β1-mediated Akt S473 phosphorylation, RhoA activation, and fibronectin upregulation in response to HG were prevented by SU-6656 and nonphosphorylatable mutant caveolin-1 Y14A. In conclusion, Src activation by HG mediates the PKC-β1/caveolin-1 association and PKC-β1 activation, which assists in caveolin-1 Y14 phosphorylation by Src kinase. The downstream effects, including Akt S473 phosphorylation, RhoA activation, and fibronectin upregulation, require caveolin-1 Y14 phosphorylation. Caveolin-1 is thus an important mediator of the profibrogenic process in diabetic renal disease.

Lipid rafts are required for signal transduction by angiotensin II receptor type 1 in neonatal glomerular mesangial cells

Angiotensin II (ANG-II) receptors (AGTRs) contribute to renal physiology and pathophysiology, but the underlying mechanisms that regulate AGTR function in glomerular mesangium are poorly understood. Here, we show that AGTR1 is the functional AGTR subtype expressed in neonatal pig glomerular mesangial cells (GMCs). Cyclodextrin (CDX)-mediated cholesterol depletion attenuated cell surface AGTR1 protein expression and ANG-II-induced intracellular Ca(2+) ([Ca(2+)]i) elevation in the cells. The COOH-terminus of porcine AGTR1 contains a caveolin (CAV)-binding motif. However, neonatal GMCs express CAV-1, but not CAV-2 and CAV-3. Colocalization and in situ proximity ligation assay detected an association between endogenous AGTR1 and CAV-1 in the cells. A synthetic peptide corresponding to the CAV-1 scaffolding domain (CSD) sequence also reduced ANG-II-induced [Ca(2+)]i elevation in the cells. Real-time imaging of cell growth revealed that ANG-II stimulates neonatal GMC proliferation. ANG-II-induced GMC growth was attenuated by EMD 66684, an AGTR1 antagonist; BAPTA, a [Ca(2+)]i chelator; KN-93, a Ca(2+)/calmodulin-dependent protein kinase II inhibitor; CDX; and a CSD peptide, but not PD 123319, a selective AGTR2 antagonist. Collectively, our data demonstrate [Ca(2+)]i-dependent proliferative effect of ANG-II and highlight a critical role for lipid raft microdomains in AGTR1-mediated signal transduction in neonatal GMCs.

Caveolin-1 deficiency protects against mesangial matrix expansion in a mouse model of type 1 diabetic nephropathy

AIMS/HYPOTHESIS

Glomerular matrix protein accumulation, mediated largely by resident mesangial cells (MCs), is central to the pathogenesis of diabetic nephropathy. We previously showed that caveolin (CAV)-1/caveolae mediate matrix upregulation by MCs in response to high glucose and TGFβ, two important pathogenic mediators of diabetic glomerular sclerosis. Here, we evaluated the in vivo role of CAV-1/caveolae in the development of diabetic nephropathy.

METHODS

Diabetes was induced in Cav1-knockout (KO) mice and their wild-type (WT) counterparts by streptozotocin injection. After 10 months, kidneys were evaluated for the development of nephropathy, including glomerular sclerosis and upregulation of matrix proteins. Parallel experiments assessing glucose-induced matrix upregulation were carried out in MCs isolated from KO mice.

RESULTS

KO diabetic mice developed hyperglycaemia and renal hypertrophy, but were protected from developing albuminuria and glomerular sclerosis compared with WT mice. KO mice were significantly protected from the upregulation of glomerular collagen I, fibronectin, connective tissue growth factor (CTGF) and TGFβ. In vitro, glucose induced collagen I A1 promoter activation and collagen I, fibronectin and CTGF protein upregulation in WT but not KO MCs. Re-expression of Cav1 in KO cells restored this response.

CONCLUSIONS/INTERPRETATION

Cav1 deletion rendered significant protection from glomerular matrix accumulation and albuminuria in a mouse model of type 1 diabetes. These studies provide a foundation for the development of renal-targeted interference with CAV-1/caveolae as a novel approach to the treatment of diabetic nephropathy.

Effects of caveolin-1 and P-ERK1/2 on Ang II-induced glomerular mesangial cell proliferation

This study explored the effects of caveolin-1, p-ERK1/2 and transient receptor potential channel 6 (TRPC6) on angiotensin II (Ang II)-induced glomerular mesangial cell (GMC) proliferation, and investigated the role of Ang II on GMC proliferation. GMC cultures were divided into Control, Ang II (Ang II 10(-7 )mol/L), PD98059 (Ang II 10(-7 )mol/L + PD98059 5 × 10(-5 )mol/L) and MβCD groups (Ang II 10(-7 )mol/L + MβCD 10(-2 )mol/L). GMCs proliferation was measured by the methyl thiazolil tetracolium and trypan blue assays. The distribution of caveolin-1, p-ERK1/2 and TRPC6 was monitored by immunocytochemistry. Real time polymerase chain reaction (PCR) was used to assess mRNA expression of caveolin-1 and TRPC6. Western blot analysis was used to assess protein expression of caveolin-1, p-ERK1/2 and TRPC6. The results showed that Ang II promoted GMC proliferation. PD98059 and MβCD blocked Ang II-induced GMC proliferation, by 31.06% and 48.96%, respectively. In comparison with the control group, the expression of p-ERK1/2 and TRPC6 was significantly higher and caveolin-1 expression was significantly lower in the Ang II group. PD98059 markedly decreased p-ERK1/2 and TRPC6 expression and increased caveolin-1 expression. MβCD decreased the expression of p-ERK1/2 and TRPC6, but had no significant effect on caveolin-1 protein expression. These findings suggested that the intact caveolae structure was associated with Ang II-induced GMC proliferation, ERK1/2 activation and TRPC6 expression. And p-ERK1/2 acted as an upstream signal molecule for TRPC6. Moreover, p-ERK1/2 and caveolin-1 appeared to be inhibited reciprocally, thus regulated GMC proliferation by regulating TRPC6 expression.

A potential role for caveolin-1 in VEGF-induced fibronectin upregulation in mesangial cells: involvement of VEGFR2 and Src

VEGF is known to be an endothelial cell mitogen that stimulates angiogenesis by promoting endothelial cell survival, proliferation, migration, and differentiation. Recent studies have suggested that VEGF may play a pivotal role in glomerular sclerosis through extracellular matrix protein (ECM) accumulation, although the signaling mechanism is still unclear. The GTPase RhoA has been implicated in VEGF-induced type IV collagen accumulation in some settings. Here we study the role of different VEGF receptors and membrane microdomain caveolae in VEGF-induced RhoA activation and fibronectin upregulation in mesangial cells (MCs). In primary rat MC, VEGF time and dose dependently increased fibronectin production. Rho pathway inhibition blocked VEGF-induced fibronectin upregulation. VEGF-induced RhoA activation was prevented by disrupting caveolae with cholesterol depletion and rescued by cholesterol repletion. VEGF stimulation led to a markedly increased VEGFR2/caveolin-1 but failed to increase VEGFR1/caveolin-1 association. VEGF also increased caveolin-1/Src association and activated Src, and Src inhibitor blocked RhoA activation and fibronectin upregulation. Src-mediated phosphorylation of caveolin-1 on Y14 has also been implicated in signaling responses. Overexpression of nonphosphorylatable caveolin-1 Y14A prevented VEGF-induced RhoA activation and fibronectin upregulation. In vivo, although VEGFR1 and VEGFR2 protein levels were both increased in the kidney cortices of diabetic rats, VEGFR2/caveolin-1 association but not VEGFR1/caveolin-1 association was significantly increased. In conclusion, VEGF-induced RhoA activation and fibronectin upregulation require caveolae and caveolin-1 interaction with VEGFR2 and Src. Interference with caveolin/-ae signaling may provide new avenues for the treatment of fibrotic renal disease.

High glucose-induced RhoA activation requires caveolae and PKCβ1-mediated ROS generation

Glomerular matrix accumulation is a hallmark of diabetic nephropathy. We previously showed that RhoA activation by high glucose in mesangial cells (MC) leads to matrix upregulation (Peng F, Wu D, Gao B, Ingram AJ, Zhang B, Chorneyko K, McKenzie R, Krepinsky JC. Diabetes 57: 1683-1692, 2008). Here, we study the mechanism whereby RhoA is activated. In primary rat MC, RhoA activation required glucose entry and metabolism. Broad PKC inhibitors (PMA, bisindolylmaleimide, Gö6976), as well as specific PKCβ blockade with an inhibitor and small interfering RNA (siRNA), prevented RhoA activation by glucose. PKCβ inhibition also abrogated reactive oxygen species (ROS) generation by glucose. The ROS scavenger N-acetylcysteine (NAC) or NADPH oxidase inhibitors apocynin and DPI prevented glucose-induced RhoA activation. RhoA and some PKC isoforms localize to caveolae. Chemical disruption of these microdomains prevented RhoA and PKCβ1 activation by glucose. In caveolin-1 knockout cells, glucose did not induce RhoA and PKCβ1 activation; these responses were rescued by caveolin-1 reexpression. Furthermore, glucose-induced ROS generation was significantly attenuated by chemical disruption of caveolae and in knockout cells. Downstream of RhoA signaling, activator protein-1 (AP-1) activation was also inhibited by disrupting caveolae, was absent in caveolin-1 knockout MC and rescued by caveolin-1 reexpression. Finally, transforming growth factor (TGF)-β1 upregulation, mediated by AP-1, was prevented by RhoA signaling inhibition and by disruption or absence of caveolae. In conclusion, RhoA activation by glucose is dependent on PKCβ1-induced ROS generation, most likely through NADPH oxidase. The activation of PKCβ1 and its downstream effects, including upregulation of TGF-β1, requires caveolae. These microdomains are thus important mediators of the profibrogenic process associated with diabetic nephropathy.

New glimpses of caveolin-1 functions in embryonic development and human diseases

Caveolin-1 (Cav-1) isoforms, including Cav-1α and Cav-1β, were identified as integral membrane proteins and the major components of caveolae. Cav-1 proteins are highly conserved during evolution from {itCaenorhabditis elegans} to human and are capable of interacting with many signaling molecules through their caveolin scaffolding domains to regulate the activities of multiple signaling pathways. Thus, Cav-1 plays crucial roles in the regulation of cellular proliferation, differentiation and apoptosis in a cell-specific and contextual manner. In addition, Cav-1 is essential for embryonic development of vertebrates owing to its regulation of BMP, Wnt, TGF-β and other key signaling molecules. Moreover, Cav-1 is mainly expressed in terminally differentiated cells and its abnormal expression is often associated with human diseases, such as tumor progression, cardiovascular diseases, fibrosis, lung regeneration, and diseases related to virus. In this review, we will further discuss the potential of Cav-1 as a target for disease therapy and multiple drug resistance.

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.

Mechanical strain-induced RhoA activation requires NADPH oxidase-mediated ROS generation in caveolae

Increased intraglomerular pressure leads to kidney fibrosis, and can be modeled by exposing glomerular mesangial cells (MC) to mechanical strain. We previously showed that RhoA mediates strain-induced matrix production. Here we investigate whether reactive oxygen species (ROS) are required for RhoA activation. Maximal RhoA activation (1 min) was inhibited by ROS scavenge or NADPH oxidase inhibition. Strain activated NADPH oxidase, with Rac1, p47(phox), and p67(phox) membrane translocation, and Rac1 activation, observed within 30 sec. Epidermal growth factor receptor (EGFR) inhibition blocked RhoA and Rac1 activation, p67(phox) membrane translocation, and ROS generation. However, EGFR activation was unaffected by ROS inhibitors, placing it upstream of ROS generation. We previously showed, using chemical disruption, that caveolae mediate strain-induced EGFR and RhoA activation. In MC from caveolin-1 knockout mice, which lack caveolae, RhoA and Rac1 activation, p67(phox) membrane translocation, and ROS generation were absent. These were rescued by caveolin-1 re-expression. ROS generation, Rac1 activation, and p67(phox) membrane translocation were also prevented by Src inhibition. They were absent in MC stably infected with caveolin-1 Y14A, a mutant resistant to Src phosphorylation. In MC, caveolae are thus important mediators of strain-induced ROS generation through NADPH oxidase, mediating a signaling cascade which results in RhoA activation.

Increased pulmonary vascular resistance and defective pulmonary artery filling in caveolin-1-/- mice

Caveolin-1, the structural and signaling protein of caveolae, is an important negative regulator of endothelial nitric oxide synthase (eNOS). We observed that mice lacking caveolin-1 (Cav1(-/-)) had twofold increased plasma NO levels but developed pulmonary hypertension. We measured pulmonary vascular resistance (PVR) and assessed alterations in small pulmonary arteries to determine the basis of the hypertension. PVR was 46% greater in Cav1(-/-) mice than wild-type (WT), and increased PVR in Cav1(-/-) mice was attributed to precapillary sites. Treatment with NG-nitro-l-arginine methyl ester (l-NAME) to inhibit NOS activity raised PVR by 42% in WT but 82% in Cav1(-/-) mice, indicating greater NO-mediated pulmonary vasodilation in Cav1(-/-) mice compared with WT. Pulmonary vasculature of Cav1(-/-) mice was also less reactive to the vasoconstrictor thromboxane A2 mimetic (U-46619) compared with WT. We observed redistribution of type I collagen and expression of smooth muscle alpha-actin in lung parenchyma of Cav1(-/-) mice compared with WT suggestive of vascular remodeling. Fluorescent agarose casting also showed markedly decreased density of pulmonary arteries and artery filling defects in Cav1(-/-) mice. Scanning electron microscopy showed severely distorted and tortuous pulmonary precapillary vessels. Thus caveolin-1 null mice have elevated PVR that is attributed to remodeling of pulmonary precapillary vessels. The elevated basal plasma NO level in Cav1(-/-) mice compensates partly for the vascular structural abnormalities by promoting pulmonary vasodilation.

TGFbeta-induced RhoA activation and fibronectin production in mesangial cells require caveolae

Glomerular sclerosis of diverse etiologies is characterized by mesangial matrix accumulation, with transforming growth factor-beta (TGFbeta) an important pathogenic factor. The GTPase RhoA mediates TGFbeta-induced matrix accumulation in some settings. Here we study the role of the membrane microdomain caveolae in TGFbeta-induced RhoA activation and fibronectin upregulation in mesangial cells (MC). In primary rat MC, TGFbeta1 time dependently increased RhoA and downstream Rho kinase activation. Rho pathway inhibition blocked TGFbeta1-induced upregulation of fibronectin transcript and protein. TGFbeta1-induced RhoA activation was prevented by disrupting caveolae with cholesterol depletion and rescued by cholesterol repletion. Compared with wild types, RhoA/Rho kinase activation was absent in MC lacking caveolae. Reexpression of caveolin-1 (and caveolae) restored these responses. Phosphorylation of caveolin-1 on Y14, effected by Src kinases, has been implicated in signaling responses. Overexpression of nonphosphorylatable caveolin-1 Y14A prevented TGFbeta1-induced RhoA activation. TGFbeta1 also activated Src, and its inhibition blocked RhoA activation. Furthermore, TGFbeta1 led to association of RhoA and caveolin-1. This was prevented by Src or TGFbeta receptor I inhibition, and by caveolin-1 Y14A overexpression. Last, fibronectin upregulation by TGFbeta1 was blocked by Src inhibition, not seen in caveolin-1 knockout MC, and restored by caveolin-1 reexpression in the latter. TGFbeta1-induced collagen I accumulation also required caveolae. TGFbeta1-mediated Smad2/3 activation, however, did not require caveolae. We conclude that RhoA/Rho kinase mediates TGFbeta-induced fibronectin upregulation. This requires caveolae and caveolin-1 interaction with RhoA. Interference with caveolin/caveolae or RhoA signaling thus represents a potential target for the treatment of fibrotic renal disease.

Heparin suppresses lipid raft-mediated signaling and ligand-independent EGF receptor activation

Heparin is well known to suppress vascular smooth muscle cell (VSMC) proliferation, and attempts to exploit this therapeutically have led to recognition of multiple pathways for heparin's anti-mitogenic actions. At low concentrations (ca. 1 microg.ml(-1)), these suppressive effects may reflect physiological activities of endogenous heparan sulfates, and appear to be rapid responses to extracellular or cell surface-associated heparin. Because heparin has been shown to influence expression of caveolin proteins, and caveolae/lipid rafts are critical structures modulating cell signaling, we examined the effect of heparin on signaling involving cholesterol-rich membrane microdomains. The VSMC line PAC-1 activates the MAP kinase Erk in response to the cholesterol-sequestering agents methyl-beta-cyclodextrin and nystatin. This follows a temporal sequence that involves Ras-GTP activation of MEK, and is independent of PKC, Src, and PI3 kinase. However, ligand-independent phosphorylation of the EGF receptor (EGFR) by removal of cholesterol precedes Ras activation, and the EGFR kinase inhibitor AG1478 blocks Erk phosphorylation, supporting occurrence of the signaling sequence EGFR-Ras-MEK-Erk. Phosphorylation of EGFR occurs predominantly in caveolin-rich microdomains as identified by Western blotting of fractions from density gradient centrifugation of membranes prepared under detergent-free conditions. In these situations, heparin inhibits phosphorylation of EGFR on the Src-dependent site Tyr(845), but not the autophosphorylation of Tyr(1173), and decreases Ras activation and Erk phosphorylation. We conclude that heparin can suppress Erk signaling in VSMC with effects on site-specific phosphorylation of EGFR localized in caveolin-enriched lipid rafts.

RhoA Activation in Mesangial Cells by Mechanical Strain Depends on Caveolae and Caveolin-1 Interaction

Increased intraglomerular pressure is an important hemodynamic determinant of glomerulosclerosis and can be modeled in vitro by exposing mesangial cells to cyclic mechanical strain. A previous study showed that RhoA mediates strain-induced production of fibronectin; herein is investigated the role of caveolae in RhoA activation. Cyclodextrin and filipin, agents that disrupt caveolae, abrogated strain-induced RhoA activation in mesangial cells. Caveolin-1 (cav-1), the defining protein of caveolae, was Y14 phosphorylated by strain, and this was inhibited by PP1, showing Src dependence. Strain also induced c-SrcY416 phosphorylation and hence activation. Strain increased RhoA association with cav-1, which was blocked by PP1. Cyclodextrin and filipin inhibited the strain-induced RhoA/cav-1 association, indicating dependence on caveolar structural integrity. Restoration of caveolae by coincubation of cyclodextrin with cholesterol rescued both RhoA activation and RhoA/cav-1 association in response to strain. Sucrose gradient detected a significant portion of RhoA in caveolae, with Src located exclusively in these domains. Finally, in cells that were infected with retrovirus that encodes the nonphosphorylatable cav-1 Y14A, RhoA/cav-1 association, RhoA activation, and fibronectin secretion in response to strain were abrogated. It is concluded that strain-induced RhoA activation depends on the integrity of caveolae and on physical association of cav-1 and RhoA. The phosphorylation of cav-1 at Y14 by Src kinases is required for this to occur. These studies define a novel function for cav-1 and caveolae as positive effectors of RhoA activation. Targeting caveolae thus may provide a new therapeutic option for glomerular sclerosis that is associated with elevated intraglomerular pressure.

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.

Caveolin-1 in mesangial cells suppresses MAP kinase activation and cell proliferation induced by bFGF and PDGF

BACKGROUND

Caveolin is a principal component of caveolae and regulates signaling in caveolae. Mesangial cells contain many caveolae, and thus manipulation of caveolin-1 expression level might be useful to control mesangial cell proliferation, which is an important aggravating factor in many renal diseases.

METHODS

In the present study, we transfected caveolin-1 cDNA to rat primary mesangial cells and MES13 cells, and examined the effects on Raf-extracellular signal-regulated protein kinase (ERK) kinase (MEK)-mitogen-activated protein (MAP) kinase pathway and cell proliferation stimulated by basic fibroblast growth factor (bFGF) and platelet-derived growth factor (PDGF). Activity of the kinases was analyzed by immunofluorescence labeling and Western blot analysis.

RESULTS

The overexpression of caveolin-1 inhibited the activation of Raf-1, MEK-1/2, and MAP kinase induced by either bFGF or PDGF. Furthermore, it suppressed the cell proliferation caused by the cytokines. The effect was specific to the Raf-MEK-MAP kinase pathway, because it did not influence activation of Smad2 induced by transforming growth factor-beta (TGF-beta). On the contrary, expression of a dominant-negative caveolin mutant, DGV-caveolin, augmented activation of MAP kinase.

CONCLUSION

The result showed that overexpression of caveolin-1 in mesangial cells suppresses MAP kinase activation and cell proliferation induced by bFGF and PDGF. Because bFGF and PDGF are two major cytokines involved in the mesangioproliferative nephritis, the result implies that introduction of caveolin-1 expression vector is a potential therapeutic tool for the disease.

Renal protection by a soy diet in obese Zucker rats is associated with restoration of nitric oxide generation

The obese Zucker rat is a valuable model for studying kidney disease associated with obesity and diabetes. Previous studies have shown that substitution of animal protein with soy ameliorates the progression of renal disease. To explore the participation of nitric oxide (NO) and caveolin-1 in this protective effect, we evaluated proteinuria, creatinine clearance, renal structural lesions, nitrites and nitrates urinary excretion (UNO(2)(-)/NO(3)V), and mRNA and protein levels of neuronal NO synthase (nNOS), endothelial NOS (eNOS), and caveolin-1 in lean and fatty Zucker rats fed with 20% casein or soy protein diet. After 160 days of feeding with casein, fatty Zucker rats developed renal insufficiency, progressive proteinuria, and renal structural lesions; these alterations were associated with an important fall of UNO(2)(-)/NO(3)V, changes in nNOS and eNOS mRNA levels, together with increased amount of eNOS and caveolin-1 present in plasma membrane proteins of the kidney. In fatty Zucker rats fed with soy, we observed that soy diet improved renal function, UNO(2)(-)/NO(3)V, and proteinuria and reduced glomerulosclerosis, tubular dilation, intersticial fibrosis, and extracapilar proliferation. Renal protection was associated with reduction of caveolin-1 and eNOS in renal plasma membrane proteins. In conclusion, our results suggest that renal protective effect of soy protein appears to be mediated by improvement of NO generation and pointed out to caveolin-1 overexpression as a potential pathophysiological mechanism in renal disease.

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.

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.

Myofibroblast differentiation: plasma membrane microdomains and cell phenotype

Myofibroblast differentiation characterizes a prominent cellular phenotype identified in experimental models of progressive kidney disease and human kidney biopsies. Mesangial cells, tubulointerstitial fibroblasts and, perhaps, tubular epithelial cells undergo myofibroblast differentiation, a process characterized by alpha-actin expression, synthesis of interstitial collagens and a growth response. Inhibition of myofibroblast differentiation could prevent kidney disease progression but may be difficult to accomplish, since inhibition of multiple signaling pathways would be required. Cell biology advances have enabled a better understanding of how information from many microenvironmental stimuli are integrated by spatial compartmentalization of extracellular receptors and cytosolic signaling molecules within specialized plasma membrane domains, such as focal adhesions and lipid rafts. We review this information and hypothesize that myofibroblast differentiation of renal cells can only proceed if the spatial arrangement of intracellular molecules, in large part determined by extracellular matrix-regulated cytoskeletal organization, permits activation of appropriate signaling pathways by soluble molecules interacting with receptors in specialized plasma membrane microdomains. If proven, this hypothesis suggests targeting key molecules within adhesion complexes and rafts (in some cases with drugs that are already clinically available) may provide more effective therapy for kidney disease progression.

Disruption of the caveolin-1 gene impairs renal calcium reabsorption and leads to hypercalciuria and urolithiasis

Using LoxP/Cre technology, we generated a knockout mouse homozygous for a null mutation in exon 2 of Cav1. In male Cav1-/- animals, we observed a dramatic increase in the incidence of urinary calcium stone formation. In 5-month-old male mice, the incidence of early urinary calculi was 67% in Cav1-/- mice compared to 19% in Cav1+/+ animals. Frank stone formation was observed in 13% of Cav1-/- males but was not seen in Cav1+/+ mice. Urine calcium concentration was significantly higher in Cav1-/- male mice compared to Cav1+/+ mice. In Cav1-/- mice, distal convoluted tubule cells were completely devoid of Cav1 and the localization of plasma membrane calcium ATPase was disrupted. Functional studies confirmed that active calcium absorption was significantly reduced in Cav1-/- compared to Cav1+/+ male mice. These results demonstrate that disruption of the Cav1 gene promotes the progressive steps required for urinary calcium stone formation and establish a new mouse model for urinary stone disease.

Endothelin-1 activates mesangial cell ERK1/2 via EGF-receptor transactivation and caveolin-1 interaction

Endothelin-1 (ET-1) stimulates glomerular mesangial cell proliferation and extracellular matrix protein transcription through an ERK1/2-dependent pathway. In this study, we determined whether ET-1 activation of glomerular mesangial cell ERK1/2 is mediated through EGF receptor (EGF-R) transactivation and whether intact caveolae are required. We showed that ET-1 stimulated tyrosine phosphorylation of the EGF-R in primary cultured, growth-arrested rat mesangial cells. In response to ET-1, ERK1/2 phosphorylation was increased by 27 +/- 1-fold and attenuated by AG-1478, a specific EGF-R inhibitor, to 9 +/- 1-fold. Moreover, filipin III and beta-cyclodextrin, two cholesterol-depleting drugs known to disrupt caveolae, significantly reduced ET-1-induced phosphorylation of ERK1/2. In addition, preincubation of mesangial cells with a myristoylated peptide that binds to the caveolin-1 scaffolding domain diminished ET-1 activation of ERK1/2. ET-1 caused interaction of caveolin-1 with phosphorylated ERK1/2 identified by coimmunoprecipitation. Activation of ERK1/2 and its interaction with caveolin-1 were reduced by AG-1478, beta-cyclodextrin, or inhibition of PKC. Phosphorylated ERK1/2 localized at focal adhesion complexes along with phospho-caveolin-1, suggesting specific sites of compartmentalization of these signaling molecules. Hence, ET-1 activates mesangial cell ERK1/2 predominantly through a pathway involving EGF-R transactivation, leading to a mechanism involving attachment to caveolin-1, presumably in caveolae.

Glomerular endothelial fenestrae in vivo are not formed from caveolae

Previous reports indicate that endothelial fenestrae in vitro can form by fusion of caveolae or caveolae-like vesicles. The principal aim of this study was to determine whether formation of glomerular endothelial cell fenestrae in vivo similarly involves caveolae and caveolin-1. Whereas caveolin-1 immunofluorescence was found around the circumference of human and mouse glomerular capillary loops, it co-localized only partially with the endothelium-specific lectin Ulex Europaeus I in human glomeruli, leaving portions of the endothelium devoid of caveolin-1. Immunogold electron microscopy, used to definitively localize caveolin-1 in glomeruli, showed that caveolin-1 was completely excluded from the fenestrated portion of the endothelium. Moreover, in caveolin-1-deficient mice, which cannot form caveolae, the ultrastructure of glomerular endothelial fenestrae appeared entirely normal. Interestingly, strong caveolin-1 immunogold labeling was observed in podocytes, where some caveolin-1 localized to filtration slits. Caveolin-1 co-immunoprecipitated with the podocyte slit diaphragm proteins nephrin and CD2AP, and dual immunofluorescence confirmed co-localization of caveolin-1 and nephrin. Nevertheless, in caveolin-1-deficient mice, podocyte ultrastructure appeared normal, and the podocyte proteins synaptopodin, nephrin, and podocin were expressed normally. In addition, blood urea nitrogen concentrations and urinary protein excretion in these mice were similar to those in wild-type mice. Thus, unlike caveolae formation, glomerular endothelial cell fenestrae formation in vivo does not require caveolin-1, ruling out the previous hypothesis that endothelial fenestrae represent fused caveolae, at least for glomerular endothelial cells. Localization of caveolin-1 to podocytes and their filtration slits is consistent with the view that the filtration slit plasma membrane represents a type of lipid raft microdomain.

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

BACKGROUND

Renal cortical/proximal tubule cholesterol accumulation, with preferential localization within plasma membrane "detergent resistant microdomains" (DRMs: rafts/caveolae), is a hallmark of the maintenance phase of acute renal failure (ARF). This study addressed two related issues: (1) Are maintenance-phase cholesterol increases accompanied by an up-regulation of caveolin, a DRM/caveolar-associated cholesterol binding protein? (2) Is DRM cholesterol/caveolin homeostasis acutely altered during the induction phase of ARF?

METHODS

Mouse kidneys were subjected to ischemia +/- reperfusion (I/R) followed by assessment of cholesterol DRM partitioning. Acute cell injury effects on potential caveolin release from isolated proximal tubules or into urine also were assessed. Finally, renal cortical/isolated proximal tubule caveolin levels were determined 18 hours after I/R or myoglobinuric ARF.

RESULTS

Acute ischemia causes a rapid shift of cholesterol into cortical DRMs (>22%). Cholesterol migration into DRMs also was observed in ATP-depleted cultured proximal tubule (HK-2) cells. Acute hypoxic or toxic tubule injury induced plasma membrane caveolin release (Western blot). By the maintenance phase of ARF, marked renal cortical/proximal tubule caveolin increases resulted.

CONCLUSIONS

Acute proximal tubular injury damages caveolar/DRM structures, as determined by cholesterol maldistribution and caveolin release. Post-injury, there is a dramatic up-regulation of renal cortical/proximal tubule caveolin, suggesting an increased caveolar mass. These findings indicate, to our knowledge for the first time, that dysregulation of caveolae/raft microdomain expression is a correlate of, and potential participant in, the induction and maintenance phases of ischemic and toxic forms of experimental ARF.