Caveolin-1 loss of function accelerates glucose transporter 4 and insulin receptor degradation in 3T3-L1 adipocytes

Caveolin 1 Regulates the Tight Junctions between Sertoli Cells and Promotes the Integrity of Blood-Testis Barrier in Yak via the FAK/ERK Signaling Pathway

Yaks, a valuable livestock species endemic to China's Tibetan plateau, have a low reproductive rate. Cryptorchidism is believed to be one of the leading causes of infertility in male yaks. In this study, we compared the morphology of the normal testis of the yak with that of the cryptorchidism, and found dysplasia of the seminiferous tubules, impaired tightness of the Sertoli cells, and a disruption of the integrity of the blood-testis barrier (BTB) in the cryptorchidism. Previous studies have shown that CAV1 significantly contributes to the regulation of cell tight junctions and spermatogenesis. Therefore, we hypothesize that CAV1 may play a regulatory role in tight junctions and BTB in Yaks Sertoli cells, thereby influencing the development of cryptorchidism. Additional analysis using immunofluorescence, qRT-PCR, and Western blotting confirmed that CAV1 expression is up-regulated in yak cryptorchidism. CAV1 over-expression plasmids and small RNA interference sequences were then transfected in vitro into yak Sertoli cells. It was furthermore found that CAV1 has a positive regulatory effect on tight junctions and BTB integrity, and that this regulatory effect is achieved through the FAK/ERK signaling pathway. Taken together, our findings, the first application of CAV1 to yak cryptorchidism, provide new insights into the molecular mechanisms of cell tight junctions and BTB. This paper suggests that CAV1 could be used as a potential therapeutic target for yak cryptorchidism and may provide insight for future investigations into the occurrence of cryptorchidism, the maintenance of a normal physiological environment for spermatogenesis and male reproductive physiology in the yak.

Regulated dynamic subcellular GLUT4 localization revealed by proximal proteome mapping in human muscle cells

Regulation of glucose transport, which is central for control of whole-body metabolism, is determined by the amount of GLUT4 glucose transporter (also known as SLC2A4) in the plasma membrane (PM) of fat and muscle cells. Physiologic signals [such as activated insulin receptor or AMP-activated protein kinase (AMPK)] increase PM GLUT4. Here, we show that the distribution of GLUT4 between the PM and interior of human muscle cells is dynamically maintained, and that AMPK promotes PM redistribution of GLUT4 by regulating exocytosis and endocytosis. Stimulation of exocytosis by AMPK is mediated by Rab10 and the Rab GTPase-activating protein TBC1D4. APEX2 proximity mapping reveals that GLUT4 traverses both PM-proximal and PM-distal compartments in unstimulated muscle cells, further supporting retention of GLUT4 by a constitutive retrieval mechanism. AMPK-stimulated translocation involves GLUT4 redistribution among the same compartments traversed in unstimulated cells, with a significant recruitment of GLUT4 from the Golgi and trans-Golgi network compartments. Our comprehensive proximal protein mapping provides an integrated, high-density, whole-cell accounting of the localization of GLUT4 at a resolution of ∼20 nm that serves as a structural framework for understanding the molecular mechanisms regulating GLUT4 trafficking downstream of different signaling inputs in a physiologically relevant cell type.

EHD2 regulates plasma membrane integrity and downstream insulin receptor signaling events

Adipocyte dysfunction is a crucial driver of insulin resistance and type 2 diabetes. We identified EH domain-containing protein 2 (EHD2) as one of the most highly upregulated genes at the early stage of adipose-tissue expansion. EHD2 is a dynamin-related ATPase influencing several cellular processes, including membrane recycling, caveolae dynamics, and lipid metabolism. Here, we investigated the role of EHD2 in adipocyte insulin signaling and glucose transport. Using C57BL6/N EHD2 knockout mice under short-term high-fat diet conditions and 3T3-L1 adipocytes we demonstrate that EHD2 deficiency is associated with deterioration of insulin signal transduction and impaired insulin-stimulated GLUT4 translocation. Furthermore, we show that lack of EHD2 is linked with altered plasma membrane lipid and protein composition, reduced insulin receptor expression, and diminished insulin-dependent SNARE protein complex formation. In conclusion, these data highlight the importance of EHD2 for the integrity of the plasma membrane milieu, insulin receptor stability, and downstream insulin receptor signaling events, involved in glucose uptake and ultimately underscore its role in insulin resistance and obesity.

GLUT4 dynamic subcellular localization is controlled by AMP kinase activation as revealed by proximal proteome mapping in human muscle cells

Regulation of glucose transport into muscle and adipocytes, central for control of whole-body metabolism, is determined by the amount of GLUT4 glucose transporter in the plasma membrane ( PM ). Physiologic signals (activated insulin receptor or AMP kinase [ AMPK ]), acutely increase PM GLUT4 to enhance glucose uptake. Here we show in kinetic studies that intracellular GLUT4 is in equilibrium with the PM in unstimulated cultured human skeletal muscle cells, and that AMPK promotes GLUT4 redistribution to the PM by regulating both exocytosis and endocytosis. AMPK-stimulation of exocytosis requires Rab10 and Rab GTPase activating protein TBC1D4, requirements shared with insulin control of GLUT4 in adipocytes. Using APEX2 proximity mapping, we identify, at high-density and high-resolution, the GLUT4 proximal proteome, revealing GLUT4 traverses both PM proximal and distal compartments in unstimulated muscle cells. These data support intracellular retention of GLUT4 in unstimulated muscle cells by a dynamic mechanism dependent on the rates of internalization and recycling. AMPK promoted GLUT4 translocation to the PM involves redistribution of GLUT4 among the same compartments traversed in unstimulated cells, with a significant redistribution of GLUT4 from the PM distal Trans Golgi Network Golgi compartments. The comprehensive proximal protein mapping provides an integrated, whole cell accounting of GLUT4's localization at a resolution of ∼20 nm, a structural framework for understanding the molecular mechanisms regulating GLUT4 trafficking downstream of different signaling inputs in physiologically relevant cell type and as such, sheds new light on novel key pathways and molecular components as potential therapeutic approaches to modulate muscle glucose uptake.

Functional basis for calmodulation of the TRPV5 calcium channel

Within the transient receptor potential (TRP) superfamily of ion channels, TRPV5 is a highly Ca²⁺ -selective channel important for active reabsorption of Ca²⁺ in the kidney. Its channel activity is controlled by a negative feedback mechanism involving calmodulin (CaM) binding. Combining advanced microscopy techniques and biochemical assays, this study characterized the dynamic lobe-specific CaM regulation. We demonstrate for the first time that functional (full-length) TRPV5 interacts with CaM in the absence of Ca²⁺ , and this interaction is intensified at increasing Ca²⁺ concentrations sensed by the CaM C-lobe that achieves channel pore blocking. Channel inactivation occurs without requiring CaM N-lobe calcification. Moreover, we show a Ca²⁺ -dependent binding stoichiometry at the single channel level. In conclusion, our study proposes a new model for CaM-dependent regulation - calmodulation - of this uniquely Ca²⁺ -selective TRP channel TRPV5 that involves apoCaM interaction and lobe-specific actions, which may be of significant physiological relevance given its role as gatekeeper of Ca²⁺ transport in the kidney. KEY POINTS: The renal Ca²⁺ channel TRPV5 is an important player in maintenance of the body's Ca²⁺ homeostasis. Activity of TRPV5 is controlled by a negative feedback loop that involves calmodulin (CaM), a protein with two Ca²⁺ -binding lobes. We investigated the dynamics of the interaction between TRPV5 and CaM with advanced fluorescence microscopy techniques. Our data support a new model for CaM-dependent regulation of TRPV5 channel activity with CaM lobe-specific actions and demonstrates Ca²⁺ -dependent binding stoichiometries. This study improves our understanding of the mechanism underlying fast channel inactivation, which is physiologically relevant given the gatekeeper function of TRPV5 in Ca²⁺ reabsorption in the kidney.

The importance of caveolin as a target in the prevention and treatment of diabetic cardiomyopathy

The diabetic population has been increasing in the past decades and diabetic cardiomyopathy (DCM), a pathology that is defined by the presence of cardiac remodeling and dysfunction without conventional cardiac risk factors such as hypertension and coronary heart diseases, would eventually lead to fatal heart failure in the absence of effective treatment. Impaired insulin signaling, commonly known as insulin resistance, plays an important role in the development of DCM. A family of integral membrane proteins named caveolins (mainly caveolin-1 and caveolin-3 in the myocardium) and a protein hormone adiponectin (APN) have all been shown to be important for maintaining normal insulin signaling. Abnormalities in caveolins and APN have respectively been demonstrated to cause DCM. This review aims to summarize recent research findings of the roles and mechanisms of caveolins and APN in the development of DCM, and also explore the possible interplay between caveolins and APN.

Interactions between Caveolin-1 polymorphism and Plant-based dietary index on metabolic and inflammatory markers among women with obesity

A series of recent studies have indicated that the Caveolin-1 (CAV-1) gene variant may be associated with metabolic and inflammatory markers and anthropometric measures. Furthermore, it has been shown that a plant-based dietary index (PDI) can elicit a positive impact on these metabolic markers. Therefore, we sought to examine whether PDI intakes may affect the relationship between CAV-1 (rs3807992) and metabolic factors, as well as serum inflammatory markers and anthropometric measures, in women with obesity. This current study consisted of 400 women with overweight and obesity, with a mean (SD) age of 36.67 ± 9.10 years. PDI was calculated by a food frequency questionnaire (FFQ). The anthropometric measurements and serum profiles were measured by standard protocols. Genotyping of the CAV-1(rs3807992) was conducted by the PCR–RFLP method. The following genotypic frequencies were found among the participants: GG (47.8%), AG (22.3%), and AA (2.3%). In comparison to GG homozygotes, risk-allele carriers (AA + AG) with higher PDI intake had lower ALT (P: 0.03), hs-CRP (P: 0.008), insulin (P: 0.01) and MCP-1 (P: 0.04). Furthermore, A-allele carriers were characterized by lower serum ALT (P: 0.04), AST (P: 0.02), insulin (P: 0.03), and TGF-β (P: 0.001) when had the higher following a healthful PDI compared to GG homozygote. Besides, risk-allele carriers who consumed higher unhealthful PDI had higher WC (P: 0.04), TC/HDL (P: 0.04), MCP-1 (P: 0.03), and galactin-3 (P: 0.04). Our study revealed that A-allele carriers might be more sensitive to PDI composition compared to GG homozygotes. Following a healthful PDI in A-allele carriers may be associated with improvements in metabolic and inflammatory markers and anthropometric measures.

Neuregulin 4 Downregulation Induces Insulin Resistance in 3T3-L1 Adipocytes through Inflammation and Autophagic Degradation of GLUT4 Vesicles

The adipokine Neuregulin 4 (Nrg4) protects against obesity-induced insulin resistance. Here, we analyze how the downregulation of Nrg4 influences insulin action and the underlying mechanisms in adipocytes. Validated shRNA lentiviral vectors were used to generate scramble (Scr) and Nrg4 knockdown (KD) 3T3-L1 adipocytes. Adipogenesis was unaffected in Nrg4 KD adipocytes, but there was a complete impairment of the insulin-induced 2-deoxyglucose uptake, which was likely the result of reduced insulin receptor and Glut4 protein. Downregulation of Nrg4 enhanced the expression of proinflammatory cytokines. Anti-inflammatory agents recovered the insulin receptor, but not Glut4, content. Proteins enriched in Glut4 storage vesicles such as the insulin-responsive aminopeptidase (IRAP) and Syntaxin-6 as well as TBC1D4, a protein involved in the intracellular retention of Glut4 vesicles, also decreased by Nrg4 KD. Insulin failed to reduce autophagy in Nrg4 KD adipocytes, observed by a minor effect on mTOR phosphorylation, at the time that proteins involved in autophagy such as LC3-II, Rab11, and Clathrin were markedly upregulated. The lysosomal activity inhibitor bafilomycin A1 restored Glut4, IRAP, Syntaxin-6, and TBC1D4 content to those found in control adipocytes. Our study reveals that Nrg4 preserves the insulin responsiveness by preventing inflammation and, in turn, benefits the insulin regulation of autophagy.

Kv1.3 Controls Mitochondrial Dynamics during Cell Cycle Progression

Simple Summary

Voltage-dependent potassium channels control the proliferation of mammalian cells. In addition, mitochondria physiology is highly dynamic during the cell cycle. The aim of this work was to investigate whether the Kv1.3 channel participates in the mitochondrial control of cell cycle progression. Our data confirmed that Kv1.3 facilitates the proliferation of preadipocytes through the control of mitochondrial dynamics. In addition, adipogenesis was also dependent on Kv1.3 expression. We shed light on the role of Kv1.3 in mitochondria and adipose tissue metabolism, contributing further to the control of cell proliferation by Kv1.3.

Abstract

The voltage-gated potassium channel Kv1.3 is a potential therapeutic target for obesity and diabetes. The genetic ablation and pharmacological inhibition of Kv1.3 lead to a lean phenotype in rodents. The mechanism of regulation of body weight and energy homeostasis involves Kv1.3 expression in different organs, including white and brown adipose tissues. Here, we show that Kv1.3 promotes the proliferation of preadipocytes through the control of mitochondrial dynamics. Kv1.3 is expressed in mitochondria exhibiting high affinity for the perinuclear population. The mitochondrial network is highly dynamic during the cell cycle, showing continuous fusion-fission events. The formation of a hyperfused mitochondrial network at the G1/S phase of the cell cycle is dependent on Kv1.3 expression. Our results demonstrate that Kv1.3 promotes preadipocyte proliferation and differentiation by controlling mitochondrial membrane potential and mitochondrial dynamics at the G1 phase of the cell cycle.

Are caveolin-1 minor alleles more likely to be risk alleles in insulin resistance mechanisms in metabolic diseases?

OBJECTIVES

Obesity and insulin resistance (IR) are interrelated in a range of ways. The IR-obesity relationship is not a cause-and-effect association. Molecular biology research has made tremendous strides in discovering contributors to find this association. Genes that control adipocyte function such as caveolin-1 (CAV1); probably interact in the pathogenesis of human IR in this context. The involvement of CAV1 in glucose/lipid homeostasis is revealed and could modify the signaling of the insulin receptor. We examined the association between CAV1 and insulin signaling in modifying dyslipidemia and fat composition in overweight and obese women with a prevalent variant in the CAV1 gene.

RESULTS

Minor allele carriers were slightly older and had higher BMI (p = 0.02), FMI (p = 0.006), and VLF (p = 0.01) values; and tended to have lower total cholesterol TC (p = 0.04), low-density lipoprotein cholesterol (LDL-C) (p = 0.001) and high-density lipoprotein cholesterol (HDL-C) (p = 0.003). HOMA-IR levels predicted fat mass index (FMI) 0.47 (0.08, 0.87), visceral fat level (VFL) 0.65 (0.23, 1.07), TC 6.82 (1.76, 11.88) and HDL-C - 1.663 (- 3.11, - 0.214) only between minor allele carriers in adjusted models. (β, CI). Our results cast a new light on the IR mechanism and future studies will elucidate the clinical relevance of CAV1-IR in patients with dyslipidemia and high fat composition.

Downregulation of Caveolin-1 and Upregulation of Deiodinase 3, Associated with Hypoxia-Inducible Factor-1α Increase, Are Involved in the Oxidative Stress of Graves' Orbital Adipocytes

Background: Even though the clinical features of Graves' orbitopathy (GO) are well known, its exact pathogenesis remains controversial. The imbalance of redox homeostasis in the connective tissue could play a crucial role leading to an inflammatory state and edema of soft orbital tissues, thus contributing to orbital hypoxia and increase in hypoxia-inducible factor (HIF)-1α. This oxidative stress appears to target the orbital cells such as fibroblasts and also adipocytes. This study aims to explore which pathways can lead to the aforementioned oxidative stress in GO adipose cells and therefore offers new plausible therapeutic targets. Methods: Orbital fat samples were obtained from patients with GO (Western blot [WB]: n = 8, immunohistochemistry [IHC]: n = 8) and from control patients (WB: n = 5, IHC: n = 3-5). They were processed for WB analysis and IHC of the antioxidants (catalase, superoxide dismutase 1) and for HIF-1α. The expression of caveolin-1 (Cav-1) and deiodinase 3 (DIO3), known to be regulated by HIF-1α, was also analyzed by WB and IHC, as well as the targets of Cav-1: glucose transporter type 4 (Glut-4), NADPH oxidase (NOX)-2, and endothelial nitric oxide synthase (eNOS). Triiodothyronine (T3) expression was also analyzed by IHC. Results: In GO adipocytes, the expression of catalase was reduced, whereas that of HIF-1α was strongly increased. A decreased local T3 supply was associated with DIO3 upregulation. The low expression of Cav-1 in GO adipocytes was associated not only with low expression of Glut-4 but also with an increased expression of NOX-2 and active eNOS phosphorylated on serine 1177. Conclusions: Cav-1 and DIO3, both sensitive to hypoxia and to the increase of HIF-1α, play a pivotal role in the oxidative stress in GO adipocytes. DIO3 regulates the cellular supply of T3, which is essential for the cell homeostasis. Cav-1 determines the cellular glucose supply through Glut-4 and regulates the activity of NOX-2 generating superoxide anions and that of eNOS generating nitric oxide (NO).

Endothelial Nox5 Expression Modulates Glucose Uptake and Lipid Accumulation in Mice Fed a High-Fat Diet and 3T3-L1 Adipocytes Treated with Glucose and Palmitic Acid

Obesity is a global health issue associated with insulin resistance and altered lipid homeostasis. It has been described that reactive oxygen species (ROS) derived from nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) activity are involved in the development of these pathologies. The present study describes the role of endothelial NOX5 expression over adipose tissue by using two experimental systems: NOX5 conditional knock-in mice fed with a high-fat diet and 3T3-L1 adipocytes cultured with conditioned media of NOX5-expressing endothelial cells previously treated with glucose and palmitic acid. Animals expressing NOX5 presented lower body weight gain and less mesenteric and epididymal adipose mass compared to control mice fed with the same diet. NOX5-expressing mice also showed significantly lower glycaemia and improved insulin-induced glucose uptake. In addition, Glut4 and Caveolin 1 (Cav1) expression were significantly increased in the adipose tissue of these animals. Likewise, 3T3-L1 adipocytes treated with conditioned media from NOX5-expressing endothelial cells, incubated with high glucose and palmitic acid, presented a reduction in lipid accumulation and an increase in glucose uptake. Moreover, a significant increase in the expression of Glut4 and Cav1 was also detected in these cells. Taken together, all these data support that, in response to a highly caloric diet, NOX5 endothelial activity may regulate glucose sensitivity and lipid homeostasis in the adipose tissue.

Caveolin-1 Is Essential for the Improvement of Insulin Sensitivity through AKT Activation during Glargine Treatment on Diabetic Mice

Insulin treatment was confirmed to reduce insulin resistance, but the underlying mechanism remains unknown. Caveolin-1 (Cav-1) is a functional protein of the membrane lipid rafts, known as caveolae, and is widely expressed in mammalian adipose tissue. There is increasing evidence that show the involvement of Cav-1 in the AKT activation, which is responsible for insulin sensitivity. Our aim was to investigate the effect of Cav-1 depletion on insulin sensitivity and AKT activation in glargine-treated type 2 diabetic mice. Mice were exposed to a high-fat diet and subject to intraperitoneal injection of streptozotocin to induce diabetes. Next, glargine was administered to treat T2DM mice for 3 weeks (insulin group). The expression of Cav-1 was then silenced by injecting lentiviral-vectored short hairpin RNA (shRNA) through the tail vein of glargine-treated T2DM mice (CAV1-shRNA group), while scramble virus injection was used as a negative control (Ctrl-shRNA group). The results showed that glargine was able to upregulate the expression of PI3K and activate serine phosphorylation of AKT through the upregulation of Cav-1 expression in paraepididymal adipose tissue of the insulin group. However, glargine treatment could not activate AKT pathway in Cav-1 silenced diabetic mice. These results suggest that Cav-1 is essential for the activation of AKT and improving insulin sensitivity in type 2 diabetic mice during glargine treatment.

Lifestyle intervention in individuals with impaired glucose regulation affects Caveolin-1 expression and DNA methylation

Aims: We investigated whether a lifestyle intervention could influence expression and DNA methylation of diabetes-related genes in patients with impaired glucose regulation (IGR), the results were compared to bariatric surgery, considering it an intensive change. Methods: Twenty participants with IGR had adipose tissue biopsy and blood collected pre- and post-lifestyle (6 months) intervention; 12 obese patients had subcutaneous fat taken before and after bariatric surgery. RNA/DNA was extracted from all samples and underwent qPCR. DNA was bisulphite converted and 12 CpG sites of Caveolin-1 (CAV1) promoter were pyrosequenced. Results: lifestyle intervention resulted in opposite direction changes in fat tissue and blood for CAV1 expression and DNA methylation and these changes were correlated between tissues, while no significative differences were found in CAV1 expression after bariatric surgery. Conclusions: Our findings suggest a role for CAV1 in modulating adipocyte function as a consequence of lifestyle changes, as exercises and diet. These results may provide insights into new therapeutic targets for diabetes prevention.

Caveolin-1 stabilizes ATP7A, a copper transporter for extracellular SOD, in vascular tissue to maintain endothelial function

Caveolin-1 (Cav-1) is a scaffolding protein and a major component of caveolae/lipid rafts. Previous reports have shown that endothelial dysfunction in Cav-1-deficient (Cav-1-/-) mice is mediated by elevated oxidative stress through endothelial nitric oxide synthase (eNOS) uncoupling and increased NADPH oxidase. Oxidant stress is the net balance of oxidant generation and scavenging, and the role of Cav-1 as a regulator of antioxidant enzymes in vascular tissue is poorly understood. Extracellular SOD (SOD3) is a copper (Cu)-containing enzyme that is secreted from vascular smooth muscle cells/fibroblasts and subsequently binds to the endothelial cells surface, where it scavenges extracellular [Formula: see text] and preserves endothelial function. SOD3 activity is dependent on Cu, supplied by the Cu transporter ATP7A, but whether Cav-1 regulates the ATP7A-SOD3 axis and its role in oxidative stress-mediated vascular dysfunction has not been studied. Here we show that the activity of SOD3, but not SOD1, was significantly decreased in Cav-1-/- vessels, which was rescued by re-expression of Cav-1 or Cu supplementation. Loss of Cav-1 reduced ATP7A protein, but not mRNA, and this was mediated by ubiquitination of ATP7A and proteasomal degradation. ATP7A bound to Cav-1 and was colocalized with SOD3 in caveolae/lipid rafts or perinucleus in vascular tissues or cells. Impaired endothelium-dependent vasorelaxation in Cav-1-/- mice was rescued by gene transfer of SOD3 or by ATP7A-overexpressing transgenic mice. These data reveal an unexpected role of Cav-1 in stabilizing ATP7A protein expression by preventing its ubiquitination and proteasomal degradation, thereby increasing SOD3 activity, which in turn protects against vascular oxidative stress-mediated endothelial dysfunction.

Caveolae as Potential Hijackable Gates in Cell Communication

Caveolae are membrane microdomains described in many cell types involved in endocytocis, transcytosis, cell signaling, mechanotransduction, and aging. They are found at the interface with the extracellular environment and are structured by caveolin and cavin proteins. Caveolae and caveolins mediate transduction of chemical messages via signaling pathways, as well as non-chemical messages, such as stretching or shear stress. Various pathogens or signals can hijack these gates, leading to infectious, oncogenic and even caveolin-related diseases named caveolinopathies. By contrast, preclinical and clinical research have fallen behind in their attempts to hijack caveolae and caveolins for therapeutic purposes. Caveolae involvement in human disease is not yet fully explored or understood and, of all their scaffold proteins, only caveolin-1 is being considered in clinical trials as a possible biomarker of disease. This review briefly summarizes current knowledge about caveolae cell signaling and raises the hypothesis whether these microdomains could serve as hijackable “gatekeepers” or “gateways” in cell communication. Furthermore, because cell signaling is one of the most dynamic domains in translating data from basic to clinical research, we pay special attention to translation of caveolae, caveolin, and cavin research into clinical practice.

Effect of Electro-acupuncture on Expression of IRS-1/PI3K/GLUT4 Pathway in Ovarian Granulosa Cells of Infertile Patients with Polycystic Ovary Syndrome-Insulin Resistance of Phlegm-Dampness Syndrome

OBJECTIVE

To evaluate the effect of electro-acupuncture (EA) in infertile patients with phlegm-dampness polycystic ovary syndrome-insulin resistance (PCOS-IR).

METHODS

Seventy-six PCOS-IR patients who underwnet in vitro fertilization and embryo transfer (IVF-ET) were equally assigned to two groups according to a random digital table: the EA group and the control group, with 38 cases in each group. Before undergoing IVF, the two groups were treated with EA or pseudo-acupuncture, respectively, for 3 menstrual cycles. The intervention was 25 min twice a week until the day of oocyte collection. The selected acupoints were Zhongwan (RN 12), Tianshu (ST 25), Daheng (SP 15), Daimai (GB 26), Qihai (CV 6), Guanyuan (CV 4), and bilateral points including Xuehai (SP 10), Fenglong (ST 40), Zusanli (ST 36), and Yinlingquan (SP 9). Evaluation of phlegm-dampness syndrome score and IR score were carried out before and after treatment. Additionally, the number of oocytes retrieved, transplantable embryo rate, high-quality embryo rate, clinical pregnancy rate and live birth rate were compared between the two groups. Real-time polymerase chain reaction analysis was used to monitor the mRNA expression of the insulin receptor substrate (IRS-1), phosphatidylinositiol 3-kinase (PI3K) and glucose transport factor 4 (GLUT4) in ovarian granulosa cells.

RESULTS

EA treatment reduced the phlegm-dampness syndrome score as well as the IR scores compared with the control group (P<0.05). No significant differences in the number of oocytes retrieved and clinical pregnancy rate between the two groups (P>0.05). Moreover, the transplantable embryo rate [49.0% (284/580) vs. 41.9% (273/652)], high-quality embryo rate [36.6% (104/284) vs. 27.8% (76/273)], and live birth rate [50% (19/38) vs. 26.3% (10/38)] in the EA group were significantly higher than in the control group (P<0.05). Gene expression analyses revealed significantly elevated IRS-1, PI3K and GLUT4 mRNA in ovarian granulosa cells of the EA group compared with the control group (P<0.05).

CONCLUSIONS

EA may ameliorate the effects of phlegm-dampness syndrome and ovarian IR in PCOS-IR patients. Mechanistically, this effect might be through an upregulation of the IRS-1/PI3K/GLUT4 signaling pathway, which may result in improved oocyte quality and embryonic development potential. (Registration No. ChiCTR1800015453).

Caveolin-3 deficiency associated with the dystrophy P104L mutation impairs skeletal muscle mitochondrial form and function

BACKGROUND

Caveolin-3 (Cav3) is the principal structural component of caveolae in skeletal muscle. Dominant pathogenic mutations in the Cav3 gene, such as the Limb Girdle Muscular Dystrophy-1C (LGMD1C) P104L mutation, result in substantial loss of Cav3 and myopathic changes characterized by muscle weakness and wasting. We hypothesize such myopathy may also be associated with disturbances in mitochondrial biology. Herein, we report studies assessing the effects of Cav3 deficiency on mitochondrial form and function in skeletal muscle cells.

METHODS

L6 myoblasts were stably transfected with Cav3^P104L or expression of native Cav3 repressed by shRNA or CRISPR/Cas9 genome editing prior to performing fixed/live cell imaging of mitochondrial morphology, subcellular fractionation and immunoblotting, or analysis of real time mitochondrial respiration. Skeletal muscle from wild-type and Cav3^-/- mice was processed for analysis of mitochondrial proteins by immunoblotting.

RESULTS

Caveolin-3 was detected in mitochondrial-enriched membranes isolated from mouse gastrocnemius muscle and L6 myoblasts. Expression of Cav3^P104L in L6 myoblasts led to its targeting to the Golgi and loss of native Cav3 (>95%), including that associated with mitochondrial membranes. Cav3^P104L reduced mitochondrial mass and induced fragmentation of the mitochondrial network that was associated with significant loss of proteins involved in mitochondrial biogenesis, respiration, morphology, and redox function [i.e. PGC1α, succinate dehyrdogenase (SDHA), ANT1, MFN2, OPA1, and MnSOD). Furthermore, Cav3^P104L myoblasts exhibited increased mitochondrial cholesterol and loss of cardiolipin. Consistent with these changes, Cav3^P104L expression reduced mitochondrial respiratory capacity and increased myocellular superoxide production. These morphological, biochemical, and functional mitochondrial changes were phenocopied in myoblasts in which Cav3 had been silenced/knocked-out using shRNA or CRISPR. Reduced mitochondrial mass, PGC1α, SDHA, ANT1, and MnSOD were also demonstrable in Cav3^-/- mouse gastrocnemius. Strikingly, Cav3 re-expression in Cav3KO myoblasts restored its mitochondrial association and facilitated reformation of a tubular mitochondrial network. Significantly, re-expression also mitigated changes in mitochondrial superoxide, cholesterol, and cardiolipin content and recovered cellular respiratory capacity.

CONCLUSIONS

Our results identify Cav3 as an important regulator of mitochondrial homeostasis and reveal that Cav3 deficiency in muscle cells associated with the Cav3^P104L mutation invokes major disturbances in mitochondrial respiration and energy status that may contribute to the pathology of LGMD1C.

Role of Caveolin-1 in Diabetes and Its Complications

It is estimated that in 2017 there were 451 million people with diabetes worldwide. These figures are expected to increase to 693 million by 2045; thus, innovative preventative programs and treatments are a necessity to fight this escalating pandemic disorder. Caveolin-1 (CAV1), an integral membrane protein, is the principal component of caveolae in membranes and is involved in multiple cellular functions such as endocytosis, cholesterol homeostasis, signal transduction, and mechanoprotection. Previous studies demonstrated that CAV1 is critical for insulin receptor-mediated signaling, insulin secretion, and potentially the development of insulin resistance. Here, we summarize the recent progress on the role of CAV1 in diabetes and diabetic complications.

Aquaglyceroporins Are Differentially Expressed in Beige and White Adipocytes

Browning of white adipocytes has been proposed as a powerful strategy to overcome metabolic complications, since brown adipocytes are more catabolic, expending energy as a heat form. However, the biological pathways involved in the browning process are still unclear. Aquaglyceroporins are a sub-class of aquaporin water channels that also permeate glycerol and are involved in body energy homeostasis. In the adipose tissue, aquaporin-7 (AQP7) is the most representative isoform, being crucial for white adipocyte fully differentiation and glycerol metabolism. The altered expression of AQP7 is involved in the onset of obesity and metabolic disorders. Herein, we investigated if aquaglyceroporins are implicated in beige adipocyte differentiation, similar to white cells. Thus, we optimized a protocol of murine 3T3-L1 preadipocytes browning that displayed increased beige and decreased white adipose tissue features at both gene and protein levels and evaluated aquaporin expression patterns along the differentiation process together with cellular lipid content. Our results revealed that AQP7 and aquaporin-9 (AQP9) expression was downregulated throughout beige adipocyte differentiation compared to white differentiation, which may be related to the beige physiological role of heat production from oxidative metabolism, contrasting with the anabolic/catabolic lipid metabolism requiring glycerol gateways occurring in white adipose cells.

Metabolic regulation through the endosomal system

The endosomal system plays an essential role in cell homeostasis by controlling cellular signaling, nutrient sensing, cell polarity and cell migration. However, its place in the regulation of tissue, organ and whole body physiology is less well understood. Recent studies have revealed an important role for the endosomal system in regulating glucose and lipid homeostasis, with implications for metabolic disorders such as type 2 diabetes, hypercholesterolemia and non-alcoholic fatty liver disease. By taking insights from in vitro studies of endocytosis and exploring their effects on metabolism, we can begin to connect the fields of endosomal transport and metabolic homeostasis. In this review, we explore current understanding of how the endosomal system influences the systemic regulation of glucose and lipid metabolism in mice and humans. We highlight exciting new insights that help translate findings from single cells to a wider physiological level and open up new directions for endosomal research.

Caveolin-1 Variant Is Associated With the Metabolic Syndrome in Kuwaiti Children

Caveolin-1 (CAV1) variants have been suggested to be associated with obesity and related metabolic disorders, but information based on human studies is limited. In the present study, we aimed to investigate the potential association between the CAV1 rs1997623 C/A variant and metabolic syndrome (MetS) in Kuwaiti children. DNA from saliva samples collected from 1313 Kuwaiti children (mean age: 12 years) were genotyped using the TaqMan SNP genotyping assay. The classification of MetS was based on the presence/absence of four indicators; (1) central obesity, (2) elevated systolic or diastolic blood pressure, (3) low salivary high-density lipoprotein cholesterol (HDLC), and (4) high salivary glucose. In this study, children with MetS scored ≥3, children in the intermediate metabolic group scored 1 or 2 and children without MetS scored 0. About one-third of the children were obese. A total of 246 children (18.7%) were classified as having MetS; 834 children (63.5%) were in the intermediate metabolic group, and 233 children (17.7%) had no indication of MetS. Obesity was highly prevalent in the MetS group (91.9%) while 26.8% of children were obese in the intermediate metabolic group. None of the children were obese in the group without MetS. Analysis of the CAV1 rs1997623 variant revealed a significant association of the A-allele (p = 0.01, Odds Ratio (OR) = 1.66) and the heterozygous CA-genotype (p = 0.005, OR = 1.88) with MetS. Consistently, the A-allele (p = 0.002, OR = 1.71) and CA-genotype (p = 0.005, OR = 1.70) also showed significant association with the intermediate metabolic group. Furthermore, the A-allele (p = 0.01, OR = 1.33) and the CA-genotype (p = 0.008, OR = 1.55) were associated with low levels of saliva HDLC. Individuals who were heterozygous or homozygous for the variant (CA/AA) showed significantly lower levels of high HDLC compared to those harboring the CC-genotype (p = 0.023). Our study revealed a novel association of the CAV1 rs1997623 variant with the MetS and with low saliva HDLC levels in young Kuwaiti children and indicated the need for further in-depth studies to unravel the role of CAV1 gene in the genetic etiology of MetS.

Caveolar targeting links Kv1.3 with the insulin-dependent adipocyte physiology

The voltage-dependent potassium channel Kv1.3 participates in peripheral insulin sensitivity. Genetic ablation of Kv1.3 triggers resistance to diet-induced weight gain, thereby pointing to this protein as a pharmacological target for obesity and associated type II diabetes. However, this role is under intense debate because Kv1.3 expression in adipose tissue raises controversy. We demonstrated that Kv1.3 is expressed in white adipose tissue from humans and rodents. Moreover, other channels, such as Kv1.1, Kv1.2, Kv1.4 and especially Kv1.5, from the same Shaker family are also present. Although elevated insulin levels and adipogenesis remodel the Kv phenotype, which could lead to multiple heteromeric complexes, Kv1.3 markedly participates in the insulin-dependent regulation of glucose uptake in mature adipocytes. Adipocyte differentiation increased the expression of Kv1.3, which is targeted to caveolae by molecular interactions with caveolin 1. Using a caveolin 1-deficient 3T3-L1 adipocyte cell line, we demonstrated that the localization of Kv1.3 in caveolar raft structures is important for proper insulin signaling. Insulin-dependent phosphorylation of the channel occurs at the onset of insulin-mediated signaling. However, when Kv1.3 was spatially outside of these lipid microdomains, impaired phosphorylation was exhibited. Our data shed light on the putative role of Kv1.3 in weight gain and insulin-dependent responses contributing to knowledge about adipocyte physiology.

The phosphodiesterase 5 inhibitor tadalafil regulates lipidic homeostasis in human skeletal muscle cell metabolism

PURPOSE

Tadalafil seems to ameliorate insulin resistance and glucose homeostasis in humans. We have previously reported that tadalafil targets human skeletal muscle cells with an insulin (I)-like effect. We aim to evaluate in human fetal skeletal muscle cells after tadalafil or I: (i) expression profile of I-regulated genes dedicated to cellular energy control, glycolitic activity or microtubule formation/vesicle transport, as GLUT4, PPARγ, HK2, IRS-1, KIF1C, and KIFAP3; (ii) GLUT4, Flotillin-1, and Caveolin-1 localization, all proteins involved in energy-dependent cell trafficking; (iii) activation of I-targeted paths, as IRS-1, PKB/AKT, mTOR, P70/S6K. Free fatty acids intracellular level was measured. Sildenafil or a cGMP synthetic analog were used for comparison; PDE5 and PDE11 gene expression was evaluated in human fetal skeletal muscle cells.

METHODS

RTq-PCR, PCR, western blot, free fatty acid assay commercial kit, and lipid stain non-fluorescent assay were used.

RESULTS

Tadalafil upregulated I-targeted investigated genes with the same temporal pattern as I (GLUT4, PPARγ, and IRS-1 at 3 h; HK2, KIF1C, KIFAP3 at 12 h), re-localized GLUT4 in cell sites positively immune-decorated for Caveolin-1 and Flotillin-1, suggesting the involvement of lipid rafts, induced specific residue phosphorylation of IRS-1/AKT/mTOR complex in association with free fatty acid de novo synthesis. Sildenafil or GMP analog did not affect GLUT4 trafficking or free fatty acid levels.

CONCLUSION

In human fetal skeletal muscle cells tadalafil likely favors energy storage by modulating lipid homeostasis via IRS-1-mediated mechanisms, involving activation of I-targeted genes and intracellular cascade related to metabolic control. Those data provide some biomolecular evidences explaining, in part, tadalafil-induced favorable control of human metabolism shown by clinical studies.

Causes and mechanisms of adipocyte enlargement and adipose expansion

Adipose tissue plays a significant role in whole body energy homeostasis. Obesity-associated diabetes, fatty liver and metabolic syndrome are closely linked to adipose stress and dysfunction. Genetic predisposition, overeating and physical inactivity influence the expansion of adipose tissues. Under conditions of constant energy surplus, adipocytes become hypertrophic and adipose tissues undergo hyperplasia so as to increase their lipid storage capacity, thereby keeping circulating blood glucose and fatty acids below toxic levels. Nonetheless, adipocytes have a saturation point where they lose capacity to store more lipids. At this stage, when adipocytes are fully lipid-engorged, they express stress signals. Adipose depots (particularly visceral compartments) from obese individuals with a severe metabolic phenotype are characterized by the high proportion of hypertrophic adipocytes. This review focuses on the mechanisms of adipocyte enlargement in relation to adipose fatty acid and cholesterol metabolism, and considers how this may be related to adipose dysfunction.

Frequency of common polymorphisms in ( ) gene in adults with high serum triglycerides from Colombian Caribbean Coast

Background

Caveolin 1 gene (CAV1) has been associated with insulin resistance, metabolic syndrome and hypertension in humans. Also, it has been related to high serum triglycerides in rodents, however there is little evidence of this relation in humans.

Aim

To describe frequencies of common variations in CAV1 in adults with high serum triglycerides.

Methods

A case-control study was carried out with adults from Colombian Caribbean Coast. A whole blood sample was employed to measure serum concentrations of triglycerides, glucose, total cholesterol and HDLc. Six common Single Nucleotide Polymorphism (SNP) in CAV1 were genotyped (rs926198, rs3779512, rs10270569, rs11773845, rs7804372 and rs1049337). Allelic and genotypic frequencies were determined by direct count and Hardy-Weinberg Equilibrium (HWE) was assessed. Case and control groups were compared with null-hypothesis tests.

Results

A total of 220 cases and 220 controls were included. For rs3779512 an excess in homozygotes frequency was found within case group (40.4% (GG), 41.3% (GT) and 18.1% (TT); F_is=0.13, p=0.03). Another homozygotes excess among case group was found in rs7804372 (59.5% (TT), 32.3% (TA) and 8.2% (AA); F_is= 0.12, p= 0.04). In rs1049337, cases also showed an excess in homozygotes frequency (52.7% (CC), 35.0% (CT) and 12.3% (TT); F_is= 0.16, p= 0.01). Finally, for rs1049337 there were differences in genotype distribution between case and control groups (p <0.05).

Conclusion

An increased frequency of homozygote genotypes was found in subjects with high serum triglycerides. These findings suggest that minor alleles for SNPs rs3779512, rs7804372 and rs1049337 might be associated to higher risk of hypertriglyceridemia.

Connecting Alzheimer's disease to diabetes: Underlying mechanisms and potential therapeutic targets

Alzheimer's disease (AD) is a risk factor for type 2 diabetes and vice versa, and a growing body of evidence indicates that these diseases are connected both at epidemiological, clinical and molecular levels. Recent studies have begun to reveal common pathogenic mechanisms shared by AD and type 2 diabetes. Impaired neuronal insulin signaling and endoplasmic reticulum (ER) stress are present in animal models of AD, similar to observations in peripheral tissue in T2D. These findings shed light into novel diabetes-related mechanisms leading to brain dysfunction in AD. Here, we review the literature on selected mechanisms shared between these diseases and discuss how the identification of such mechanisms may lead to novel therapeutic targets in AD. This article is part of the Special Issue entitled 'Metabolic Impairment as Risk Factors for Neurodegenerative Disorders.'

Cavin-2 is a specific marker for detection of well-differentiated liposarcoma

Caveolae are cholesterol enriched invaginations of the plasma membrane involved in a variety of processes, including glucose and fatty acids absorption, cell transduction and mechanoprotection. The biogenesis and function of caveolae depend on the activity of Caveolin (Cav-1, -2 and -3) and Cavin (Cavin-1, -2, -3 and -4) protein families. Since the membrane Cavin-2 protein was reported to play a key role in caveolae formation of adipocytes, in this work we have used a multidisciplinary approach to investigate its expression in liposarcoma (LPS), an adipocytic soft tissue sarcoma affecting adults. Data obtained through an in silico and immunohistochemical analysis suggest that Cavin-2, along with Cavin-1, Cav-1 and Cav-2, is mostly expressed in the least aggressive LPS subtype, namely well-differentiated LPS, while is almost undetectable in the more aggressive myxoid, pleomorphic and dedifferentiated LPS tumors. Accordingly, in vitro analysis confirmed that Cavin-2 expression increases in LPS tumor cell lines during differentiation as compared to proliferation, as detected by immunoblotting and immunofluorescence analysis. Overall, these data suggest that Cavin-2 represents a useful marker for discriminating the degree of differentiation in LPS tumors.

Changes in the plasma membrane in metabolic disease: impact of the membrane environment on G protein-coupled receptor structure and function

Drug development targeting GPCRs often utilizes model heterologous cell expression systems, reflecting an implicit assumption that the membrane environment has little functional impact on these receptors or on their responsiveness to drugs. However, much recent data have illustrated that membrane components can have an important functional impact on intrinsic membrane proteins. This review is directed toward gaining a better understanding of the structure of the plasma membrane in health and disease, and how this organelle can influence GPCR structure, function and regulation. It is important to recognize that the membrane provides a potential mode of lateral allosteric regulation of GPCRs and can affect the effectiveness of drugs and their biological responses in various disease states, which can even vary among individuals across the population. The type 1 cholecystokinin receptor is reviewed as an exemplar of a class A GPCR that is affected in this way by changes in the plasma membrane.

LINKED ARTICLES

This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

Comprehensive functional screening of miRNAs involved in fat cell insulin sensitivity among women

The key pathological link between obesity and type 2 diabetes is insulin resistance, but the molecular mechanisms are not entirely identified. micro-RNAs (miRNA) are dysregulated in obesity and may contribute to insulin resistance. Our objective was to detect and functionally investigate miRNAs linked to insulin sensitivity in human subcutaneous white adipose tissue (scWAT). Subjects were selected based on the insulin-stimulated lipogenesis response of subcutaneous adipocytes. Global miRNA profiling was performed in abdominal scWAT of 18 obese insulin-resistance (OIR), 21 obese insulin-sensitive (OIS), and 9 lean women. miRNAs demonstrating differential expression between OIR and OIS women were overexpressed in human in vitro-differentiated adipocytes followed by assessment of lipogenesis and identification of miRNA targets by measuring mRNA/protein expression and 3'-untranslated region analysis. Eleven miRNAs displayed differential expression between OIR and OIS states. Overexpression of miR-143-3p and miR-652-3p increased insulin-stimulated lipogenesis in human in vitro differentiated adipocytes and directly or indirectly affected several genes/proteins involved in insulin signaling at transcriptional or posttranscriptional levels. Adipose expression of miR-143-3p and miR-652-3p was positively associated with insulin-stimulated lipogenesis in scWAT independent of body mass index. In conclusion, miR-143-3p and miR-652-3p are linked to scWAT insulin resistance independent of obesity and influence insulin-stimulated lipogenesis by interacting at different steps with insulin-signaling pathways.

Potential role for the VDR agonist elocalcitol in metabolic control: Evidences in human skeletal muscle cells

Vitamin D plays a pivotal role to maintain skeletal muscle integrity and health. Vitamin D deficiency characterizes inflammatory myopathy (IM) and diabetes, often overlapping diseases involving skeletal muscle damage. Vitamin D receptor (VDR) agonists likely exert beneficial effects in both IM and metabolic disturbances. We aim to evaluate in vitro the effect of elocalcitol, a non-hypercalcemic VDR agonist, on the biomolecular metabolic machinery of human skeletal muscle cells (Hfsmc), vs. insulin (I). We analyzed GLUT4, Flotillin-1, Caveolin-3 and Caveolin-1 cell expression/localization; mTOR, AKT, ERK and 4E-BP1 phosphorylation; IL-6 myokine release; VDR expression. We investigated in vivo vitamin D status in IM subjects, evaluating VDR muscular expression and serum vitamin D with metabolism-related parameters, as glycemia, triglycerides, cholesterol, resistin and adiponectin. In Hfsmc, elocalcitol exerted an I-like effect, promoting GLUT4 re-localization in Flotillin-1, Caveolin-3 and Caveolin-1 positive sites and mTOR, AKT, ERK, 4E-BP1 activation; it enhanced IL-6 myokine release. IM subjects, all normoglycemic, showed VDR/vitamin D deficiency that, together with high lipidemic and resistin profile, possibly increases the risk to develop metabolic diseases. VDR agonists as elocalcitol may be therapeutic tools for skeletal muscle integrity/function maintenance, an indispensable condition for health homeostasis.

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.

Versatile Functions of Caveolin-1 in Aging-related Diseases

Caveolin-1 (Cav-1) is a trans-membrane protein that is a major component of the caveolae structure on the plasma membrane. Cav-1 is involved in the regulation of various cellular processes, including cell growth, differentiation, endocytosis, and in particular it has been implied in cellular senescence. Here we review current knowledge about Cav-1 in cellular signaling and discuss the role of Cav-1 in aging-related diseases.

Disruption of Lipid Raft Function Increases Expression and Secretion of Monocyte Chemoattractant Protein-1 in 3T3-L1 Adipocytes

The adipocyte is unique in its capacity to store lipids. In addition to triglycerides, the adipocyte stores a significant amount of cholesterol. Moreover, obese adipocytes are characterized by a redistribution of cholesterol with depleted cholesterol in the plasma membrane, suggesting that cholesterol perturbation may play a role in adipocyte dysfunction. We used methyl-β-cyclodextrin (MβCD), a molecule with high affinity for cholesterol, to rapidly deplete cholesterol level in differentiated 3T3-L1 adipocytes. We tested whether this perturbation altered adipocyte secretion of monocyte chemoattractant protein-1 (MCP-1), a chemokine that is elevated in obesity and is linked to obesity-associated chronic diseases. Depletion of cholesterol by MβCD increased MCP-1 secretion as well as the mRNA and protein levels, suggesting perturbation at biosynthesis and secretion. Pharmacological inhibition revealed that NF-κB, but not MEK, p38 and JNK, was involved in MβCD-stimulated MCP-1 biosynthesis and secretion in adipocytes. Finally, another cholesterol-binding drug, filipin, also induced MCP-1 secretion without altering membrane cholesterol level. Interestingly, both MβCD and filipin disturbed the integrity of lipid rafts, the membrane microdomains enriched in cholesterol. Thus, the depletion of membrane cholesterol in obese adipocytes may result in dysfunction of lipid rafts, leading to the elevation of proinflammatory signaling and MCP-1 secretion in adipocytes.

Caveolin 1 Modulates Aldosterone-Mediated Pathways of Glucose and Lipid Homeostasis

Background

Overactivation of the aldosterone and mineralocorticoid receptor (MR) pathway is associated with hyperglycemia and dyslipidemia. Caveolin 1 (cav‐1) is involved in glucose/lipid homeostasis and may modulate MR signaling. We investigated the interplay between cav‐1 and aldosterone signaling in modulating insulin resistance and dyslipidemia in cav‐1–null mice and humans with a prevalent variant in the CAV1 gene.

Methods and Results

In mouse studies, cav‐1 knockout mice exhibited higher levels of homeostatic model assessment of insulin resistance, cholesterol, and resistin and lower ratios of high‐ to low‐density lipoprotein (all P<0.001 versus wild type). Moreover, cav‐1 knockout mice displayed hypertriglyceridemia and higher mRNA levels for resistin, retinol binding protein 4, NADPH oxidase 4, and aldose reductase in liver and/or fat tissues. MR blockade with eplerenone significantly decreased glycemia (P<0.01), total cholesterol (P<0.05), resistin (P<0.05), and described enzymes, with no effect on insulin or triglycerides. In the human study, we analyzed the CAV1 gene polymorphism rs926198 in 556 white participants; 58% were minor allele carriers and displayed higher odds of insulin resistance (odds ratio 2.26 [95% CI 1.40–3.64]) and low high‐density lipoprotein (odds ratio 1.54 [95% CI 1.01–3.37]). Aldosterone levels correlated with higher homeostatic model assessment of insulin resistance and resistin and lower high‐density lipoprotein only in minor allele carriers. CAV1 gene expression quantitative trait loci data revealed lower cav‐1 expression in adipose tissues by the rs926198 minor allele.

Conclusions

Our findings in mice and humans suggested that decreased cav‐1 expression may activate the effect of aldosterone/MR signaling on several pathways of glycemia, dyslipidemia, and resistin. In contrast, hyperinsulinemia and hypertriglyceridemia are likely mediated by MR‐independent mechanisms. Future human studies will elucidate the clinical relevance of MR blockade in patients with genotype‐mediated cav‐1 deficiency.

Phosphatidic Acid Produced by RalA-activated PLD2 Stimulates Caveolae-mediated Endocytosis and Trafficking in Endothelial Cells

Caveolae are the primary route for internalization and transendothelial transport of macromolecules, such as insulin and albumin. Caveolae-mediated endocytosis is activated by Src-dependent caveolin-1 (Cav-1) phosphorylation and subsequent recruitment of dynamin-2 and filamin A (FilA), which facilitate vesicle fission and trafficking, respectively. Here, we tested the role of RalA and phospholipase D (PLD) signaling in the regulation of caveolae-mediated endocytosis and trafficking. The addition of albumin to human lung microvascular endothelial cells induced the activation of RalA within minutes, and siRNA-mediated down-regulation of RalA abolished fluorescent BSA uptake. Co-immunoprecipitation studies revealed that albumin induced the association between RalA, Cav-1, and FilA; however, RalA knockdown with siRNA did not affect FilA recruitment to Cav-1, suggesting that RalA was not required for FilA and Cav-1 complex formation. Rather, RalA probably facilitates caveolae-mediated endocytosis by activating downstream effectors. PLD2 was shown to be activated by RalA, and inhibition of PLD2 abolished Alexa-488-BSA uptake, indicating that phosphatidic acid (PA) generated by PLD2 may facilitate caveolae-mediated endocytosis. Furthermore, using a PA biosensor, GFP-PASS, we observed that BSA induced an increase in PA co-localization with Cav-1-RFP, which could be blocked by a dominant negative PLD2 mutant. Total internal reflection fluorescence microscopy studies of Cav-1-RFP also showed that fusion of caveolae with the basal plasma membrane was dependent on PLD2 activity. Thus, our results suggest that the small GTPase RalA plays an important role in promoting invagination and trafficking of caveolae, not by potentiating the association between Cav-1 and FilA but by stimulating PLD2-mediated generation of phosphatidic acid.

Increased caveolin-1 in intervertebral disc degeneration facilitates repair

Background

Preceding intervertebral disc (IVD) degeneration, the cell phenotype in the nucleus pulposus (NP) shifts from notochordal cells (NCs) to chondrocyte-like cells (CLCs). Microarray analysis showed a correlation between caveolin-1 expression and the phenotypic transition of NCs to CLCs. With a clinical directive in mind, the aim of this study was to determine the role of caveolin-1 in IVD degeneration. As a scaffolding protein, caveolin-1 influences several signaling pathways, and transforming growth factor (TGF)-β receptors have been demonstrated to colocalize with caveolin-1. Therefore, the hypothesis of this study was that caveolin-1 facilitates repair by enhancing TGF-β signaling in the IVD.

Methods

Protein expression (caveolin-1, apoptosis, progenitor cell markers, extracellular matrix, and phosphorylated Smad2 [pSmad2]) was determined in IVDs of wild-type (WT) and caveolin-1-null mice and canine IVDs of different degeneration grades (immunofluorescence, immunohistochemistry, and TUNEL assay). Canine/human CLC microaggregates were treated with chondrogenic medium alone or in combination with caveolin-1 scaffolding domain (CSD) peptide and/or caveolin-1 silencing RNA. After 28 days, gene and protein expression profiles were determined.

Results

The NP of WT mice was rich in viable NCs, whereas the NP of caveolin-1-null mice contained more collagen-rich extracellular matrix and fewer cells, together with increased progenitor cell marker expression, pSmad2 TGF-β signaling, and high apoptotic activity. During canine IVD degeneration, caveolin-1 expression and apoptotic activity increased. In vitro caveolin-1 silencing decreased the CLC microaggregate glycosaminoglycan (GAG) content, which could be rescued by CSD treatment. Furthermore, CSD increased TGF-β/pSmad2 signaling at gene and protein expression levels and enhanced the anabolic effects of TGF-β₁, reflected in increased extracellular matrix deposition by the CLCs.

Conclusions

Caveolin-1 plays a role in preservation of the NC phenotype. Additionally, it may be related to CLC apoptosis, given its increased expression in degenerated IVDs. Nevertheless, CSD enhanced CLC GAG deposition in vitro, and hence the increased caveolin-1 expression during IVD degeneration may also facilitate an ultimate attempt at repair. Further studies are needed to investigate how caveolin-1 modifies other signaling pathways and facilitates IVD repair.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-016-0960-y) contains supplementary material, which is available to authorized users.

Caveolin interaction governs Kv1.3 lipid raft targeting

The spatial localization of ion channels at the cell surface is crucial for their functional role. Many channels localize in lipid raft microdomains, which are enriched in cholesterol and sphingolipids. Caveolae, specific lipid rafts which concentrate caveolins, harbor signaling molecules and their targets becoming signaling platforms crucial in cell physiology. However, the molecular mechanisms involved in such spatial localization are under debate. Kv1.3 localizes in lipid rafts and participates in the immunological response. We sought to elucidate the mechanisms of Kv1.3 surface targeting, which govern leukocyte physiology. Kv1 channels share a putative caveolin-binding domain located at the intracellular N-terminal of the channel. This motif, lying close to the S1 transmembrane segment, is situated near the T1 tetramerization domain and the determinants involved in the Kvβ subunit association. The highly hydrophobic domain (FQRQVWLLF) interacts with caveolin 1 targeting Kv1.3 to caveolar rafts. However, subtle variations of this cluster, putative ancillary associations and different structural conformations can impair the caveolin recognition, thereby altering channel’s spatial localization. Our results identify a caveolin-binding domain in Kv1 channels and highlight the mechanisms that govern the regulation of channel surface localization during cellular processes.

Quasi-Steady-State Analysis based on Structural Modules and Timed Petri Net Predict System's Dynamics: The Life Cycle of the Insulin Receptor

The insulin-dependent activation and recycling of the insulin receptor play an essential role in the regulation of the energy metabolism, leading to a special interest for pharmaceutical applications. Thus, the recycling of the insulin receptor has been intensively investigated, experimentally as well as theoretically. We developed a time-resolved, discrete model to describe stochastic dynamics and study the approximation of non-linear dynamics in the context of timed Petri nets. Additionally, using a graph-theoretical approach, we analyzed the structure of the regulatory system and demonstrated the close interrelation of structural network properties with the kinetic behavior. The transition invariants decomposed the model into overlapping subnetworks of various sizes, which represent basic functional modules. Moreover, we computed the quasi-steady states of these subnetworks and demonstrated that they are fundamental to understand the dynamic behavior of the system. The Petri net approach confirms the experimental results of insulin-stimulated degradation of the insulin receptor, which represents a common feature of insulin-resistant, hyperinsulinaemic states.

Functional characterization of retromer in GLUT4 storage vesicle formation and adipocyte differentiation

Insulin-stimulated translocation of glucose transporter 4 (GLUT4) storage vesicles (GSVs), the specialized intracellular compartments within mature adipocytes, to the plasma membrane (PM) is a fundamental cellular process for maintaining glucose homeostasis. Using 2 independent adipocyte cell line models, human primary Simpson-Golabi-Behmel syndrome and mouse 3T3-L1 fibroblast cell lines, we demonstrate that the endosome-associated protein-sorting complex retromer colocalizes with GLUT4 on the GSVs by confocal microscopy in mature adipocytes. By use of both confocal microscopy and differential ultracentrifugation techniques, retromer is redistributed to the PM of mature adipocytes upon insulin stimulation. Furthermore, stable knockdown of the retromer subunit-vacuolar protein-sorting 35, or the retromer-associated protein sorting nexin 27, by lentivirus-delivered small hairpin RNA impaired the adipogenesis process when compared to nonsilence control. The knockdown of retromer decreased peroxisome proliferator activated receptor γ expression during differentiation, generating adipocytes with decreased levels of GSVs, lipid droplet accumulation, and insulin-stimulated glucose uptake. In conclusion, our study demonstrates a role for retromer in the GSV formation and adipogenesis.

Caveolins in cardioprotection - translatability and mechanisms

Translation of preclinical treatments for ischaemia-reperfusion injury into clinical therapies has been limited by a number of factors. This review will focus on a single mode of cardiac protection related to a membrane scaffolding protein, caveolin, which regulates protective signalling as well as myocyte ultrastructure in the setting of ischaemic stress. Factors that have limited the clinical translation of protection will be considered specifically in terms of signalling and structural defects. The potential of caveolin to overcome barriers to protection with the ultimate hope of clinical translation will be discussed.

Role of aquaglyceroporins and caveolins in energy and metabolic homeostasis

Aquaglyceroporins and caveolins are submicroscopic integral membrane proteins that are particularly abundant in many mammalian cells. Aquaglyceroporins (AQP3, AQP7, AQP9 and AQP10) encompass a subfamily of aquaporins that allow the movement of water, but also of small solutes, such as glycerol, across cell membranes. Glycerol constitutes an important metabolite as a substrate for de novo synthesis of triacylglycerols and glucose as well as an energy substrate to produce ATP via the mitochondrial oxidative phosphorylation. In this sense, the control of glycerol influx/efflux in metabolic organs by aquaglyceroporins plays a crucial role with the dysregulation of these glycerol channels being associated with metabolic diseases, such as obesity, insulin resistance, non-alcoholic fatty liver disease and cardiac hypertrophy. On the other hand, caveolae have emerged as relevant plasma membrane sensors implicated in a wide range of cellular functions, including endocytosis, apoptosis, cholesterol homeostasis, proliferation and signal transduction. Caveolae-coating proteins, namely caveolins and cavins, can act as scaffolding proteins within caveolae by concentrating signaling molecules involved in free fatty acid and cholesterol uptake, proliferation, insulin signaling or vasorelaxation, among others. The importance of caveolae in whole-body homeostasis is highlighted by the link between homozygous mutations in genes encoding caveolins and cavins with metabolic diseases, such as lipodystrophy, dyslipidemia, muscular dystrophy and insulin resistance in rodents and humans. The present review focuses on the role of aquaglyceroporins and caveolins on lipid and glucose metabolism, insulin secretion and signaling, energy production and cardiovascular homeostasis, outlining their potential relevance in the development and treatment of metabolic diseases.

Polymerase I and transcript release factor (PTRF) regulates adipocyte differentiation and determines adipose tissue expandability

Impaired adipogenesis renders an adipose tissue unable to expand, leading to lipotoxicity and conditions such as diabetes and cardiovascular disease. While factors important for adipogenesis have been studied extensively, those that set the limits of adipose tissue expansion remain undetermined. Feeding a Western-type diet to apolipoprotein E2 knock-in mice, a model of metabolic syndrome, produced 3 groups of equally obese mice: mice with normal glucose tolerance, hyperinsulinemic yet glucose-tolerant mice, and prediabetic mice with impaired glucose tolerance and reduced circulating insulin. Using proteomics, we compared subcutaneous adipose tissues from mice in these groups and found that the expression of PTRF (polymerase I and transcript release factor) associated selectively with their glucose tolerance status. Lentiviral and pharmacologically overexpressed PTRF, whose function is critical for caveola formation, compromised adipocyte differentiation of cultured 3T3-L1cells. In human adipose tissue, PTRF mRNA levels positively correlated with markers of lipolysis and cellular senescence. Furthermore, a negative relationship between telomere length and PTRF mRNA levels was observed in human subcutaneous fat. PTRF is associated with limited adipose tissue expansion underpinning the key role of caveolae in adipocyte regulation. Furthermore, PTRF may be a suitable adipocyte marker for predicting pathological obesity and inform clinical management.

The role of the Niemann-Pick disease, type C1 protein in adipocyte insulin action

The Niemann-Pick disease, type C1 (NPC1) gene encodes a transmembrane protein involved in cholesterol efflux from the lysosome. SNPs within NPC1 have been associated with obesity and type 2 diabetes, and mice heterozygous or null for NPC1 are insulin resistant. However, the molecular mechanism underpinning this association is currently undefined. This study aimed to investigate the effects of inhibiting NPC1 function on insulin action in adipocytes. Both pharmacological and genetic inhibition of NPC1 impaired insulin action. This impairment was evident at the level of insulin signalling and insulin-mediated glucose transport in the short term and decreased GLUT4 expression due to reduced liver X receptor (LXR) transcriptional activity in the long-term. These data show that cholesterol homeostasis through NPC1 plays a crucial role in maintaining insulin action at multiple levels in adipocytes.

Expression of Caveolin 1 is enhanced by DNA demethylation during adipocyte differentiation. status of insulin signaling

Caveolin 1 (Cav-1) is an essential constituent of adipocyte caveolae which binds the beta subunit of the insulin receptor (IR) and is implicated in the regulation of insulin signaling. We have found that, during adipocyte differentiation of 3T3-L1 cells the promoter, exon 1 and first intron of the Cav-1 gene undergo a demethylation process that is accompanied by a strong induction of Cav-1 expression, indicating that epigenetic mechanisms must have a pivotal role in this differentiation process. Furthermore, IR, PKB-Akt and Glut-4 expression are also increased during the differentiation process suggesting a coordinated regulation with Cav-1. Activation of Cav-1 protein by phosphorylation arises during the differentiation process, yet in fully mature adipocytes insulin is no longer able to significantly increase Cav-1 phosphorylation. However, these long-term differentiated cells are still able to respond adequately to insulin, increasing IR and PKB-Akt phosphorylation and glucose uptake. The activation of Cav-1 during the adipocyte differentiation process could facilitate the maintenance of insulin sensitivity by these fully mature adipocytes isolated from additional external stimuli. However, under the influence of physiological conditions associated to obesity, such as chronic inflammation and hypoxia, insulin sensitivity would finally be compromised.

An association of metabolic syndrome constellation with cellular membrane caveolae

Metabolic syndrome (MetS) is a cluster of metabolic abnormalities that can predispose an individual to a greater risk of developing type-2 diabetes and cardiovascular diseases. The cluster includes abdominal obesity, dyslipidemia, hypertension, and hyperglycemia - all of which are risk factors to public health. While searching for a link among the aforementioned malaises, clues have been focused on the cell membrane domain caveolae, wherein the MetS-associated active molecules are colocalized and interacted with to carry out designated biological activities. Caveola disarray could induce all of those individual metabolic abnormalities to be present in animal models and humans, providing a new target for therapeutic strategy in the management of MetS.