芦丁对β淀粉样肽纤维化及细胞毒性的抑制作用

目的探索芦丁对Aβ25-35肽段淀粉样纤维化及纤维细胞毒性的抑制作用。方法在pH值为7.4、温度37℃孵育Aβ25-35肽,采用硫黄素(thioflavin T,ThT)荧光和透射电子显微镜检测多肽的淀粉样纤维化;以淀粉样纤维处理PC12细胞建立的细胞损伤模型,MTT法检测细胞存活率,以评估芦丁对β淀粉样纤维细胞毒性的抑制作用。结果 Aβ25-35肽段在pH值为7.4、温度37℃条件下,经孵育60h左右形成淀粉样纤维;芦丁抑制Aβ淀粉样纤维的形成,破坏纤维结构,并降低纤维诱导的细胞损害。结论芦丁能够抑制Aβ25-35的淀粉样纤维化和破坏成熟纤维结构,并降低Aβ纤维的细胞毒性。     

Signal transduction mechanisms for the survival and death of neurons and muscle cells: modulation by membrane lipid rafts and their abnormality in the disorders of the nervous system]

Recent advances have accumulated evidence that membrane lipid rafts or caveola play an essential role in cell-cell communications and signal transduction across membranes. The main constituents of lipid rafts include cholesterol, sphingomyelin, and glycosphingolipids such GM1 ganglioside. Many receptor-type tyrosine kinases and GPI-anchored proteins are now known to be the residents of lipid rafts. Therefore, it has been postulated that there are some direct or indirect interactions between these signaling molecules and lipids within lipid rafts, but no definite evidence has been available. In this study, we explored the molecular interactions of receptor-type tyrosine kinase, Trk, which essential for the neuronal survival and differentiation and for lipids, especially gangliosides. We also examined how the chemical depletion of another main lipid, cholesterol, affects the cellular function of muscle cells and its outcome. The data clearly indicate that 1) chemical and genetical depletion of gangliosides resulted in the impairment of the Trk-dependent protein kinase cascade. 2) depletion of intracellular cholesterol induced tyrosine phosphorylations of several cellular proteins including the p110 catalytic subunit of phosphatidylinositol-3 kinase and phospholipase C-gamma and the destruction of lipid rafts resulting in the development of apoptotic cell death of muscle cells.     

丹酚酸B体外抑制淀粉样β蛋白的纤维形成及其细胞毒作用(英文)

目的:观察丹酚酸B对淀粉样β蛋白的纤维形成及其细胞毒作用的影响。方法:将不同浓度丹酚酸B与淀粉样β蛋白(1-40)在25℃共同孵育,于不同时间取样品电镜观察纤维形成。用MTT法观察此不同时间点淀粉样β蛋白(1-40)对PC12细胞的毒性作用。另将淀粉样β蛋白(25-35)预先老化7d,用MTT法观察此老化蛋白对PC12细胞的毒性及丹酚酸B的作用。结果:丹酚酸B10—100nmol/L可以完全抑制淀粉样β蛋白(1-40)25℃放置30h的纤维形成,对淀粉样β蛋白(1-40)25℃放置48及100h的纤维形成也有明显抑制作用。MTT法显示,经与丹酚酸B共同孵育的淀粉样β蛋白(1-40)明显较未与丹酚酸B孵育的淀粉样β蛋白对PC12细胞的毒性小。丹酚酸B1μmol/L可明显抑制预先老化的淀粉样β蛋白(25-35)对PC12细胞的毒性作用。结论:丹酚酸B可抑制淀粉样β蛋白的老化及纤维形成,同时可直接抑制老化淀粉样β蛋白对PC12细胞的毒性作用。 (责任编辑 吴民淑)     

Methyl-β-cyclodextrin induces programmed cell death in chronic myeloid leukemia cells and, combined with imatinib, produces a synergistic downregulation of ERK/SPK1 signaling

Lipid rafts mediate several survival signals in the development of chronic myeloid leukemia (CML). Methyl-β-cyclodextrin (MβCD) is an inhibitor specifically designed to disrupt lipid rafts in cells by depleting the cholesterol component. We hypothesize that treatment of CML cells with MβCD and imatinib could reduce imatinib resistance. Apoptotic and autophagic cell death was assayed using annexin V-propidium iodide double staining, immunoblotting, and immunocytochemistry. We next investigated whether MβCD could enhance the cytotoxicity of imatinib in imatinib-sensitive and imatinib-resistant K562 cells. Extracellular signal-regulated kinase/sphingosine kinase 1 signaling downstream of lipid raft-activated signaling pathways was significantly inhibited by treatment of cells with a combination of MβCD and imatinib compared with treatment with either agent alone. MβCD induces programmed cell death in CML cells, and its antileukemia action is synergistic with that of imatinib.     

Role of lipid rafts in trimeric G protein-mediated signal transduction

Lipid rafts and caveolae are microdomains in the cell membranes, which contain cholesterol, glycolipids, and sphingomyelin. While caveolae are relatively stable because caveolin, an integral protein, supports the structure, lipid rafts are considered to be unstable, being dynamically produced and degraded. Recent studies have reported that lipid rafts contain many signaling molecules, such as glycosylphosphatidylinositol-anchored proteins, acylated proteins, G-protein-coupled receptors (GPCRs), trimeric and small G-proteins and their effectors, suggesting that the lipid rafts have an important role in receptor-mediated signal transduction. Therefore drugs that modify the composition of lipid rafts might influence the efficacy of cellular signal transduction. In this review, we demonstrate the role of lipid rafts in GPCR-G-protein signaling and also present our recent results showing that the wasp toxin mastoparan modifies G(q/11)-mediated phospholipase C activation through the interaction with gangliosides in lipid rafts.     

Ganglioside GD3 as a raft component in cell death regulation

Subcellular organelles such as mitochondria, endoplasmic reticulum and the Golgi complex are involved in the progression of cell death program. Recent evidence unveils that Fas ligand-mediated apoptosis induces scrambling of mitochondrial and secretory organelles via a global alteration of membrane traffic that is modulated by apical caspases. On the basis of the biochemical nature of lipid rafts, composed by sphingolipids, including gangliosides and sphingomyelin, cholesterol and signaling proteins, it has been suggested that they are part of this traffic and can participate in cell remodelling leading to cell death program execution. Although detected in various cell types, the role of lipid rafts in apoptosis has been mostly studied in T cells, where the physiological apoptotic program occurs through CD95/Fas. In this review, the possible contribution of lipid rafts to the cascade of events leading to T cell apoptosis after CD95/Fas ligation is summarized. We focused on the paradigmatic component of rafts GD3, which can proceed from the cell plasma membrane (and/or from trans Golgi network) to the mitochondria via a microtubule-dependent mechanism. This transport may be regulated by CLIPR-59, a new CLIP-170-related protein, involved in the regulation of microtubule dynamics. Particular attention has been given to mitochondrial raft-like microdomains, which may represent preferential sites where key reactions take place. Indeed, GD3, by interacting with mitochondrial raft-like microdomains, may trigger specific events involved in the apoptogenic program, including mitochondria hyperpolarization and depolarization, fission-associated changes, megapore formation and release of apoptogenic factors. These findings introduce an additional task for identifying new molecular target(s) of anti-cancer agents.     

Disruption of lipid rafts enhances activity of botulinum neurotoxin serotype A

Botulinum neurotoxin serotype A (BoNT/A), one of seven serotypes of botulinum neurotoxin, is taken up by neurons of the peripheral nervous system. Within the neurons it catalyzes cleavage of the synaptosomal-associated protein having a mass of 25 kDa, SNAP-25, thereby blocking neurotransmission. BoNT/A has been shown to interact with SV2, as well as gangliosides that are often found in lipid rafts. Lipid rafts are microdomains that can be found on the outer leaflet of the plasma membrane and are enriched in cholesterol and glycosphingolipids. To determine whether lipid rafts are needed for BoNT/A activity, those associated with the plasma membranes of murine N2a neuroblastoma cells were disrupted using either methyl-β-cyclodextrin or filipin. Disruption of cholesterol-containing lipid rafts by either reagent did not prevent the action of BoNT/A on N2a cells, in fact activity was enhanced. While our results indicate that disruption of lipid rafts enhances BoNT/A activity, disruption of clathrin-dependent endocytosis appeared to be inhibitory.     

Lipid rafts and their analytical methods

Microdomains in cell membranes consist of caveolae and lipid rafts, in which cholesterol, glycolipids, and sphingomyelin are concentrated. While caveolae are relatively stable because caveolin, an integral protein, supports the structure, lipid rafts are unstable, being dynamically produced and degraded. In lipid rafts, flotillin is assumed to be one of the specifically located proteins. Since microdomains contain several signaling molecules, such as transmembrane receptors, they have an important role in receptor-medicated signal transduction. Caveolae or lipid rafts are known to be resistant to non-ionic detergents, such as Triton X-100. Because of this property, they are separated as the detergent-resistant membranes when the Triton X-100-treated cell lysate is subjected to sucrose gradient centrifugation. On the other hand, cholesterol is an essential molecule to maintain microdomain structure. When the cells are treated with cholesterol removing agents, such as methyl-beta-cyclodextrin and filipin, the microdomain in cell membranes is disrupted. Thus, the cholesterol removing agents are utilized to determine whether the microdomain is involved in certain cellular/physiological responses. Recently, green fluorescent protein-tagged protein is used to analyze the localization of the protein in lipid rafts in intact cells. Research on lipid rafts will be helpful for understanding the detailed mechanism of signal transduction and to clarify the molecular basis of several diseases.     

Mastoparan changes the cellular localization of Galphaq/11 and Gbeta through its binding to ganglioside in lipid rafts

Although it is known that mastoparan, a wasp venom toxin, directly activates Gi/o, mastoparan-induced biological responses are not always explained by this mechanism. For instance, we have demonstrated previously that mastoparan suppressed phosphoinositide hydrolysis induced by carbachol in human astrocytoma cells (FEBS Lett 206:91-94, 1990). In the present study, we examined whether mastoparan affected phosphoinositide hydrolysis by interacting with lipid rafts in PC-12 cells. Mastoparan inhibited UTP-induced increase in [Ca2+]i and phosphoinositide hydrolysis in a concentration-dependent manner. UTP-induced phosphoinositide hydrolysis occurred in lipid rafts, because methyl-beta-cyclodextrin, a disrupting regent of lipid rafts, inhibited the hydrolysis. Mastoparan changed the localization of Galphaq/11 and Gbeta together with cholesterol from lipid rafts to nonraft fractions or cytosol. These changes were inhibited by ganglioside mixtures, suggesting that mastoparan interacts with gangliosides in lipid rafts. In fact, ganglioside mixtures and neuraminidase, but not sialic acid, attenuated the inhibitory effect of mastoparan on phosphoinositide hydrolysis. Furthermore, fluorescence intensity of tyrosine residue of [Tyr3]mastoparan was potentiated by ganglioside mixtures, suggesting the direct binding of mastoparan to gangliosides. Mastoparan caused cytotoxicity of PC-12 cells in a concentration-dependent manner, determined by LDH release. The mastoparan-induced cytotoxicity was significantly inhibited by neuraminidase or gangliosides. The order of inhibitory potency of gangliosides was GT1b approximately GD1b > GD1a > GM1 > GQ1b, but asialo-GM1 and sialic acid were inactive. These results suggest that mastoparan initially binds to gangliosides in lipid rafts and then it inhibits phosphoinositide hydrolysis by changing the localization of Galphaq/11 and Gbeta in lipid rafts.     

丹酚酸B和银杏叶提取物EGb 761对β-淀粉样蛋白神经毒性抑制作用的比较

目的比较丹酚酸B(Sal B)和银杏叶提取物EGb 761对β-淀粉样蛋白(β-AP)纤维形成及细胞毒作用的影响。方法运用硫黄素T(ThT)荧光法和电子显微镜技术分析Sal B和EGb 761对β-AP_1-40聚集和纤维形成的影响；另将β-AP_25-35预先老化7 d，用MTT法和流式细胞仪检测两种提取物对此老化蛋白造成的PC12细胞毒性的保护作用，用荧光法观察β-AP_25-35作用后细胞内活性氧含量的变化以及两种提取物的作用。结果Sal B和EGb 761都可有效地抑制β-AP_1-40纤维的形成，明显抑制老化的β-AP_25-35对PC12细胞的毒性作用，降低β-AP_25-35造成的细胞内活性氧含量的增加。Sal B的有效剂量远远低于EGb 761。结论Sal B对β-AP神经毒性的抑制作用强于EGb 761。     

Gangliosides induce autophagic cell death in astrocytes

Background and purpose:

Gangliosides, sialic acid-containing glycosphingolipids, abundant in brain, are involved in neuronal function and disease, but the precise molecular mechanisms underlying their physiological or pathological activities are poorly understood. In this study, the pathological role of gangliosides in the extracellular milieu with respect to glial cell death and lipid raft/membrane disruption was investigated.

Experimental approach:

We determined the effect of gangliosides on astrocyte death or survival using primary astrocyte cultures and astrocytoma/glioma cell lines as a model. Signalling pathways of ganglioside-induced autophagic cell death of astrocytes were examined using pharmacological inhibitors and biochemical and genetic assays.

Key results:

Gangliosides induced autophagic cell death in based on the following observations. Incubation of the cells with a mixture of gangliosides increased a punctate distribution of fluorescently labelled microtubule-associated protein 1 light chain 3 (GFP-LC3), the ratio of LC3-II/LC3-I and LC3 flux. Gangliosides also increased the formation of autophagic vacuoles as revealed by monodansylcadaverine staining. Ganglioside-induced cell death was inhibited by either a knockdown of beclin-1/Atg-6 or Atg-7 gene expression or by 3-methyladenine, an inhibitor of autophagy. Reactive oxygen species (ROS) were involved in ganglioside-induced autophagic cell death of astrocytes, because gangliosides induced ROS production and ROS scavengers decreased autophagic cell death. In addition, lipid rafts played an important role in ganglioside-induced astrocyte death.

Conclusions and implications:

Gangliosides released under pathological conditions may induce autophagic cell death of astrocytes, identifying a neuropathological role for gangliosides.     

Lipid raft: A floating island of death or survival

Lipid rafts are microdomains of the plasma membrane enriched in cholesterol and sphingolipids, and play an important role in the initiation of many pharmacological agent-induced signaling pathways and toxicological effects. The structure of lipid rafts is dynamic, resulting in an ever-changing content of both lipids and proteins. Cholesterol, as a major component of lipid rafts, is critical for the formation and configuration of lipid raft microdomains, which provide signaling platforms capable of activating both pro-apoptotic and anti-apoptotic signaling pathways. A change of cholesterol level can result in lipid raft disruption and activate or deactivate raft-associated proteins, such as death receptor proteins, protein kinases, and calcium channels. Several anti-cancer drugs are able to suppress growth and induce apoptosis of tumor cells through alteration of lipid raft contents via disrupting lipid raft integrity.     

Increased β-amyloid deposition in Tg-SWDI transgenic mouse brain following in vivo lead exposure

Previous studies in humans and animals have suggested a possible association between lead (Pb) exposure and the etiology of Alzheimer's disease (AD). Animals acutely exposed to Pb display an over-expressed amyloid precursor protein (APP) and the ensuing accumulation of beta-amyloid (Aβ) in brain extracellular spaces. This study was designed to examine whether in vivo Pb exposure increased brain concentrations of Aβ, resulting in amyloid plaque deposition in brain tissues. Human Tg-SWDI APP transgenic mice, which genetically over-express amyloid plaques at age of 2–3 months, received oral gavages of 50 mg/kg Pb acetate once daily for 6 weeks; a control group of the same mouse strain received the same molar concentration of Na acetate. ELISA results revealed a significant increase of Aβ in the CSF, brain cortex and hippocampus. Immunohistochemistry displayed a detectable increase of amyloid plaques in brains of Pb-exposed animals. Neurobehavioral test using Morris water maze showed an impaired spatial learning ability in Pb-treated mice, but not in C57BL/6 wild type mice with the same age. In vitro studies further uncovered that Pb facilitated Aβ fibril formation. Moreover, the synchrotron X-ray fluorescent studies demonstrated a high level of Pb present in amyloid plaques in mice exposed to Pb in vivo. Taken together, these data indicate that Pb exposure with ensuing elevated Aβ level in mouse brains appears to be associated with the amyloid plaques formation. Pb apparently facilitates Aβ fibril formation and participates in deposition of amyloid plaques.     

7β-Hydroxycholesterol-induced energy stress leads to sequential opposing signaling responses and to death of C6 glioblastoma cells

7β-Hydroxycholesterol cytotoxicity has been shown in vivo and in vitro to be dependent on the accumulation of its esters. We show in our study, using a detergent-free raft preparation and LC/MS lipid content analysis, that membrane microdomains isolated from 7β-hydroxycholesterol-treated C6 cells have a reduced cholesterol: cholesterol ester ratio and accumulate 7keto-hydroxycholesterol, 7β-hydroxycholesterol and 7β-hydroxycholesterol esters. These modifications in lipid content are accompanied by a redistribution of flotillin-1 in the lipid rafts. Transient increases of AMPK phosphorylation and mitochondrial activity during the first 12 h of 7β-hydroxycholesterol treatment indicate that C6 cells undergo energy stress and increase oxidative phosphorylation. Even so, ATP levels are maintained during 15 h until glucose uptake decreases. The cell's answers to raft modifications and energy stress are sequential activations of different signaling pathways such as ERK, AMPK and PI3K/Akt. These pathways, known to be activated under energy stress conditions, are transiently activated at 6 h (ERK, AMPK) and 12 h (Akt) of treatment respectively suggesting a shift from cell survival to cell proliferation. The persistence of 7β-hydroxycholesterol-induced stress led after 24 h to P38 activation, loss of GSK3β activation and to cell death. Finally we demonstrate that the observed signaling responses depend on 7β-hydroxycholesterol esterification, confirming that esterification of 7β-hydroxycholesterol is essential for cytotoxicity.     

Pinpointing Proline Substitution to be Responsible for the Loss of Amyloidogenesis in IAPP

Human islet amyloid polypeptide (hIAPP) is highly amyloidogenic, whereas its homologs in rodents are non‐amyloidogenic. This observed non‐amyloidogenecity of rodent IAPP has been attributed to substitutions by proline in a region of IAPP that forms the core of the fibril. By employing molecular dynamics simulation, we have analyzed effects of position‐specific proline substitution on amyloidogenesis of the core region of the hIAPP fibril (22–28). We depict that substitution to proline at the 25th position is primarily responsible for the loss of amyloidogenecity of the peptide. In addition, 25th and 26th double mutation to proline and valine has been observed to show significant fibril destabilizing ability. On the contrary, substitution at 28th position to proline has the least ability to destabilize the amyloid fibril. Results obtained from this study are particularly important to design variants of the existing antihyperglycemic drug with minimalistic mutation approach for use in patients with diabetes.     

Small transthyretin (TTR) ligands as possible therapeutic agents in TTR amyloidoses

In transthyretin (TTR) amyloidosis TTR variants deposit as amyloid fibrils giving origin, in most cases, to peripheral polyneuropathy, cardiomyopathy, carpal tunnel syndrome and/or amyloid deposition in the eye. More than eighty TTR variants are known, most of them being pathogenic. The mechanism of TTR fibril formation is still not completely elucidated. However it is widely accepted that the amino acid substitutions in the TTR variants contribute to a destabilizing effect on the TTR tetramer molecule, which in particular conditions dissociate into non native monomeric intermediates that aggregate and polymerize in amyloid fibrils that further elongate. Since this is a multi-step process there is the possibility to impair TTR amyloid fibril formation at different stages of the process namely by tetramer stabilization, inhibition of fibril formation or fibril disruption. Till now the only efficient therapy available is liver transplant when performed in an early phase of the onset of the disease symptoms. Since this is a very invasive therapy alternatives are desirable. In that sense, several compounds have been proposed to impair amyloid formation or disruption. Based on the proposed mechanism for TTR amyloid fibril formation we discuss the action of some of the proposed TTR stabilizers such as derivatives of some NSAIDs (diflunisal, diclofenac, flufenamic acid, and derivatives) and the action of amyloid disrupters such as 4'-iodo-4'-deoxydoxorubicin (I-DOX) and tetracyclines. Among all these compounds, TTR stabilizers seem to be the most interesting since they would impair very early the process of amyloid formation and could also have a prophylactic effect.     

Cholesterol regulates micro-opioid receptor-induced beta-arrestin 2 translocation to membrane lipid rafts

μ-Opioid receptor (OPRM1) is mainly localized in lipid raft microdomains but internalizes through clathrin-dependent pathways. Our previous studies demonstrated that disruption of lipid rafts by cholesterol-depletion reagent blocked the agonist-induced internalization of OPRM1 and G protein-dependent signaling. The present study demonstrated that reduction of cholesterol level decreased and culturing cells in excess cholesterol increased the agonist-induced internalization and desensitization of OPRM1, respectively. Further analyses indicated that modulation of cellular cholesterol level did not affect agonist-induced receptor phosphorylation but did affect membrane translocation of β-arrestins. The translocation of β-arrestins was blocked by cholesterol reduction, and the effect could be reversed by incubating with cholesterol. OptiPrep gradient separation of lipid rafts revealed that excess cholesterol retained more receptors in lipid raft domains and facilitated the recruitment of β-arrestins to these microdomains upon agonist activation. Moreover, excess cholesterol could evoke receptor internalization and protein kinase C-independent extracellular signal-regulated kinases activation upon morphine treatment. Therefore, these results suggest that cholesterol not only can influence OPRM1 localization in lipid rafts but also can effectively enhance the recruitment of β-arrestins and thereby affect the agonist-induced trafficking and agonist-dependent signaling of OPRM1.     

Cholesterol-lowering drugs inhibit lectin-like oxidized low-density lipoprotein-1 receptor function by membrane raft disruption

Lectin-like oxidized low-density lipoprotein (LOX-1), the primary receptor for oxidized low-density lipoprotein (ox-LDL) in endothelial cells, is up-regulated in atherosclerotic lesions. Statins are the principal therapeutic agents for cardiovascular diseases and are known to down-regulate LOX-1 expression. Whether the effect on the LOX-1 receptor is related to statin-mediated cholesterol-lowering activity is unknown. We investigate the requirement of cholesterol for LOX-1-mediated lipid particle internalization, trafficking, and processing and the role of statins as inhibitors of LOX-1 function. Disruption of cholesterol-rich membrane microdomains by acute exposure of cells to methyl-β-cyclodextrin or chronic exposure to different statins (lovastatin and atorvastatin) led to a spatial disorganization of LOX-1 in plasma membranes and a marked loss of specific LOX-1 function in terms of ox-LDL binding and internalization. Subcellular fractionation and immunochemical studies indicate that LOX-1 is naturally present in caveolae-enriched lipid rafts and, by cholesterol reduction, the amount of LOX-1 in this fraction is highly decreased (≥60%). In contrast, isoprenylation inhibition had no effect on the distribution and function of LOX-1 receptors. Furthermore, in primary cultures from atherosclerotic human aorta lesions, we confirm the presence of LOX-1 in caveolae-enriched lipid rafts and demonstrate that lovastatin treatment led to down-regulation of LOX-1 in lipid rafts and rescue of the ox-LDL-induced apoptotic phenotype. Taken together, our data reveal a previously unrecognized essential role of membrane cholesterol for LOX-1 receptor activity and suggest that statins protect vascular endothelium against the adverse effect of ox-LDL by disruption of membrane rafts and impairment of LOX-1 receptor function.     

The insecticide 1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane (DDT) alters the membrane raft location of the TSH receptor stably expressed in Chinese hamster ovary cells

DDT is a highly lipophilic molecule known to deplete membrane rafts of their phosphoglycolipid and cholesterol contents. However, we have recently shown that DDT can also alter the thyroid homeostasis by inhibiting TSH receptor (TSHr) internalization. The present study was undertaken to verify whether DDT goitrogenic effects are due to the insecticide acting directly on TSHr or via alteration of the membrane rafts hosting the receptor itself. Our results demonstrate that, in CHO-TSHr transfected cells, TSHr is activated in the presence of TSH, while it is inhibited following DDT exposure. DDT can also reduce the endocytic vesicular traffic, alter the extension of multi-branched microvilli along their plasma membranes and induce TSHr shedding in vesicular forms. To verify whether TSHr displacement might depend on DDT altering the raft constitution of CHO-TSHr cell membranes the extent of TSHr and lipid raft co-localization was examined by confocal microscopy. Evidence shows that receptor/raft co-localization increased significantly upon exposure to TSH, while receptors and lipid rafts become dislodged on opposite cell poles in DDT-exposed CHO-TSHr cells. As a control, under similar culturing conditions, diphenylethylene, which is known to be a lipophilic substance that is structurally related to DDT, did not affect the extent of TSHr and lipid raft co-localization in CHO-TSHr cells treated with TSH. These findings corroborate and extend our view that, in CHO cells, the DDT disrupting action on TSHr is primarily due to the insecticide acting on membranes to deplete their raft cholesterol content, and that the resulting inhibition on TSHr internalization is due to receptor dislodgement from altered raft microdomains of the plasma membrane.     

The gut-to-breast connection - interdependence of sterols and sphingolipids in multidrug resistance and breast cancer therapy

Almost all classes of bioactive lipids such as cholesterol and cholesterol derivatives, phospholipids and lysophospholipids, eicosanoids, and sphingolipids are critically involved in tumorigenesis. However, a systematic analysis of the distinct tumorigenic functions of lipids is rare. As a general principle, lipids either act directly by binding to receptors and other cell signaling proteins in growth control, or indirectly by regulating membrane organization such as the formation of membrane microdomains (lipid rafts) that modulate receptor or other membrane protein function. Lipid rafts are known to be formed by cholesterol and the sphingolipids or ceramide derivatives sphingomyelin and glucosylceramide (cholesterol-sphingomyelin-glucosylceramide or CSG rafts). In this review, we discuss the interconnection of sphingolipids with cholesterol and its derivatives in breast cancer drug resistance. Bile acids are cholesterol derivatives that are first synthesized in the liver (primary bile acids) and then metabolized by intestinal bacteria giving rise to secondary bile acids. They activate farnesoid X receptor (FXR), which inhibits cholesterol conversion to primary bile acids and induces the expression of drug resistance proteins. We introduce a novel model by which bile acid-mediated activation of FXR may promote the formation of CSG lipid rafts that trans-activate drug resistance proteins in breast cancer. Since breast cancer stem cells express high levels of drug resistance proteins, our model predicts that serum bile acids promote breast cancer stem cell survival and metastasis. Our model also predicts that FXR antagonists in combination with sphingolipid biosynthesis inhibitors may be promising candidates for novel drugs in lipid therapy of breast cancer.