Recent advances in membrane microdomains: rafts, caveolae, and intracellular cholesterol trafficking

Goat miR-92a-3p Targets APOL6 Gene to Regulate the Differentiation of Intramuscular Precursor Adipocytes

The quality of lamb meat is positively correlated with intramuscular fat content. In recent years, a large number of studies have shown that miRNAs play an important role in the proliferation and differentiation of adipocytes. In this study, we aimed to explore the effect of miR-92a-3p on the differentiation of goat intramuscular preadipocytes. The results showed that the expression level of miR-92a-3p was low in the early stage of differentiation, reached the highest level on the third day of differentiation, and then decreased. And miR-92a-3p can inhibit the accumulation of lipid droplets and down-regulate the determinants of adipogenic differentiation. Mechanistically, by predicting target genes, we found that miR-92a-3p affects the differentiation of goat intramuscular preadipocytes and the accumulation of lipid droplets by regulating the expression of goat gene APOL6. This study provides important new information to better understand the relationship between miRNAs and the differentiation of goat intramuscular preadipocytes, thus providing a new reference for goat intramuscular adipogenesis.

The Potential of Biologically Active Brazilian Plant Species as a Strategy to Search for Molecular Models for Mosquito Control

Natural products are a valuable source of biologically active compounds and continue to play an important role in modern drug discovery due to their great structural diversity and unique biological properties. Brazilian biodiversity is one of the most extensive in the world and could be an effective source of new chemical entities for drug discovery. Mosquitoes are vectors for the transmission of dengue, Zika, chikungunya, yellow fever, and many other diseases of public health importance. These diseases have a major impact on tropical and subtropical countries, and their incidence has increased dramatically in recent decades, reaching billions of people at risk worldwide. The prevention of these diseases is mainly through vector control, which is becoming more difficult because of the emergence of resistant mosquito populations to the chemical insecticides. Strategies to provide efficient and safe vector control are needed, and secondary metabolites from plant species from the Brazilian biodiversity, especially Cerrado, that are biologically active for mosquito control are herein highlighted. Also, this is a literature revision of targets as insights to promote advances in the task of developing active compounds for vector control. In view of the expansion and occurrence of arboviruses diseases worldwide, scientific reviews on bioactive natural products are important to provide molecular models for vector control and contribute with effective measures to reduce their incidence.

Phosphatidylserine biosynthesis pathways in lipid homeostasis: Toward resolution of the pending central issue for decades

Enzymatic control of lipid homeostasis in the cell is a vital element in the complex organization of life. Phosphatidylserine (PS) is an essential anionic phospholipid of cell membranes, and conducts numerous roles for their structural and functional integrity. In mammalian cells, two distinct enzymes phosphatidylserine synthases-1 (PSS1) and -2 (PSS2) in the mitochondria-associated membrane (MAM) in the ER perform de novo synthesis of PS. It is based on base-exchange reactions of the preexisting dominant phospholipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE). While PSS2 specifically catalyzes the reaction "PE → PS," whether or not PSS1 is responsible for the same reaction along with the reaction "PC → PS" remains unsettled despite its fundamental impact on the major stoichiometry. We propose here that a key but the only report that appeared to have put scientists on hold for decades in answering to this issue may be viewed consistently with other available research reports; PSS1 utilizes the two dominant phospholipid classes at a similar intrinsic rate. In this review, we discuss the issue in view of the current information for the enzyme machineries, membrane structure and dynamics, intracellular network of lipid transport, and PS synthesis in health and disease. Resolution of the pending issue is thus critical in advancing our understanding of roles of the essential anionic lipid in biology, health, and disease.

Unidirectional Regulation of Vimentin Intermediate Filaments to Caveolin-1

Both the mechanosensitive vimentin cytoskeleton and endocytic caveolae contribute to various active processes such as cell migration, morphogenesis, and stress response. However, the crosstalk between these two systems has remained elusive. Here, we find that the subcellular expression between vimentin and caveolin-1 is mutual exclusive, and vimentin filaments physically arrest the cytoplasmic motility of caveolin-1 vesicles. Importantly, vimentin depletion increases the phosphorylation of caveolin-1 on site Tyr14, and restores the compromised cell migration rate and directionality caused by caveolin-1 deprivation. Moreover, upon hypo-osmotic shock, vimentin-knockout recovers the reduced intracellular motility of caveolin-1 vesicles. In contrary, caveolin-1 depletion shows no effect on the expression, phosphorylation (on sites Ser39, Ser56, and Ser83), distribution, solubility, and cellular dynamics of vimentin filaments. Taken together, our data reveals a unidirectional regulation of vimentin to caveolin-1, at least on the cellular level.

Lipids and Membrane Microdomains: The Glycerolipid and Alkylphosphocholine Class of Cancer Chemotherapeutic Drugs

Synthetic antitumor lipids are metabolically stable lysophosphatidylcholine derivatives, encompassing a class of non-mutagenic drugs that selectively target cancerous cells. In this chapter we review the literature as relates to the clinical efficacy of these antitumor lipid drugs and how our understanding of their mode of action has evolved alongside key advances in our knowledge of membrane structure, organization, and function. First, the history of the development of this class of drugs is described, providing a summary of clinical outcomes of key members including edelfosine, miltefosine, perifosine, erufosine, and erucylphosphocholine. A detailed description of the biophysical properties of these drugs and specific drug-lipid interactions which may contribute to the selectivity of the antitumor lipids for cancer cells follows. An updated model on the mode of action of these lipid drugs as membrane disorganizing agents is presented. Membrane domain organization as opposed to targeting specific proteins on membranes is discussed. By altering membranes, these antitumor lipids inhibit many survival pathways while activating pro-apoptotic signals leading to cell demise.

Vimentin intermediate filaments function as a physical barrier during intracellular trafficking of caveolin-1

Both the cytoskeletal intermediate filaments (IFs) and cytoplasmic caveolae contribute to active processes such as cell migration, morphogenesis and vesicular trafficking, but the interplay between these two systems has remained elusive. Here, we find that vimentin and nestin IFs interact with caveolae central component caveolin-1 (CAV-1) and importantly, restrain the intracellular trafficking of CAV-1 positive vesicles by serving as a physical barrier. Consequently, CAV-1 vesicles show less density and mobility in vimentin IFs enriched region, which is a substrate stiffness independent process. Moreover, depletion of vimentin IFs releases the slow movement proportion of CAV-1 positive vesicles and thus increases their cytoplasmic dynamics, whereas the expression of caveolae-associated protein CAV-1, CAV-2 and cavin-1 were unaffected. Collectively, these results reveal a negative role of IFs in regulating the trafficking of intracellular CAV-1 vesicles in live cells.

Thermoresponsive nanocomposite gel for local drug delivery to suppress the growth of glioma by inducing autophagy

Although the treatments of malignant glioma include surgery, radiotherapy and chemotherapy by oral drug administration, the prognosis of patients with glioma remains very poor. We developed a polyethylene glycol-dipalmitoylphosphatidyle- thanoiamine (mPEG-DPPE) calcium phosphate nanoparticles (NPs) injectable thermoresponsive hydrogel (nanocomposite gel) that could provide a sustained and local delivery of paclitaxel (PTX) and temozolomide (TMZ). In addition, the proportion of PTX and TMZ for the optimal synergistic antiglioma effect on C6 cells was determined to be 1:100 (w/w) by the Chou and Talalay method. Our results clearly indicated that the autophagy induced by PTX:TMZ NPs plays an important role in regulating tumor cell death, while autophagy inhibition dramatically reverses the antitumor effect of PTX:TMZ NPs, suggesting that antiproliferative autophagy occurs in response to PTX:TMZ NPs treatment. The antitumor efficacy of the PTX:TMZ NP-loaded gel was evaluated in situ using C6 tumor-bearing rats, and the PTX:TMZ NP-loaded gel exhibited superior antitumor performance. The antitumor effects of the nanocomposite gel in vivo were shown to correlate with autophagic cell death in this study. The in vivo results further confirmed the advantages of such a strategy. The present study may provide evidence supporting the development of nanomedicine for potential clinical application.

The impact of impaired macrophage functions in cystic fibrosis disease progression

The underlying cause of morbidity in cystic fibrosis (CF) is the decline in lung function, which results in part from chronic inflammation. Inflammation and infection occur early in infancy in CF and the role of innate immune defense in CF has been highlighted in the last years. Once thought simply to be consumers of bacteria, macrophages have emerged as highly sensitive immune cells that are located at the balance point between inflammation and resolution of this inflammation in CF pathophysiology. In order to assess the potential role of macrophage in CF, we review the evidence that: (1) CF macrophage has a dysregulated inflammatory phenotype; (2) CF macrophage presents altered phagocytosis capacity and bacterial killing; and (3) lipid disorders in CF macrophage affect its function. These alterations of macrophage weaken innate defense of CF patients and may be involved in CF disease progression and lung damage.

Caveolae regulate vasoconstriction of conduit arteries to angiotensin II in hindlimb unweighted rats

Weightlessness induces the functional remodelling of arteries, but the changes to angiotensin II (Ang II)-elicited vasoconstriction and the underlying mechanism have never been reported. Caveolae are invaginations of the cell membrane crucial for the contraction of vascular smooth muscle cells, so we investigated the adaptation of Ang II-elicited vasoconstriction to simulated weightlessness and the role of caveolae in it. The 4 week hindlimb unweighted (HU) rat was used to simulate the effects of weightlessness. Ang II-elicited vasoconstriction was measured by isometric force recording. The morphology of caveolae was examined by transmission electron microscope. The binding of the angiotensin II type 1 receptor (AT1 ) and caveolin-1 (cav-1) was examined by coimmunoprecipitation and Western blot. We found that the maximal developing force (E(max)) of Ang II-elicited vasoconstriction was decreased in abdominal aorta by 30.6%, unchanged in thoracic aorta and increased in carotid artery by 17.9% after HU, while EC50 of the response was increased in all three arteries (P < 0.05). AT1 desensitization upon activation was significantly reduced by HU in all three arteries, as was the number of caveolae (P < 0.05). Furthermore, Ang II promoted the binding of AT1 and cav-1 significantly in control but not HU arteries. Both the number of caveolae and the binding of AT1 and cav-1 in HU arteries were restored by cholesterol pretreatment which also reinstated the change in EC50 as well as the level of AT1 desensitization. These results indicate that modified caveolae in vascular smooth muscle cells could interfere with the binding of AT1 and cav-1 mediating the adaptation of Ang II-elicited vasoconstriction to HU.

Itraconazole suppresses the growth of glioblastoma through induction of autophagy: involvement of abnormal cholesterol trafficking

Glioblastoma is one of the most aggressive human cancers with poor prognosis, and therefore a critical need exists for novel therapeutic strategies for management of glioblastoma patients. Itraconazole, a traditional antifungal drug, has been identified as a novel potential anticancer agent due to its inhibitory effects on cell proliferation and tumor angiogenesis; however, the molecular mechanisms involved are still unclear. Here, we show that itraconazole inhibits the proliferation of glioblastoma cells both in vitro and in vivo. Notably, we demonstrate that treatment with itraconazole induces autophagic progression in glioblastoma cells, while blockage of autophagy markedly reverses the antiproliferative activities of itraconazole, suggesting an antitumor effect of autophagy in response to itraconazole treatment. Functional studies revealed that itraconazole retarded the trafficking of cholesterol from late endosomes and lysosomes to the plasma membrane by reducing the levels of SCP2, resulting in repression of AKT1-MTOR signaling, induction of autophagy, and finally inhibition of cell proliferation. Together, our studies provide new insights into the molecular mechanisms regarding the antitumor activities of itraconazole, and may further assist both the pharmacological investigation and rational use of itraconazole in potential clinical applications.

Cholesterol as a causative factor in Alzheimer's disease: a debatable hypothesis

High serum/plasma cholesterol levels have been suggested as a risk factor for Alzheimer's disease (AD). Some reports, mostly retrospective epidemiological studies, have observed a decreased prevalence of AD in patients taking the cholesterol lowering drugs, statins. The strongest evidence causally linking cholesterol to AD is provided by experimental studies showing that adding/reducing cholesterol alters amyloid precursor protein (APP) and amyloid beta-protein (Ab) levels. However, there are problems with the cholesterol-AD hypothesis. Cholesterol levels in serum/plasma and brain of AD patients do not support cholesterol as a causative factor in AD.Prospective studies on statins and AD have largely failed to show efficacy. Even the experimental data are open to interpretation given that it is well-established that modification of cholesterol levels has effects on multiple proteins, not only amyloid precursor protein and Ab. The purpose of this review, therefore, was to examine the above-mentioned issues, discuss the pros and cons of the cholesterol-AD hypothesis, involvement of other lipids in the mevalonate pathway, and consider that AD may impact cholesterol homeostasis.

Mixed modeling of meta-analysis P-values (MixMAP) suggests multiple novel gene loci for low density lipoprotein cholesterol

Informing missing heritability for complex disease will likely require leveraging information across multiple SNPs within a gene region simultaneously to characterize gene and locus-level contributions to disease phenotypes. To this aim, we introduce a novel strategy, termed Mixed modeling of Meta-Analysis P-values (MixMAP), that draws on a principled statistical modeling framework and the vast array of summary data now available from genetic association studies, to test formally for locus level association. The primary inputs to this approach are: (a) single SNP level p-values for tests of association; and (b) the mapping of SNPs to genomic regions. The output of MixMAP is comprised of locus level estimates and tests of association. In application of MixMAP to summary data from the Global Lipids Gene Consortium, we suggest twelve new loci (PKN, FN1, UGT1A1, PPARG, DMDGH, PPARD, CDK6, VPS13B, GAD2, GAB2, APOH and NPC1) for low-density lipoprotein cholesterol (LDL-C), a causal risk factor for cardiovascular disease and we also demonstrate the potential utility of MixMAP in small data settings. Overall, MixMAP offers novel and complementary information as compared to SNP-based analysis approaches and is straightforward to implement with existing open-source statistical software tools.

Role of cholesterol in APP metabolism and its significance in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is a complex multifactorial neurodegenerative disorder believed to be initiated by accumulation of amyloid β (Aβ)-related peptides derived from proteolytic processing of amyloid precursor protein (APP). Research over the past two decades provided a mechanistic link between cholesterol and AD pathogenesis. Genetic polymorphisms in genes regulating the pivotal points in cholesterol metabolism have been suggested to enhance the risk of developing AD. Altered neuronal membrane cholesterol level and/or subcellular distribution have been implicated in aberrant formation, aggregation, toxicity, and degradation of Aβ-related peptides. However, the results are somewhat contradictory and we still do not have a complete understanding on how cholesterol can influence AD pathogenesis. In this review, we summarize our current understanding on the role of cholesterol in regulating the production/function of Aβ-related peptides and also examine the therapeutic potential of regulating cholesterol homeostasis in the treatment of AD pathology.

Quantification of cholesterol solubilized in bile salt micellar aqueous solutions using (13)C nuclear magnetic resonance

In this work, we develop a methodology to quantitatively follow the solubilization of cholesterol on glycodeoxycholic acid (GDCA) micelles using (13)C nuclear magnetic resonance (NMR). The amount of solubilized cholesterol enriched in (13)C at position 4, [4-(13)C]cholesterol, was quantified from the area of its resonance, at 44.5 ppm, using the CH(2) groups from GDCA as an internal reference. The loading of the micelles with cholesterol leads to a quantitative upper field shift of most carbons in the nonpolar surface of GDCA, and this was used to follow the solubilization of unlabeled cholesterol. The solubilization followed a pseudo first-order kinetics with a characteristic time constant of 3.6 h, and the maximum solubility of cholesterol in 50 mM total lipid (GDCA + cholesterol) is 3.0 ± 0.1mM, corresponding to a mean occupation number per micelle ≥1. The solubilization profile indicates that the affinity of cholesterol for the GDCA micelles is unaffected by the presence of the solute, leading essentially to full solubilization up to the saturation limit. The relaxation times of GDCA carbons at 50mM give information regarding its aggregation and indicate that GDCA is associated in small micelles (hydrodynamic [Rh] = 1.1 nm) without any evidence for formation of larger secondary micelles. This was confirmed by dynamic light scattering results.

Development and use of ceramide nanoliposomes in cancer

Integration of C₆-ceramide into stealth pegylated nanoliposomes has led to the development of a promising preclinical therapeutic alone and in combination with other agents for treatment of cancer. Ceramide itself has been implicated as a bioactive lipid second messenger mediating cell senescence, cell cycle arrest, and apoptosis. Recent lipidomic analyses have demonstrated that specific ceramide species are differentially metabolized in individual cancers. Therapeutics that increase ceramide levels in cancer tissues have shown increased cell death and tumor inhibition. However, the use of ceramide itself as therapeutic has been problematic due to its inherent hydrophobicity and insolubility, therefore limiting the application for intravenous administration. Pegylated nanoliposomes eliminate this issue and are able to enhance the intracellular delivery of ceramide to cancer cells.

Proteolipidic composition of exosomes changes during reticulocyte maturation

During the orchestrated process leading to mature erythrocytes, reticulocytes must synthesize large amounts of hemoglobin, while eliminating numerous cellular components. Exosomes are small secreted vesicles that play an important role in this process of specific elimination. To understand the mechanisms of proteolipidic sorting leading to their biogenesis, we have explored changes in the composition of exosomes released by reticulocytes during their differentiation, in parallel to their physical properties. By combining proteomic and lipidomic approaches, we found dramatic alterations in the composition of the exosomes retrieved over the course of a 7-day in vitro differentiation protocol. Our data support a previously proposed model, whereby in reticulocytes the biogenesis of exosomes involves several distinct mechanisms for the preferential recruitment of particular proteins and lipids and suggest that the respective prominence of those pathways changes over the course of the differentiation process.

Neuronal glycoprotein M6a induces filopodia formation via association with cholesterol-rich lipid rafts

A neuronal integral membrane glycoprotein M6a has been suggested to be involved in a number of biological processes, including neuronal remodeling and differentiation, trafficking of mu-opioid receptors, and Ca(2+) transportation. Moreover, pathological situations such as chronic stress in animals and depression in humans have been associated with alterations in M6a sequence and expression. The mechanism of action of M6a is essentially unknown. In this work, we analyze the relevance of M6a distribution in plasma membrane, namely its lipid microdomain targeting, for its biological function in filopodia formation. We demonstrate that M6a is localized in membrane microdomains compatible with lipid rafts in cultured rat hippocampal neurons. Removal of cholesterol from neuronal membranes with methyl-β-cyclodextrin decreases M6a-induced filopodia formation, an effect that is reversed by the addition of cholesterol. Inhibition of Src kinases and MAPK prevents filopodia formation in M6a-over-expressing neurons. Src-deficient SYF cells over-expressing M6a fail to promote filopodia formation. Taken together, our findings reveal that the association of M6a with lipid rafts is important for its role in filopodia formation and Src and MAPK kinases participate in M6a signal propagation.

Lysosomes, cholesterol and atherosclerosis

Cholesterol-engorged macrophage foam cells are a critical component of the atherosclerotic lesion. Reducing the sterol deposits in lesions reduces clinical events. Sterol accumulations within lysosomes have proven to be particularly hard to mobilize out of foam cells. Moreover, excess sterol accumulation in lysosomes has untoward effects, including a complete disruption of lysosome function. Recently, we demonstrated that treatment of sterol-engorged macrophages in culture with triglyceride-containing particles can reverse many of the effects of cholesterol on lysosomes and dramatically reduce the sterol burden in these cells. This article describes what is known about lysosomal sterol engorgement, discusses the possible mechanisms by which triglyceride could produce its effects, and evaluates the possible positive and negative effects of reducing the lysosomal cholesterol levels in foam cells.

Glycosylphosphatidylinositol anchor analogues sequester cholesterol and reduce prion formation

A hallmark of prion diseases is the conversion of the host-encoded prion protein (PrP(C) where C is cellular) into an alternatively folded, disease-related isoform (PrP(Sc), where Sc is scrapie), the accumulation of which is associated with synapse degeneration and ultimately neuronal death. The formation of PrP(Sc) is dependent upon the presence of PrP(C) in specific, cholesterol-sensitive membrane microdomains, commonly called lipid rafts. PrP(C) is targeted to these lipid rafts because it is attached to membranes via a glycosylphosphatidylinositol anchor. Here, we show that treatment of prion-infected neuronal cell lines (ScN2a, ScGT1, or SMB cells) with synthetic glycosylphosphatidylinositol analogues, glucosamine-phosphatidylinositol (glucosamine-PI) or glucosamine 2-O-methyl inositol octadecyl phosphate, reduced the PrP(Sc) content of these cells in a dose-dependent manner. In addition, ScGT1 cells treated with glucosamine-PI did not transmit infection following intracerebral injection to mice. Treatment with glucosamine-PI increased the cholesterol content of ScGT1 cell membranes and reduced activation of cytoplasmic phospholipase A(2) (PLA(2)), consistent with the hypothesis that the composition of cell membranes affects key PLA(2)-dependent signaling pathways involved in PrP(Sc) formation. The effect of glucosamine-PI on PrP(Sc) formation was also reversed by the addition of platelet-activating factor. Glucosamine-PI caused the displacement of PrP(C) from lipid rafts and reduced expression of PrP(C) at the cell surface, putative sites for PrP(Sc) formation. We propose that treatment with glucosamine-PI modifies local micro-environments that control PrP(C) expression and activation of PLA(2) and subsequently inhibits PrP(Sc) formation.

A glycosylphosphatidylinositol analogue reduced prion-derived peptide mediated activation of cytoplasmic phospholipase A2, synapse degeneration and neuronal death

The pathogenesis of prion diseases includes synapse degeneration and neuronal death. Here we report that pre-treatment with glucosamine-phosphatidylinositol (glucosamine-PI), a synthetic analogue of the glycosylphosphatidylinositol (GPI) anchor that attaches the prion protein (PrP(C)) to plasma membranes, increased the resistance of cultured cortical neurones to the toxic effects of the prion-derived peptide PrP82-146. Pre-treatment with glucosamine-PI reduced the PrP82-146 induced activation of cytoplasmic phospholipase A(2) (cPLA(2)), activation of caspase-3 and synapse degeneration. The addition of glucosamine-PI significantly increased the amount of cholesterol within neuronal membranes consistent with the hypothesis that GPI anchors sequester cholesterol. Whereas in untreated neurones PrP82-146 was found within lipid rafts, in glucosamine-PI treated neurones most PrP82-146 was found in the normal cell membrane and was rerouted into the lysosomes. Complex GPI anchors isolated from PrP(C), Thy-1 or CD55 were also protective against PrP82-146. We conclude that glucosamine-PI, or isolated GPI anchors, can modify local membrane micro-environments that are important in the initiation of signalling events that mediate PrP82-146 induced neurodegeneration.

Membrane organization and regulation of cellular cholesterol homeostasis

An excess of intracellular free cholesterol (Chol) is cytotoxic, and its homeostasis is crucial for cell viability. Apolipoprotein A-I (apoA-I) is a highly efficient Chol acceptor because it activates complex cellular pathways that tend to mobilize and export Chol from cellular depots. We hypothesize that membrane composition and/or organization is strongly involved in Chol homeostasis. To test this hypothesis, we constructed a cell line overexpressing stearoyl coenzyme A (CoA) desaturase (SCD cells), which modifies plasma membrane (PM) composition by the enrichment of monounsaturated fatty acids, and determined this effect on membrane properties, cell viability, and Chol homeostasis. PM in SCD cells has a higher ratio of phospholipids to sphingomyelin and is slightly enriched in Chol. These cells showed an increase in the ratio of cholesteryl esters to free Chol; they were more resistant to Chol toxicity, and they exported more caveolin than control cells. The data suggest that cell functionality is preserved by regulating membrane fluidity and Chol exportation and storage.

Caveolin, sterol carrier protein-2, membrane cholesterol-rich microdomains and intracellular cholesterol trafficking

While the existence of membrane lateral microdomains has been known for over 30 years, interest in these structures accelerated in the past decade due to the discovery that cholesterol-rich microdomains serve important biological functions. It is increasingly appreciated that cholesterol-rich microdomains in the plasma membranes of eukaryotic cells represent an organizing nexus for multiple cellular proteins involved in transmembrane nutrient uptake (cholesterol, fatty acid, glucose, etc.), cell-signaling, immune recognition, pathogen entry, and many other roles. Despite these advances, however, relatively little is known regarding the organization of cholesterol itself in these plasma membrane microdomains. Although a variety of non-sterol markers indicate the presence of microdomains in the plasma membranes of living cells, none of these studies have demonstrated that cholesterol is enriched in these microdomains in living cells. Further, the role of cholesterol-rich membrane microdomains as targets for intracellular cholesterol trafficking proteins such as sterol carrier protein-2 (SCP-2) that facilitate cholesterol uptake and transcellular transport for targeting storage (cholesterol esters) or efflux is only beginning to be understood. Herein, we summarize the background as well as recent progress in this field that has advanced our understanding of these issues.

Lipid raft-mediated regulation of G-protein coupled receptor signaling by ligands which influence receptor dimerization: a computational study

G-protein coupled receptors (GPCRs) are the largest family of cell surface receptors; they activate heterotrimeric G-proteins in response to ligand stimulation. Although many GPCRs have been shown to form homo- and/or heterodimers on the cell membrane, the purpose of this dimerization is not known. Recent research has shown that receptor dimerization may have a role in organization of receptors on the cell surface. In addition, microdomains on the cell membrane termed lipid rafts have been shown to play a role in GPCR localization. Using a combination of stochastic (Monte Carlo) and deterministic modeling, we propose a novel mechanism for lipid raft partitioning of GPCRs based on reversible dimerization of receptors and then demonstrate that such localization can affect GPCR signaling. Modeling results are consistent with a variety of experimental data indicating that lipid rafts have a role in amplification or attenuation of G-protein signaling. Thus our work suggests a new mechanism by which dimerization-inducing or inhibiting characteristics of ligands can influence GPCR signaling by controlling receptor organization on the cell membrane.

Coupled stochastic spatial and non-spatial simulations of ErbB1 signaling pathways demonstrate the importance of spatial organization in signal transduction

Background

The ErbB family of receptors activates intracellular signaling pathways that control cellular proliferation, growth, differentiation and apoptosis. Given these central roles, it is not surprising that overexpression of the ErbB receptors is often associated with carcinogenesis. Therefore, extensive laboratory studies have been devoted to understanding the signaling events associated with ErbB activation.

Methodology/Principal Findings

Systems biology has contributed significantly to our current understanding of ErbB signaling networks. However, although computational models have grown in complexity over the years, little work has been done to consider the spatial-temporal dynamics of receptor interactions and to evaluate how spatial organization of membrane receptors influences signaling transduction. Herein, we explore the impact of spatial organization of the epidermal growth factor receptor (ErbB1/EGFR) on the initiation of downstream signaling. We describe the development of an algorithm that couples a spatial stochastic model of membrane receptors with a nonspatial stochastic model of the reactions and interactions in the cytosol. This novel algorithm provides a computationally efficient method to evaluate the effects of spatial heterogeneity on the coupling of receptors to cytosolic signaling partners.

Conclusions/Significance

Mathematical models of signal transduction rarely consider the contributions of spatial organization due to high computational costs. A hybrid stochastic approach simplifies analyses of the spatio-temporal aspects of cell signaling and, as an example, demonstrates that receptor clustering contributes significantly to the efficiency of signal propagation from ligand-engaged growth factor receptors.

Molecular cloning and characterization ofBombyx moristerol carrier protein x/sterol carrier protein 2 (SCPx/SCP2) gene

Cholesterol transport is a very important process in insect. We have isolated the Bombyx mori sterol carrier protein x (BmSCPx) cDNA and sterol carrier protein 2 (BmSCP2) cDNA: a 1.7 kb clone encoding SCPx, a 3-ketoacyl CoA thiolase, and 0.6 kb clone presumably encoding SCP2, which is thought to be an intracellular lipid transfer protein. Interestingly, the identical gene SCPx/SCP2 encodes the two types of transcripts by alternative splicing mechanism in Bombyx mori. The SCPx mRNA spans two exons in genome, and conceptual translation of the SCPx cDNA encodes a protein of 536 amino acids, which contains a thiolase domain and a SCP2 domain. Whereas the SCP2 mRNA partly lakes the first exon, and the SCP2 is a 146 amino acids containing a SCP2 domain only. Both BmSCPx and BmSCP2 have a putative peroxisomal targeting signal in the C-terminal region. BmSCPx shares 94 and 72% similarity to Spodoptera littoralis SCPx and human SCPx, respectively. RT-PCR analysis reveals that transcripts of BmSCP2 were detected in all tissues analyzed. BmSCPx transcription expressed only in midgut and malpighian tubules. However, the BmSCPx and BmSCP2 express strong in midgut during the last instar larvae. The tissue-specific expression pattern of BmSCPx and BmSCP2 is consistent with a role for these proteins in cholesterol metabolism. The results suggest that SCPx/SCP2 may play a key role in sterol absorption and intracellular carrier in silkworm.

Fluorescent sterols monitor cell penetrating peptide Pep-1 mediated uptake and intracellular targeting of cargo protein in living cells

Although cell-penetrating peptides (CPP) facilitate endocytic uptake of proteins, little is known regarding the extent to which CPPs facilitate protein cargo exit from endocytic vesicles for targeting to other intracellular sites. Since the plasma membrane and less so intracellular membranes contain cholesterol, the fluorescent sterol analogues dansyl-cholestanol (DChol) and dehydroergosterol (DHE) were used to monitor the uptake and intracellular distribution of fluorescent-tagged acyl coenzyme A binding protein (ACBP) into COS-7 cells and rat hepatoma cells. Confocal microscopy colocalized DChol and Texas Red-ACBP (TR-ACBP) with markers for the major endocytosis pathways, especially fluorescent-labeled cholera toxin (marker of ganglioside GM1 in plasma membrane lipid rafts) and dextran (macropinocytosis marker), but less so with transferrin (clathrin-mediated endocytosis marker). These findings were confirmed by multiphoton laser scanning microscopy colocalization of TR-ACBP with DHE (naturally-fluorescent sterol) and by double immunofluorescence labeling of native endogenous ACBP. Serum greatly and Pep-1 further 2.4-fold facilitated uptake of TR-ACBP, but neither altered the relative proportion of TR-ACBP colocalized with membranes/organelles (nearly 80%) vs cytoplasm and/or nucleoplasm (20%). Interestingly, Pep-1 selectively increased TR-ACBP associated with mitochondria while concomitantly decreasing that in endoplasmic reticulum. In summary, fluorescent sterols (DChol, DHE) were useful markers for comparing the distributions of both transported and endogenous proteins. Pep-1 modestly enhanced the translocation and altered the intracellular targeting of exogenous-delivered (TR-ACBP) in living cells.

Fluorescence techniques using dehydroergosterol to study cholesterol trafficking

Cholesterol itself has very few structural/chemical features suitable for real-time imaging in living cells. Thus, the advent of dehydroergosterol [ergosta-5,7,9(11),22-tetraen-3beta-ol, DHE] the fluorescent sterol most structurally and functionally similar to cholesterol to date, has proven to be a major asset for real-time probing/elucidating the sterol environment and intracellular sterol trafficking in living organisms. DHE is a naturally occurring, fluorescent sterol analog that faithfully mimics many of the properties of cholesterol. Because these properties are very sensitive to sterol structure and degradation, such studies require the use of extremely pure (>98%) quantities of fluorescent sterol. DHE is readily bound by cholesterol-binding proteins, is incorporated into lipoproteins (from the diet of animals or by exchange in vitro), and for real-time imaging studies is easily incorporated into cultured cells where it co-distributes with endogenous sterol. Incorporation from an ethanolic stock solution to cell culture media is effective, but this process forms an aqueous dispersion of DHE crystals which can result in endocytic cellular uptake and distribution into lysosomes which is problematic in imaging DHE at the plasma membrane of living cells. In contrast, monomeric DHE can be incorporated from unilamellar vesicles by exchange/fusion with the plasma membrane or from DHE-methyl-beta-cyclodextrin (DHE-MbetaCD) complexes by exchange with the plasma membrane. Both of the latter techniques can deliver large quantities of monomeric DHE with significant distribution into the plasma membrane. The properties and behavior of DHE in protein-binding, lipoproteins, model membranes, biological membranes, lipid rafts/caveolae, and real-time imaging in living cells indicate that this naturally occurring fluorescent sterol is a useful mimic for probing the properties of cholesterol in these systems.

Structure of dehydroergosterol monohydrate and interaction with sterol carrier protein-2

Dehydroergosterol [ergosta-5,7,9(11),22-tetraen-3beta-ol] is a naturally-occurring, fluorescent sterol utilized extensively to probe membrane cholesterol distribution, cholesterol-protein interactions, and intracellular cholesterol transport both in vitro and in vivo. In aqueous solutions, the low solubility of dehydroergosterol results in the formation of monohydrate crystals similar to cholesterol. Low temperature X-ray diffraction analysis reveals that dehydroergosterol monohydrate crystallizes in the space group P2(1) with four molecules in the unit cell and monoclinic crystal parameters a = 9.975(1) A, b = 7.4731(9) A, c = 34.054(4) A, and beta = 92.970(2) degrees somewhat similar to ergosterol monohydrate. The molecular arrangement is in a slightly closer packed bilayer structure resembling cholesterol monohydrate. Since dehydroergosterol fluorescence emission undergoes a quantum yield enhancement and red-shift of its maximum wavelength when crystallized, formation or disruption of microcrystals was monitored with high sensitivity using cuvette-based spectroscopy and multi-photon laser scanning imaging microscopy. This manuscript reports on the dynamical effect of sterol carrier protein-2 (SCP-2) interacting between aqueous dispersions of dehydroergosterol monohydrate microcrystal donors and acceptors consisting not only of model membranes but also vesicles derived from plasma membranes isolated by biochemical fractionation and affinity purification from Madin-Darby canine kidney cells. Furthermore, this study provides real-time measurements of the effect of increased SCP-2 levels on the rate of disappearance of dehydroergosterol microcrystals in living cells.

Endothelial lipase promotes apolipoprotein AI-mediated cholesterol efflux in THP-1 macrophages

OBJECTIVE

Endothelial lipase (EL) is expressed by macrophages within atherosclerotic lesions. We investigated the influence of EL expression on cholesterol efflux in macrophages.

METHODS AND RESULTS

The present study used lentivirus to introduce either EL shRNA for loss-of-function studies or EL cDNA for gain-of-function studies to investigate the role of EL in apoAI-mediated cholesterol efflux. ApoAI-mediated cholesterol efflux was decreased after EL suppression, but increased with EL overexpression in free cholesterol labeled and acLDL loaded THP-1 macrophages. Similar findings were observed in THP-1 macrophages after exogenous EL addition and in transfected 293 cells. EL-related apoAI-mediated cholesterol efflux decreased after treatment with heparin or catalytic inactivation (S149A mutation or tetrahydrolipstatin) alone, and completely inhibited in combination. Furthermore, EL expression did not change ABCA1 expression, but was positively correlated with apoAI binding to macrophages and 293 cells. This effect was mitigated after heparin treatment but not influenced by catalytic inactivation via tetrahydrolipstatin or the S149A mutation. Moreover, EL expression was positively associated with lysophosphatidylcholine production and inversely with phosphatidylcholine, phosphatidylethanolamine, and sphingomyelin levels. Lysophosphatidylcholine treatment dose-dependently stimulated apoAI-mediated cholesterol efflux in THP-1 macrophages.

CONCLUSIONS

EL appears to promote apoAI-mediated cholesterol efflux through catalytic and noncatalytic-dependent mechanisms.

Use of Bodipy-labeled sphingolipid and cholesterol analogs to examine membrane microdomains in cells

Much evidence has accumulated to show that cellular membranes such as the plasma membrane, contain multiple "microdomains" of differing lipid and protein composition and function. These domains are sometimes enriched in cholesterol and sphingolipids and are believed to be important structures for the regulation of many biological and pathological processes. This review focuses on the use of fluorescent (Bodipy) labeled analogs of sphingolipids and cholesterol to study such domains. We discuss the similarities between the behavior of Bodipy-cholesterol and natural cholesterol in artificial bilayers and in cultured cells, and the use of Bodipy-sphingolipid analogs to visualize membrane domains in living cells based on the concentration-dependent monomer-excimer fluorescence properties of the Bodipy-fluorophore. The use of Bodipy-D-erythro-lactosylceramide is highlighted for detection of domains on the plasma membrane and endosome membranes, and the importance of the sphingolipid stereochemistry in modulating domain formation is discussed. Finally, we suggest that Bodipy-sphingolipids may be useful in future studies to examine the relationship between membrane domains at the cell surface and domains enriched in other lipids and proteins on the inner leaflet of the plasma membrane.

Genome-wide expression analysis of cells expressing gain of function mutant D374Y-PCSK9

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a key regulator of serum cholesterol. The possibility that PCSK9 also functions in other pathways needs to be addressed. We have transfected HepG2 cells with mutant D374Y-PCSK9 or control vector. Gene expression signatures were determined using the Affymetrix GeneChip technology, and the expression pattern of selected genes was confirmed by quantitative real-time polymerase chain reaction (qRT-PCR). Data was normalized and analyzed using a model-based background adjustment for oligonucleotide expression arrays, then filtered based upon expression within treatments group, and subjected to moderated t-statistics. Five hundred twenty transcripts had altered expression levels between D374Y-PCSK9 and control vector. Among the 520 probes on our top list, 312 were found to have an assigned Gene Ontology (GO) term, and 96 were found in the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. Genome-wide expression profiling revealed that "steroid biosynthesis," "sterol metabolism," and "cholesterol biosynthsis" were affected by D374Y-PCSK9. Also, the GO biological process terms "response to stresss," "response to virus," "response to unfolded protein," and "immune response" were influenced by D374Y-PCSK9. Our results suggest that D374Y-PCSK9 results in up-regulation of genes involved in sterol biosynthesis and down-regulation of stress-response genes and specific inflammation pathways.

Docosahexaenoic and eicosapentaenoic acids increase prion formation in neuronal cells

Background

The transmissible spongiform encephalopathies, otherwise known as prion diseases, occur following the conversion of the cellular prion protein (PrP^C) to an alternatively folded, disease-associated isoform (PrP^Sc). Recent studies suggest that this conversion occurs via a cholesterol-sensitive process, as cholesterol synthesis inhibitors reduced the formation of PrP^Sc and delayed the clinical phase of scrapie infection. Since polyunsaturated fatty acids also reduced cellular cholesterol levels we tested their effects on PrP^Sc formation in three prion-infected neuronal cell lines (ScGT1, ScN2a and SMB cells).

Results

We report that treatment with docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) or the cholesterol synthesis inhibitor simvastatin reduced the amounts of free cholesterol in membrane extracts from prion-infected neuronal cells. Simvastatin reduced cholesterol production while DHA and EPA promoted the conversion of free cholesterol to cholesterol esters. Crucially, while simvastatin reduced PrP^Sc formation, both DHA and EPA significantly increased the amounts of PrP^Sc in these cells. Unlike simvastatin, the effects of DHA and EPA on PrP^Sc content were not reversed by stimulation of cholesterol synthesis with mevalonate. Treatment of ScGT1 cells with DHA and EPA also increased activation of cytoplasmic phospholipase A₂ and prostaglandin E₂ production. Finally, treatment of neuronal cells with DHA and EPA increased the amounts of PrP^C expressed at the cell surface and significantly increased the half-life of biotinylated PrP^C.

Conclusion

We report that although treatment with DHA or EPA significantly reduced the free cholesterol content of prion-infected cells they significantly increased PrP^Sc formation in three neuronal cell lines. DHA or EPA treatment of infected cells increased activation of phospholipase A₂, a key enzyme in PrP^Sc formation, and altered the trafficking of PrP^C. PrP^C expression at the cell surface, a putative site for the PrP^Sc formation, was significantly increased, and the rate at which PrP^C was degraded was reduced. Cholesterol depletion is seen as a potential therapeutic strategy for prion diseases. However, these results indicate that a greater understanding of the precise relationship between membrane cholesterol distribution, PrP^C trafficking, cell activation and PrP^Sc formation is required before cholesterol manipulation can be considered as a prion therapeutic.

Larvicidal activity of sterol carrier protein-2 inhibitor in four species of mosquitoes

A previous report has shown that mosquito sterol carrier protein-2 inhibitors (SCPIs) are larvicidal to larvae of the yellowfever mosquito, Aedes aegypti (L.) (J. Lipid Res. 46: 650-657, 2005). In the current study, we tested SCPI-1 in an additional four mosquito species for larvicidal activities: Culex pipiens pipiens, Anopheles gambiae, Culex restuans, and Aedes vexans. Cholesterol accumulation in SCPI-treated Ae. aegypti fourth instars was examined. SCPI-1 is lethal to all tested mosquito species, with the LC50 value ranging from 5.2 to 15 microM when treatments started at the first to third instar. However, LC50 values increase to from 5.2 to 38.7 microM in treatments started at first and fourth instar, respectively. The results indicate that the lethal effect of SCPI-1 decreases with the growth of larvae, which suggests that SCPI-1 is more effective before the larvae reach final growth period (the last instar). SCPI-1 suppressed cholesterol uptake in Ae. aegypti fourth instars, suggesting that one of the modes of action of SCPI-1 is via reduction in cholesterol absorption.

Cholesterol sulphate sulphohydrolase of human placenta lysosomal membrane

In this paper we report that the activity of cholesterol sulphate sulphohydrolase (CHS-ase) is associated with the lysosomal membranes. The procedure of purification of CHS-ase from human placenta lysosomes was elaborated. The purified enzyme is highly specific to cholesterol sulphate (specific activity 2126.60+/-940.90 nmol min(-1) mg protein(-1)) and acts optimally at pH 3.4. The K(M) value for the hydrolysis of cholesterol sulphate is 3.6+/-0.95 x 10(-5)mol/l. The isoelectric point (pI) has the value 5.7, molecular weight estimated by SDS-PAGE electrophoresis is 38 kDa. The described enzyme may be involved in a regulation of cholesterol and cholesterol sulphate levels in the lysosomal membrane.

Comparison of expression profiles induced by dust mite in airway epithelia reveals a common pathway

BACKGROUND

Airway epithelial cells have shown to be active participants in the defense against pathogens by producing signaling and other regulatory molecules in response to the encounter.

METHODS

In previous manuscripts, we have studied the effect of house dust mite (HDM) extract on both an epithelial cell-line (H292) and primary nasal epithelial cell. When we compare these responses we conclude that the H292 cells more closely resemble nasal epithelium of healthy controls (share 107 probe-sets) than of allergic individuals (share 17 probe-sets).

RESULTS

Interestingly, probably because of an absent intraindividual variation between samples, more probe-sets (8280) change expression significantly in H292 than in either healthy (555) or allergic (401) epithelium.

CONCLUSIONS

A direct comparison of all the responses in these epithelial cells reveals a core-response to HDM of just 29 genes. These genes (CCL20, IL-8, CXCL2, CXCL1, IL-1B, AREG, TNFAIP3, HBEGF, PTGS2, BMP2, LDLR, PLAUR, PLAU, NFKB2, NFKB1, JUN, ATF3, EGR1, NPC1, TICAM1, EPHA2, CTGF, DUSP1, SPRY1, TLR-3, complement factor C3, IVNS1ABP, SerpinB3, and PSAT1) have described links with allergy or inflammation and may even describe the well-established relationship between viral infections and allergic exacerbations or allergy development.

Sequestration of free cholesterol in cell membranes by prions correlates with cytoplasmic phospholipase A2 activation

Background

The transmissible spongiform encephalopathies (TSEs), otherwise known as the prion diseases, occur following the conversion of the normal cellular prion protein (PrP^C) to an alternatively folded isoform (PrP^Sc). The accumulation of PrP^Sc within the brain leads to neurodegeneration through an unidentified mechanism. Since many neurodegenerative disorders including prion, Parkinson's and Alzheimer's diseases may be modified by cholesterol synthesis inhibitors, the effects of prion infection on the cholesterol balance within neuronal cells were examined.

Results

We report the novel observation that prion infection altered the membrane composition and significantly increased total cholesterol levels in two neuronal cell lines (ScGT1 and ScN2a cells). There was a significant correlation between the concentration of free cholesterol in ScGT1 cells and the amounts of PrP^Sc. This increase was entirely a result of increased amounts of free cholesterol, as prion infection reduced the amounts of cholesterol esters in cells. These effects were reproduced in primary cortical neurons by the addition of partially purified PrP^Sc, but not by PrP^C. Crucially, the effects of prion infection were not a result of increased cholesterol synthesis. Stimulating cholesterol synthesis via the addition of mevalonate, or adding exogenous cholesterol, had the opposite effect to prion infection on the cholesterol balance. It did not affect the amounts of free cholesterol within neurons; rather, it significantly increased the amounts of cholesterol esters. Immunoprecipitation studies have shown that cytoplasmic phospholipase A₂ (cPLA₂) co-precipitated with PrP^Sc in ScGT1 cells. Furthermore, prion infection greatly increased both the phosphorylation of cPLA₂ and prostaglandin E₂ production.

Conclusion

Prion infection, or the addition of PrP^Sc, increased the free cholesterol content of cells, a process that could not be replicated by the stimulation of cholesterol synthesis. The presence of PrP^Sc increased solubilisation of free cholesterol in cell membranes and affected their function. It increased activation of the PLA₂ pathway, previously implicated in PrP^Sc formation and in PrP^Sc-mediated neurotoxicity. These observations suggest that the neuropathogenesis of prion diseases results from PrP^Sc altering cholesterol-sensitive processes. Furthermore, they raise the possibility that disturbances in membrane cholesterol are major triggering events in neurodegenerative diseases.

GnRH-mediated DAN production regulates the transcription of the GnRH receptor in gonadotrope cells

The primary function of gonadotropin-releasing hormone (GnRH) is the regulation of pituitary gonadotropin hormone gene transcription, biosynthesis and release. These effects are mediated through intracellular mobilization of Ca2+ and activation of PKC isoforms and MAP kinases. We show here that DAN (differential screening-selected gene aberrative in neuroblastoma) which is a secreted bone morphogenic protein (BMP) antagonist belonging to the TGFbeta protein superfamily, is controlled by GnRH in murine gonadotrope cells. Acute GnRH stimulation induced a rapid, 27-fold, elevation of DAN mRNA, accompanied by an approximate 3-fold increase in the amount of mature DAN glycoprotein in the cell cytoplasm and in DAN secretion into the culture medium. Incubation of L beta T2 cells in DAN-containing medium altered the levels of a number of cellular proteins. Two of these were identified as the steroidogenic acute regulatory protein (StAR) and the actin-related protein 2/3 complex subunits 2 (p34-ARC) which are primarily involved in steroidogenesis and cytoskeleton remodelling, respectively. DAN caused an approximate 2-fold specific elevation in the cytoplasmic levels of both these proteins in L beta T2 cells. We further tested the effects of DAN on classical GnRH effects viz. gonadotropin and GnRH receptor gene expression. Co-transfection of L beta T2 cells with DAN and gonadotropin subunit promoter luciferase reporter genes had no effect on GnRH stimulation of alpha GSU and LH beta or on the additive GnRH and activin induction of FSH beta subunit transcription. However, co-transfection of DAN markedly inhibited the synergistic activation of GnRH and activin on GnRH receptor gene expression thus implicating DAN as a novel autocrine/paracrine factor that modulates GnRH function in pituitary gonadotropes.

Plasma LDL cholesterol has no impact on P-glycoprotein (MDR1/ABCB1) activity in human peripheral blood mononuclear cells

Several studies have demonstrated that the adenosine triphosphate-binding cassette transporter P-glycoprotein (P-gp) is at least partly located in cholesterol- and sphingolipid-enriched parts of the plasma membrane called "lipid rafts" and that modification of cellular cholesterol content has an impact on the activity of P-gp in vitro and ex vivo. Cholesterol modulation in vitro does not closely reflect the in vivo situation. The aim of our study was therefore to investigate whether differences in individual plasma low-density lipoprotein (LDL) cholesterol levels in humans have an impact on cholesterol content in peripheral blood mononuclear cells (PBMCs) and thereby on individual activity of P-gp. PBMCs of 20 ambulatory patients with elevated LDL cholesterol (173.9 +/- 22.4 mg/dl; range 151.0-234.4 mg/dl) and 28 controls (125.2 +/- 16.9 mg/dl; range 74.6-149.6 mg/dl) were isolated. Cellular cholesterol was measured by an enzymatic fluorimetric assay, efflux activity of P-gp in PBMCs was determined by a flow cytometric method (rhodamine123 efflux), and messenger ribonucleic acid expression was quantified by reverse transcriptase real-time polymerase chain reaction (RT-PCR). There was no difference in cellular cholesterol or P-gp activity between the two groups suggesting that high plasma LDL cholesterol concentration as observed in dyslipidemic patients does not correlate with cellular cholesterol content or P-gp activity in PBMCs. There was, however, a significant negative relationship between age and P-gp efflux activity indicating that P-gp activity in PBMCs decreases with advancing age. These results need further confirmation because investigation of age dependency of P-gp activity was not the primary aim of the study.

A new N-terminal recognition domain in caveolin-1 interacts with sterol carrier protein-2 (SCP-2)

Although plasma membrane domains, such as caveolae, provide an organizing principle for signaling pathways and cholesterol homeostasis in the cell, relatively little is known regarding specific mechanisms, whereby intracellular lipid-binding proteins are targeted to caveolae. Therefore, the interaction between caveolin-1 and sterol carrier protein-2 (SCP-2), a protein that binds and transfers both cholesterol and signaling lipids (e.g., phosphatidylinositides and sphingolipids), was examined by yeast two-hybrid, in vitro binding and fluorescence resonance energy transfer (FRET) analyses. Results of the in vivo and in vitro assays identified for the first time the N-terminal amino acids (aa) 1-32 amphipathic alpha helix of SCP-2 functionally interacted with caveolin-1. This interaction was independent of the classic caveolin-1 scaffolding domain, in which many signaling proteins interact. Instead, SCP-2 bound caveolin-1 through a new domain identified in the N-terminal domain of caveolin-1 between aa 34-40. Modeling studies suggested that electrostatic interactions between the SCP-2 N-terminal aa 1-32 amphipathic alpha-helical domain (cationic, positively charged face) and the caveolin-1 N-terminal aa 33-59 alpha helix (anionic, negatively charged face) may significantly contribute to this interaction. These findings provide new insights on how SCP-2 enhances cholesterol retention within the cell as well as regulates the distribution of signaling lipids, such as phosphoinositides and sphingolipids, at plasma membrane caveolae.

Identification of two sterol carrier protein-2 like genes in the yellow fever mosquito, Aedes aegypti

Two genes encoding sterol carrier protein-2 like proteins are identified from fourth instar cDNAs of the yellow fever mosquito, Aedes aegypti. The predicted AeSCP-2like1 (AeSCP-2L1) and AeSCP-2like2 (AeSCP-2L2) proteins are small, acidic and lacking the peroxisomal targeting sequence at the C-termini. Purified recombinant AeSCP-2L1 and -2L2 bind to cholesterol with a Kd of 5.4 x 10(-6) M and 2.6 x 10(-6) M, respectively. The Kd values of AeSCP-2L1 and -2L2 to palmitic acid are 3.7 x 10(-7) M and 2.6 x 10(-7) M, respectively. Both genes are expressed predominantly in gut tissues. The transcripts of the AeSCP-2L1 gene are only detected in larval stages, whereas AeSCP-2L2 is expressed in larval and adult stages. AeSCP-2L2 transcription increases within 5 h after a bloodmeal and stays at high levels during vitellogenesis. In in vitro larval gut tissue cultures, AeSCP-2L1 transcripts were increased in the presence of juvenile hormone III, whereas AeSCP-2L2 mRNA levels increased in the presence 20-hydroxylecdysone. The results suggest that transcription of AeSCP-2L1 and -2L2 genes are regulated differently through the mosquito life cycle.

Sterol carrier protein-2: new roles in regulating lipid rafts and signaling

Sterol carrier protein-2 (SCP-2) was independently discovered as a soluble protein that binds and transfers cholesterol as well as phospholipids (nonspecific lipid transfer protein, nsLTP) in vitro. Physiological functions of this protein are only now beginning to be resolved. The gene encoding SCP-2 also encodes sterol carrier protein-x (SCP-x) arising from an alternate transcription site. In vitro and in vivo SCP-x serves as a peroxisomal 3-ketoacyl-CoA thiolase in oxidation of branched-chain lipids (cholesterol to form bile acids; branched-chain fatty acid for detoxification). While peroxisomal SCP-2 facilitates branched-chain lipid oxidation, the role(s) of extraperoxisomal (up to 50% of total) are less clear. Studies using transfected fibroblasts overexpressing SCP-2 and hepatocytes from SCP-2/SCP-x gene-ablated mice reveal that SCP-2 selectively remodels the lipid composition, structure, and function of lipid rafts/caveolae. Studies of purified SCP-2 and in cells show that SCP-2 has high affinity for and selectively transfers many lipid species involved in intracellular signaling: fatty acids, fatty acyl CoAs, lysophosphatidic acid, phosphatidylinositols, and sphingolipids (sphingomyelin, ceramide, mono-di-and multi-hexosylceramides, gangliosides). SCP-2 selectively redistributes these signaling lipids between lipid rafts/caveolae and intracellular sites. These findings suggest SCP-2 serves not only in cholesterol and phospholipid transfer, but also in regulating multiple lipid signaling pathways in lipid raft/caveolae microdomains of the plasma membrane.