Glut-4 is translocated to both caveolae and non-caveolar lipid rafts, but is partially internalized through caveolae in insulin-stimulated adipocytes

ANGPT2/CAV1 regulates albumin transcytosis of glomerular endothelial cells under high glucose exposure and is impaired by losartan

BACKGROUND

Microalbuminuria is a common clinical symptom that manifests in the early stages of diabetic kidney disease (DKD) and is also the main feature of glomerular endothelial cells (GECs) injury. There is increasing evidence that the transcytosis of albumin across GECs is closely related to the formation of albuminuria. Our previous studies have shown that angiopoietin 2 (ANGPT2) can inhibit albumin transcytosis across renal tubular epithelial cells by activating caveolin 1 (CAV1) phosphorylation during high glucose (HG) exposure. The role of ANGPT2 in albumin transcytosis across GECs remains unclear. Losartan significantly reduces albuminuria, but the mechanism has not been clarified.

METHODS

We established an in vitro albumin transcytosis model to investigate the change in albumin transcytosis across human renal glomerular endothelial cells (hrGECs) under normal glucose (NG), high glucose (HG) and losartan intervention. We knocked down ANGPT2 and CAV1 to evaluate their roles in albumin transcytosis across hrGECs and verified the relationship between them. In vivo, DKD mouse models were established and treated with different doses of losartan. Immunohistochemistry and Western blot were used to detect the expression of ANGPT2 and CAV1.

RESULTS

In vitro, the transcytosis of albumin across hrGECs was significantly increased under high glucose stimulation, and losartan inhibited this process. The expression of ANGPT2 and CAV1 were both increased in hrGECs under HG conditions and losartan intervention reduced the expression of them. Moreover, ANGPT2 downregulation reduced albumin transcytosis in hrGECs by regulating CAV1 expression. In vivo, the expression of ANGPT2 and CAV1 in the glomerulus was both increased significantly in DKD mice. Compared with DKD mice, losartan treatment reduced albuminuria and decreased the expression of ANGPT2 and CAV1 in a dose-dependent manner.

CONCLUSIONS

ANGPT2 exacerbated albumin transcytosis across GECs by increasing CAV1 expression during HG exposure, thereby increasing albuminuria. Losartan reduces albumin transcytosis and albuminuria formation in DKD by inhibiting the upregulation of ANGPT2 under HG conditions. Our findings suggest that ANGPT2 and CAV1 may be novel therapeutic targets for diabetic albuminuria. In addition, we provide new evidence to elaborate on the mechanism of losartan in the development of DKD.

Inhibiting Phosphatidylcholine Remodeling in Adipose Tissue Increases Insulin Sensitivity

Cell membrane phosphatidylcholine (PC) composition is regulated by lysophosphatidylcholine acyltransferase (LPCAT); changes in membrane PC saturation are implicated in metabolic disorders. Here, we identified LPCAT3 as the major isoform of LPCAT in adipose tissue and created adipocyte-specific Lpcat3-knockout mice to study adipose tissue lipid metabolism. Transcriptome sequencing and plasma adipokine profiling were used to investigate how LPCAT3 regulates adipose tissue insulin signaling. LPCAT3 deficiency reduced polyunsaturated PCs in adipocyte plasma membranes, increasing insulin sensitivity. LPCAT3 deficiency influenced membrane lipid rafts, which activated insulin receptors and AKT in adipose tissue, and attenuated diet-induced insulin resistance. Conversely, higher LPCAT3 activity in adipose tissue from ob/ob, db/db, and high-fat diet-fed mice reduced insulin signaling. Adding polyunsaturated PCs to mature human or mouse adipocytes in vitro worsened insulin signaling. We suggest that targeting LPCAT3 in adipose tissue to manipulate membrane phospholipid saturation is a new strategy to treat insulin resistance.

ARTICLE HIGHLIGHTS

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

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

Lipid rafts as potential mechanistic targets underlying the pleiotropic actions of polyphenols

Polyphenols have attracted a lot of global attention due to their diverse biological actions against cancer, obesity, and cardiovascular diseases. Although extensive research has been carried out to elucidate the mechanisms of pleiotropic actions of polyphenols, this remains unclear. Lipid rafts are distinct nanodomains enriched in cholesterol and sphingolipids, present in the inner and outer leaflets of cell membranes, forming functional platforms for the regulation of cellular processes and diseases. Recent studies focusing on the interaction between polyphenols and cellular lipid rafts shed new light on the pleiotropic actions of polyphenols. Polyphenols are postulated to interact with lipid rafts in two ways: first, they interfere with the structural integrity of lipid rafts, by disrupting their structure and clustering of the ordered domains; second, they modulate the downstream signaling pathways mediated by lipid rafts, by binding to receptor proteins associated with lipid rafts, such as the 67 kDa laminin receptor (67LR), epidermal growth factor receptor (EGFR), and others. This study aims to elaborate the mechanism of interaction between polyphenols and lipid rafts, and describe pleiotropic preventive effects of polyphenols.

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.

Comparative study of testosterone and vitamin D analogue, elocalcitol, on insulin-controlled signal transduction pathway regulation in human skeletal muscle cells

PURPOSE

Skeletal muscle (Skm) plays a key role in regulating energetic metabolism through glucose homeostasis. Several hormones such as Testosterone (T) and Vitamin D (VD) have been shown to affect energy-dependent cell trafficking by determining Insulin (I)-like effects.

AIM

To elucidate possible hormone-related differences on muscular metabolic control, we analyzed and compared the effects of T and elocalcitol (elo), a VD analogue, on the activation of energy-dependent cell trafficking, metabolism-related-signal transduction pathways and transcription of gene downstream targets.

METHODS

Human fetal skeletal muscle cells (Hfsmc) treated with T or elo were analyzed for GLUT4 localization, phosphorylation/activation status of AKT, ERK1/2, IRS-1 signaling and c-MYC protein expression.

RESULTS

T, similar to elo, induced GLUT4 protein translocation likely in lipid raft microdomains. While both T and elo induced a rapid IRS-1 phosphorylation, the following dynamic in phosphorylation/activation of AKT and ERK1/2 signaling was different. Moreover, T but not elo increased c-MYC protein expression.

CONCLUSIONS

All together, our evidence indicates that whether both T and elo are able to affect upstream I-like pathway, they differently determine downstream effects in I-dependent cascade, suggesting diverse physiological roles in mediating I-like response in human skeletal muscle.

Phospholipid transfer protein (PLTP) deficiency attenuates high fat diet induced obesity and insulin resistance

Increased phospholipid transfer protein (PLTP) activity has been found to be associated with obesity, and metabolic syndrome in humans. However, whether or not PLTP has a direct effect on insulin sensitivity and obesity is largely unknown. Here we analyzed the effect by using PLTP knockout (PLTP^-/-) mouse model. Although, PLTP^-/- mice have normal body-weight-gain under chow diet, these mice were protected from high-fat-diet-induced obesity and insulin resistance, compared with wild type mice. In order to understand the mechanism, we evaluated insulin receptor and Akt activation and found that PLTP deficiency significantly enhanced phosphorylated insulin receptor and Akt levels in high-fat-diet fed mouse livers, adipose tissues, and muscles after insulin stimulation, while total Akt and insulin receptor levels were unchanged. Moreover, we found that the PLTP deficiency induced significantly more GLUT4 protein in the plasma membranes of adipocytes and muscle cells after insulin stimulation. Finally, we found that PLTP-deficient hepatocytes had less sphingomyelins and free cholesterols in the lipid rafts and plasma membranes than that of controls and this may provide a molecular basis for PLTP deficiency-mediated increase in insulin sensitivity. We have concluded that PLTP deficiency leads to an improvement in tissue and whole-body insulin sensitivity through modulating lipid levels in the plasma membrane, especially in the lipid rafts.

Role of the Endocytosis of Caveolae in Intracellular Signaling and Metabolism

Caveolae are 60-80 nm invaginated plasma membrane (PM) nanodomains, with a specific lipid and protein composition, which assist and regulate multiple processes in the plasma membrane-ranging from the organization of signalling complexes to the mechanical adaptation to changes in PM tension. However, since their initial descriptions, these structures have additionally been found tightly linked to internalization processes, mechanoadaptation, to the regulation of signalling events and of endosomal trafficking. Here, we review caveolae biology from this perspective, and its implications for cell physiology and disease.

The caveolae-associated coiled-coil protein, NECC2, regulates insulin signalling in Adipocytes

Adipocyte dysfunction in obesity is commonly associated with impaired insulin signalling in adipocytes and insulin resistance. Insulin signalling has been associated with caveolae, which are coated by large complexes of caveolin and cavin proteins, along with proteins with membrane-binding and remodelling properties. Here, we analysed the regulation and function of a component of caveolae involved in growth factor signalling in neuroendocrine cells, neuroendocrine long coiled-coil protein-2 (NECC2), in adipocytes. Studies in 3T3-L1 cells showed that NECC2 expression increased during adipogenesis. Furthermore, NECC2 co-immunoprecipitated with caveolin-1 (CAV1) and exhibited a distribution pattern similar to that of the components of adipocyte caveolae, CAV1, Cavin1, the insulin receptor and cortical actin. Interestingly, NECC2 overexpression enhanced insulin-activated Akt phosphorylation, whereas NECC2 downregulation impaired insulin-induced phosphorylation of Akt and ERK2. Finally, an up-regulation of NECC2 in subcutaneous and omental adipose tissue was found in association with human obesity and insulin resistance. This effect was also observed in 3T3-L1 adipocytes exposed to hyperglycaemia/hyperinsulinemia. Overall, the present study identifies NECC2 as a component of adipocyte caveolae that is regulated in response to obesity and associated metabolic complications, and supports the contribution of this protein as a molecular scaffold modulating insulin signal transduction at these membrane microdomains.

Modulation of glucose metabolism by a natural compound from Chloranthus japonicus via activation of AMP-activated protein kinase

AMP-activated protein kinase (AMPK) is a key sensor and regulator of glucose metabolism. Here, we demonstrated that shizukaol F, a natural compound isolated from Chloranthus japonicus, can activate AMPK and modulate glucose metabolism both in vitro and in vivo. Shizukaol F increased glucose uptake in differentiated C2C12 myotubes by stimulating glucose transporter-4 (GLUT-4) membraned translocation. Treatment of primary mouse hepatocytes with shizukaol F decreased the expression of phosphoenolpyruvate carboxykinase 2 (PEPCK), glucose-6-phosphatase (G6Pase) and suppressed hepatic gluconeogenesis. Meanwhile, a single oral dose of shizukaol F reduced gluconeogenesis in C57BL/6 J mice. Further studies indicated that shizukaol F modulates glucose metabolism mainly by AMPKa phosphorylation activity. In addition, we also found that shizukaol F depolarizes the mitochondrial membrane and inhibits respiratory complex I, which may result in AMPK activation. Our results highlight the potential value of shizukaol F as a possible treatment of metabolic syndrome.

Effects of high glucose on caveolin-1 and insulin signaling in 3T3-L1 adipocytes

Adipocytes exposed to high glucose concentrations exhibit impaired metabolic function, including an increase of oxidative and proinflammatory factors that might favor the development of insulin resistance. Caveolin-1 (Cav-1) is a key mediator of the insulin transduction pathway whose expression is significantly enhanced during adipocyte differentiation. In this work, we studied the effects of high glucose concentration on the regulation of Cav-1 expression and activation and its relation to the insulin signaling pathway during the adipogenic process and in long-term differentiated adipocytes. Both, long-term high glucose exposure during adipogenesis and short-term glucose incubation of mature adipocytes, promoted triglyceride accumulation in 3T3-L1 cells. The short-term exposure of mature adipocytes to high glucose significantly reduced the sensitivity to insulin of Cav-1, insulin receptor (IR) and potein kinase B (AKT-2) phosphorylation, as well as insulin-induced deoxyglucose uptake. Adipocytes differentiated in the presence of high glucose lost Cav-1 and IR response to insulin-stimulated phosphorylation, but maintained the insulin sensitivity of AKT-2 phosphorylation and deoxyglucose uptake. Although long-term high glucose exposure increased DNA methylation in Cav-1 promoter, Cav-1 expression was not affected. Moreover, these cells showed an increase of Cav-1, IR and AKT-2 protein content, pointing to an adaptive response induced by the long-term high glucose exposure.

A prevalent caveolin-1 gene variant is associated with the metabolic syndrome in Caucasians and Hispanics

CONTEXT AND OBJECTIVE

We examined whether a prevalent caveolin-1 gene (CAV1) variant, previously related to insulin resistance, is associated with metabolic syndrome (MetS).

PATIENTS AND METHODS

We included subjects genotyped for the CAV1 variant rs926198 from two cohorts: 735 Caucasians from the HyperPATH multicenter study, and 810 Hispanic participants from the HTN-IR cohort.

RESULTS

Minor allele carriers from HyperPATH cohort (57% of subjects) had higher Framingham risk scores, higher odds of diabetes (10.7% vs 5.7%, p=0.016), insulin resistance (44.3% vs 35.1%, p=0.022), low HDL (49.3% vs 39.6%, p=0.018) and MetS (33% vs 20.5%, p<0.001) but similar BMI. Consistently, minor allele carriers exhibited higher odds of MetS, even when adjusted for confounders and relatedness (OR 2.83 (1.73-4.63), p<0.001). The association with MetS was replicated in the Hispanic cohort HTN-IR (OR 1.61, [1.06-2.44], p=0.025). Exploratory analyses suggest that MetS risk is modified by a CAV1 variant-BMI status interaction, whereby the minor allele carrier status strongly predicted MetS (OR 3.86 [2.05-7.27], p<0.001) and diabetes (OR 2.27 [1.07-4.78], p=0.03) in non-obese, but not in obese subjects. In addition, we observed a familial aggregation for MetS diagnosis in minor allele carriers.

CONCLUSION

The prevalent CAV1 gene variant rs926198 is associated with MetS in separate Caucasian and Hispanic cohorts. These findings appear to be driven by an interaction between the genetic marker and obesity status, suggesting that the CAV1 variant may improve risk profiling in non-obese subjects. Additional studies are needed to confirm the clinical implications of our results.

Impact of keratin intermediate filaments on insulin-mediated glucose metabolism regulation in the liver and disease association

In all cells, a tight regulation exists between glucose uptake and utilization to prevent diseases related to its perturbed metabolism. In insulin-targeted cells, such as hepatocytes, proper glucose utilization requires an elaborate interplay between the insulin receptor, the glucose transporter, and mitochondria that involves the participation of actin microfilaments and microtubules. In addition, there is increasing evidence of an involvement of the third cytoskeletal network provided by intermediate filaments (IFs). Keratins belong to the multigene family of IF proteins, coordinately expressed as distinct pairs within the context of epithelial cell differentiation. Hepatocyte IFs are made up of the [keratin (K)8/K18] pair only, whereas pancreatic β-cell IFs additionally include small amounts of K7. There are accumulating examples of K8/K18 involvement in the glucose-insulin cross-talk, including the modulation of plasma glucose levels, insulin release from pancreatic β-cells, and insulin-mediated glucose uptake and glycogen production in hepatocytes after a K8/K18 loss. This review integrates the mechanistic features that support such an impact of K8/K18 IFs on insulin-dependent glucose metabolism regulation in liver and its implication in glucose- or insulin-associated diseases.

Adiponectin is released via a unique regulated exocytosis pathway from a pre-formed vesicle pool on insulin stimulation

Adiponectin, a hormone secreted from adipocytes and released at a high rate into the circulation, plays a pivotal role in maintaining insulin sensitivity at the whole-body level. Despite the importance of this adipokine in metabolic homoeostasis, the mechanism of its secretion from adipocytes remains largely unclear. In the present study, we investigated the subcellular localization of adiponectin, and its secretion regulation in 3T3-L1-differentiated adipocytes, using biochemical methods and fluorescence microscopic imaging. We show that adiponectin is localized in vesicular compartments with no apparent overlap with the Golgi apparatus or endosomes. Moreover, adiponectin-containing vesicles are enriched in two distinct pools: one at the plasma membrane (PM) and the other co-fractionating with endoplasmic reticulum membranes. When viewed under a total internal refection fluorescence microscope, a subset of adiponectin-Venus vesicles is readily observed in proximity to PMs, and could be released in response to insulin. Insulin-stimulated adiponectin release appears to be from a pre-existing pool of vesicles, and is not dependent on new protein synthesis, because adiponectin mRNA levels remain unchanged over a 6-h period of insulin treatment, and inhibition of protein synthesis has no effect on adiponectin release. Disruption of insulin signalling by inhibitors of phosphoinositide 3-kinase and protein kinase B (Akt)-1/2 abrogates the stimulated release of adiponectin. Taken together, our results show that adiponectin is stored in a unique vesicular compartment, and released through a regulated exocytosis pathway that is dependent on insulin signalling.

The cell biology of fat expansion

Adipose tissue is a complex, multicellular organ that profoundly influences the function of nearly all other organ systems through its diverse metabolite and adipokine secretome. Adipocytes are the primary cell type of adipose tissue and play a key role in maintaining energy homeostasis. The efficiency with which adipose tissue responds to whole-body energetic demands reflects the ability of adipocytes to adapt to an altered nutrient environment, and has profound systemic implications. Deciphering adipocyte cell biology is an important component of understanding how the aberrant physiology of expanding adipose tissue contributes to the metabolic dysregulation associated with obesity.

Insight into nanoparticle cellular uptake and intracellular targeting

Collaborative efforts from the fields of biology, materials science, and engineering are leading to exciting progress in the development of nanomedicines. Since the targets of many therapeutic agents are localized in subcellular compartments, modulation of nanoparticle-cell interactions for efficient cellular uptake through the plasma membrane and the development of nanomedicines for precise delivery to subcellular compartments remain formidable challenges. Cellular internalization routes determine the post-internalization fate and intracellular localization of nanoparticles. This review highlights the cellular uptake routes most relevant to the field of non-targeted nanomedicine and presents an account of ligand-targeted nanoparticles for receptor-mediated cellular internalization as a strategy for modulating the cellular uptake of nanoparticles. Ligand-targeted nanoparticles have been the main impetus behind the progress of nanomedicines towards the clinic. This strategy has already resulted in remarkable progress towards effective oral delivery of nanomedicines that can overcome the intestinal epithelial barrier. A detailed overview of the recent developments in subcellular targeting as a novel platform for next-generation organelle-specific nanomedicines is also provided. Each section of the review includes prospects, potential, and concrete expectations from the field of targeted nanomedicines and strategies to meet those expectations.

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.

RhoA localization with caveolin-1 regulates vascular contractions to serotonin

Vascular smooth muscle contraction occurs following an initial response to an increase in intracellular calcium concentration and a sustained response following increases in the sensitivity of contractile proteins to calcium (calcium sensitization). This latter process is regulated by the rhoA/rho kinase pathway and activated by serotonin. In multiple cell types, signaling molecules compartmentalize within caveolae to regulate their activation. We hypothesized that serotonin differentially compartmentalizes rhoA within caveolar versus noncaveolar lipid rafts to regulate sustained vascular contractions. To test this hypothesis, we measured aortic contractions in response to serotonin in wild-type (WT) and cav-1-deficient mice (cav-1 KO). RhoA-dependent contractions in response to serotonin were markedly augmented in arteries from cav-1 KO mice despite a modest reduction in rhoA expression compared with WT. We found that under basal conditions, rhoA in WT arteries was primarily localized within high-density sucrose gradient fractions but temporally shifted to low-density fractions in response to serotonin. In contrast, rhoA in cav-1 KO arteries was primarily in low-density fractions and shifted to high-density fractions in a similar timeframe as that seen in WT mice. We conclude that localization of rhoA to caveolar versus noncaveolar lipid rafts differentially regulates its activation and contractions to rhoA-dependent agonists with greater activation associated with its localization to noncaveolar rafts. Disruption of rhoA localization within caveolae may contribute to increased activation and enhanced vascular contractions in cardiovascular disease.

Lipid raft redox signaling: molecular mechanisms in health and disease

Lipid rafts, the sphingolipid and cholesterol-enriched membrane microdomains, are able to form different membrane macrodomains or platforms upon stimulations, including redox signaling platforms, which serve as a critical signaling mechanism to mediate or regulate cellular activities or functions. In particular, this raft platform formation provides an important driving force for the assembling of NADPH oxidase subunits and the recruitment of other related receptors, effectors, and regulatory components, resulting, in turn, in the activation of NADPH oxidase and downstream redox regulation of cell functions. This comprehensive review attempts to summarize all basic and advanced information about the formation, regulation, and functions of lipid raft redox signaling platforms as well as their physiological and pathophysiological relevance. Several molecular mechanisms involving the formation of lipid raft redox signaling platforms and the related therapeutic strategies targeting them are discussed. It is hoped that all information and thoughts included in this review could provide more comprehensive insights into the understanding of lipid raft redox signaling, in particular, of their molecular mechanisms, spatial-temporal regulations, and physiological, pathophysiological relevances to human health and diseases.

Human cholesteryl ester transfer protein enhances insulin-mediated glucose uptake in adipocytes

AIMS

Adipose tissue plays an important role in the pathogenesis of insulin resistance, obesity, and Type-2 diabetes. Human adipocytes express abundant cholesteryl ester transfer protein (CETP). However, the function and role of CETP in regulating lipoprotein metabolism are mostly unknown. In this study, we examined whether CETP affected the insulin-mediated responses in adipocytes.

MAIN METHODS

Because mouse 3T3-L1 preadipocytes do not express CETP, we established a stable cell line expressing human CETP by transfecting the cells with pcDNA3.1/human CETP. We used a standard approach to differentiate the cells into mature adipocytes, and we examined the cholesterol balance and insulin responses.

KEY FINDINGS

The human CETP stable cell line expressed stable levels of CETP without affecting the expression of either peroxisome proliferator-activated receptor-gamma (PPARγ) or glucose transporter-4 (GLUT4) throughout cell differentiation. CETP expression significantly increased the level of both total and free cholesterol in the mature adipocytes. Upon insulin stimulation, CETP expressing cells had significantly higher protein kinase B (Akt) phosphorylation and 2-(3)H-deoxyglucose uptake, as compared with 3T3-L1 cells and cells transfected with control vector.

SIGNIFICANCE

Human CETP expression increased cellular cholesterol levels and enhanced insulin-stimulated Akt phosphorylation and glucose uptake in adipocytes. Thus, CETP may modulate glucose metabolism and insulin action in addition to its effects on lipoprotein metabolism.

Flotillins play an essential role in Niemann-Pick C1-like 1-mediated cholesterol uptake

Dietary absorption is a major way for mammals to obtain cholesterol, which is mediated by Niemann-Pick C1-like 1 (NPC1L1) via vesicular endocytosis. One fundamental question in this process is how free cholesterol is efficiently taken up through the internalization of NPC1L1. Using exogenously expressed NPC1L1-EGFP, we show that the lipid raft proteins flotillins associate with NPC1L1 and their localization is regulated by NPC1L1 during intracellular trafficking. Furthermore, flotillins are essential for NPC1L1-mediated cellular cholesterol uptake, biliary cholesterol reabsorption, and the regulation of lipid levels in mice. Together with NPC1L1, they form cholesterol-enriched membrane microdomains, which function as carriers for bulk of cholesterol. The hypocholesterolemic drug ezetimibe disrupts the association between NPC1L1 and flotillins, which blocks the formation of the cholesterol-enriched microdomains. Our findings reveal a functional role of flotillins in NPC1L1-mediated cholesterol uptake and elucidate the formation of NPC1L1-flotillins-postive cholesterol-enriched membrane microdomains as a mechanism for efficient cholesterol absorption.

Cholesterol imbalance in adipocytes: a possible mechanism of adipocytes dysfunction in obesity

Studies of the past decade have increased our understanding of the role of adipose tissue dysfunction in obesity and obesity-related insulin resistance and type 2 diabetes. Although adipose tissue is the body's largest pool of free cholesterol, adipocytes have limited activity in cholesterol synthetic pathway. Thus, the majority of adipocyte cholesterol originates from circulating lipoproteins. To maintain cholesterol homeostasis, adipocytes have developed multiple pathways for cholesterol efflux. Several transcriptional factors, such as sterol regulatory element-binding proteins and liver X receptors may be responsible for the regulation of cholesterol homeostasis in adipocytes. Most notably, because altering cholesterol balance profoundly modifies adipocyte metabolism in a way resembling that seen in hypertrophied adipocytes, cholesterol imbalance is recognized as a characteristic for enlarged adipocytes per se in the obese state. In addition, plasma membrane cholesterol normalization by chromium picolinate can fully restore insulin-stimulated glucose transport, further supporting the role of the adipocyte cholesterol imbalance in obesity and insulin resistance.

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

Caveolae are a specialized type of lipid rafts that are stabilized by oligomers of caveolin protein. Caveolae are particularly enriched in adipocytes. Here we analyzed the effects of caveolin-1 knockdown and caveolae ablation on adipocyte function. To this end, we obtained several multiclonal mouse 3T3-L1 cell lines with a reduced expression of caveolin-1 (95% reduction) by a small interfering RNA approach using lentiviral vectors. Control cell lines were obtained by lentiviral infection with lentiviral vectors encoding appropriate scrambled RNAs. Caveolin-1 knockdown adipocytes showed a drastic reduction in the number of caveolae (95% decrease) and cholera toxin labeling was reorganized in dynamic plasma membrane microdomains. Caveolin-1 depletion caused a specific decrease in glucose transporter 4 (GLUT4) and insulin receptor protein levels. This reduction was not the result of a generalized defect in adipocyte differentiation or altered gene expression but was explained by faster degradation of these proteins. Caveolin-1 knockdown adipocytes showed reductions in insulin-stimulated glucose transport, insulin-triggered GLUT4 recruitment to the cell surface, and insulin receptor activation. In all, our data indicate that caveolin-1 loss of function reduces maximal insulin response through lowered stability and diminished expression of insulin receptors and GLUT4. We propose that caveolin-1/caveolae control insulin action in adipose cells.

The differential protein and lipid compositions of noncaveolar lipid microdomains and caveolae

Morphologically, caveolae and lipid rafts are two different membrane structures. They are often reported to share similar lipid and protein compositions, and are considered to be two subtypes of membrane lipid microdomains. By modifying sucrose density gradient flotation centrifugation, which is used to isolate lipid microdomains, we were able to separate caveolae and noncaveolar lipid microdomains into two distinct fractions. The caveolar membranes are membrane vesicles of 100-nm diameter, enriched with caveolin-1 and flotillin-1. The noncaveolar lipid microdomains are amorphous membranes and most likely the coalescence of heterogeneous lipid rafts. They are depleted of caveolin-1 and are more enriched with cholesterol and sphingolipids than the caveolae. Many membrane proteins, such as insulin-like growth factor-1 receptor (membrane receptor), aquaporin-1 (membrane transporter), Thy-1 and N-cadherin (glycosylphosphatidylinositol-anchored membrane protein and membrane glycoprotein), are specifically associated with noncaveolar lipid microdomains, but not with caveolae. These results indicate that the lipid and protein compositions of caveolae differ from those of noncaveolar lipid microdomains. The difference in their protein compositions implies that these two membrane microdomains may have different cellular functions.

The activity of the epithelial sodium channels is regulated by caveolin-1 via a Nedd4-2-dependent mechanism

It has recently been shown that the epithelial Na(+) channel (ENaC) is compartmentalized in caveolin-rich lipid rafts and that pharmacological depletion of membrane cholesterol, which disrupts lipid raft formation, decreases the activity of ENaC. Here we show, for the first time, that a signature protein of caveolae, caveolin-1 (Cav-1), down-regulates the activity and membrane surface expression of ENaC. Physical interaction between ENaC and Cav-1 was also confirmed in a coimmunoprecipitation assay. We found that the effect of Cav-1 on ENaC requires the activity of Nedd4-2, a ubiquitin protein ligase of the Nedd4 family, which is known to induce ubiquitination and internalization of ENaC. The effect of Cav-1 on ENaC requires the proline-rich motifs at the C termini of the beta- and gamma-subunits of ENaC, the binding motifs that mediate interaction with Nedd4-2. Taken together, our data suggest that Cav-1 inhibits the activity of ENaC by decreasing expression of ENaC at the cell membrane via a mechanism that involves the promotion of Nedd4-2-dependent internalization of the channel.

Liquid chromatography electrospray ionization and matrix-assisted laser desorption ionization tandem mass spectrometry for the analysis of lipid raft proteome of monocytes

Lipid rafts are dynamic assemblies of cholesterol and glycolipid that form detergent-insoluble microdomains within membrane lipid bilayers. Because rafts can be separated by flotation on sucrose gradients, interrogation by mass spectrometry (MS) provides a valuable new insight into lipid raft function. Here we combine liquid chromatography (LC) electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) MS/MS to corroborate and extend our previous description of lipid raft proteomes derived from the monocytic cell line THP-1. Interestingly, LC-ESI and MALDI MS/MS identify largely non-overlapping, and therefore, potentially complementary protein populations. Using the combined approach, we detected 277 proteins compared to 52 proteins obtained with the original gel-based MALDI MS. We confirmed the presence of 47 of the original 52 proteins demonstrating the consistency of the lipid raft preparations. We demonstrated by immunoblotting that Rac 1 and Rac 2, two of the 52 proteins we failed to confirm, were indeed absent from the lipid raft fractions. The majority of new proteins were cytoskeletal proteins and their regulators, proteins implicated in membrane fusion and vesicular trafficking or signaling molecules. Our results therefore, confirm and extend previous evidence indicating lipid rafts of monocytic cells are specialized for cytoskeletal assembly and vesicle trafficking. Of particular interest, we detected SNAP-23, basigin, Glut-4 and pantophysin in lipid rafts. Since these proteins are implicated in both vesicular trafficking and gamete fusion, lipid rafts may play a common role in these processes. It is evident that the combination of LC-ESI and LC-MALDI MS/MS increases the proteome coverage which allows better understanding of the lipid raft function.