Cholesterol depletion disrupts caveolae and insulin receptor signaling for metabolic control via insulin receptor substrate-1, but not for mitogen-activated protein kinase control

Treadmill exercise supplemented by OPN promote axon regeneration through the IGF-1R/Akt/mTOR signaling pathway

Regeneration of the corticospinal tract (CST) is considered a therapeutic target to achieve improved recovery of motor function after spinal cord injury (SCI), which is an incurable CNS damage that affects millions of people. Exercise training is effective in improving multiple functions in spinal cord-injured patients. However, the effects of exercise training on axon regeneration have not been sufficiently reported. Osteopontin (OPN) has great potential application as a neuroprotective agent for the repair of the nervous system. Studies have shown that the extent of axon regeneration strongly correlates with the expression of OPN. Our previous studies demonstrated that treadmill exercise supplemented by OPN enhances motor function recovery, but axon regeneration is still limited. Extending the treadmill exercise for 12 weeks, we observed promoted axon regeneration, motor function improvement, and signaling pathway activation in mice with SCI after supplementing OPN. Axon regeneration was observed with an anterograde tracer, motor function recovery was evaluated by animal ethology and electrophysiology, and the levels of IGF-1R/Akt/mTOR signaling pathway were evaluated. The results showed that the CST of C5 crushed mice regenerated and formed synaptic connections with neurons after treadmill exercise supplemented by OPN, the horizontal ladder and cylinder rearing test of injured limbs were improved, motor evoked potential also suggested enhanced nerve conduction, and the expression of p-IR, p-Akt, and p-S6 were increased. And the improvements were more obvious than that of the exercise group. Collectively, our study found that treadmill exercise supplemented by OPN promote axon regeneration and motor function through the IGF-1R/Akt/mTOR signaling pathways, and these improvements can be inhibited by rapamycin and Methyl-β-CD(M-B-CD).

ACAT1/SOAT1 maintains adipogenic ability in preadipocytes by regulating cholesterol homeostasis

Maintaining cholesterol homeostasis is critical for preserving adipocyte function during the progression of obesity. Despite this, the regulatory role of cholesterol esterification in governing adipocyte expandability has been understudied. Acyl-coenzyme A (CoA):cholesterol acyltransferase/Sterol O-acyltransferase 1 (ACAT1/SOAT1) is the dominant enzyme to synthesize cholesteryl ester in most tissues. Our previous study demonstrated that knockdown of either ACAT1 or ACAT2 impaired adipogenesis. However, the underlying mechanism of how ACAT1 mediates adipogenesis remains unclear. Here, we reported that ACAT1 is the dominant isoform in white adipose tissue of both humans and mice, and knocking out ACAT1 reduced fat mass in mice. Furthermore, ACAT1-deficiency inhibited the early stage of adipogenesis via attenuating PPARγ pathway. Mechanistically, ACAT1 deficiency inhibited SREBP2-mediated cholesterol uptake and thus reduced intracellular and plasma membrane cholesterol levels during adipogenesis. Replenishing cholesterol could rescue adipogenic master gene-Pparγ's-transcription in ACAT1-deficient cells during adipogenesis. Finally, overexpression of catalytically functional ACAT1, not the catalytic-dead ACAT1, rescued cholesterol levels and efficiently rescued the transcription of PPARγ as well as the adipogenesis in ACAT1-deficient preadipocytes. In summary, our study revealed the indispensable role of ACAT1 in adipogenesis via regulating intracellular cholesterol homeostasis.

Loss of HD-PTP function results in lipodystrophy, defective cellular signaling and altered lipid homeostasis

His domain protein tyrosine phosphatase (HD-PTP; also known as PTPN23) facilitates function of the endosomal sorting complexes required for transport (ESCRTs) during multivesicular body (MVB) formation. To uncover its role in physiological homeostasis, embryonic lethality caused by a complete lack of HD-PTP was bypassed through generation of hypomorphic mice expressing reduced protein, resulting in animals that are viable into adulthood. These mice exhibited marked lipodystrophy and decreased receptor-mediated signaling within white adipose tissue (WAT), involving multiple prominent pathways including RAS/MAPK, phosphoinositide 3-kinase (PI3K)/AKT and receptor tyrosine kinases (RTKs), such as EGFR. EGFR signaling was dissected in vitro to assess the nature of defective signaling, revealing decreased trans-autophosphorylation and downstream effector activation, despite normal EGF binding. This corresponds to decreased plasma membrane cholesterol and increased lysosomal cholesterol, likely resulting from defective endosomal maturation necessary for cholesterol trafficking and homeostasis. The ESCRT components Vps4 and Hrs have previously been implicated in cholesterol homeostasis; thus, these findings expand knowledge on which ESCRT subunits are involved in cholesterol homeostasis and highlight a non-canonical role for HD-PTP in signal regulation and adipose tissue homeostasis.

Lipid droplets, autophagy, and ageing: A cell-specific tale

Lipid droplets are the essential organelle for storing lipids in a cell. Within the variety of the human body, different cells store, utilize and release lipids in different ways, depending on their intrinsic function. However, these differences are not well characterized and, especially in the context of ageing, represent a key factor for cardiometabolic diseases. Whole body lipid homeostasis is a central interest in the field of cardiometabolic diseases. In this review we characterize lipid droplets and their utilization via autophagy and describe their diverse fate in three cells types central in cardiometabolic dysfunctions: adipocytes, hepatocytes, and macrophages.

Mechanoregulatory Cholesterol Oxidase-Functionalized Nanoscale Metal-Organic Framework Stimulates Pyroptosis and Reinvigorates T Cells

Cancer cells alter mechanical tension in their cell membranes. New interventions to regulate cell membrane tension present a potential strategy for cancer therapy. Herein, the increase of cell membrane tension by cholesterol oxidase (COD) via cholesterol depletion in vitro and the design of a COD-functionalized nanoscale metal-organic framework, Hf-TBP/COD, for cholesterol depletion and mechanoregulation of tumors in vivo, are reported. COD is found to deplete cholesterol and disrupt the mechanical properties of lipid bilayers, leading to decreased cell proliferation, migration, and tolerance to oxidative stress. Hf-TBP/COD increases mechanical tension of plasma membranes and osmotic fragility of cancer cells, which induces influx of calcium ions, inhibits cell migration, increases rupturing propensity for effective caspase-1 mediated pyroptosis, and decreases tolerance to oxidative stress. In the tumor microenvironment, Hf-TBP/COD downregulates multiple immunosuppressive checkpoints to reinvigorate T cells and enhance T cell infiltration. Compared to Hf-TBP, Hf-TBP/COD improves anti-tumor immune response and tumor growth inhibition from 54.3% and 79.8% to 91.7% and 95% in a subcutaneous triple-negative breast cancer model and a colon cancer model, respectively.

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.

IGF-1 Combined with OPN Promotes Neuronal Axon Growth in Vitro Through the IGF-1R/Akt/mTOR Signaling Pathway in Lipid Rafts

This study aims to investigate the effect of insulin-like growth factor 1 (IGF-1) combined with osteopontin (OPN) on the protein expression levels and growth of neuronal axons and its possible mechanism. In this study, IGF-1 combined with OPN promoted neuronal axon growth through the IGF-1R/Akt/mTOR signaling pathway in lipid rafts, and the effect was better than that of either agent alone. This effect was suppressed when given the mTOR inhibitor rapamycin or the lipid raft cholesterol extraction agent methyl-β-cyclodextrin (M-β-CD). Rapamycin could inhibit the expression of phosphorylated ribosomal S6 protein (p-S6) and phosphorylated protein kinase B (p-Akt) and limit axon growth. In addition to the above effects, M-β-CD significantly downregulated the expression of phosphorylated insulin-like growth factor 1 receptor (p-IR). To further investigate the changes in lipid rafts when stimulated by different recombinant proteins, membrane lipid rafts were isolated to observe the changes by western blot. The expression levels of insulin-like growth factor 1 receptor (IR) and P-IR in the IGF-1 combined with OPN group were the highest. When M-β-CD was administered to the lipid rafts of neurons, the enrichment of IR by IGF-1 combined with OPN was weakened, and the p-IR was decreased. Our study found that IGF-1 combined with OPN could promote axon growth by activating the IGF-1R/Akt/mTOR signaling pathway in neuronal lipid rafts.

Caveolae Microdomains Mediate STAT5 Signaling Induced by Insulin in MCF-7 Breast Cancer Cells

Caveolae are small plasma membrane invaginations constituted for membrane proteins namely caveolins and cytosolic proteins termed cavins, which can occupy up to 50% of the surface of mammalian cells. The caveolae have been involved with a variety of cellular processes including regulation of cellular signaling. Insulin is a hormone that mediates a variety of physiological processes through activation of insulin receptor (IR), which is a tyrosine kinase receptor expressed in all mammalian tissues. Insulin induces activation of signal transducers and activators of transcription (STAT) family members including STAT5. In this study, we demonstrate, for the first time, that insulin induces phosphorylation of STAT5 at tyrosine-694 (STAT5-Tyr(P)⁶⁹⁴), STAT5 nuclear accumulation and an increase in STAT5-DNA complex formation in MCF-7 breast cancer cells. Insulin also induces nuclear accumulation of STAT5-Tyr(P)⁶⁹⁴, caveolin-1, and IR in MCF-7 cells. STAT5 nuclear accumulation and the increase of STAT5-DNA complex formation require the integrity of caveolae and microtubule network. Moreover, insulin induces an increase and nuclear accumulation of STAT5-Tyr(P)⁶⁹⁴ in MDA-MB-231 breast cancer cells. In conclusion, results demonstrate that caveolae and microtubule network play an important role in STAT5-Tyr(P)⁶⁹⁴, STAT5 nuclear accumulation and STAT5-DNA complex formation induced by insulin in breast cancer cells.

The regulation of adipocyte growth in white adipose tissue

Adipocytes can increase in volume up to a thousand-fold, storing excess calories as triacylglycerol in large lipid droplets. The dramatic morphological changes required of adipocytes demands extensive cytoskeletal remodeling, including lipid droplet and plasma membrane expansion. Cell growth-related signalling pathways are activated, stimulating the production of sufficient amino acids, functional lipids and nucleotides to meet the increasing cellular needs of lipid storage, metabolic activity and adipokine secretion. Continued expansion gives rise to enlarged (hypertrophic) adipocytes. This can result in a failure to maintain growth-related homeostasis and an inability to cope with excess nutrition or respond to stimuli efficiently, ultimately leading to metabolic dysfunction. We summarize recent studies which investigate the functional and cellular structure remodeling of hypertrophic adipocytes. How adipocytes adapt to an enlarged cell size and how this relates to cellular dysfunction are discussed. Understanding the healthy and pathological processes involved in adipocyte hypertrophy may shed light on new strategies for promoting healthy adipose tissue expansion.

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.

Lactose-Appended Hydroxypropyl-β-Cyclodextrin Lowers Cholesterol Accumulation and Alleviates Motor Dysfunction in Niemann-Pick Type C Disease Model Mice

Niemann-Pick disease type C (NPC) is characterized by the accumulation of glycolipids such as free cholesterol, sphingomyelin, and gangliosides in late endosomes/lysosomes (endolysosomes) due to abnormalities in the membrane proteins NPC1 or NPC2. The main symptoms of NPC caused by free cholesterol accumulation in various tissues vary depending on the time of onset, but hepatosplenomegaly and neurological symptoms accompanied by decreased motor, cognitive, and mental functions are observed in all age groups. However, the efficacy of NPC treatment remains limited. Herein, we have fabricated lactose-appended hydroxypropyl-β-cyclodextrin (Lac-HPβCD) and evaluated its lowering effects on cholesterol accumulation in NPC model mice. We reveal that Lac-HPβCD lowers cholesterol accumulation in the liver and spleen by reducing the amount of free cholesterol. Moreover, Lac-HPβCD reduces the amount of free cholesterol in the cerebrum and slightly alleviates motor dysfunction. These results suggest that Lac-HPβCD has potential for the treatment of NPC.

Statins Aggravate the Risk of Insulin Resistance in Human Muscle

Beside their beneficial effects on cardiovascular events, statins are thought to contribute to insulin resistance and type-2 diabetes. It is not known whether these effects are long-term events from statin-treatment or already triggered with the first statin-intake. Skeletal muscle is considered the main site for insulin-stimulated glucose uptake and therefore, a primary target for insulin resistance in the human body. We analyzed localization and expression of proteins related to GLUT4 mediated glucose uptake via AMPKα or AKT in human skeletal muscle tissue from patients with statin-intake >6 months and in primary human myotubes after 96 h statin treatment. The ratio for AMPKα activity significantly increased in human skeletal muscle cells treated with statins for long- and short-term. Furthermore, the insulin-stimulated counterpart, AKT, significantly decreased in activity and protein level, while GSK3ß and mTOR protein expression reduced in statin-treated primary human myotubes, only. However, GLUT4 was normally distributed whereas CAV3 was internalized from plasma membrane around the nucleus in statin-treated primary human myotubes. Statin-treatment activates AMPKα-dependent glucose uptake and remains active after long-term statin treatment. Permanent blocking of its insulin-dependent counterpart AKT activation may lead to metabolic inflexibility and insulin resistance in the long run and may be a direct consequence of statin-treatment.

It Takes More than Two to Tango: Complex, Hierarchal, and Membrane-Modulated Interactions in the Regulation of Receptor Tyrosine Kinases

The search for an understanding of how cell fate and motility are regulated is not a purely scientific undertaking, but it can also lead to rationally designed therapies against cancer. The discovery of tyrosine kinases about half a century ago, the subsequent characterization of certain transmembrane receptors harboring tyrosine kinase activity, and their connection to the development of human cancer ushered in a new age with the hope of finding a treatment for malignant diseases in the foreseeable future. However, painstaking efforts were required to uncover the principles of how these receptors with intrinsic tyrosine kinase activity are regulated. Developments in molecular and structural biology and biophysical approaches paved the way towards better understanding of these pathways. Discoveries in the past twenty years first resulted in the formulation of textbook dogmas, such as dimerization-driven receptor association, which were followed by fine-tuning the model. In this review, the role of molecular interactions taking place during the activation of receptor tyrosine kinases, with special attention to the epidermal growth factor receptor family, will be discussed. The fact that these receptors are anchored in the membrane provides ample opportunities for modulatory lipid-protein interactions that will be considered in detail in the second part of the manuscript. Although qualitative and quantitative alterations in lipids in cancer are not sufficient in their own right to drive the malignant transformation, they both contribute to tumor formation and also provide ways to treat cancer. The review will be concluded with a summary of these medical aspects of lipid-protein interactions.

Statins Are Associated With Increased Insulin Resistance and Secretion

Objective

Statin treatment reduces the risk of atherosclerotic cardiovascular disease but is associated with a modest increased risk of type 2 diabetes, especially in those with insulin resistance or prediabetes. Our objective was to determine the physiological mechanism for the increased type 2 diabetes risk.

Approach and Results

We conducted an open-label clinical trial of atorvastatin 40 mg daily in adults without known atherosclerotic cardiovascular disease or type 2 diabetes at baseline. The co-primary outcomes were changes at 10 weeks versus baseline in insulin resistance as assessed by steady-state plasma glucose during the insulin suppression test and insulin secretion as assessed by insulin secretion rate area under the curve (ISRAUC) during the graded-glucose infusion test. Secondary outcomes included glucose and insulin, both fasting and during oral glucose tolerance test. Of 75 participants who enrolled, 71 completed the study (median age 61 years, 37% women, 65% non-Hispanic White, median body mass index, 27.8 kg/m2). Atorvastatin reduced LDL (low-density lipoprotein)-cholesterol (median decrease 53%, P<0.001) but did not change body weight. Compared with baseline, atorvastatin increased insulin resistance (steady-state plasma glucose) by a median of 8% (P=0.01) and insulin secretion (ISRAUC) by a median of 9% (P<0.001). There were small increases in oral glucose tolerance test glucoseAUC (median increase, 0.05%; P=0.03) and fasting insulin (median increase, 7%; P=0.01).

Conclusions

In individuals without type 2 diabetes, high-intensity atorvastatin for 10 weeks increases insulin resistance and insulin secretion. Over time, the risk of new-onset diabetes with statin use may increase in individuals who become more insulin resistant but are unable to maintain compensatory increases in insulin secretion.

Biological activity of 2α,3β,23-trihydroxyolean-12-ene on glucose homeostasis

We studied the effect and the mechanisms of action of 2α,3β,23-trihydroxyolean-12-ene (THO), from Croton heterodoxus Baill. (Euphorbiaceae), in glucose uptake in hyperglycemic rats. The effect of in vivo pretreatment with THO in hyperglycemic rats was analyzed. The in vitro effects of THO were observed in adipocytes and in adipose tissue. THO reduced glycemia, in part by increasing serum insulin and augmenting the disposal of glucose as glycogen in hepatocytes but did not change the serum concentration of glucagon-like peptide-1. THO increased glucose uptake in adipocytes and in adipose tissue by a mechanism dependent on phosphatidylinositol 3-kinase vesicular traffic and on the process of vesicle fusion at the plasma membrane in regions containing cholesterol, indicating the involvement of glucose transporter-4 (GLUT4). This triterpene may act solely via the activation and translocation of GLUT4 (rather than via nuclear actions, such as upregulation of GLUT4 synthesis), since THO did not alter the amount of GLUT4 mRNA or the content of GLUT4. Consistent with these data, the stimulatory effect of this triterpene on the quantity of GLUT4 in the membrane fraction was dependent upon p38 phosphorylation. In this experimental model, orally administered 10 mg/kg THO did not modulate extracellular serum lactate dehydrogenase. In conclusion, THO decreases hyperglycemia by increasing serum insulin and hepatic glycogen content. The THO mechanism of action on adipose tissue for glucose uptake is suggested to be via GLUT4 translocation stimulation mediated by a p38-dependent mechanism. THO is a potential antihyperglycemic agent that acts in a target tissue for glucose homeostasis.

Primary cilia and lipid raft dynamics

Primary cilia, antenna-like structures of the plasma membrane, detect various extracellular cues and transduce signals into the cell to regulate a wide range of functions. Lipid rafts, plasma membrane microdomains enriched in cholesterol, sphingolipids and specific proteins, are also signalling hubs involved in a myriad of physiological functions. Although impairment of primary cilia and lipid rafts is associated with various diseases, the relationship between primary cilia and lipid rafts is poorly understood. Here, we review a newly discovered interaction between primary cilia and lipid raft dynamics that occurs during Akt signalling in adipogenesis. We also discuss the relationship between primary cilia and lipid raft-mediated Akt signalling in cancer biology. This review provides a novel perspective on primary cilia in the regulation of lipid raft dynamics.

Phospholipid exchange shows insulin receptor activity is supported by both the propensity to form wide bilayers and ordered raft domains

Insulin receptor (IR) is a membrane tyrosine kinase that mediates the response of cells to insulin. IR activity has been shown to be modulated by changes in plasma membrane lipid composition, but the properties and structural determinants of lipids mediating IR activity are poorly understood. Here, using efficient methyl-alpha-cyclodextrin mediated lipid exchange, we studied the effect of altering plasma membrane outer leaflet phospholipid composition upon the activity of IR in mammalian cells. After substitution of endogenous lipids with lipids having an ability to form liquid ordered (Lo) domains (sphingomyelins) or liquid disordered (Ld) domains (unsaturated phosphatidylcholines (PCs)), we found that the propensity of lipids to form ordered domains is required for high IR activity. Additional substitution experiments using a series of saturated PCs showed that IR activity increased substantially with increasing acyl chain length, which increases both bilayer width and the propensity to form ordered domains. Incorporating purified IR into alkyl maltoside micelles with increasing hydrocarbon lengths also increased IR activity, but more modestly than by increasing lipid acyl chain length in cells. These results suggest that the ability to form Lo domains as well as wide bilayer width contributes to increased IR activity. Inhibition of phosphatases showed that some of the lipid dependence of IR activity upon lipid structure reflected protection from phosphatases by lipids that support Lo domain formation. These results are consistent with a model in which a combination of bilayer width and ordered domain formation modulates IR activity via IR conformation and accessibility to phosphatases.

Cell cycle dependence on the mevalonate pathway: Role of cholesterol and non-sterol isoprenoids

The mevalonate pathway is responsible for the synthesis of isoprenoids, including sterols and other metabolites that are essential for diverse biological functions. Cholesterol, the main sterol in mammals, and non-sterol isoprenoids are in high demand by rapidly dividing cells. As evidence of its importance, many cell signaling pathways converge on the mevalonate pathway and these include those involved in proliferation, tumor-promotion, and tumor-suppression. As well as being a fundamental building block of cell membranes, cholesterol plays a key role in maintaining their lipid organization and biophysical properties, and it is crucial for the function of proteins located in the plasma membrane. Importantly, cholesterol and other mevalonate derivatives are essential for cell cycle progression, and their deficiency blocks different steps in the cycle. Furthermore, the accumulation of non-isoprenoid mevalonate derivatives can cause DNA replication stress. Identification of the mechanisms underlying the effects of cholesterol and other mevalonate derivatives on cell cycle progression may be useful in the search for new inhibitors, or the repurposing of preexisting cholesterol biosynthesis inhibitors to target cancer cell division. In this review, we discuss the dependence of cell division on an active mevalonate pathway and the role of different mevalonate derivatives in cell cycle progression.

Decreased caveolae in AGPAT2 lacking adipocytes is independent of changes in cholesterol or sphingolipid levels: A whole cell and plasma membrane lipidomic analysis of adipogenesis

BACKGROUND

Adipocytes from lipodystrophic Agpat2^-/- mice have impaired adipogenesis and fewer caveolae. Herein, we examined whether these defects are associated with changes in lipid composition or abnormal levels of caveolae-associated proteins. Lipidome changes were quantified in differentiated Agpat2^-/- adipocytes to identify lipids with potential adipogenic roles.

METHODS

Agpat2^-/- and wild type brown preadipocytes were differentiated in vitro. Plasma membrane was purified by ultracentrifugation. Number of caveolae and caveolae-associated proteins, as well as sterol, sphingolipid, and phospholipid lipidome were determined across differentiation.

RESULTS

Differentiated Agpat2^-/- adipocytes had decreased caveolae number but conserved insulin signaling. Caveolin-1 and cavin-1 levels were equivalent between Agpat2^-/- and wild type adipocytes. No differences in PM cholesterol and sphingolipids abundance were detected between genotypes. Levels of phosphatidylserine at day 10 of differentiation were increased in Agpat2^-/- adipocytes. Wild type adipocytes had increased whole cell triglyceride, diacylglycerol, phosphatidylglycerol, phosphatidic acid, lysophosphatidylcholine, lysophosphatidylethanolamine, and trihexosyl ceramide, and decreased 24,25-dihydrolanosterol and sitosterol, as a result of adipogenic differentiation. By contrast, adipogenesis did not modify whole cell neutral lipids but increased lysophosphatidylcholine, sphingomyelin, and trihexosyl ceramide levels in Agpat2^-/- adipocytes. Unexpectedly, adipogenesis decreased PM levels of main phospholipids in both genotypes.

CONCLUSION

In Agpat2^-/- adipocytes, decreased caveolae is not associated with changes in PM cholesterol nor sphingolipid levels; however, increased PM phosphatidylserine content may be implicated. Abnormal lipid composition is associated with the adipogenic abnormalities of Agpat2 -/- adipocytes but does not prevent insulin signaling.

Variable Expressivity and Allelic Heterogeneity in Type 2 Familial Partial Lipodystrophy: The p.(Thr528Met) LMNA Variant

Type 2 familial partial lipodystrophy, or Dunnigan disease, is a metabolic disorder characterized by abnormal subcutaneous adipose tissue distribution. This rare condition results from variants principally affecting exons 8 and 11 of the LMNA gene. In this study, five FPLD2-diagnosed patients carrying the c.1583C>T, p.(Thr528Met) variant in exon 9 of the LMNA gene and with obvious clinical heterogeneity were evaluated. Specific polymorphisms in LMNA and in PPARG were also detected. Exhaustive clinical course, physical examination, biochemical features and family history were recorded, along with the assessment of anthropometric features and body composition by dual-energy X-ray absorptiometry. Preadipocytes obtained from a T528M patient were treated with the classic adipose differentiation medium with pioglitazone. Various adipogenes were evaluated by real-time PCR, and immunofluorescence was used to study intracellular localization of emerin, lamin A and its precursors. As demonstrated with Oil red O staining, the preadipocytes of the T528M patient failed to differentiate, the expression of various adipogenic genes was reduced in the lipodystrophic patient and immunofluorescence studies showed an accumulation of farnesylated prelamin A in T528M cells. We conclude that the T528M variant in LMNA could lead to FPLD2, as the adipogenic machinery is compromised.

Live-cell monitoring of protein localization to membrane rafts using protein-fragment complementation

The plasma membrane consists of a variety of discrete domains differing from the surrounding membrane in composition and properties. Selective partitioning of protein to these microdomains is essential for membrane functioning and integrity. Studying the nanoscale size and dynamic nature of the membrane microdomains requires advanced imaging approaches with a high spatiotemporal resolution and, consequently, expensive and specialized equipment, unavailable for most researchers and unsuited for large-scale studies. Thus, understanding of protein partitioning to the membrane microdomains in health and disease is still hampered by the lack of inexpensive live-cell approaches with an appropriate spatial resolution. Here, we have developed a novel approach based on Gaussia princeps luciferase protein-fragment complementation assay to quantitively investigate protein partitioning to cholesterol and sphingomyelin-rich domains, sometimes called ‘lipid rafts’, in intact living cells with a high-spatial resolution. In the assay, the reporter construct, carrying one half of the luciferase protein, is targeted to lipid microdomains through the fused acetylation motif from Src-family kinase Fyn. A protein of interest carries the second half of the luciferase protein. Together, this serves as a reversible real-time sensor of raft recruitment for the studied protein. We demonstrated that the assay can efficiently detect the dynamic alterations in raft localization of two disease-associated proteins: Akt and APP. Importantly, this method can be used in high-throughput screenings and other large-scale studies in living cells. This inexpensive, and easy to implement raft localization assay will benefit all researchers interested in protein partitioning in rafts.

Primary cilia-dependent lipid raft/caveolin dynamics regulate adipogenesis

Primary cilia play a pivotal role in signal transduction and development and are known to serve as signaling hubs. Recent studies have shown that primary cilium dysfunction influences adipogenesis, but the mechanisms are unclear. Here, we show that mesenchymal progenitors C3H10T1/2 depleted of trichoplein, a key regulator of cilium formation, have significantly longer cilia than control cells and fail to differentiate into adipocytes. Mechanistically, the elongated cilia prevent caveolin-1- and/or GM3-positive lipid rafts from being assembled around the ciliary base where insulin receptor proteins accumulate, thereby inhibiting the insulin-Akt signaling. We further generate trichoplein knockout mice, in which adipogenic progenitors display elongated cilia and impair the lipid raft dynamics. The knockout mice on an extended high-fat diet exhibit reduced body fat and smaller adipocytes than wild-type (WT) mice. Overall, our results suggest a role for primary cilia in regulating adipogenic signal transduction via control of the lipid raft dynamics around cilia.

The Placenta as a Target for Alcohol During Pregnancy: The Close Relation with IGFs Signaling Pathway

Alcohol is one of the most consumed drugs in the world, even during pregnancy. Its use is a risk factor for developing adverse outcomes, e.g. fetal death, miscarriage, fetal growth restriction, and premature birth, also resulting in fetal alcohol spectrum disorders. Ethanol metabolism induces an oxidative environment that promotes the oxidation of lipids and proteins, triggers DNA damage, and advocates mitochondrial dysfunction, all of them leading to apoptosis and cellular injury. Several organs are altered due to this harmful behavior, the brain being one of the most affected. Throughout pregnancy, the human placenta is one of the most important organs for women's health and fetal development, as it secretes numerous hormones necessary for a suitable intrauterine environment. However, our understanding of the human placenta is very limited and even more restricted is the knowledge of the impact of toxic substances in its development and fetal growth. So, could ethanol consumption during this period have wounding effects in the placenta, compromising proper fetal organ development? Several studies have demonstrated that alcohol impairs various signaling cascades within G protein-coupled receptors and tyrosine kinase receptors, mainly through its action on insulin and insulin-like growth factor 1 (IGF-1) signaling pathway. This last cascade is involved in cell proliferation, migration, and differentiation and in placentation. This review tries to examine the current knowledge and gaps in our existing understanding of the ethanol effects in insulin/IGFs signaling pathway, which can explain the mechanism to elucidate the adverse actions of ethanol in the maternal-fetal interface of mammals.

Predictive network modeling in human induced pluripotent stem cells identifies key driver genes for insulin responsiveness

Insulin resistance (IR) precedes the development of type 2 diabetes (T2D) and increases cardiovascular disease risk. Although genome wide association studies (GWAS) have uncovered new loci associated with T2D, their contribution to explain the mechanisms leading to decreased insulin sensitivity has been very limited. Thus, new approaches are necessary to explore the genetic architecture of insulin resistance. To that end, we generated an iPSC library across the spectrum of insulin sensitivity in humans. RNA-seq based analysis of 310 induced pluripotent stem cell (iPSC) clones derived from 100 individuals allowed us to identify differentially expressed genes between insulin resistant and sensitive iPSC lines. Analysis of the co-expression architecture uncovered several insulin sensitivity-relevant gene sub-networks, and predictive network modeling identified a set of key driver genes that regulate these co-expression modules. Functional validation in human adipocytes and skeletal muscle cells (SKMCs) confirmed the relevance of the key driver candidate genes for insulin responsiveness.

Insulin resistance is characterized by a defective response (“resistance”) to normal insulin concentrations to uptake the glucose present in the blood, and is the underlying condition that leads to type 2 diabetes (T2D) and increases the risk of cardiovascular disease. It is estimated that 25–33% of the US population are insulin resistant enough to be at risk of serious clinical consequences. For more than a decade, large population studies have tried to discover the genes that participate in the development of insulin resistance, but without much success. It is now increasingly clear that the complex genetic nature of insulin resistance requires novel approaches centered in patient specific cellular models. To fill this gap, we have generated an induced pluripotent stem cell (iPSC) library from individuals with accurate measurements of insulin sensitivity, and performed gene expression and key driver analyses. Our work demonstrates that iPSCs can be used as a revolutionary technology to model insulin resistance and to discover key genetic drivers. Moreover, they can develop our basic knowledge of the disease, and are ultimately expected to increase the therapeutic targets to treat insulin resistance and type 2 diabetes.

Microvillar dynamic in renal tubular epithelial cells mediated by insulin/PLCγ signal pathway

Significant cellular morphology changes in renal tubules were observed in diabetes patients and animal models. However, the interaction between insulin and tubular epithelial cells microvillar structure remains obscure. To understand microvillar dynamics, we used Scanning Ion Conductance Microscope to visualize microvillar in the living cell. Here, we found two layers of microvilli on the tubular epithelial cell surface: short compact microvilli and netlike long microvilli. Insulin treatment could increase microvilli length and density. This process was mediated by the PI₃K/PLCγ signaling pathway, other than the PI₃K/Arp2/3 signal pathway. In conclusion, our findings present a novel insulin signaling transduction mechanism, which contributes to understanding renal tubular epithelial cell microvilli dynamic regulation.

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.

Regulation of caveolae through cholesterol-depletion-dependent tubulation mediated by PACSIN2

The membrane-shaping ability of PACSIN2 (also known as syndapin II), which is mediated by its F-BAR domain, has been shown to be essential for caveolar morphogenesis, presumably through the shaping of the caveolar neck. Caveolar membranes contain abundant cholesterol. However, the role of cholesterol in PACSIN2-mediated membrane deformation remains unclear. Here, we show that the binding of PACSIN2 to the membrane can be negatively regulated by cholesterol. We prepared reconstituted membranes based on the lipid composition of caveolae. The reconstituted membrane with cholesterol had a weaker affinity for the F-BAR domain of PACSIN2 than a membrane without cholesterol. Consistent with this, upon depletion of cholesterol from the plasma membrane, PACSIN2 localized at tubules that had caveolin-1 at their tips, suggesting that cholesterol inhibits membrane tubulation mediated by PACSIN2. The tubules induced by PACSIN2 could be representative of an intermediate of caveolae endocytosis. Consistent with this, the removal of caveolae from the plasma membrane upon cholesterol depletion was diminished in the PACSIN2-deficient cells. These data suggest that PACSIN2-mediated caveolae internalization is dependent on the amount of cholesterol, providing a mechanism for cholesterol-dependent regulation of caveolae.This article has an associated First Person interview with the first author of the paper.

Caveolae and Lipid Rafts in Endothelium: Valuable Organelles for Multiple Functions

Caveolae are flask-shaped invaginations of the plasma membrane found in numerous cell types and are particularly abundant in endothelial cells and adipocytes. The lipid composition of caveolae largely matches that of lipid rafts microdomains that are particularly enriched in cholesterol, sphingomyelin, glycosphingolipids, and saturated fatty acids. Unlike lipid rafts, whose existence remains quite elusive in living cells, caveolae can be clearly distinguished by electron microscope. Despite their similar composition and the sharing of some functions, lipid rafts appear more heterogeneous in terms of size and are more dynamic than caveolae. Following the discovery of caveolin-1, the first molecular marker as well as the unique scaffolding protein of caveolae, we have witnessed a remarkable increase in studies aimed at investigating the role of these organelles in cell functions and human disease. The goal of this review is to discuss the most recent studies related to the role of caveolae and caveolins in endothelial cells. We first recapitulate the major embryological processes leading to the formation of the vascular tree. We next discuss the contribution of caveolins and cavins to membrane biogenesis and cell response to extracellular stimuli. We also address how caveolae and caveolins control endothelial cell metabolism, a central mechanism involved in migration proliferation and angiogenesis. Finally, as regards the emergency caused by COVID-19, we propose to study the caveolar platform as a potential target to block virus entry into endothelial cells.

Pleiotropic use of Statins as non-lipid-lowering drugs

Statins, known as HMG-CoA reductase (HMGCR) inhibitors, have primarily been utilized for metabolic and angiographic medical applications because of their cholesterol-lowering effects. Similar to other drugs, statins may also induce a series of potential side effects. Statins inhibit the HMGCR (rate-limiting enzyme) activity in early stages of mevalonate pathway and then indirectly affect a number of intermediate products, including non-sterol isoprenoids (coenzyme Q10, dolichol etc.), which can result in impaired functions of body organs. Recently, scores of studies have uncovered additional functional mechanisms of statins in other diseases, such as diabetes mellitus, nervous system diseases, coronary heart disease, inflammation and cancers. This review aims to summarize the positive and adverse mechanisms of statin therapy. Statin care should be taken in the treatment of many diseases including cancers. Since the underlying mechanisms are not fully elucidated, future studies should spend more time and efforts on basic research to explore the mechanisms of statins.

A Role for the Insulin Receptor in the Suppression of Dengue Virus and Zika Virus in Wolbachia-Infected Mosquito Cells

Wolbachia-infected mosquitoes are refractory to super-infection with arthropod-borne pathogens, but the role of host cell signaling proteins in pathogen-blocking mechanisms remains to be elucidated. Here, we use an antibody microarray approach to provide a comprehensive picture of the signaling response of Aedes aegypti-derived cells to Wolbachia. This approach identifies the host cell insulin receptor as being downregulated by the bacterium. Furthermore, siRNA-mediated knockdown and treatment with a small-molecule inhibitor of the insulin receptor kinase concur to assign a crucial role for this enzyme in the replication of dengue and Zika viruses in cultured mosquito cells. Finally, we show that the production of Zika virus in Wolbachia-free live mosquitoes is impaired by treatment with the selective inhibitor mimicking Wolbachia infection. This study identifies Wolbachia-mediated downregulation of insulin receptor kinase activity as a mechanism contributing to the blocking of super-infection by arboviruses.

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.

Impaired glucagon suppression and reduced insulin sensitivity in subjects with prediabetes undergoing atorvastatin therapy

OBJECTIVE

Statin therapy has been linked to an increased risk of type 2 diabetes in high-risk populations; however, the pathophysiology of this association remains to be clarified. We investigated glucagon suppression and its relationship with insulin resistance in prediabetic subjects undergoing atorvastatin therapy; in addition, we studied molecular insulin signaling in pancreatic α-cells exposed to atorvastatin in vitro.

DESIGN AND METHODS

Fifty subjects with prediabetes were divided into two groups based on atorvastatin therapy. All subjects underwent an oral glucose tolerance test. Early (0-30 min), late (30-120 min) and overall (0-120 min) glucagon suppression were evaluated. Insulin sensitivity was estimated by the insulin sensitivity index (ISI0-120). Insulin signaling pathway and insulin-mediated glucagon suppression were investigated in pancreatic αTC1-6 cells chronically exposed (24 or 48 h) to atorvastatin (100 ng/mL).

RESULTS

Individuals on statin therapy (n = 26) showed a significantly reduced early (0-30 min) (P = 0.003) and overall (0-120 min) (P = 0.01) glucagon suppression compared with controls (n = 24). In multivariate regression analysis, early glucagon suppression (0-30 min) exhibited a significant correlation with statin therapy. Regression analysis showed a significant association between ISI 0-120 and early0-30 (r = 0.33, P < 0.05) and overall0-120 (r = 0.38, P < 0.05) glucagon suppression. Moreover, in αTC1-6 cells atorvastatin treatment affected insulin-mediated glucagon suppression, insulin receptor phosphorylation and IRS-1-AKT pathway signaling.

CONCLUSIONS

Prediabetic patients undergoing statin therapy exhibit impaired glucagon suppression associated with lower insulin sensitivity. Our data revealed a new molecular aspect behind the deregulation of insulin sensitivity secondary to statin exposure.

Lipid mediators of insulin signaling in diabetic kidney disease

Diabetic kidney disease (DKD) affects ∼40% of patients with diabetes and is associated with high mortality rates. Among different cellular targets in DKD, podocytes, highly specialized epithelial cells of the glomerular filtration barrier, are injured in the early stages of DKD. Both clinical and experimental data support the role of preserved insulin signaling as a major contributor to podocyte function and survival. However, little is known about the key modulators of podocyte insulin signaling. This review summarizes the novel knowledge that intracellular lipids such as cholesterol and sphingolipids are major determinants of podocyte insulin signaling. In particular, the implications of these lipids on DKD development, progression, and treatment will be addressed.

Lateral interaction of cholesterol with a semifluorinated amphiphile at the air-water interface

Lipid rafts consisting mainly of sphingomyelin and cholesterol (Ch) on biomembrane surfaces are deeply related to cellular processes such as protein trafficking and signal transduction. During the processes, the raft microdomains affect the fluidty of biological membranes, which is controlled to large extents by Ch. In this paper, we have investigated the interaction between Ch and a semiflurinated alcohol (F6H9OH) from the aspect of a fluidty control using surface chemistry. The two-component Langmuir monolayer at the air-water interface was characterized by the surface pressure (π)-molecular area (A) and surface potential (ΔV)-A isotherms. The compressibility modulus and excess Gibbs free energy of mixing were calculated from the π-A isotherms. And also the two-dimensional phase diagram was constructed on the basis of phase transition pressures and monolayer collapse pressures. Furthermore, the phase behavior of binary monolayers was visualized with fluorescence microscopy (in situ) and atomic force microscopy (ex situ). The result here indicates a possibility of fluidity control of Ch-related membranes by arranging the fluorination degree of the constituent lipids.

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.

In vivo Efficacy and Safety Evaluation of Lactosyl-β-cyclodextrin as a Therapeutic Agent for Hepatomegaly in Niemann-Pick Type C Disease

Niemann-Pick type C disease (NPC) is a fatal, autosomal recessive disorder, which causes excessive accumulation of free cholesterol in endolysosomes, resulting in progressive hepatomegaly and neurodegeneration. Currently, 2-hydroxypropyl-β-cyclodextrin (HP-β-CyD) is used at a high dose for the treatment of NPC, risking lung toxicity and hearing loss during treatment. One method to reduce the required dose of HP-β-CyD for the treatment of hepatomegaly is to actively deliver β-cyclodextrin (β-CyD) to hepatocytes. Previously, we synthesized lactosyl-β-CyD (Lac-β-CyD) and demonstrated that it lowers cholesterol in NPC model liver cells. In the present study, we studied the efficacy and safety of Lac-β-CyD treatment of hepatomegaly in Npc1^−/− mice. After subcutaneous administration, Lac-β-CyD accumulated in the liver and reduced hepatomegaly with greater efficacy than HP-β-CyD. In addition, subcutaneous administration of a very high dose of Lac-β-CyD was less toxic to the lungs than HP-β-CyD. Notably, the accumulation of intracellular free cholesterol in endolysosomes of NPC-like liver cells was significantly lower after administration of Lac-β-CyD than after treatment with HP-β-CyD. In conclusion, these results suggest that Lac-β-CyD is a candidate for the effective treatment of hepatomegaly in NPC.

Akt-ing Up Just About Everywhere: Compartment-Specific Akt Activation and Function in Receptor Tyrosine Kinase Signaling

The serine/threonine kinase Akt is a master regulator of many diverse cellular functions, including survival, growth, metabolism, migration, and differentiation. Receptor tyrosine kinases are critical regulators of Akt, as a result of activation of phosphatidylinositol-3-kinase (PI3K) signaling leading to Akt activation upon receptor stimulation. The signaling axis formed by receptor tyrosine kinases, PI3K and Akt, as well as the vast range of downstream substrates is thus central to control of cell physiology in many different contexts and tissues. This axis must be tightly regulated, as disruption of PI3K-Akt signaling underlies the pathology of many diseases such as cancer and diabetes. This sophisticated regulation of PI3K-Akt signaling is due in part to the spatial and temporal compartmentalization of Akt activation and function, including in specific nanoscale domains of the plasma membrane as well as in specific intracellular membrane compartments. Here, we review the evidence for localized activation of PI3K-Akt signaling by receptor tyrosine kinases in various specific cellular compartments, as well as that of compartment-specific functions of Akt leading to control of several fundamental cellular processes. This spatial and temporal control of Akt activation and function occurs by a large number of parallel molecular mechanisms that are central to regulation of cell physiology.

Use of virus-like particles as a native membrane model to study the interaction of insulin with the insulin receptor

There is emerging evidence of the utility of virus-like particles (VLPs) as a novel model for the study of receptor-ligand interactions in a native plasma membrane environment. VLPs consist of a viral core protein encapsulated by portions of the cell membrane with membrane proteins and receptors expressed in their native conformation. VLPs can be generated in mammalian cells by transfection with the retroviral core protein (gag). In this study, we used Chinese hamster ovary (CHO T10) cells stably overexpressing the insulin receptor (IR) to generate IR bearing VLPs. The diameter and size uniformity of VLPs were estimated by dynamic light scattering and morphological features examined by scanning electron microscopy. The presence of high affinity IR on VLPs was demonstrated by competitive binding assays (K_D: 2.3 ± 0.4 nM, n = 3), which was similar to that on the parental CHO T10 cells (K_D: 2.1 ± 0.4 nM, n = 3). We also report that increases or decreases in membrane cholesterol content by treatment with methyl-β-cyclodextrin (MBCD) or cholesterol pre-loaded methyl-β-cyclodextrin (cMBCD), respectively, substantially decreased insulin binding (> 30%) to both VLPs and cells, and we speculate this is due to a change in receptor disposition. We suggest that this novel finding of decreases in insulin binding in response to changes in membrane cholesterol content may largely account for the unexplained decreases in insulin signalling events previously reported elsewhere. Finally, we propose VLPs as a viable membrane model for the study of insulin-IR interactions in a native membrane environment.

Cholesterol Depletion by MβCD Enhances Cell Membrane Tension and Its Variations-Reducing Integrity

Cholesterol depletion by methyl-β-cyclodextrin (MβCD) remodels the plasma membrane’s mechanics in cells and its interactions with the underlying cytoskeleton, whereas in red blood cells, it is also known to cause lysis. Currently it’s unclear if MβCD alters membrane tension or only enhances membrane-cytoskeleton interactions—and how this relates to cell lysis. We map membrane height fluctuations in single cells and observe that MβCD reduces temporal fluctuations robustly but flattens spatial membrane undulations only slightly. Utilizing models explicitly incorporating membrane confinement besides other viscoelastic factors, we estimate membrane mechanical parameters from the fluctuations’ frequency spectrum. This helps us conclude that MβCD enhances membrane tension and does so even on ATP-depleted cell membranes where this occurs despite reduction in confinement. Additionally, on cholesterol depletion, cell membranes display higher intracellular heterogeneity in the amplitude of spatial undulations and membrane tension. MβCD also has a strong impact on the cell membrane’s tenacity to mechanical stress, making cells strongly prone to rupture on hypo-osmotic shock with larger rupture diameters—an effect not hindered by actomyosin perturbations. Our study thus demonstrates that cholesterol depletion increases membrane tension and its variability, making cells prone to rupture independent of the cytoskeletal state of the cell.

Sterol structure dependence of insulin receptor and insulin-like growth factor 1 receptor activation

The plasma membrane is a dynamic environment with a complex composition of lipids, proteins, and cholesterol. Areas enriched in cholesterol and sphingolipids are believed to form lipid rafts, domains of highly ordered lipids. The unique physical properties of these domains have been proposed to influence many cellular processes. Here, we demonstrate that the activation of insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF1R) depends critically on the structures of membrane sterols. IR and IGF1R autophosphorylation in vivo was inhibited by cholesterol depletion, and autophosphorylation was restored by the replacement with exogenous cholesterol. We next screened a variety of sterols for effects on IR activation. The ability of sterols to support IR autophosphorylation was strongly correlated to the propensity of the sterols to form ordered domains. IR autophosphorylation was fully restored by the incorporation of ergosterol, dihydrocholesterol, 7-dehydrocholesterol, lathosterol, desmosterol, and allocholesterol, partially restored by epicholesterol, and not restored by lanosterol, coprostanol, and 4-cholesten-3-one. These data support the hypothesis that the ability to form ordered domains is sufficient for a sterol to support ligand-induced activation of IR and IGF1R in intact mammalian cells.

Metastasis and chemoresistance in CD133 expressing pancreatic cancer cells are dependent on their lipid raft integrity

Metabolic rewiring is an integral part of tumor growth. Among metabolic pathways, the Mevalonic-Acid-Pathway (MVAP) plays a key role in maintaining membrane architecture through cholesterol synthesis, thereby affecting invasiveness. In the current study, we show for the first time that CD133^Hi pancreatic tumor initiating cells (TIC) have increased expression of MVAP enzymes, cholesterol-content and Caveolin expression. Further, we show that CD133 in these cells is localized in the lipid-rafts (characterized by Cav-1-cholesterol association). Disruption of lipid-rafts by either depleting Cav-1 or by inhibiting MVAP by lovastatin decreased metastatic-potential and chemoresistance in CD133^Hi cells while not affecting the CD133^lo cells. Additionally, disruption of lipid-raft results in deregulation of FAK-signaling, decreasing invasiveness in pancreatic-TICs. Furthermore, this also inhibits ABC-transporter activity resulting in sensitizing TICs to standard chemotherapeutic agents. Repurposing existing drugs for new clinical applications is one of the safest and least resource intensive approaches to improve therapeutic options. In this context, our study is extremely timely as it shows that targeting lipid-rafts with statins can sensitize the normally resistant pancreatic TIC^Hi-cells to standard chemotherapy and decrease metastasis, thereby defining a novel strategy for targeting the TIC^Hi-PDAC.

Intermittent hypoxia regulates vasoactive molecules and alters insulin-signaling in vascular endothelial cells

Vascular dysfunction and insulin resistance (IR) are associated with obstructive sleep apnea (OSA), which is characterized by frequent episodes of nocturnal intermittent hypoxia (IH). While it is recognized that the balance between vasoconstrictive (endothelin-1) and vasodilatory molecules (nitric oxide, NO) determine vascular profile, molecular mechanisms contributing to vascular dysfunction and IR in OSA are not completely understood. Caveolin-1 is a membrane protein which regulates endothelial nitric oxide synthase (eNOS) activity which is responsible for NO generation and cellular insulin-signaling. Hence, we examined the effects of IH on caveolin-1, eNOS, and endothelin-1 in human coronary artery endothelial cells in the context of IR. Chronic 3-day IH exposure up-regulated caveolin-1 and endothelin-1 expression while reducing NO. Also, IH altered insulin-mediated activation of AKT but not ERK resulting in increased endothelin-1 transcription. Similarly, caveolin-1 overexpression attenuated basal and insulin-stimulated NO synthesis along with impaired insulin-dependent activation of AKT and eNOS, with no effect on insulin-stimulated ERK1/2 phosphorylation and endothelin-1 transcription. Our data suggest that IH contributes to a vasoconstrictive profile and to pathway-selective vascular IR, whereby insulin potentiates ET-1 expression. Moreover, IH may partly mediate its effects on NO and insulin-signaling via upregulating caveolin-1 expression.

Effect of hypoxia on caveolae-related protein expression and insulin signaling in adipocytes

Obesity is characterized by hypertrophy and hyperplasia of adipose tissue, which have been related to the development of hypoxia and insulin resistance. On the other hand, caveolin-1 (Cav-1), one of the main proteins of caveolae, promotes insulin receptor (IR) phosphorylation and the subsequent activation of insulin signaling. In this work we investigated the effect of hypoxia on Cav-1 regulation and the status of insulin signaling in 3T3-L1 adipocytes. Our results showed that hypoxia inhibited adipogenesis and insulin signaling in adipocytes. Furthermore, 48 h of hypoxia reduced insulin-induced glucose uptake while increased basal glucose uptake. This result was consistent with the upregulation of glucose transporter GLUT1 and the downregulation of GLUT4, which also showed defective translocation to plasma membrane when adipocytes were stimulated with insulin. In addition, the expression of caveolae-related proteins was reduced by hypoxia and chromatin immunoprecipitation assay demonstrated that Cav-1 transcription was directly regulated by HIF-1. These results strengthen the role of caveolae in insulin signaling and help to explain adipocyte response to hypoxia.

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.