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

Transcription factor specificity protein (SP) family in renal physiology and diseases

Dysregulated specificity proteins (SPs), members of the C2H2 zinc-finger family, are crucial transcription factors (TFs) with implications for renal physiology and diseases. This comprehensive review focuses on the role of SP family members, particularly SP1 and SP3, in renal physiology and pathology. A detailed analysis of their expression and cellular localization in the healthy human kidney is presented, highlighting their involvement in fatty acid metabolism, electrolyte regulation, and the synthesis of important molecules. The review also delves into the diverse roles of SPs in various renal diseases, including renal ischemia/reperfusion injury, diabetic nephropathy, renal interstitial fibrosis, and lupus nephritis, elucidating their molecular mechanisms and potential as therapeutic targets. The review further discusses pharmacological modulation of SPs and its implications for treatment. Our findings provide a comprehensive understanding of SPs in renal health and disease, offering new avenues for targeted therapeutic interventions and precision medicine in nephrology.

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.

Molecular regulation and clinical significance of caveolin-1 methylation in chronic lung diseases

Chronic lung diseases represent a largely global burden whose pathogenesis remains largely unknown. Research increasingly suggests that epigenetic modifications, especially DNA methylation, play a mechanistic role in chronic lung diseases. DNA methylation can affect gene expression and induce various diseases. Of the caveolae in plasma membrane of cell, caveolin-1 (Cav-1) is a crucial structural constituent involved in many important life activities. With the increasingly advanced progress of genome-wide methylation sequencing technologies, the important impact of Cav-1 DNA methylation has been discovered. The present review overviews the biological characters, functions, and structure of Cav-1; epigenetic modifications of Cav-1 in health and disease; expression and regulation of Cav-1 DNA methylation in the respiratory system and its significance; as well as clinical potential as disease-specific biomarker and targets for early diagnosis and therapy.

Role of caveolin-1 in epidermal stem cells during burn wound healing in rats

Local transplantation of stem cells has therapeutic effects on skin damage but cannot provide satisfactory wound healing. Studies on the mechanisms underlying the therapeutic effects of stem cells on skin wound healing will be needed. Hence, in the present study, we explored the role of Caveolin-1 in epidermal stem cells (EpiSCs) in the modulation of wound healing. We first isolated EpiSCs from mouse skin tissues and established stable EpiSCs with overexpression of Caveolin-1 using a lentiviral construct. We then evaluated the epidermal growth factor (EGF)-induced cell proliferation ability using cell counting Kit-8 (CCK-8) assay and assessed EpiSC pluripotency by examining Nanog mRNA levels in EpiSCs. Furthermore, we treated mice with skin burn injury using EpiSCs with overexpression of Caveolin-1. Histological examinations were conducted to evaluate re-epithelialization, wound scores, cell proliferation and capillary density in wounds. We found that overexpression of Caveolin-1 in EpiSCs promoted EGF-induced cell proliferation ability and increased wound closure in a mouse model of skin burn injury. Histological evaluation demonstrated that overexpression of Caveolin-1 in EpiSCs promoted re-epithelialization in wounds, enhanced cellularity, and increased vasculature, as well as increased wound scores. Taken together, our results suggested that Caveolin-1 expression in the EpiSCs play a critical role in the regulation of EpiSC proliferation ability and alteration of EpiSC proliferation ability may be an effective approach in promoting EpiSC-based therapy in skin wound healing.

Is There a Potential Therapeutic Role for Caveolin-1 in Fibrosis?

Fibrosis is a process of dysfunctional wound repair, described by a failure of tissue regeneration and excessive deposition of extracellular matrix, resulting in tissue scarring and subsequent organ deterioration. There are a broad range of stimuli that may trigger, and exacerbate the process of fibrosis, which can contribute to the growing rates of morbidity and mortality. Whilst the process of fibrosis is widely described and understood, there are no current standard treatments that can reduce or reverse the process effectively, likely due to the continuing knowledge gaps surrounding the cellular mechanisms involved. Several cellular targets have been implicated in the regulation of the fibrotic process including membrane domains, ion channels and more recently mechanosensors, specifically caveolae, particularly since these latter contain various signaling components, such as members of the TGFβ and MAPK/ERK signaling pathways, all of which are key players in the process of fibrosis. This review explores the anti-fibrotic influences of the caveola, and in particular the key underpinning protein, caveolin-1, and its potential as a novel therapeutic target.

Interleukin (IL)-6 modulates transforming growth factor-β receptor I and II (TGF-βRI and II) function in epidermal keratinocytes

It been shown that IL-6 modulates TGF-β1 expression in fibroblasts, however, what role IL-6 plays concerning TGF-βR expression and function in skin is unknown. Therefore, the aim of this study was to investigate the mechanism by which IL-6 might modulates TGF-β receptors in skin. Skin from WT, IL-6 over-expressing mice and IL-6 treated keratinocyte cultures was analysed for TGF-βRI and TGF-βRII expression via histology, PCR and flow cytometry. Receptor function was assessed by cell migration, bromodeoxyuridine (BrdU) proliferation assays, and Smad7 expression and Smad2/3 phosphorylation. Receptor localization within the membrane was determined by co-immunoprecipitation. IL-6 overexpression and treatment increased TGF-βRII expression in the epidermis. IL-6 treatment of keratinocytes induced TGF-βRI and II expression and augmented TGF-β1-induced function as demonstrated through increased migration and decreased proliferation. Additionally, IL-6 treatment of keratinocytes altered receptor activity as indicated by altered Smad2/3 phosphorylation and increased Smad7 and membrane localization. These results suggest that IL-6 regulates keratinocyte function by modulating TGF-βRI and II expression and signal transduction via trafficking of the receptor to lipid raft pools.

Caveolin-1 Plays an Important Role in the Differentiation of Bone Marrow-Derived Mesenchymal Stem Cells into Cardiomyocytes

OBJECTIVES

Accumulating evidence has demonstrated that bone marrow-derived mesenchymal stem cells (BMSCs) may transdifferentiate into cardiomyocytes, making BMSCs a promising source of cardiomyocytes for transplantation. However, little is known about the molecular mechanisms underlying myogenic conversion of BMSCs.

METHODS

This study was designed to investigate the functional role of caveolin-1 in the cardiomyocyte differentiation of BMSCs and to explore the potential underlying molecular mechanisms.

RESULTS

BMSC differentiation was induced by treatment with 10 μM 5-azacytidine, and immunofluorescence assay showed that the expression of cardiomyocyte marker cardiac troponin T (cTnT) was significantly increased compared with a control group. Meanwhile, an increased caveolin-1 expression was found during the 5-azacytidine-induced BMSC differentiation. Additionally, the role of caveolin-1 in the differentiation process was then studied by using caveolin-1 siRNAs. We found that silencing caveolin-1 during induction remarkably enhanced the expression of cardiomyocyte marker genes, including cTnT, Nkx2.5 (cardiac-specific transcription factor), α-cardiac actin and α-myosin heavy chain (α-MHC). Moreover, we observed that downregulation of caveolin-1 was accompanied by inhibition of signal transducer and activator of transcription 3 (STAT3) phosphorylation.

CONCLUSIONS

Taken together, these findings demonstrate that caveolin-1 plays an important role in the differentiation of BMSCs into cardiomyocytes in conjunction with the STAT3 pathway.

Caveolin-1 in Breast Cancer: Single Molecule Regulation of Multiple Key Signaling Pathways

Caveolin-1 is a 22-kD trans-membrane protein enriched in particular plasma membrane invaginations known as caveolae. Cav-1 expression is often dysregulated in human breast cancers, being commonly upregulated in cancer cells and downregulated in stromal cells. As an intracellular scaffolding protein, Cav-1, is involved in several vital biological regulations including endocytosis, transcytosis, vesicular transport, and signaling pathways. Several pathways are modulated by Cav-1 including estrogen receptor, EGFR, Her2/neu, TGFβ, and mTOR and represent as major drivers in mammary carcinogenesis. Expression and role of Cav-1 in breast carcinogenesis is highly variable depending on the stage of tumor development as well as context of the cell. However, recent data have shown that downregulation of Cav-1 expression in stromal breast tumors is associated with frequent relapse, resistance to therapy, and poor outcome. Modification of Cav-1 expression for translational cancer therapy is particularly challenging since numerous signaling pathways might be affected. This review focuses on present understanding of Cav-1 in breast carcinogenesis and its potential role as a new biomarker for predicting therapeutic response and prognosis as well as new target for therapeutic manipulation.

Bioinformatics Analysis of Proteome Changes in Calu-3 Cell Infected by Influenza A Virus (H5N1)

<b><i>Aim:</i></b> This paper aimed to identify the differentially expressed proteins (DEPs) in Calu-3 cells infected by influenza A virus (IAV) subtype H5N1. <b><i>Methods:</i></b> We downloaded proteome data (BTO: 0000762) from the Proteomics Identifications database and identified the DEPs in the IAV-infected Calu-3 cells. Then we constructed a protein-protein interaction network and a transcriptional regulatory network of the proteins. Finally, we performed gene ontology (GO) analysis to study the IAV infection at a functional level. <b><i>Results:</i></b> A total of 4 protein groups between the normal cells and the Calu-3 cells infected by IAV, severe acute respiratory syndrome or swine influenza were identified. In the networks, we found 5 significant proteins including FAN, CPSF2, AGO1, AGO2 and PAX5. In addition, we demonstrated those proteins were associated with GO terms such as phosphate metabolic process, calcium ion transport, cell division and regulation of cell motion. STAT1, NS2, CD5, NCKX6 and PDGFB were significant DEPs in these GO terms. <b><i>Conclusions:</i></b> By referring to the previous studies, we suggest that proteins including FAN, CPSF2, AGO1, AGO2, PAX5, STAT1 and PDGFB can be used as therapeutic targets of IAV infection.

Regulation of signaling interactions and receptor endocytosis in growing blood vessels

Blood vessels and the lymphatic vasculature are extensive tubular networks formed by endothelial cells that have several indispensable functions in the developing and adult organism. During growth and tissue regeneration but also in many pathological settings, these vascular networks expand, which is critically controlled by the receptor EphB4 and the ligand ephrin-B2. An increasing body of evidence links Eph/ephrin molecules to the function of other receptor tyrosine kinases and cell surface receptors. In the endothelium, ephrin-B2 is required for clathrin-dependent internalization and full signaling activity of VEGFR2, the main receptor for vascular endothelial growth factor. In vascular smooth muscle cells, ephrin-B2 antagonizes clathrin-dependent endocytosis of PDGFRβ and controls the balanced activation of different signal transduction processes after stimulation with platelet-derived growth factor. This review summarizes the important roles of Eph/ephrin molecules in vascular morphogenesis and explains the function of ephrin-B2 as a molecular hub for receptor endocytosis in the vasculature.

Cell proliferation in in vivo-like three-dimensional cell culture is regulated by sequestration of ERK1/2 to lipid rafts

OBJECTIVES

Regulatory mechanisms of cell proliferation have been extensively studied as they represent major challenges when dealing with pathologies such as fibrosis, tumourigenesis or tissue regeneration. Numerous in vitro studies still exploit conventional, two-dimensional cell cultures where cells are forced to adhere to unnaturally stiff and flat surfaces of culture dishes. In the living organism, however, each cell is in contact with components of the extracellular matrix and/or neighbouring cells, thus creating a complex three-dimensional (3D) tissue structure. The current paper describes a native 3D culture of cells, based on the GD25β1 fibroblast cell line, and its use for investigating cell proliferation in in vivo-like conditions.

MATERIALS AND METHODS

Four-day post-confluent culture of GD25β1 fibroblasts resulted in formation of a 3D system of cells embedded in naturally synthesized extracellular matrix. Morphological characterization of the culture was performed by histochemistry, immunohistochemistry and immunofluorescence. Viability/proliferation was assayed by MTT testing, FACS analysis and Western blotting for determination of expression levels and activation status of the relevant signalling molecules.

RESULTS

GD25b1 fibroblasts, grown as 3D culture, gave rise to tissue-like structures characterized by low level of apoptosis, low senescence and development of 3D matrix adhesions, typical of living tissues. Transition to three-dimensionality led to a switch from exponential to linear culture growth, accompanied by accumulation of activated ERK1/2 into caveolin-containing raft domains. Disruption of raft domains as well as reverse transition from 3D back to monolayer culture led to release of phosphorylated ERK1/2 from rafts, activation of cyclin D1 expression and increase in proliferation levels.

CONCLUSIONS

These results imply that under in vivo-like conditions, cells might achieve reduction of their proliferation level by sequestering activated ERK1/2 to lipid rafts.

Ephrin-B2 controls PDGFRβ internalization and signaling

Ephrin-B2 is essential for supporting mural cells; namely, pericytes and vascular smooth muscle cells (VSMCs). Nakayama et al. find that ephrin-B2 controls platelet-derived growth factor receptor β (PDGFRβ) distribution in the VSMC plasma membrane, endocytosis, and signaling. VSMCs lacking ephrin-B2 exhibited a redistribution of PDGFRβ from caveolin-positive to clathrin-associated membrane fractions and enhanced PDGF-B-induced PDGFRβ internalization. Mice lacking ephrin-B2 in vascular smooth muscle developed vessel wall defects and aortic aneurysms. These results suggest that ephrin-B2 is an important regulator of PDGFRβ endocytosis in mural cells.

B-class ephrins, ligands for EphB receptor tyrosine kinases, are critical regulators of growth and patterning processes in many organs and species. In the endothelium of the developing vasculature, ephrin-B2 controls endothelial sprouting and proliferation, which has been linked to vascular endothelial growth factor (VEGF) receptor endocytosis and signaling. Ephrin-B2 also has essential roles in supporting mural cells (namely, pericytes and vascular smooth muscle cells [VSMCs]), but the underlying mechanism is not understood. Here, we show that ephrin-B2 controls platelet-derived growth factor receptor β (PDGFRβ) distribution in the VSMC plasma membrane, endocytosis, and signaling in a fashion that is highly distinct from its role in the endothelium. Absence of ephrin-B2 in cultured VSMCs led to the redistribution of PDGFRβ from caveolin-positive to clathrin-associated membrane fractions, enhanced PDGF-B-induced PDGFRβ internalization, and augmented downstream mitogen-activated protein (MAP) kinase and c-Jun N-terminal kinase (JNK) activation but impaired Tiam1–Rac1 signaling and proliferation. Accordingly, mutant mice lacking ephrin-B2 expression in vascular smooth muscle developed vessel wall defects and aortic aneurysms, which were associated with impaired Tiam1 expression and excessive activation of MAP kinase and JNK. Our results establish that ephrin-B2 is an important regulator of PDGFRβ endocytosis and thereby acts as a molecular switch controlling the downstream signaling activity of this receptor in mural cells.

The role of caveolae in the pathophysiology of lung diseases

Caveolae are flask-shaped plasma membrane invaginations formed by constitutive caveolin proteins and regulatory cavin proteins. Caveolae harbor a range of signaling components such as receptors, ion channels and regulatory molecules. There is now increasing evidence that caveolins and cavins play an important role in a variety of diseases. However, the mechanisms by which these caveolar proteins affect lung health and disease are still under investigation, with emerging data suggesting complex roles in disease pathophysiology. This review summarizes the current state of understanding of how caveolar proteins contribute to lung structure and function and how their altered expression and/or function can influence lung diseases.

The less-often-traveled surface of stem cells: caveolin-1 and caveolae in stem cells, tissue repair and regeneration

Stem cells are an important resource for tissue repair and regeneration. While a great deal of attention has focused on derivation and molecular regulation of stem cells, relatively little research has focused on how the subcellular structure and composition of the cell membrane influences stem cell activities such as proliferation, differentiation and homing. Caveolae are specialized membrane lipid rafts coated with caveolin scaffolding proteins, which can regulate cholesterol transport and the activity of cell signaling receptors and their downstream effectors. Caveolin-1 is involved in the regulation of many cellular processes, including growth, control of mitochondrial antioxidant levels, migration and senescence. These activities are of relevance to stem cell biology, and in this review evidence for caveolin-1 involvement in stem cell biology is summarized. Altered stem and progenitor cell populations in caveolin-1 null mice suggest that caveolin-1 can regulate stem cell proliferation, and in vitro studies with isolated stem cells suggest that caveolin-1 regulates stem cell differentiation. The available evidence leads us to hypothesize that caveolin-1 expression may stabilize the differentiated and undifferentiated stem cell phenotype, and transient downregulation of caveolin-1 expression may be required for transition between the two. Such regulation would probably be critical in regenerative applications of adult stem cells and during tissue regeneration. We also review here the temporal changes in caveolin-1 expression reported during tissue repair. Delayed muscle regeneration in transgenic mice overexpressing caveolin-1 as well as compromised cardiac, brain and liver tissue repair and delayed wound healing in caveolin-1 null mice suggest that caveolin-1 plays an important role in tissue repair, but that this role may be negative or positive depending on the tissue type and the nature of the repair process. Finally, we also discuss how caveolin-1 quiescence-inducing activities and effects on mitochondrial antioxidant levels may influence stem cell aging.

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.

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.

Caveolin-1 as a novel indicator of wound-healing capacity in aged human corneal epithelium

Excess caveolin-1 has been reported to play a role in age-dependent hyporesponsiveness to growth factors in vitro. Therefore, we hypothesized that caveolin-1-dependent hyporesponsiveness to growth factors in aged corneal epithelial cells might be responsible for delayed wound healing in vivo. To test this hypothesis, we evaluated corneal wound-healing time by vital staining using fluorescein after laser epithelial keratomileusis (LASEK). We compared wound-healing times in young, middle-aged and elderly patients. We also examined caveolin-1 levels and other aging markers, such as p53 and p21, in the corneal epithelium. Elderly patients generally had higher caveolin-1 levels in the corneal epithelia than young patients. There were, however, variations among individuals with increased caveolin-1 in some young patients and decreased levels in some elderly patients. Wound-healing time after LASEK correlated well with the corneal caveolin-1 status. Therefore, we suggest that caveolin-1 status might be responsible for delayed wound healing in elderly patients after LASEK. Caveolin-1 status might be a regulator for wound-healing capacity and a novel target for in vivo adjustment.

Angiogenic growth factors inhibit chondrocyte ageing in osteoarthritis: potential involvement of catabolic stress-induced overexpression of caveolin-1 in cellular ageing

OBJECTIVE

Recently, attention has been attracted by the finding that overexpression of caveolin-1 induces cellular senescence in age-related diseases. We aimed to ascertain whether angiogenic growth factors (AGFs) can inhibit interleukin (IL)-1beta-induced senescence in human chondrocytes by downregulation of caveolin-1.

METHODS

We investigated the intracellular signalling pathways involved in chondrocyte ageing. Human chondrocytes were isolated from the articular cartilage of patients undergoing arthroplastic knee surgery in osteoarthritis (OA). Chondrocytes were stimulated with or without IL-1beta (10 ng/mL) in the presence or absence of vascular endothelial growth factor, basic fibroblast growth factor or hepatocyte growth factor (20 ng/mL). After 72-h incubation, we observed the expression of caveolin-1 in human chondrocytes by immunohistochemistry, and analysed the protein levels of caveolin-1 by Western blot. We examined the time-course of phosphorylation patterns of mammalian mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3-K) by Western blot, and used several specific protein kinase inhibitors to evaluate the involvement of the intracellular signalling pathways. Also, chondrocyte replicative lifespan was analyzed in the presence or absence of AGFs.

RESULTS

Treatment with AGFs inhibited IL-1beta-induced overexpression of caveolin-1 in human OA chondrocytes. Treatment with AGFs all down-regulated protein levels of IL-1beta-accelerated expression of caveolin-1 in chondrocytes. IL-1beta significantly decreased the cellular replicative lifespan in chondrocytes. Treatment with AGFs prevented the IL-1beta-induced shortening of chondrocyte replicative lifespan. The specific inhibitors for MAPK/extracellular signal-regulated kinase and PI3-K cancelled the AGF-induced downregulation of overexpression of caveolin-1.

CONCLUSION

Our results suggest that AGFs downregulated IL-1beta-induced chondrocyte ageing and overexpression of caveolin-1 in human chondrocytes, which is mediated by kinase cascades involving the p42/44 MAP kinase and PI3-K/Akt signalling pathways.

Differential involvement of caveolin-1 in brown adipocyte signaling: impaired beta3-adrenergic, but unaffected LPA, PDGF and EGF receptor signaling

Caveolae and caveolin have been implicated as being involved in the signal transduction of many receptors, including the EGF, PDGF, LPA and beta3-adrenergic receptors. To investigate the role of caveolin-1 (Cav1) in these signaling pathways in brown adipose tissue, primary brown adipocyte cultures from Cav1-ablated mice and wild-type mice were investigated. In pre-adipocytes, Cav1-ablation affected neither the G-protein coupled LPA receptor signaling to Erk1/2, nor the receptor tyrosine kinases PDGF- or EGF-receptor signaling to Erk1/2. Mature primary Cav1-/- brown adipocytes accumulated lipids and expressed aP2 to the same extent as did wild-type cells. However, the cAMP levels induced by the beta3-adrenergic receptor agonist CL316,243 were lower in the Cav1-/- cultures, with an unchanged EC50 for CL316,243. Also the response to the direct adenylyl cyclase agonist forskolin was reduced. Thus, in brown adipocytes, Cav1 is apparently required for an intact response to adenylyl cyclase-linked agonists/activators, whereas other signaling pathways examined function without Cav1.

Developmental iodine deficiency and hypothyroidism impair neural development, up-regulate caveolin-1 and down-regulate synaptophysin in rat hippocampus

Developmental iodine deficiency leads to inadequate thyroid hormone, which damages the hippocampus. In the present study, we implicate hippocampal caveolin-1 and synaptophysin in developmental iodine deficiency and hypothyroidism. Two developmental rat models were established: pregnant rats were administered either an iodine-deficient diet or propylthiouracil (PTU)-adulterated (5 p.p.m. or 15 p.p.m.) drinking water from gestational day 6 until postnatal day (PN) 28. Nissl staining and the levels of caveolin-1 and synaptophysin in several hippocampal subregions were assessed on PN14, PN21, PN28 and PN42. The results obtained show that surviving cells in the iodine-deficient and PTU-treated rats were lower than in controls. Up-regulation of caveolin-1 and down-regulation of synaptophysin were observed in the iodine-deficient and PTU-treated rats. Our findings implicate decreases in the number of surviving cells and alterations in the levels of caveolin-1 and synaptophysin in the impairments in neural development induced by developmental iodine deficiency and hypothyroidism.

Tissue-type plasminogen activator deficiency attenuates peritoneal fibrosis in mice

Peritoneal fibrosis (PF) is an important complication of peritoneal dialysis therapy. The present study was performed to examine the mechanisms of PF in view of the plasminogen activator (PA)/plasmin/matrix metalloproteinase (MMP) cascade. PF was induced in tissue-type PA (tPA) deficient mice and wild-type mice by intraperitoneal injection of chlorhexidine gluconate. Mice were killed on day 21, and tissue samples were taken. Histopathological studies were performed. Plasmin activity, gelatinases activity, and the levels of tPA, transforming growth factor-β1 (TGF-β1), and MMP-2 mRNA were determined. Protein levels of MMP-3, tissue inhibitor of metalloproteinases (TIMP)-1, -2, and -3, phospho-Smad3, membrane-type 1 (MT1)-MMP, and MT3-MMP were also studied. On day 21, tPA +/+ mice showed severe PF, whereas tPA −/− mice showed milder change. Submesothelial basement membranes were dissolved in tPA +/+ mice while they were relatively preserved in tPA −/− mice. The levels of macrophage infiltration, staining for α-smooth muscle actin (α-SMA) and collagen type III, and vascular density were all significantly lower in tPA −/− mice than in tPA +/+ mice. The levels of plasmin activity, pro- and active MMP-2, mRNA expression of tPA and TGF-β1, and phospho-Smad3 protein were also lower in tPA −/− mice. No difference was observed between the two groups concerning the protein levels of MMP-3, TIMP-1, TIMP-2, TIMP-3, MT1-MMP, or MT3-MMP. These results indicate that the presence of tPA enhances inflammation, angiogenesis, and fibrogenesis in the peritoneum of the PF model mice. Activation of the PA/plasmin/MMP cascade may play a pivotal role in the pathogenesis of PF.

Protective effect of Lycium barbarum polysaccharide 4 on kidneys in streptozotocin-induced diabetic rats

Lycium barbarum polysaccharide (LBP) has been shown to have hypoglycemic and antioxidative properties, although its mode of action is yet unknown. Because oxidative stress is implicated in the pathogenesis of diabetic nephropathy, we evaluated the protective effect of LBP-4, the major active component of Lycium barbarum, on the defensive antioxidative mechanism in kidneys in a streptozotocin-induced diabetic rat model. Moreover, we investigated the effects of LBP-4 on the activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) in isolated mesangial cells. The role of protein kinase C (PKC)-dependent and -independent pathways in LBP-4-reduced ERK1/2 was studied by bisindolylmaleimide (BIM) IV, an inhibitor of PKC. Diabetic rats treated with LBP-4 (10 mg/kg) for 8 weeks showed increased activity of antioxidant enzymes and increased scavenging of oxygen radicals, while the activity of PKC in the renal cortex was maintained at a physiological level. The decreased activation of ERK1/2 in mesangial cells, through the involvement of PKC, could explain the protective mechanism in kidneys of diabetic rats treated with LBP-4.

Caveolin-1 restoration by cholesterol enhances the inhibitory effect of simvastatin on arginine vasopressin-induced cardiac fibroblasts proliferation

Caveolin-1 (cav1) has been implicated in the regulation of cell growth, and its expression can be regulated by cellular cholesterol content. In this study, we examined the effect of manipulating cellular cholesterol content on cav1 expression and the proliferation of adult rat cardiac fibroblasts (CFs) in the presence of arginine vasopressin (AVP). We found that AVP concentration-dependently down-regulated the expression of cav1 protein. Cav1 antisense treatment enhanced the proliferatory effect of AVP. Simvastatin, a HMG-CoA reductase inhibitor, further down-regulated cav1 protein, whereas repleting cells with cholesterol increased cav1 protein and enhanced the anti-growth effect of simvastatin. Our results provide a novel finding that cholesterol restoration may confer an additional inhibitory effect over simvastatin on AVP-induced CFs proliferation through cholesterol-cav1 interaction.

Mechanisms of endocytosis

Endocytic mechanisms control the lipid and protein composition of the plasma membrane, thereby regulating how cells interact with their environments. Here, we review what is known about mammalian endocytic mechanisms, with focus on the cellular proteins that control these events. We discuss the well-studied clathrin-mediated endocytic mechanisms and dissect endocytic pathways that proceed independently of clathrin. These clathrin-independent pathways include the CLIC/GEEC endocytic pathway, arf6-dependent endocytosis, flotillin-dependent endocytosis, macropinocytosis, circular doral ruffles, phagocytosis, and trans-endocytosis. We also critically review the role of caveolae and caveolin1 in endocytosis. We highlight the roles of lipids, membrane curvature-modulating proteins, small G proteins, actin, and dynamin in endocytic pathways. We discuss the functional relevance of distinct endocytic pathways and emphasize the importance of studying these pathways to understand human disease processes.

Regulation of brain endothelial cells migration and angiogenesis by P-glycoprotein/caveolin-1 interaction

We have investigated the involvement of P-glycoprotein (P-gp)/caveolin-1 interaction in the regulation of brain endothelial cells (EC) migration and tubulogenesis. P-gp overexpression in MDCK-MDR cells was correlated with enhanced cell migration whereas treatment with P-gp inhibitors CsA or PSC833 reduced it. Transfection of RBE4 rat brain endothelial cells with mutated versions of MDR1, in the caveolin-1 interaction motif, decreased the interaction between P-gp and caveolin-1, enhanced P-gp transport activity and cell migration. Moreover, down-regulation of caveolin-1 in RBE4 cells by siRNA against caveolin-1 stimulated cell migration. Interestingly, the inhibition of P-gp/caveolin-1 interaction increased also EC tubulogenesis. Furthermore, decrease of P-gp expression by siRNA inhibited EC tubulogenesis. These data indicate that the level of P-gp/caveolin-1 interaction can modulate brain endothelial angiogenesis and P-gp dependent cell migration.

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.

p42/p44 MAP kinase activation is localized to caveolae-free membrane domains in airway smooth muscle

Caveolae are abundant plasma membrane invaginations in airway smooth muscle that may function as preorganized signalosomes by sequestering and regulating proteins that control cell proliferation, including receptor tyrosine kinases (RTKs) and their signaling effectors. We previously demonstrated, however, that p42/p44 MAP kinase, a critical effector for cell proliferation, does not colocalize with RTKs in caveolae of quiescent airway myocytes. Therefore, we investigated the subcellular sites of growth factor-induced MAP kinase activation. In quiescent myocytes, though epidermal growth factor receptor (EGFR) was almost exclusively found in caveolae, p42/p44 MAP kinase, Grb2, and Raf-1 were absent from these membrane domains. EGF induced concomitant phosphorylation of caveolin-1 and p42/p44 MAP kinase; however, EGF did not promote the localization of p42/p44 MAP kinase, Grb2, or Raf-1 to caveolae. Interestingly, stimulation of muscarinic M(2) and M(3) receptors that were enriched in caveolae-deficient membranes also induced p42/p44 MAP kinase phosphorylation, but this occurred in the absence of caveolin-1 phosphorylation. This suggests that the localization of receptors to caveolae and interaction with caveolin-1 is not directly required for p42/p44 MAP kinase phosphorylation. Furthermore, we found that EGF exposure induced rapid translocation of EGFR from caveolae to caveolae-free membranes. EGFR trafficking coincided temporally with EGFR and p42/p44 MAP kinase phosphorylation. Collectively, this indicates that although caveolae sequester some receptors associated with p42/p44 MAP kinase activation, the site of its activation is associated with caveolae-free membrane domains. This reveals that directed trafficking of plasma membrane EGFR is an essential element of signal transduction leading to p42/p44 MAP kinase activation.

Caveolin-1: a critical regulator of lung fibrosis in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive chronic disorder characterized by activation of fibroblasts and overproduction of extracellular matrix (ECM). Caveolin-1 (cav-1), a principal component of caveolae, has been implicated in the regulation of numerous signaling pathways and biological processes. We observed marked reduction of cav-1 expression in lung tissues and in primary pulmonary fibroblasts from IPF patients compared with controls. We also demonstrated that cav-1 markedly ameliorated bleomycin (BLM)-induced pulmonary fibrosis, as indicated by histological analysis, hydroxyproline content, and immunoblot analysis. Additionally, transforming growth factor beta1 (TGF-beta1), the well-known profibrotic cytokine, decreased cav-1 expression in human pulmonary fibroblasts. cav-1 was able to suppress TGF-beta1-induced ECM production in cultured fibroblasts through the regulation of the c-Jun N-terminal kinase (JNK) pathway. Interestingly, highly activated JNK was detected in IPF- and BLM-instilled lung tissue samples, which was dramatically suppressed by ad-cav-1 infection. Moreover, JNK1-null fibroblasts showed reduced smad signaling cascades, mimicking the effects of cav-1. This study indicates a pivotal role for cav-1 in ECM regulation and suggests a novel therapeutic target for patients with pulmonary fibrosis.

Different Effects of Growth Factors on Human Renal Early Distal Tubular Cells in vitro

BACKGROUND

In the kidney, recovery from tubular damage requires regenerative mechanisms leading to re-epithelialization of the injured tubules. Current evidence supports the para- or autocrine role of growth factors in repair and regeneration of ischemic or nephrotoxic experimental acute renal failure.

METHODS

We evaluated the effects of EGF, HGF, IGF-1, and bFGF on human renal thick ascending limb and distal convoluted cells (TALDC) in vitro. TALDC were isolated by immunomagnetic separation and cultured. Signal transduction of the growth factors was evaluated by Western blot of ERK1/2 MAP-K phosphorylation. Cell proliferation was measured by MTT assay and a fluorometric assay.

RESULTS

A significant, dose- and time-dependent phosphorylation of ERK1/2 could be detected exclusively after stimulation with EGF. No other growth factor induced a significant MAPK phosphorylation. In the same manner, proliferation assays showed a significant growth-promoting effect of EGF. Neither HGF, nor IGF-1 or bFGF showed a stimulative effect on TALDC proliferation.

CONCLUSION

The present study highlights the effects of growth factors on cultured TALDC and supports the hypothesis that in vivo EGF plays a para- or autocrine role during renal repair mechanisms.

Gut-enriched Krüppel-like factor interaction with Smad3 inhibits myofibroblast differentiation

Gut-enriched Krüppel-like factor (GKLF) has been reported to partially inhibit alpha-smooth muscle actin (alpha-SMA) gene transcription by competing for binding to the TGF-beta control element (TCE) with known activators such as Sp1 and other Krüppel-like factors. This incomplete inhibition via the TCE suggests an additional mechanism, which was evaluated in this study. The results showed that an alpha-SMA promoter mutated in the TCE remained susceptible to inhibition by GKLF in rat lung fibroblasts consistent with the existence of an additional TCE-independent mechanism. Since TGF-beta- induced alpha-SMA expression is Smad3-dependent, potential interaction between GKLF and Smad3 was examined as a basis for this additional inhibitory mechanism. Co-immunoprecipitation and yeast two-hybrid assays revealed that GKLF could bind Smad3 through the Smad3 MH2 domain. Electrophoretic mobility shift assays and ChIP assay indicated that this GKLF-Smad3 interaction inhibited Smad3 binding to the Smad3-binding element (SBE) in the alpha-SMA promoter, and the activity of an SBE containing artificial promoter. Further analysis using smad3(-/-) fibroblasts confirmed that the TCE-independent inhibition by GKLF was dependent on Smad3. These data taken together suggest that in addition to inhibition via the TCE, GKLF represses alpha-SMA gene expression by interacting with Smad3 to prevent Smad3 binding to the SBE. It represents the first evidence to directly link GKLF with Smad3, a key intracellular mediator of TGF-beta signaling, which should lead to a clearer understanding of the mechanism of how GKLF regulates TGF-beta-induced gene expression.

Caveolin-1 functions as a novel Cdc42 guanine nucleotide dissociation inhibitor in pancreatic beta-cells

The cycling of the small Rho family GTPase Cdc42 is required for insulin granule exocytosis, although the regulatory proteins involved in Cdc42 cycling in pancreatic beta-cells are unknown. Here we demonstrate that the caveolar protein caveolin-1 (Cav-1) is a Cdc42-binding protein in beta-cells. Cav-1 associated with Cdc42-VAMP2-bound granules present near the plasma membrane under basal conditions. However, stimulation with glucose induced the dissociation of Cav-1 from Cdc42-VAMP2 complexes, coordinate with the timing of Cdc42 activation. Analyses of the Cav-1 scaffolding domain revealed a motif conserved in guanine nucleotide dissociation inhibitors (GDIs), which suggested a novel role for Cav-1 as a Cdc42 GDI in beta-cells. The novel role was further supported by: 1) in vitro binding analyses that demonstrated a direct interaction between Cav-1 and Cdc42; 2) GST-Cdc42 interaction assays showing preferential Cav-1 binding to GDP-Cdc42 over that of GTP-Cdc42; 3) Cav-1 depletion studies resulting in an inappropriate 40% induction of activated Cdc42 in the absence of stimuli and also a 40% increase in basal insulin release from both MIN6 cells and islets. Expression of wild-type Cav-1 in Cav-1-depleted cells restored basal level secretion to normal, whereas expression of a scaffolding domain mutant of Cav-1 failed to normalize secretion. Taken together, these data suggest that Cav-1 functions as a Cdc42 GDI in beta-cells, maintaining Cdc42 in an inactive state and regulating basal secretion in the absence of stimuli. Through its interaction with the Cdc42-VAMP2-bound insulin granule complex, Cav-1 may contribute to the specific targeting of granules to "active sites" of exocytosis organized by caveolae.

Role of caveolin-1 in p42/p44 MAP kinase activation and proliferation of human airway smooth muscle

Chronic airways diseases, including asthma, are associated with an increased airway smooth muscle (ASM) mass, which may contribute to chronic airway hyperresponsiveness. Increased muscle mass is due, in part, to increased ASM proliferation, although the precise molecular mechanisms for this response are not completely clear. Caveolae, which are abundant in smooth muscle cells, are membrane microdomains where receptors and signaling effectors can be sequestered. We hypothesized that caveolae and caveolin-1 play an important regulatory role in ASM proliferation. Therefore, we investigated their role in p42/p44 MAPK signaling and proliferation using human ASM cell lines. Disruption of caveolae using methyl-beta-cyclodextrin and small interfering (si)RNA-knockdown of caveolin-1 caused spontaneous p42/p44 MAPK activation; additionally, caveolin-1 siRNA induced ASM proliferation in mitogen deficient conditions, suggesting a key role for caveolae and caveolin-1 in maintaining quiescence. Moreover, caveolin-1 accumulates twofold in myocytes induced to a contractile phenotype compared with proliferating ASM cells. Caveolin-1 siRNA failed to increase PDGF-induced p42/p44 MAPK activation and cell proliferation, however, indicating that PDGF stimulation actively reversed the antimitogenic control by caveolin-1. Notably, the PDGF induced loss of antimitogenic control by caveolin-1 coincided with a marked increase in caveolin-1 phosphorylation. Furthermore, the strong association of PDGF receptor-beta with caveolin-1 that exists in quiescent cells was rapidly and markedly reduced with agonist addition. This suggests a dynamic relationship in which mitogen stimulation actively reverses caveolin-1 suppression of p42/p44 MAPK signal transduction. As such, caveolae and caveolin-1 coordinate PDGF receptor signaling, leading to myocyte proliferation, and inhibit constitutive activity of p42/p44 MAPK to sustain cell quiescence.

IGF-1 induces rat glomerular mesangial cells to accumulate triglyceride

Rat glomerular mesangial cells (MC) become lipid-laden foam cells when they are exposed to IGF-1. IGF-1 increased accumulation of triglyceride (TG) 2.5-fold in MC after 7 days. TG accumulation resulted from enhanced macropinocytosis and decreased efflux secondary to a 40-50% reduction in peroxisome proliferator-activated receptor (PPAR)-delta (PPARdelta). There was no evidence of primary or secondary changes in cholesterol or TG synthesis, increased uptake by LDL or scavenger receptors, or reduced efflux via ATP-binding cassette A-1. Although the lipid moiety taken up can be influenced by the concentration of cholesterol or TG in the medium, in standard medium MC preferentially accumulate TG. TG-rich MC foam cells fail to contract in response to angiotensin II (Berfield AK, Andress DL, and Abrass CK. Kidney Int 62: 1229-1237, 2002); however, their migratory response to IGF binding protein-5 is unaffected. This differs from cholesterol loading, which impairs both phagocytosis and migration. These findings have important implications for understanding the mechanisms that contribute to lipid accumulation in MC and the functional consequences of different forms of foam cells. These observations are relevant to understanding vascular disease and progressive renal diseases that are accelerated by abnormalities in lipid metabolism.

Interleukin-6 regulation of transforming growth factor (TGF)-beta receptor compartmentalization and turnover enhances TGF-beta1 signaling

Transforming growth factor (TGF)-beta1 is a key cytokine involved in the pathogenesis of fibrosis in many organs, whereas interleukin (IL)-6 plays an important role in the regulation of inflammation. Recent reports demonstrate interaction between the two cytokines in disease states. We have assessed the effect of IL-6 on TGF-beta1 signaling and defined the mechanism by which this occurred. Stimulation of Smad-responsive promoter (SBE)4-Lux activity by TGF-beta1 was significantly greater in the presence of IL-6 than that induced by TGF-beta1 alone. Augmented TGF-beta1 signaling following the addition of IL-6 appeared to be mediated through binding to the cognate IL-6 receptor, the presence of which was confirmed by fluorescence-activated cell sorting and Stat-specific signaling. TGF-beta1 receptors internalize by both caveolin-1 (Cav-1) lipid raft and early endosome antigen 1 (EEA-1) non-lipid raft pathways, with non-lipid raft-associated internalization increasing TGF-beta1 signaling. Affinity labeling of TGF-beta1 receptors demonstrated that IL-6 stimulation resulted in increased partitioning of TGF-beta receptors to the non-lipid raft fraction. There was no change in expression of Cav-1; however, following IL-6 stimulation, co-immunoprecipitation demonstrated decreased association of IL-6 receptor with Cav-1. Increased TGF-beta1-dependent Smad signaling by IL-6 was significantly attenuated by inhibition of clathrin-mediated endocytosis and augmented by depletion of membrane cholesterol. These results indicate that IL-6 increased trafficking of TGF-beta1 receptors to non-lipid raft-associated pools results in augmented TGF-beta1 Smad signaling.