Cholesterol-induced conformational change in SCAP enhanced by Insig proteins and mimicked by cationic amphiphiles

Imipramine Administration in Brucella abortus 2308-Infected Mice Restores Hippocampal Serotonin Levels, Muscle Strength, and Mood, and Decreases Spleen CFU Count

Brucellosis infection causes non-specific symptoms such as fever, chills, sweating, headaches, myalgia, arthralgia, anorexia, fatigue, and mood disorders. In mouse models, it has been associated with increased levels of IL-6, TNF-α, and IFN-γ, a decrease in serotonin and dopamine levels within the hippocampus, induced loss of muscle strength and equilibrium, and increased anxiety and hopelessness. Imipramine (ImiP), a tricyclic antidepressant, is used to alleviate neuropathic pain. This study evaluated the effects of ImiP on Balb/c mice infected with Brucella abortus 2308 (Ba) at 14- and 28-days post-infection. Serum levels of six cytokines (IFN-γ, IL-6, TNF-α, IL-12, MCP-1. and IL-10) were assessed by FACS, while the number of bacteria in the spleen was measured via CFU. Serotonin levels in the hippocampus were analyzed via HPLC, and behavioral tests were conducted to assess strength, equilibrium, and mood. Our results showed that mice infected with Brucella abortus 2308 and treated with ImiP for six days (Im6Ba14) had significantly different outcomes compared to infected mice (Ba14) at day 14 post-infection. The mood was enhanced in the forced swimming test (FST) (p < 0.01), tail suspension test (TST) (p < 0.0001), and open-field test (p < 0.0001). Additionally, there was an increase in serotonin levels in the hippocampus (p < 0.001). Furthermore, there was an improvement in equilibrium (p < 0.0001) and muscle strength (p < 0.01). Lastly, there was a decrease in IL-6 levels (p < 0.05) and CFU count in the spleen (p < 0.0001). At 28 days, infected mice that received ImiP for 20 days (Im20Ba28) showed preservation of positive effects compared to infected mice (Ba28). These effects include the following: (1) improved FST (p < 0.0001) and TST (p < 0.0001); (2) better equilibrium (p < 0.0001) and muscle strength (p < 0.0001); (3) decreased IL-6 levels (p < 0.05); and (4) reduced CFU count in the spleen (p < 0.0001). These findings suggest the potential for ImiP to be used as an adjuvant treatment for the symptoms of brucellosis, which requires future studies.

Low abundance of insulin-induced gene 1 contributes to SREBP-1c processing and hepatic steatosis in dairy cows with severe fatty liver

Fatty liver is a major metabolic disorder of high-producing dairy cows during the transition period. In nonruminants, it is well established that insulin-induced gene 1 (INSIG1) plays a crucial role in regulating hepatic lipogenesis by controlling the anchoring of sterol regulatory element-binding protein 1 (SREBP-1) on the endoplasmic reticulum along with SREBP cleavage-activating protein (SCAP). Whether the INSIG1-SCAP-SREBP-1c transport axis is affected in cows experiencing fatty liver is unknown. Thus, the aim of this study was to investigate the potential role of INSIG1-SCAP-SREBP-1c axis in the progression of fatty liver in dairy cows. For in vivo experiments, 24 dairy cows at the start of their fourth lactation (median; range 3-5) and 8 d in milk (median; range 4-12 d) were selected into a healthy group [n = 12; triglyceride (TG) content <1%] and a severe fatty liver group (n = 12; TG content >10%) according to their hepatic TG content. Blood samples were collected for detecting serum concentrations of free fatty acids, β-hydroxybutyrate, and glucose. Compared with healthy cows, cows with severe fatty liver had higher serum concentrations of β-hydroxybutyrate and free fatty acids and lower concentration of glucose. Liver biopsies were used to detect the status of INSIG1-SCAP-SREBP-1c axis, and the mRNA expression of SREBP-1c-target lipogenic genes acetyl-CoA carboxylase α (ACACA), fatty acid synthase (FASN), and diacylglycerol acyltransferase 1 (DGAT1). Cows with severe fatty liver had lower protein expression of INSIG1 in the hepatocyte endoplasmic reticulum fraction, greater protein expression of SCAP and precursor SREBP-1c in the hepatocyte Golgi fraction, and greater protein expression of mature SREBP-1c in the hepatocyte nuclear fraction. In addition, the mRNA expression of SREBP-1c-target lipogenic genes ACACA, FASN, and DGAT1 was greater in the liver of dairy cows with severe fatty liver. In vitro experiments were conducted on hepatocytes isolated from 5 healthy 1-d-old female Holstein calves, and hepatocytes from each calf were run independently. First, hepatocytes were treated with 0, 200, or 400 μM palmitic acid (PA) for 12 h. Exogenous PA treatment decreased INSIG1 protein abundance, enhanced the endoplasmic reticulum to Golgi export of SCAP-precursor SREBP-1c complex and the nuclear translocation of mature SREBP-1c, all of which was associated with increased transcriptional activation of lipogenic genes and TG synthesis. Second, hepatocytes were transfected with INSIG1-overexpressing adenovirus for 48 h and treated with 400 μM PA 12 h before the end of transfection. Overexpressing INSIG1 inhibited PA-induced SREBP-1c processing, upregulation of lipogenic genes, and TG synthesis in hepatocytes. Overall, the present in vivo and in vitro results indicated that the low abundance of INSIG1 contributed to SREBP-1c processing and hepatic steatosis in dairy cows. Thus, the INSIG1-SCAP-SREBP-1c axis may be a novel target for treatment of fatty liver in dairy cows.

ABCA9, an ER cholesterol transporter, inhibits breast cancer cell proliferation via SREBP-2 signaling

The association between cholesterol metabolism and cancer development and progression has been recently highlighted. However, the role and function of many cholesterol transporters remain largely unknown. Here, we focused on the ATP-binding cassette subfamily A member 9 (ABCA9) transporter given that its expression is significantly downregulated in both canine mammary tumors and human breast cancers, which in breast cancer patients correlates with poor prognosis. We found that ABCA9 is mainly present in the endoplasmic reticulum (ER) and is responsible for promoting cholesterol accumulation in this structure. Accordingly, ABCA9 inhibited sterol-regulatory element binding protein-2 (SREBP-2) translocation from the ER to the nucleus, a crucial step for cholesterol synthesis, resulting in the downregulation of cholesterol synthesis gene expression. ABCA9 expression in breast cancer cells attenuated cell proliferation and reduced their colony-forming abilities. We identified ABCA9 expression to be regulated by Forkhead box O1 (FOXO1). Inhibition of PI3K induced enhanced ABCA9 expression through the activation of the PI3K-Akt-FOXO1 pathway in breast cancer cells. Altogether, our study suggests that ABCA9 functions as an ER cholesterol transporter that suppresses cholesterol synthesis via the inhibition of SREBP-2 signaling and that its restoration halts breast cancer cell proliferation. Our findings provide novel insight into the vital role of ABCA9 in breast cancer progression.

Obstructive Sleep Apnoea and Lipid Metabolism: The Summary of Evidence and Future Perspectives in the Pathophysiology of OSA-Associated Dyslipidaemia

Obstructive sleep apnoea (OSA) is associated with cardiovascular and metabolic comorbidities, including hypertension, dyslipidaemia, insulin resistance and atherosclerosis. Strong evidence suggests that OSA is associated with an altered lipid profile including elevated levels of triglyceride-rich lipoproteins and decreased levels of high-density lipoprotein (HDL). Intermittent hypoxia; sleep fragmentation; and consequential surges in the sympathetic activity, enhanced oxidative stress and systemic inflammation are the postulated mechanisms leading to metabolic alterations in OSA. Although the exact mechanisms of OSA-associated dyslipidaemia have not been fully elucidated, three main points have been found to be impaired: activated lipolysis in the adipose tissue, decreased lipid clearance from the circulation and accelerated de novo lipid synthesis. This is further complicated by the oxidisation of atherogenic lipoproteins, adipose tissue dysfunction, hormonal changes, and the reduced function of HDL particles in OSA. In this comprehensive review, we summarise and critically evaluate the current evidence about the possible mechanisms involved in OSA-associated dyslipidaemia.

Lipid Metabolism in Glioblastoma: From De Novo Synthesis to Storage

Glioblastoma (GBM) is the most lethal primary brain tumor. With limited therapeutic options, novel therapies are desperately needed. Recent studies have shown that GBM acquires large amounts of lipids for rapid growth through activation of sterol regulatory element-binding protein 1 (SREBP-1), a master transcription factor that regulates fatty acid and cholesterol synthesis, and cholesterol uptake. Interestingly, GBM cells divert substantial quantities of lipids into lipid droplets (LDs), a specific storage organelle for neutral lipids, to prevent lipotoxicity by increasing the expression of diacylglycerol acyltransferase 1 (DGAT1) and sterol-O-acyltransferase 1 (SOAT1), which convert excess fatty acids and cholesterol to triacylglycerol and cholesteryl esters, respectively. In this review, we will summarize recent progress on our understanding of lipid metabolism regulation in GBM to promote tumor growth and discuss novel strategies to specifically induce lipotoxicity to tumor cells through disrupting lipid storage, a promising new avenue for treating GBM.

Ammonia stimulates SCAP/Insig dissociation and SREBP-1 activation to promote lipogenesis and tumour growth

Tumorigenesis is associated with elevated glucose and glutamine consumption, but how cancer cells can sense their levels to activate lipid synthesis is unknown. Here, we reveal that ammonia, released from glutamine, promotes lipogenesis via activation of sterol regulatory element-binding proteins (SREBPs), endoplasmic reticulum-bound transcription factors that play a central role in lipid metabolism. Ammonia activates the dissociation of glucose-regulated, N-glycosylated SREBP-cleavage-activating protein (SCAP) from insulin-inducible gene protein (Insig), an endoplasmic reticulum-retention protein, leading to SREBP translocation and lipogenic gene expression. Notably, 25-hydroxycholesterol blocks ammonia to access its binding site on SCAP. Mutating aspartate D428 to alanine prevents ammonia binding to SCAP, abolishes SREBP-1 activation and suppresses tumour growth. Our study characterizes the unknown role, opposite to sterols, of ammonia as a key activator that stimulates SCAP-Insig dissociation and SREBP-1 activation to promote tumour growth and demonstrates that SCAP is a critical sensor of glutamine, glucose and sterol levels to precisely control lipid synthesis.

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.

The ER cholesterol sensor SCAP promotes CARTS biogenesis at ER-Golgi membrane contact sites

In response to cholesterol deprivation, SCAP escorts SREBP transcription factors from the endoplasmic reticulum to the Golgi complex for their proteolytic activation, leading to gene expression for cholesterol synthesis and uptake. Here, we show that in cholesterol-fed cells, ER-localized SCAP interacts through Sac1 phosphatidylinositol 4-phosphate (PI4P) phosphatase with a VAP-OSBP complex, which mediates counter-transport of ER cholesterol and Golgi PI4P at ER-Golgi membrane contact sites (MCSs). SCAP knockdown inhibited the turnover of PI4P, perhaps due to a cholesterol transport defect, and altered the subcellular distribution of the VAP-OSBP complex. As in the case of perturbation of lipid transfer complexes at ER-Golgi MCSs, SCAP knockdown inhibited the biogenesis of the trans-Golgi network-derived transport carriers CARTS, which was reversed by expression of wild-type SCAP or a Golgi transport-defective mutant, but not of cholesterol sensing-defective mutants. Altogether, our findings reveal a new role for SCAP under cholesterol-fed conditions in the facilitation of CARTS biogenesis via ER-Golgi MCSs, depending on the ER cholesterol.

High-content, targeted RNA-seq screening in organoids for drug discovery in colorectal cancer

Organoids allow the recapitulation of intestinal homeostasis and cancerogenesis in vitro; however, RNA sequencing (RNA-seq)-based methods for drug screens are missing. We develop targeted organoid sequencing (TORNADO-seq), a high-throughput, high-content drug discovery platform that uses targeted RNA-seq to monitor the expression of large gene signatures for the detailed evaluation of cellular phenotypes in organoids. TORNADO-seq is a fast, highly reproducible time- and cost-effective ($5 per sample) method that can probe cell mixtures and their differentiation state in the intestinal system. We apply this method to isolate drugs that enrich for differentiated cell phenotypes and show that these drugs are highly efficacious against cancer compared to wild-type organoids. Furthermore, TORNADO-seq facilitates in-depth insight into the mode of action of these drugs. Our technology can easily be adapted to many other systems and will allow for more systematic, large-scale, and quantitative approaches to study the biology of complex cellular systems.

Active cholesterol 20 years on

This review considers the following hypotheses, some well-supported and some speculative. Almost all of the sterol molecules in plasma membranes are associated with bilayer phospholipids in complexes of varied strength and stoichiometry. These complexes underlie many of the material properties of the bilayer. The small fraction of cholesterol molecules exceeding the binding capacity of the phospholipids is thermodynamically active and serves diverse functions. It circulates briskly among the cell membranes, particularly through contact sites linking the organelles. Active cholesterol provides the upstream feedback signal to multiple mechanisms governing plasma membrane homeostasis, pegging the sterol level to a threshold set by its phospholipids. Active cholesterol could also be the cargo for various inter-organelle transporters and the form excreted from cells by reverse transport. Furthermore, it is integral to the function of caveolae; a mediator of Hedgehog regulation; and a ligand for the binding of cytolytic toxins to membranes. Active cholesterol modulates a variety of plasma membrane proteins-receptors, channels and transporters-at least in vitro.

Lipids, inflammasomes, metabolism, and disease

Inflammasomes are multi-protein complexes that regulate the cleavage of cysteine protease caspase-1, secretion of inflammatory cytokines, and induction of inflammatory cell death, pyroptosis. Several members of the nod-like receptor family assemble inflammasome in response to specific ligands. An exception to this is the NLRP3 inflammasome which is activated by structurally diverse entities. Recent studies have suggested that NLRP3 might be a sensor of cellular homeostasis, and any perturbation in distinct metabolic pathways results in the activation of this inflammasome. Lipid metabolism is exceedingly important in maintaining cellular homeostasis, and it is recognized that cells and tissues undergo extensive lipid remodeling during activation and disease. Some lipids are involved in instigating chronic inflammatory diseases, and new studies have highlighted critical upstream roles for lipids, particularly cholesterol, in regulating inflammasome activation implying key functions for inflammasomes in diseases with defective lipid metabolism. The focus of this review is to highlight how lipids regulate inflammasome activation and how this leads to the progression of inflammatory diseases. The key roles of cholesterol metabolism in the activation of inflammasomes have been comprehensively discussed. Besides, the roles of oxysterols, fatty acids, phospholipids, and lipid second messengers are also summarized in the context of inflammasomes. The overriding theme is that lipid metabolism has numerous but complex functions in inflammasome activation. A detailed understanding of this area will help us develop therapeutic interventions for diseases where dysregulated lipid metabolism is the underlying cause.

Emerging role of Insig-1 in lipid metabolism and lipid disorders

Growing evidence has demonstrated that Insig-1 is intricately involved in lipid metabolism regulation and the progression of lipid disorders. Our review summarizes updated information on the role and underlying mechanisms of Insig-1 in lipid metabolism dyshomeostasis and lipid disorders. As a member of the insulin-induced gene family, insulin-induced gene 1 (Insig-1) is a six-span transmembrane protein embedded in the endoplasmic reticulum (ER) membrane. Insig-1 is widely involved in the maintenance of intracellular lipid metabolism homeostasis by controlling the activation of sterol regulatory element-binding proteins (SREBPs) and the degradation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR). Growing experimental and clinical data have identified that Insig-1 reduces lipid accumulation in hepatocytes to relieve the development of nonalcoholic fatty liver disease (NAFLD), downregulates the plasma level of free cholesterol and protects β cells against lipotoxicity to alleviate diabetic dyslipidemia. In addition, Insig-1 suppresses adipogenesis and inhibits the differentiation of preadipocytes to prevent the occurrence of obesity. Insig-1 is a key regulatory factor that maintains intracellular lipid metabolism homeostasis and is a promising therapeutic target for lipid disorders.

The Chemical Potential of Plasma Membrane Cholesterol: Implications for Cell Biology

Cholesterol is abundant in plasma membranes and exhibits a variety of interactions throughout the membrane. Chemical potential accounts for thermodynamic consequences of molecular interactions, and quantifies the effective concentration (i.e., activity) of any substance participating in a process. We have developed, to our knowledge, the first method to measure cholesterol chemical potential in plasma membranes. This was accomplished by complexing methyl-β-cyclodextrin with cholesterol in an aqueous solution and equilibrating it with an organic solvent containing dissolved cholesterol. The chemical potential of cholesterol was thereby equalized in the two phases. Because cholesterol is dilute in the organic phase, here activity and concentration were equivalent. This equivalence allowed the amount of cholesterol bound to methyl-β-cyclodextrin to be converted to cholesterol chemical potential. Our method was used to determine the chemical potential of cholesterol in erythrocytes and in plasma membranes of nucleated cells in culture. For erythrocytes, the chemical potential did not vary when the concentration was below a critical value. Above this value, the chemical potential progressively increased with concentration. We used standard cancer lines to characterize cholesterol chemical potential in plasma membranes of nucleated cells. This chemical potential was significantly greater for highly metastatic breast cancer cells than for nonmetastatic breast cancer cells. Chemical potential depended on density of the cancer cells. A method to alter and fix the cholesterol chemical potential to any value (i.e., a cholesterol chemical potential clamp) was also developed. Cholesterol content did not change when cells were clamped for 24-48 h. It was found that the level of activation of the transcription factor STAT3 increased with increasing cholesterol chemical potential. The cholesterol chemical potential may regulate signaling pathways.

Tumor metastasis to lymph nodes requires YAP-dependent metabolic adaptation

In cancer patients, metastasis of tumors to sentinel lymph nodes (LNs) predicts disease progression and often guides treatment decisions. The mechanisms underlying tumor LN metastasis are poorly understood. By using comparative transcriptomics and metabolomics analyses of primary and LN-metastatic tumors in mice, we found that LN metastasis requires that tumor cells undergo a metabolic shift toward fatty acid oxidation (FAO). Transcriptional coactivator yes-associated protein (YAP) is selectively activated in LN-metastatic tumors, leading to the up-regulation of genes in the FAO signaling pathway. Pharmacological inhibition of FAO or genetic ablation of YAP suppressed LN metastasis in mice. Several bioactive bile acids accumulated to high levels in the metastatic LNs, and these bile acids activated YAP in tumor cells, likely through the nuclear vitamin D receptor. Inhibition of FAO or YAP may merit exploration as a potential therapeutic strategy for mitigating tumor metastasis to LNs.

Proteostatic Tactics in the Strategy of Sterol Regulation

In eukaryotes, the synthesis and uptake of sterols undergo stringent multivalent regulation. Both individual enzymes and transcriptional networks are controlled to meet changing needs of the many sterol pathway products. Regulation is tailored by evolution to match regulatory constraints, which can be very different in distinct species. Nevertheless, a broadly conserved feature of many aspects of sterol regulation is employment of proteostasis mechanisms to bring about control of individual proteins. Proteostasis is the set of processes that maintain homeostasis of a dynamic proteome. Proteostasis includes protein quality control pathways for the detection, and then the correction or destruction, of the many misfolded proteins that arise as an unavoidable feature of protein-based life. Protein quality control displays not only the remarkable breadth needed to manage the wide variety of client molecules, but also extreme specificity toward the misfolded variants of a given protein. These features are amenable to evolutionary usurpation as a means to regulate proteins, and this approach has been used in sterol regulation. We describe both well-trod and less familiar versions of the interface between proteostasis and sterol regulation and suggest some underlying ideas with broad biological and clinical applicability.

Lipid metabolism reprogramming and its potential targets in cancer

Reprogramming of lipid metabolism is a newly recognized hallmark of malignancy. Increased lipid uptake, storage and lipogenesis occur in a variety of cancers and contribute to rapid tumor growth. Lipids constitute the basic structure of membranes and also function as signaling molecules and energy sources. Sterol regulatory element-binding proteins (SREBPs), a family of membrane-bound transcription factors in the endoplasmic reticulum, play a central role in the regulation of lipid metabolism. Recent studies have revealed that SREBPs are highly up-regulated in various cancers and promote tumor growth. SREBP cleavage-activating protein is a key transporter in the trafficking and activation of SREBPs as well as a critical glucose sensor, thus linking glucose metabolism and de novo lipid synthesis. Targeting altered lipid metabolic pathways has become a promising anti-cancer strategy. This review summarizes recent progress in our understanding of lipid metabolism regulation in malignancy, and highlights potential molecular targets and their inhibitors for cancer treatment.

Neurodegenerative diseases, infectious pathologies and liquid crystals: Hypothesis of a common information vector involving a multidisciplinary approach

The existence of an information vector common to very different pathologies is the hypothesis of one of us, the argumentation and discussion of which we present here. It is a mesomorphic state of material called liquid crystal. The liquid-ordered (Lo) phase, made up of membrane rafts mediated by cholesterol, lies at the center of our concept. This mesophase is either preexistent and then modified by the pathogenic process, or initiated by the latter. The most notable disorders involved are Alzheimer's, Parkinson's, Charcot and Creutzfeldt-Jakob diseases, flu-like illnesses and acquired immunodeficiency syndrome (AIDS), although this list may well be extended to include other anisotropic, birefringent amyloid proteinopathies, which have properties compatible with those of liquid crystals. Incidentally, numerous conventional infectious pathologies can also induce a mesomorphic state in cell membranes. It has already been established that mesophases contain the chemical information transmitted from the intramolecular microscopic level, where covalent bonds are applied. Information is then transmitted at the intermolecular macroscopic level, where it is made up of informed, self-organized collections. Electrostatic interactions, coordination of metallic ions, van der Waals forces and donor-acceptor interactions of hydrogen bonding all come into play. These reactions are produced notably in the nanodomains enriched by cholesterol and sphingolipids. Lipids in the cell membrane are where the phase separations favoring elastic hydrodynamic instabilities conducive to the Lo phase take place. In addition, perturbations of the mesomorphic states of membrane rafts due, for example, to lipid dysfunction-even mild ones-with an intracerebral or generalized location could bring about a displacement of thermodynamic equilibrium favoring the initiation and progression of the pathologies under consideration here. Indeed, the most recent work has rendered our hypothesis highly probable. Moreover, our hypothesis is supported by medical and biological observations arising essentially from biophysics and widely documented in the literature. Thus, these facts expand the number of diagnostic and therapeutic perspectives that could be evoked and perhaps even demand exploration.

microRNA-29 mediates a novel negative feedback loop to regulate SCAP/SREBP-1 and lipid metabolism

The membrane-bound transcription factors, SREBPs (sterol regulatory element-binding proteins), play a central role in regulating lipid metabolism. The transcriptional activation of SREBPs requires the key protein SCAP (SREBP-cleavage activating protein) to translocate their precursors from the endoplasmic reticulum to the Golgi for subsequent proteolytic activation, a process tightly regulated by a cholesterol-mediated negative feedback loop. Our previous work showed that the SCAP/SREBP-1 pathway is significantly upregulated in human glioblastoma (GBM), the most deadly brain cancer, and that glucose-mediated N-glycosylation of SCAP is a prerequisite step for SCAP/SREBP trafficking. More recently, we demonstrated that microRNA-29 (miR-29) mediates a previously unrecognized negative feedback loop in SCAP/SREBP-1 signaling to control lipid metabolism. We found that SREBP-1, functioning as a transcription factor, promotes the expression of the miR-29 family members, miR-29a, -29b and -29c. In turn, the miR-29 isoforms reversely repress the expression of SCAP and SREBP-1. Moreover, treatment with miR-29 mimics effectively suppressed GBM tumor growth by inhibiting SCAP/SREBP-1 and de novo lipid synthesis. These findings, recently published in Cell Reports, strongly suggest that delivery of miR-29 in vivo may be a promising approach to treat cancer and metabolic diseases by suppressing SCAP/SREBP-1-regulated lipid metabolism.

Structural Stringency of Cholesterol for Membrane Protein Function Utilizing Stereoisomers as Novel Tools: A Review

Cholesterol is an important lipid in the context of membrane protein function. The function of a number of membrane proteins, including G protein-coupled receptors (GPCRs) and ion channels, has been shown to be dependent on membrane cholesterol. However, the molecular mechanism underlying such regulation is still being explored. In some cases, specific interaction between cholesterol and the protein has been implicated. In other cases, the effect of cholesterol on the membrane properties has been attributed for the regulation of protein function. In this article, we have provided an overview of experimental approaches that are useful for determining the degree of structural stringency of cholesterol for membrane protein function. In the process, we have highlighted the role of immediate precursors in cholesterol biosynthetic pathway in the function of membrane proteins. Special emphasis has been given to the application of stereoisomers of cholesterol in deciphering the structural stringency required for regulation of membrane protein function. A comprehensive examination of these processes would help in understanding the molecular basis of cholesterol regulation of membrane proteins in subtle details.

Necroptosis-like Neuronal Cell Death Caused by Cellular Cholesterol Accumulation

Aberrant cellular accumulation of cholesterol is associated with neuronal lysosomal storage disorders such as Niemann-Pick disease Type C (NPC). We have shown previously that l-norephedrine (l-Nor), a sympathomimetic amine, induces necrotic cell death associated with massive cytoplasmic vacuolation in SH-SY5Y human neuroblastoma cells. To reveal the molecular mechanism underling necrotic neuronal cell death caused by l-Nor, we examined alterations in the gene expression profile of cells during l-Nor exposure. DNA microarray analysis revealed that the gene levels for cholesterol transport (LDL receptor and NPC2) as well as cholesterol biosynthesis (mevalonate pathway enzymes) are increased after exposure to 3 mm l-Nor for ∼6 h. Concomitant with this observation, the master transcriptional regulator of cholesterol homeostasis, SREBP-2, is activated by l-Nor. The increase in cholesterol uptake as well as biosynthesis is not accompanied by an increase in cholesterol in the plasma membrane, but rather by aberrant accumulation in cytoplasmic compartments. We also found that cell death by l-Nor can be suppressed by nec-1s, an inhibitor of a regulated form of necrosis, necroptosis. Abrogation of SREBP-2 activation by the small molecule inhibitor betulin or by overexpression of dominant-negative SREBP-2 efficiently reduces cell death by l-Nor. The mobilization of cellular cholesterol in the presence of cyclodextrin also suppresses cell death. These results were also observed in primary culture of striatum neurons. Taken together, our results indicate that the excessive uptake as well as synthesis of cholesterol should underlie neuronal cell death by l-Nor exposure, and suggest a possible link between lysosomal cholesterol storage disorders and the regulated form of necrosis in neuronal cells.

Endoplasmic Reticulum-Associated Degradation and Lipid Homeostasis

The endoplasmic reticulum is the port of entry for proteins into the secretory pathway and the site of synthesis for several important lipids, including cholesterol, triacylglycerol, and phospholipids. Protein production within the endoplasmic reticulum is tightly regulated by a cohort of resident machinery that coordinates the folding, modification, and deployment of secreted and integral membrane proteins. Proteins failing to attain their native conformation are degraded through the endoplasmic reticulum-associated degradation (ERAD) pathway via a series of tightly coupled steps: substrate recognition, dislocation, and ubiquitin-dependent proteasomal destruction. The same ERAD machinery also controls the flux through various metabolic pathways by coupling the turnover of metabolic enzymes to the levels of key metabolites. We review the current understanding and biological significance of ERAD-mediated regulation of lipid metabolism in mammalian cells.

Dysregulation of the Low-Density Lipoprotein Receptor Pathway Is Involved in Lipid Disorder-Mediated Organ Injury

The low-density lipoprotein receptor (LDLR) pathway is a negative feedback system that plays important roles in the regulation of plasma and intracellular cholesterol homeostasis. To maintain a cholesterol homeostasis, LDLR expression is tightly regulated by sterol regulatory element-binding protein-2 (SREBP-2) and SREBP cleavage-activating protein (SCAP) in transcriptional level and by proprotein convertase subtilisin/kexin type 9 (PCSK9) in posttranscriptional level. The dysregulation of LDLR expression results in abnormal lipid accumulation in cells and tissues, such as vascular smooth muscle cells, hepatic cells, renal mesangial cells, renal tubular cells and podocytes. It has been demonstrated that inflammation, renin-angiotensin system (RAS) activation, and hyperglycemia induce the disruption of LDLR pathway, which might contribute to lipid disorder-mediated organ injury (atherosclerosis, non-alcoholic fatty liver disease, kidney fibrosis, etc). The mammalian target of rapamycin (mTOR) pathway is a critical mediator in the disruption of LDLR pathway caused by pathogenic factors. The mTOR complex1 activation upregulates LDLR expression at the transcriptional and posttranscriptional levels, consequently resulting in lipid deposition. This paper mainly reviews the mechanisms for the dysregulation of LDLR pathway and its roles in lipid disorder-mediated organ injury under various pathogenic conditions. Understanding these mechanisms leading to the abnormality of LDLR expression contributes to find potential new drug targets in lipid disorder-mediated diseases.

Cholesterol homeostasis: How do cells sense sterol excess?

Cholesterol is vital in mammals, but toxic in excess. Consequently, elaborate molecular mechanisms have evolved to maintain this sterol within narrow limits. How cells sense excess cholesterol is an intriguing area of research. Cells sense cholesterol, and other related sterols such as oxysterols or cholesterol synthesis intermediates, and respond to changing levels through several elegant mechanisms of feedback regulation. Cholesterol sensing involves both direct binding of sterols to the homeostatic machinery located in the endoplasmic reticulum (ER), and indirect effects elicited by sterol-dependent alteration of the physical properties of membranes. Here, we examine the mechanisms employed by cells to maintain cholesterol homeostasis.

Active membrane cholesterol as a physiological effector

Sterols associate preferentially with plasma membrane sphingolipids and saturated phospholipids to form stoichiometric complexes. Cholesterol in molar excess of the capacity of these polar bilayer lipids has a high accessibility and fugacity; we call this fraction active cholesterol. This review first considers how active cholesterol serves as an upstream regulator of cellular sterol homeostasis. The mechanism appears to utilize the redistribution of active cholesterol down its diffusional gradient to the endoplasmic reticulum and mitochondria, where it binds multiple effectors and directs their feedback activity. We have also reviewed a broad literature in search of a role for active cholesterol (as opposed to bulk cholesterol or lipid domains such as rafts) in the activity of diverse membrane proteins. Several systems provide such evidence, implicating, in particular, caveolin-1, various kinds of ABC-type cholesterol transporters, solute transporters, receptors and ion channels. We suggest that this larger role for active cholesterol warrants close attention and can be tested easily.

Rapid proteasomal elimination of 3-hydroxy-3-methylglutaryl-CoA reductase by interferon-γ in primary macrophages requires endogenous 25-hydroxycholesterol synthesis

Interferons (IFNs) play a central role in immunity and emerging evidence suggests that IFN-signalling coordinately regulates sterol biosynthesis in macrophages, via Sterol Regulatory Element-Binding Protein (SREBP) dependent and independent pathways. However, the precise mechanisms and kinetic steps by which IFN controls sterol biosynthesis are as yet not fully understood. Here, we elucidate the molecular circuitry governing how IFN controls the first regulated step in the mevalonate-sterol pathway, 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), through the synthesis of 25-Hydroxycholesterol (25-HC) from cholesterol by the IFN-inducible Cholesterol-25-Hydroxylase (CH25H). We show for the first 30-min of IFN stimulation of macrophages the rate of de novo synthesis of the Ch25h transcript is markedly increased but by 120-min becomes transcriptionally curtailed, coincident with induction of the Activating Transcription Factor 3 (ATF3) repressor. We demonstrate ATF3 induction by Toll-like receptors is strictly dependent on IFN-signalling. While the SREBP-pathway dependent rates of de novo transcription of Hmgcr are relatively unchanged in the first 90-min of IFN treatment, we find HMGCR enzyme levels undergo a rapid proteasomal-mediated degradation, defining a previously unappreciated SREBP-independent mechanism for IFN-action. These events precede a sustained marked reduction in Hmgcr RNA levels involving SREBP-dependent mechanisms. We demonstrate that HMGCR proteasomal-degradation by IFN strictly requires the synthesis of endogenous 25-HC and functionally couples HMGCR to CH25H to coordinately suppress sterol biosynthesis. In conclusion, we quantitatively delineate proteomic and transcriptional levels of IFN-mediated control of HMGCR, the primary enzymatic step of the mevalonate-sterol biosynthesis pathway, providing a foundational framework for mathematically modelling the therapeutic outcome of immune-metabolic pathways.

Sterol Regulatory Element Binding Transcription Factor-1 Gene Variation and Medication Load Influence White Matter Structure in Schizophrenia

BACKGROUND

Diffusion tensor imaging (DTI) studies have shown a widespread disruption of white matter (WM) microstructure in schizophrenia. Furthermore, higher fractional anisotropy (FA) has been consistently correlated with the severity of psychotic symptoms. Antipsychotic drugs (APDs) affect lipid homeostasis. Gene polymorphisms in sterol regulatory element binding transcription factor (SREBF)-1 and SREBF-2 have been associated with schizophrenia.

METHODS

In a sample of 65 patients affected by chronic schizophrenia, we investigated the effect of ongoing APD medication, SREBF-1 rs11868035 polymorphism and SREBF-2 rs1052717 polymorphism on the WM microstructure, using tract-based spatial statistics with threshold-free cluster enhancement.

RESULTS

We reported increased FA associated with the risk rs11868035 G/G genotype in several WM tracts, mainly located in the left hemisphere, and opposite effects of the APD medication load, with reduced FA and generally increased diffusivity. These opposite effects overlapped in the forceps minor, cingulum, uncinate fasciculus, the superior and inferior longitudinal fasciculi, the corticospinal tract, inferior fronto-occipital fasciculus and the anterior thalamic radiation.

CONCLUSION

We suggest that changes of WM structure could be an as yet poorly explored biomarker of the effects of APDs, to be further investigated in prospective studies correlating long-term clinical effects with changes of DTI measures in specific WM tracts contributing to the functional integrity of the brain. © 2015 S. Karger AG, Basel.

Transcriptional control of hepatic lipid metabolism by SREBP and ChREBP

The liver is a central organ that controls systemic energy homeostasis and nutrient metabolism. Dietary carbohydrates and lipids, and fatty acids derived from adipose tissue are delivered to the liver, and utilized for gluconeogenesis, lipogenesis, and ketogenesis, which are tightly regulated by hormonal and neural signals. Hepatic lipogenesis is activated primarily by insulin that is secreted from the pancreas after a high-carbohydrate meal. Sterol regulatory element binding protein-1c (SREBP-1c) and carbohydrate-responsive element-binding protein (ChREBP) are major transcriptional regulators that induce key lipogenic enzymes to promote lipogenesis in the liver. Sterol regulatory element binding protein-1c is activated by insulin through complex signaling cascades that control SREBP-1c at both transcriptional and posttranslational levels. Carbohydrate-responsive element-binding protein is activated by glucose independently of insulin. Here, the authors attempt to summarize the current understanding of the molecular mechanism for the transcriptional regulation of hepatic lipogenesis, focusing on recent studies that explore the signaling pathways controlling SREBPs and ChREBP.

Topics in transcriptional control of lipid metabolism: from transcription factors to gene-promoter polymorphisms

The central dogma of biology (DNA>>RNA>>Protein) has remained as an extremely useful scaffold to guide the study of molecular regulation of cellular metabolism. Molecular regulation of cellular metabolism has been pursued from an individual enzyme to a global assessment of protein function at the genomic (DNA), transcriptomic (RNA) and translation (Protein) levels. Details of a key role by inhibitory small RNAs and post-translational processing of cellular proteins on a whole cell/global basis are now just emerging. Below we emphasize the role of transcription factors (TF) in regulation of adipogenesis and lipogenesis. Additionally we have also focused on emerging additional TF that may also have hitherto unrecognized roles in adipogenesis and lipogenesis as compared to our present understanding. It is generally recognized that SNPs in structural genes can affect the final structure/function of a given protein. The implications of SNPs located in the non-transcribed promoter region on transcription have not been examined as extensively at this time. Here we have also summarized some emerging results on promoter SNPs for lipid metabolism and related cellular processes.

Imipramine is an orally active drug against both antimony sensitive and resistant Leishmania donovani clinical isolates in experimental infection

Background

In an endeavor to find an orally active and affordable antileishmanial drug, we tested the efficacy of a cationic amphiphilic drug, imipramine, commonly used for the treatment of depression in humans. The only available orally active antileishmanial drug is miltefosine with long half life and teratogenic potential limits patient compliance. Thus there is a genuine need for an orally active antileishmanial drug. Previously it was shown that imipramine, a tricyclic antidepressant alters the protonmotive force in promastigotes, but its in vivo efficacy was not reported.

Methodology/Principal Findings

Here we show that the drug is highly active against antimony sensitive and resistant Leishmania donovani in both promastigotes and intracellular amastigotes and in LD infected hamster model. The drug was found to decrease the mitochondrial transmembrane potential of Leishmania donovani (LD) promastigotes and purified amastigotes after 8 h of treatment, whereas miltefosine effected only a marginal change even after 24 h. The drug restores defective antigen presenting ability of the parasitized macrophages. The status of the host protective factors TNF α, IFN γ and iNOS activity increased with the concomitant decrease in IL 10 and TGF β level in imipramine treated infected hamsters and evolution of matured sterile hepatic granuloma. The 10-day therapeutic window as a monotherapy, showing about 90% clearance of organ parasites in infected hamsters regardless of their SSG sensitivity.

Conclusions

This study showed that imipramine possibly qualifies for a new use of an old drug and can be used as an effective orally active drug for the treatment of Kala-azar.

The disease Kala-azar or visceral leishmaniasis is still a big problem in the Indian subcontinent. The antimonials were used for the chemotherapy of Kala-azar but with time its efficacy has reduced dramatically. The newer version of orally active drug miltefosine has been introduced, but its efficacy has decreased considerably as relapse cases are on the rise. Other drugs like liposomal form of amphotericin B is expensive and the patients require hospitalization. Thus there is a genuine need for an orally active antileishmanial drug. There are reports that the cationic amphiphilic molecule, imipramine, a drug used for the treatment of depression in humans, kills the promastigotes of Leishmania donovani. We tested the efficacy of imipramine in experimental infection in hamster and mouse model. Our study showed that the drug is highly effective against antimony sensitive and antimony resistant Leishmania donovani infected hamsters as well as mouse and offered almost sterile cure.

Dual functions of Insig proteins in cholesterol homeostasis

The molecular mechanism of how cells maintain cholesterol homeostasis has become clearer for the understanding of complicated association between sterol regulatory element-binding proteins (SREBPs), SREBP cleavage-activating protein (SCAP), 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) and Insuin induced-genes (Insigs). The pioneering researches suggested that SREBP activated the transcription of genes encoding HMG-CoA reductase and all of the other enzymes involved in the synthesis of cholesterol and lipids. However, SREBPs can not exert their activities alone, they must form a complex with another protein, SCAP in the endoplasmic reticulum (ER) and translocate to Golgi. Insigs are sensors and mediators that regulate cholesterol homeostasis through binding to SCAP and HMG-CoA reductase in diverse tissues such as adipose tissue and liver, as well as the cultured cells. In this article, we aim to review on the dual functions of Insig protein family in cholesterol homeostasis.

Cholesterol through the looking glass: ability of its enantiomer also to elicit homeostatic responses

How cholesterol is sensed to maintain homeostasis has been explained by direct binding to a specific protein, Scap, or through altering the physical properties of the membrane. The enantiomer of cholesterol (ent-cholesterol) is a valuable tool in distinguishing between these two models because it shares nonspecific membrane effects with native cholesterol (nat-cholesterol), but not specific binding interactions. This is the first study to compare ent- and nat-cholesterol directly on major molecular parameters of cholesterol homeostasis. We found that ent-cholesterol suppressed activation of the master transcriptional regulator of cholesterol metabolism, SREBP-2, almost as effectively as nat-cholesterol. Importantly, ent-cholesterol induced a conformational change in the cholesterol-sensing protein Scap in isolated membranes in vitro, even when steps were taken to eliminate potential confounding effects from endogenous cholesterol. Ent-cholesterol also accelerated proteasomal degradation of the key cholesterol biosynthetic enzyme, squalene monooxygenase. Together, these findings provide compelling evidence that cholesterol maintains its own homeostasis not only via direct protein interactions, but also by altering membrane properties.

Activation mobilizes the cholesterol in the late endosomes-lysosomes of Niemann Pick type C cells

A variety of intercalating amphipaths increase the chemical activity of plasma membrane cholesterol. To test whether intracellular cholesterol can be similarly activated, we examined NPC1 and NPC2 fibroblasts, since they accumulate large amounts of cholesterol in their late endosomes and lysosomes (LE/L). We gauged the mobility of intracellular sterol from its appearance at the surface of the intact cells, as determined by its susceptibility to cholesterol oxidase and its isotope exchange with extracellular 2-(hydroxypropyl)-β-cyclodextrin-cholesterol. The entire cytoplasmic cholesterol pool in these cells was mobile, exchanging with the plasma membrane with an apparent half-time of ∼3–4 hours, ∼4–5 times slower than that for wild type human fibroblasts (half-time ∼0.75 hours). The mobility of the intracellular cholesterol was increased by the membrane-intercalating amphipaths chlorpromazine and 1-octanol. Chlorpromazine also promoted the net transfer of LE/L cholesterol to serum and cyclodextrin. Surprisingly, the mobility of LE/L cholesterol was greatly stimulated by treating intact NPC cells with glutaraldehyde or formaldehyde. Similar effects were seen with wild type fibroblasts in which the LE/L cholesterol pool had been expanded using U18666A. We also showed that the cholesterol in the intracellular membranes of fixed wild-type fibroblasts was mobile; it was rapidly oxidized by cholesterol oxidase and was rapidly replenished by exogenous sterol. We conclude that a) the cholesterol in NPC cells can exit the LE/L (and the extensive membranous inclusions therein) over a few hours; b) this mobility is stimulated by the activation of the cholesterol with intercalating amphipaths; c) intracellular cholesterol is even more mobile in fixed cells; and d) amphipaths that activate cholesterol might be useful in treating NPC disease.

Overweight induced by chronic risperidone exposure is correlated with overexpression of the SREBP-1c and FAS genes in mouse liver

Weight gain and metabolic disturbances, such as dyslipidemia and hyperglycaemia, are common side effects of most antipsychotic drugs, including risperidone. The aim of this study was to investigate the effects of chronic treatment with risperidone on body weight, fat accumulation, liver weight, and hepatic expression of key genes involved in lipid metabolism in female mice. We also addressed the mechanism of risperidone induction of metabolic side effects by exploring its effect on lipid and cholesterol metabolism in primary cultures of rat hepatocytes. Eleven weeks of treatment with long-acting risperidone (12.5 mpk/week) resulted in a significant weight gain associated with an increase of liver and adipose tissue weight. These effects were positively correlated with hepatic mRNA induction of two key genes involved in lipogenesis: sterol regulatory element binding protein-1c (SREBP-1c) and fatty acid synthase (FAS). Furthermore, in line with these in vivo results, risperidone elicited significant inductions of SREBP-1 maturation and FAS mRNA expression in primary cultures of rat hepatocytes associated with an increase of free fatty acid, triacylglycerol, and phospholipid synthesis as assessed by acetate incorporation. The current investigations underscore the usefulness of a mouse model to study the weight gain observed with risperidone treatment in humans. This study shows that risperidone induces similar effects in the liver (in vivo) and in hepatocyte cell cultures (in vitro) on the expression of key genes and/or proteins that control lipid metabolism. This suggests that risperidone could alter lipid metabolism in the liver and induce weight gain in a way that is partly independent of its action on the central nervous system.

Disruption of the acyl-CoA-binding protein gene delays hepatic adaptation to metabolic changes at weaning

The acyl-CoA-binding protein (ACBP)/diazepam binding inhibitor is an intracellular protein that binds C(14)-C(22) acyl-CoA esters and is thought to act as an acyl-CoA transporter. In vitro analyses have indicated that ACBP can transport acyl-CoA esters between different enzymatic systems; however, little is known about the in vivo function in mammalian cells. We have generated mice with targeted disruption of ACBP (ACBP(-/-)). These mice are viable and fertile and develop normally. However, around weaning, the ACBP(-/-) mice go through a crisis with overall weakness and a slightly decreased growth rate. Using microarray analysis, we show that the liver of ACBP(-/-) mice displays a significantly delayed adaptation to weaning with late induction of target genes of the sterol regulatory element-binding protein (SREBP) family. As a result, hepatic de novo cholesterogenesis is decreased at weaning. The delayed induction of SREBP target genes around weaning is caused by a compromised processing and decreased expression of SREBP precursors, leading to reduced binding of SREBP to target sites in chromatin. In conclusion, lack of ACBP interferes with the normal metabolic adaptation to weaning and leads to delayed induction of the lipogenic gene program in the liver.

Insulin-induced gene: a new regulator in lipid metabolism

Insulin-induced genes (Insigs) including Insig-1 and Insig-2, are proteins that mediate sterol regulation of sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP) and 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase). Insigs perform distinct tasks in the regulation of these effectors: they promote the endoplasmic reticulum (ER) retention of SCAP, but ubiquitin-mediated degradation of HMG-CoA reductase. Through these activities, Insig-1 and Insig-2 influence cholesterol metabolism, lipogenesis, and glucose homeostasis in diverse tissues such as adipose tissue and liver. In this article, we focus on the functions, expression and regulation, gene polymorphisms of Insigs, and their deficiency with diseases.

Ergosterol regulates sterol regulatory element binding protein (SREBP) cleavage in fission yeast

In fission yeast, the endoplasmic reticulum membrane-bound proteins Sre1 and Scp1, orthologs of mammalian sterol regulatory element binding protein (SREBP) and Scap, monitor sterol synthesis as an indirect measure of oxygen supply. When cellular oxygen levels are low, sterol synthesis is inhibited, and the Sre1-Scp1 complex responds by increasing transcription of genes required for adaptation to hypoxia. Sre1 and Scp1 are believed to detect a blockage in sterol synthesis by monitoring levels of particular sterols, but the evidence concerning which sterol signals this condition is unclear. Here, we demonstrate that Sre1-Scp1 senses ergosterol. Processing experimental data with a mathematical model of Sre1 and Scp1 function reveals a clear quantitative relationship between ergosterol concentration in the endoplasmic reticulum and Sre1 activation. Based on this relationship, we predict that the Sre1-Scp1 complex exists under "active" and "inactive" states and that the transition between these states is cooperatively mediated by ergosterol.

Coordination of lipid metabolism in membrane biogenesis

Bilayer synthesis during membrane biogenesis involves the concerted assembly of multiple lipid species, requiring coordination of the level of lipid synthesis, uptake, turnover, and subcellular distribution. In this review, we discuss some of the salient conclusions regarding the coordination of lipid synthesis that have emerged from work in mammalian and yeast cells. The principal instruments of global control are a small number of transcription factors that target a wide range of genes encoding enzymes that operate in a given metabolic pathway. Critical in mammalian cells are sterol regulatory element binding proteins (SREBPs) that stimulate expression of genes for the uptake and synthesis of cholesterol and fatty acids. From work with Saccharomyces cerevisiae, much has been learned about glycerophospholipid and ergosterol regulation through Ino2p/Ino4p and Upc2p transcription factors, respectively. Lipid supply is fine-tuned through a multitude of negative feedback circuits initiated by both end products and intermediates of lipid synthesis pathways. Moreover, there is evidence that the diversity of membrane lipids is maintained through cross-regulatory effects, whereby classes of lipids activate the activity of enzymes operating in another metabolic branch.

Sterol-protein interactions in cholesterol and bile acid synthesis

Cholesterol and other cholesterol related metabolites, oxysterols, and bile acids, establish specific interactions with enzymes and other proteins involved in cholesterol and bile acid homeostasis, triggering a variety of biological responses. The substrate-enzyme binding represents the best-characterized type of complementary interaction between proteins and small molecules. Key enzymes in the pathway that converts cholesterol to bile acids belong to the cytochrome P450 superfamily. In contrast to the majority of P450 enzymes, those acting on cholesterol and related metabolites exhibit higher stringency with respect to substrate molecules. This stringency, coupled with the specificity of the reactions, dictates the chemical features of intermediate metabolites (oxysterols) and end products (bile acids). Both oxysterols and bile acids have emerged in recent years as new signalling molecules due to their ability to interact and activate nuclear receptors, and consequently to regulate the transcription of genes involved in cholesterol and bile acid homeostasis and metabolism, but also in glucose and fatty acid metabolism. Interestingly, other proteins function as bile acid or sterol receptors. New findings indicate that bile acids also interact with a membrane G protein-coupled receptor, triggering a signalling cascade that ultimately promote energy expenditure. On the other end, cholesterol and side chain oxysterols establish specific interactions with different proteins residing in the endoplasmic reticulum that result in controlled protein degradation and/or trafficking to the Golgi and the nucleus. These regulatory pathways converge and contribute to adapt cholesterol uptake and synthesis to the cellular needs.

Antipsychotic drugs upregulate lipogenic gene expression by disrupting intracellular trafficking of lipoprotein-derived cholesterol

Antipsychotic drugs (APDs) have been reported to induce lipogenic genes. This has been proposed to contribute to their efficacy in treating schizophrenia and other psychiatric disorders, as well as the metabolic side effects often associated with these drugs. The precise mechanism for the lipogenic effects of APDs is unknown, but is believed to involve increased activation of the lipogenic transcription factors, such as sterol regulatory element binding proteins (SREBPs). In a series of experiments in a model cell line, we found that a panel of typical and atypical APDs inhibited transport of lipoprotein-derived cholesterol to the endoplasmic reticulum (ER), which houses the cholesterol homeostatic machinery. APDs belong to the class of cationic amphiphiles and as has been shown for other amphiphiles, caused lipoprotein-derived cholesterol to accumulate intracellularly, preventing it from being esterified in the ER and suppressing SREBP activation. APDs did not activate the liver X receptor, another transcription factor involved in lipogenesis. However, these drugs markedly reduced cholesterol synthesis. This paradoxical result indicates that the upregulation of SREBP-target genes by APDs may not translate to increased cellular cholesterol levels. In conclusion, we have determined that APDs disrupt intracellular trafficking and synthesis of cholesterol, which may have important clinical ramifications.

Cytotoxic effects of antipsychotic drugs implicate cholesterol homeostasis as a novel chemotherapeutic target

The reported reduction in cancer risk in those suffering from schizophrenia may be because antipsychotic medications have antineoplastic effects. In this study, 6 antipsychotic agents with a range of structural and pharmacological properties (reserpine, chlorpromazine, haloperidol, pimozide, risperidone and olanzapine), were screened for their effect on the viability of cell lines derived from lymphoblastoma, neuroblastoma, non-small cell lung cancer and breast adenocarcinoma. We aimed to determine if antipsychotic drugs in general possess cancer-specific cytotoxic potential, and whether it can be attributed to a common mode of action. With the exception of risperidone, all drugs tested displayed selective inhibition of the viability of cancer cell lines compared with normal cells. Using Affymetrix expression microarrays and quantitative real-time polymerase chain reaction, we found that for the antipsychotic drugs, olanzapine and pimozide, cytotoxicity appeared to be mediated via effects on cholesterol homeostasis. The role of cholesterol metabolism in the selective cytotoxicity of these drugs was supported by demonstration of their increased lethality when coadministered with a cholesterol synthesis inhibitor, mevastatin. Also, pimozide and olanzapine showed accelerating cytotoxic effects from 12 to 48 hr in time course studies, mirroring the time-dependent onset of cytotoxicity induced by the amphiphile, U18666A. On the basis of these results, we concluded that the Class II cationic amphiphilic properties of antipsychotic drugs contribute to their cytotoxic effects by acting on cholesterol homeostasis and altering the biophysical properties of cellular membranes, and that drugs affecting membrane-related cholesterol pathways warrant further investigation as potential augmentors of standard cancer chemotherapy.

Psychotropic drugs up-regulate the expression of cholesterol transport proteins including ApoE in cultured human CNS- and liver cells

Background

Disturbances in lipid homeostasis and myelination have been proposed in the pathophysiology of schizophrenia and bipolar disorder. We have previously shown that several antipsychotic and antidepressant drugs increase lipid biosynthesis through activation of the Sterol Regulatory Element-Binding Protein (SREBP) transcription factors, which control the expression of numerous genes involved in fatty acid and cholesterol biosynthesis. The aim of the present proof-of-principle study was to investigate whether such drugs also affect lipid transport and export pathways in cultured human CNS and liver cells.

Results

Quantitative PCR and immunoblotting were used to determine the level of lipid transport genes in human glioblastoma (GaMg) exposed to clozapine, olanzapine, haloperidol or imipramine. The effect of some of these drugs was also investigated in human astrocytoma (CCF-STTG1), neuroblastoma (SH-SY5Y) and hepatocellular carcinoma (HepG2) cells. We found significant transcriptional changes of cholesterol transport genes (ApoE, ABCA1, NPC1, NPC2, NPC1L1), which are predominantly controlled by the Liver X receptor (LXR) transcription factor. The up-regulation was observed after 24 to 48 hours of drug exposure, which is markedly delayed as compared to the drug-induced SREBP-controlled stimulation of lipid biosynthesis seen after 6 hours.

Conclusion

Our data show that stimulation of cellular lipid biosynthesis by amphiphilic psychotropic drugs is followed by a transcriptional activation of cholesterol transport and efflux pathways. Such effects may be relevant for both therapeutic effects and metabolic adverse effects of psychotropic drugs.

Identification of novel genes and pathways regulating SREBP transcriptional activity

BACKGROUND

Lipid metabolism in mammals is orchestrated by a family of transcription factors called sterol regulatory element-binding proteins (SREBPs) that control the expression of genes required for the uptake and synthesis of cholesterol, fatty acids, and triglycerides. SREBPs are thus essential for insulin-induced lipogenesis and for cellular membrane homeostasis and biogenesis. Although multiple players have been identified that control the expression and activation of SREBPs, gaps remain in our understanding of how SREBPs are coordinated with other physiological pathways.

METHODOLOGY

To identify novel regulators of SREBPs, we performed a genome-wide cDNA over-expression screen to identify proteins that might modulate the transcription of a luciferase gene driven from an SREBP-specific promoter. The results were verified through secondary biological assays and expression data were analyzed by a novel application of the Gene Set Enrichment Analysis (GSEA) method.

CONCLUSIONS/SIGNIFICANCE

We screened 10,000 different cDNAs and identified a number of genes and pathways that have previously not been implicated in SREBP control and cellular cholesterol homeostasis. These findings further our understanding of lipid biology and should lead to new insights into lipid associated disorders.

Serum cholesterol and expression of ApoAI, LXRbeta and SREBP2 in vitamin D receptor knock-out mice

Vitamin D insufficiency has been reported to be associated with increased blood cholesterol concentrations. Here we used two strains of VDR knock-out (VDR-KO) mice to study whether a lack of vitamin D action has any effect on cholesterol metabolism. In 129S1 mice, both in male and female VDR-KO mice serum total cholesterol levels were significantly higher than those in wild type (WT) mice (20.7% (P=0.05) and 22.2% (P=0.03), respectively). In addition, the serum high-density lipoprotein-bound cholesterol (HDL-C) level was 22% (P=0.03), respectively higher in male VDR-KO mice than in WT mice. The mRNA expression levels of five cholesterol metabolism related genes in livers of 129S1 mice were studied using quantitative real-time PCR (QRT-PCR): ATP-binding cassette transporter A1 (ABCA1), regulatory element binding protein (SREBP2), apolipoprotein A-I (ApoAI), low-density lipoprotein receptor (LDLR) and liver X receptor beta (LXRbeta). In the mutant male mice, the mRNA level of ApoAI and LXRbeta were 49.2% (P=0.005) and 38.8% (P=0.034) higher than in the WT mice. These changes were not observed in mutant female mice, but the female mutant mice showed 52.5% (P=0.006) decrease of SREBP2 mRNA expression compared to WT mice. Because the mutant mice were fed with a special rescue diet, we wanted to test whether the increased cholesterol levels in mutant mice were due to the diet. Both the WT and mutant NMRI mice were given the same diet for 3 weeks before the blood sampling. No difference in cholesterol or in HDL-C between WT and mutant mice was found. The results suggest that the food, gender and genetic background have an effect on the cholesterol metabolism. Although VDR seems to regulate some of the genes involved in cholesterol metabolism, its role in the regulation of serum cholesterol seems to be minimal.

Insulin enhances post-translational processing of nascent SREBP-1c by promoting its phosphorylation and association with COPII vesicles

The regulation of lipid homeostasis by insulin is mediated in part by the enhanced transcription of the gene encoding SREBP-1c (sterol regulatory element-binding protein-1c). Nascent SREBP-1c is synthesized and embedded in the endoplasmic reticulum (ER) and must be transported to the Golgi in coatomer protein II (COPII) vesicles where two sequential cleavages generate the transcriptionally active NH(2)-terminal fragment, nSREBP-1c. There is limited indirect evidence to suggest that insulin may also regulate the posttranslational processing of the nascent SREBP-1c protein. Therefore, we designed experiments to directly assess the action of insulin on the post-translational processing of epitope-tagged full-length SREBP-1c and SREBP-2 proteins expressed in cultured hepatocytes. We demonstrate that insulin treatment led to enhanced post-translational processing of SREBP-1c, which was associated with phosphorylation of ER-bound nascent SREBP-1c protein that increased affinity of the SREBP-1c cleavage-activating protein (SCAP)-SREBP-1c complex for the Sec23/24 proteins of the COPII vesicles. Furthermore, chemical and molecular inhibitors of the phosphoinositide 3-kinase pathway and its downstream kinase protein kinase B (PKB)/Akt prevented both insulin-mediated phosphorylation of nascent SREBP-1c protein and its posttranslational processing. Insulin had no effect on the proteolysis of nascent SREBP-2 under identical conditions. We also show that in vitro incubation of an active PKB/Akt enzyme with recombinant full-length SREBP-1c led to its phosphorylation. Thus, insulin selectively stimulates the processing of SREBP-1c in rat hepatocytes by enhancing the association between the SCAP-SREBP-1c complex and COPII proteins and subsequent ER to Golgi transport and proteolytic cleavage. This effect of insulin is tightly linked to phosphoinositide 3-kinase and PKB/Akt-dependent serine phosphorylation of the precursor SREBP-1c protein.

Characterization of fluorescent sterol binding to purified human NPC1

Mutations in the NPC1 gene cause Niemann-Pick type C disease, which appears to result from a defect in intracellular cholesterol trafficking. NPC1 is a member of the resistance-nodulation-cell division (RND) permease superfamily and contains a sterol-sensing domain, yet its cellular function and the identity of its substrates remain unknown. FLAG-tagged human NPC1 was purified from NPC1-expressing Chinese hamster ovary cells by solubilization in 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS), followed by affinity chromatography. Purified NPC1 in detergent solution appeared to be oligomeric as determined by gel filtration fast protein liquid chromatography and was photolabeled by an azido-cholesterol derivative. Fluorescent cholesterol analogs, including dehydroergosterol, cholestatrienol, and 22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3beta-ol (NBD-cholesterol), displayed enhanced fluorescence upon binding to NPC1 and also resulted in saturable, concentration-dependent quenching of NPC1 intrinsic Trp fluorescence. The apparent binding affinity for these three sterols was in the 0.5-6 microm range. Binding of NBD-cholesterol to NPC1 at low detergent concentration (2 mm CHAPS) was of high apparent affinity (0.5-0.6 microm) and occurred rapidly (<1 min). However, binding of a BODIPY-labeled cholesterol derivative was very slow, requiring approximately 3 h to reach equilibrium. The apparent NBD-cholesterol binding affinity was greatly reduced at higher detergent concentration. The stoichiometry of NBD-cholesterol binding to NPC1 was approximately 1. Various sterols, including native cholesterol and 25-hydroxycholesterol, inhibited NBD-cholesterol binding, suggesting that they compete for binding to the protein. Dynamic quenching studies showed that bound NBD-cholesterol was almost completely shielded from the aqueous medium, suggesting that it is buried in a deep hydrophobic pocket in NPC1. The use of fluorescent cholesterol analogs provides novel information on the molecular properties of the sterol-binding site in the full-length NPC1 protein.