Adipokine在肥胖症及胰岛素抵抗中的作用（综述）

从Adipokines与胰岛素抵抗的关系,Adipokines与肥胖者脂肪组织巨噬细胞浸润,炎症与胰岛素抵抗信号转导,内质网应激与胰岛素抵抗等方面综述了Adipokines在肥胖症及胰岛素抵抗中的作用;并着重探讨了肥胖,adipokines,炎症,胰岛素抵抗四者之间的关系。     

Aberrant lipid metabolism disrupts calcium homeostasis causing liver endoplasmic reticulum stress in obesity

The endoplasmic reticulum (ER) is the main site of protein and lipid synthesis, membrane biogenesis, xenobiotic detoxification and cellular calcium storage, and perturbation of ER homeostasis leads to stress and the activation of the unfolded protein response. Chronic activation of ER stress has been shown to have an important role in the development of insulin resistance and diabetes in obesity. However, the mechanisms that lead to chronic ER stress in a metabolic context in general, and in obesity in particular, are not understood. Here we comparatively examined the proteomic and lipidomic landscape of hepatic ER purified from lean and obese mice to explore the mechanisms of chronic ER stress in obesity. We found suppression of protein but stimulation of lipid synthesis in the obese ER without significant alterations in chaperone content. Alterations in ER fatty acid and lipid composition result in the inhibition of sarco/endoplasmic reticulum calcium ATPase (SERCA) activity and ER stress. Correcting the obesity-induced alteration of ER phospholipid composition or hepatic Serca overexpression in vivo both reduced chronic ER stress and improved glucose homeostasis. Hence, we established that abnormal lipid and calcium metabolism are important contributors to hepatic ER stress in obesity.     

Phagosomes are competent organelles for antigen cross-presentation

The ability to process microbial antigens and present them at the surface of cells is an important aspect of our innate ability to clear infections. It is generally accepted that antigens in the cytoplasm are loaded in the endoplasmic reticulum and presented at the cell surface on major histocompatibility complex (MHC) class I molecules, whereas peptides present in endo/phagocytic compartments are presented on MHC class II molecules. Despite the apparent segregation of the class I and class II pathways, antigens from intracellular pathogens including mycobacteria, Escherichia coli, Salmonella typhimurium, Brucella abortus and Leishmania, have been shown to elicit an MHC class-I-dependent CD8+ T-cell response, a process referred to as cross-presentation. The cellular mechanisms allowing the cross-presentation pathway are poorly understood. Here we show that phagosomes display the elements and properties needed to be self-sufficient for the cross-presentation of exogenous antigens, a newly ascribed function linked to phagocytosis mediated by the endoplasmic reticulum.     

Differential modulation of endotoxin responsiveness by human caspase-12 polymorphisms

Caspases mediate essential key proteolytic events in inflammatory cascades and the apoptotic cell death pathway. Human caspases functionally segregate into two distinct subfamilies: those involved in cytokine maturation (caspase-1, -4 and -5) and those involved in cellular apoptosis (caspase-2, -3, -6, -7, -8, -9 and -10). Although caspase-12 is phylogenetically related to the cytokine maturation caspases, in mice it has been proposed as a mediator of apoptosis induced by endoplasmic reticulum stress including amyloid-beta cytotoxicity, suggesting that it might contribute to the pathogenesis of Alzheimer's disease. Here we show that a single nucleotide polymorphism in caspase-12 in humans results in the synthesis of either a truncated protein (Csp12-S) or a full-length caspase proenzyme (Csp12-L). The read-through single nucleotide polymorphism encoding Csp12-L is confined to populations of African descent and confers hypo-responsiveness to lipopolysaccharide-stimulated cytokine production in ex vivo whole blood, but has no significant effect on apoptotic sensitivity. In a preliminary study, we find that the frequency of the Csp12-L allele is increased in African American individuals with severe sepsis. Thus, Csp12-L attenuates the inflammatory and innate immune response to endotoxins and in doing so may constitute a risk factor for developing sepsis.     

Macrophage migration inhibitory factor stimulates AMP-activated protein kinase in the ischaemic heart

Understanding cellular response to environmental stress has broad implications for human disease. AMP-activated protein kinase (AMPK) orchestrates the regulation of energy-generating and -consuming pathways, and protects the heart against ischaemic injury and apoptosis. A role for circulating hormones such as adiponectin and leptin in the activation of AMPK has received recent attention. Whether local autocrine and paracrine factors within target organs such as the heart modulate AMPK is unknown. Here we show that macrophage migration inhibitory factor (MIF), an upstream regulator of inflammation, is released in the ischaemic heart, where it stimulates AMPK activation through CD74, promotes glucose uptake and protects the heart during ischaemia-reperfusion injury. Germline deletion of the Mif gene impairs ischaemic AMPK signalling in the mouse heart. Human fibroblasts with a low-activity MIF promoter polymorphism have diminished MIF release and AMPK activation during hypoxia. Thus, MIF modulates the activation of the cardioprotective AMPK pathway during ischaemia, functionally linking inflammation and metabolism in the heart. We anticipate that genetic variation in MIF expression may impact on the response of the human heart to ischaemia by the AMPK pathway, and that diagnostic MIF genotyping might predict risk in patients with coronary artery disease.     

Caspase-12 mediates endoplasmic-reticulum-specific apoptosis and cytotoxicity by amyloid-beta

Apoptosis, or cellular suicide, is important for normal development and tissue homeostasis, but too much or too little apoptosis can also cause disease. The family of cysteine proteases, the so- called caspases, are critical mediators of programmed cell death, and thus far 14 family members have been identified. Some of these, such as caspase-8, mediate signal transduction downstream of death receptors located on the plasma membrane. Others, such as caspase-9, mediate apoptotic signals after mitochondrial damage. Stress in the endoplasmic reticulum (ER) can also result in apoptosis. Here we show that caspase-12 is localized to the ER and activated by ER stress, including disruption of ER calcium homeostasis and accumulation of excess proteins in ER, but not by membrane- or mitochondrial-targeted apoptotic signals. Mice that are deficient in caspase-12 are resistant to ER stress-induced apoptosis, but their cells undergo apoptosis in response to other death stimuli. Furthermore, we show that caspase-12-deficient cortical neurons are defective in apoptosis induced by amyloid-beta protein but not by staurosporine or trophic factor deprivation. Thus, caspase-12 mediates an ER-specific apoptosis pathway and may contribute to amyloid-beta neurotoxicity.     

A subfamily of stress proteins facilitates translocation of secretory and mitochondrial precursor polypeptides

Depletion of a subset of 70K stress proteins in yeast mutants shows that they are involved in the post-translational import of precursor polypeptides into both mitochondria and the lumen of the endoplasmic reticulum. The identification of such a basic function may explain the remarkable evolutionary conservation of the gene family encoding these proteins.     

内质网靶向生物活性小分子探针研究进展

内质网是真核细胞中重要的细胞器，参与多种生理过程．各种外界诱因会引发内质网应激，使细胞产生各种生物活性分子，从而导致细胞生理活性变化，引起癌症等多种疾病．生物活性物种，如活性氧、活性氮、活性硫等，具有较高的氧化还原活性和生物活性，在众多生理病理过程中起着关键作用．荧光探针技术是检测这些生物活性物种的一种较理想的手段，具有光学性质优异、定位效果突出、可以进行多功能实时原位监测等特点．因而，借助荧光成像技术监测活性物种在内质网中的变化过程，对攻克某些代谢疾病和癌症具有极大的推动作用．本文综述了近几年来可以靶向于内质网、特异性检测活性物种的荧光探针，在对其结构进行介绍的基础上，展示了其功能性和生物应用前景，并进一步阐述了定位和检测机制，最后对该类探针的未来发展进行了展望．      

Model for signal sequence recognition from amino-acid sequence of 54K subunit of signal recognition particle

Protein targeting to the endoplasmic reticulum in mammalian cells is catalysed by signal recognition particle (SRP). Cross-linking experiments have shown that the subunit of relative molecular mass 54,000 (Mr 54K; SRP54) interacts directly with signal sequences as they emerge from the ribosome. Here we present the sequence of a complementary DNA clone of SRP54 which predicts a protein that contains a putative GTP-binding domain and an unusually methionine-rich domain. The properties of this latter domain suggest that it contains the signal sequence binding site. A previously uncharacterized Escherichia coli protein has strong homology to both domains. Closely homologous GTP-binding domains are also found in the alpha-subunit of the SRP receptor (SR alpha, docking protein) in the endoplasmic reticulum membrane and in a second E. coli protein, ftsY, which resembles SR alpha. Recent work has shown that SR alpha is a GTP-binding protein and that GTP is required for the release of SRP from the signal sequence and the ribosome on targeting to the endoplasmic reticulum membrane. We propose that SRP54 and SR alpha use GTP in sequential steps of the targeting reaction and that essential features of such a pathway are conserved from bacteria to mammals.     

Aggregation and vesiculation of membrane proteins by curvature-mediated interactions

Membrane remodelling plays an important role in cellular tasks such as endocytosis, vesiculation and protein sorting, and in the biogenesis of organelles such as the endoplasmic reticulum or the Golgi apparatus. It is well established that the remodelling process is aided by specialized proteins that can sense as well as create membrane curvature, and trigger tubulation when added to synthetic liposomes. Because the energy needed for such large-scale changes in membrane geometry significantly exceeds the binding energy between individual proteins and between protein and membrane, cooperative action is essential. It has recently been suggested that curvature-mediated attractive interactions could aid cooperation and complement the effects of specific binding events on membrane remodelling. But it is difficult to experimentally isolate curvature-mediated interactions from direct attractions between proteins. Moreover, approximate theories predict repulsion between isotropically curving proteins. Here we use coarse-grained membrane simulations to show that curvature-inducing model proteins adsorbed on lipid bilayer membranes can experience attractive interactions that arise purely as a result of membrane curvature. We find that once a minimal local bending is realized, the effect robustly drives protein cluster formation and subsequent transformation into vesicles with radii that correlate with the local curvature imprint. Owing to its universal nature, curvature-mediated attraction can operate even between proteins lacking any specific interactions, such as newly synthesized and still immature membrane proteins in the endoplasmic reticulum.     

Glycoproteins form mixed disulphides with oxidoreductases during folding in living cells

The formation of intra- and interchain disulphide bonds constitutes an integral part of the maturation of most secretory and membrane-bound proteins in the endoplasmic reticulum. Evidence indicates that members of the protein disulphide isomerase (PDI) superfamily are part of the machinery needed for proper oxidation and isomerization of disulphide bonds. Models based on in vitro studies predict that the formation of mixed disulphide bonds between oxidoreductase and substrate is intermediate in the generation of the native intrachain disulphide bond in the substrate polypeptide. Whether this is how thiol oxidoreductases work inside the endoplasmic reticulum is not clear. Nor has it been established which of the many members of the PDI superfamily interacts directly with newly synthesized substrate proteins, because transient mixed disulphides have never been observed in the mammalian endoplasmic reticulum during oxidative protein folding. Here we describe the mechanisms involved in co- and post-translational protein oxidation in vivo. We show that the endoplasmic-reticulum-resident oxidoreductases PDI and ERp57 are directly involved in disulphide oxidation and isomerization, and, together with the lectins calnexin and calreticulin, are central in glycoprotein folding in the endoplasmic reticulum of mammalian cells.     

内质网应激对心肌梗死后心力衰竭大鼠心肌组织的作用

目的 研究内质网应激对心肌梗死后心力衰竭大鼠心肌组织的作用。方法 结扎大鼠左冠状动脉前降支建立心肌梗死后心力衰竭(心衰)模型,大鼠随机分为：假手术组、手术组术后2周组、4周组、6周组和8周组,测量各组大鼠的血流动力学指标,用荧光定量PCR方法检测各组大鼠的内质网分子伴侣GRP78蛋白,内质网应激相关凋亡蛋白CHOP、Caspase-12、Caspase-3、Bcl-2在心肌组织中的表达量。结果 与假手术组相比较,手术组大鼠左室收缩压(LVSP)、左室压最大上升和下降速率(±dp/dt max)明显低于假手术组(P≤0.05 或 P≤0.01);左室舒张末压(LVEDP)则高于假手术组(P≤0.05)。手术组大鼠心肌组织中GRP78、CHOP、Caspase-12、Caspase-3的表达量随术后饲养时间的增长而增高(P<0.05或P≤0.01),且表达量与术后饲养时间呈正相关(P≤0.01);心肌组织中Bcl-2的表达量随术后饲养时间的增长逐渐降低(P≤0.01),表达量与术后饲养时间呈负相关(P≤0.01)。结论 内质网应激激活了相关的细胞凋亡通路,使得心肌细胞凋亡。     

An ARF-GEF acting at the Golgi and in selective endocytosis in polarized plant cells

Circumstantial evidence suggests that intracellular membrane trafficking pathways diversified independently in the plant kingdom, but documented examples are rare. ARF-GEFs (guanine-nucleotide exchange factors for ADP-ribosylation factor GTPases) are essential for vesicular trafficking in all eukaryotic kingdoms, but of the eight ARF-GEF families, only the ancestral BIG and GBF types are found in plants. Whereas fungal and animal GBF proteins perform conserved functions at the Golgi, the Arabidopsis thaliana GBF protein GNOM is thought to act in only the process of recycling from endosomes. We now show that the related Arabidopsis GBF protein GNOM-LIKE1 (GNL1) has an ancestral function at the Golgi but is also required for selective internalization from the plasma membrane in the presence of brefeldin A (BFA). We identified gnl1 mutants that accumulated biosynthetic and recycling endoplasmic reticulum markers in enlarged internal compartments. Notably, in the absence of functional GNL1, Golgi stacks were rendered sensitive to the selective ARF-GEF inhibitor BFA, which caused them to fuse with the endoplasmic reticulum. Furthermore, in BFA-treated gnl1 roots, the internalization of a polar plasma-membrane marker, the auxin efflux carrier PIN2, was selectively inhibited. Thus, GNL1 is a BFA-resistant GBF protein that functions with a BFA-sensitive ARF-GEF both at the Golgi and in selective endocytosis, but not in recycling from endosomes. We propose that the evolution of endocytic trafficking in plants was accompanied by neofunctionalization within the GBF family, whereas in other kingdoms it occurred independently by elaboration of additional ARF-GEF families.     

A substrate-specific inhibitor of protein translocation into the endoplasmic reticulum

The segregation of secretory and membrane proteins to the mammalian endoplasmic reticulum is mediated by remarkably diverse signal sequences that have little or no homology with each other. Despite such sequence diversity, these signals are all recognized and interpreted by a highly conserved protein-conducting channel composed of the Sec61 complex. Signal recognition by Sec61 is essential for productive insertion of the nascent polypeptide into the translocation site, channel gating and initiation of transport. Although subtle differences in these steps can be detected between different substrates, it is not known whether they can be exploited to modulate protein translocation selectively. Here we describe cotransin, a small molecule that inhibits protein translocation into the endoplasmic reticulum. Cotransin acts in a signal-sequence-discriminatory manner to prevent the stable insertion of select nascent chains into the Sec61 translocation channel. Thus, the range of substrates accommodated by the channel can be specifically and reversibly modulated by a cell-permeable small molecule that alters the interaction between signal sequences and the Sec61 complex.     

Retrograde transport of endocytosed Shiga toxin to the endoplasmic reticulum.

Shiga toxin and some other protein toxins that act on targets in the cytosol have previously been shown to enter the trans-Golgi network. Transport by this route may be necessary for translocation of the toxin to the cytosol and for intoxication, but it is not known whether the enzymatically active part of the toxins actually enters the cytosol from the trans-Golgi network. It has been suggested that such toxins are transported in a retrograde manner to the endoplasmic reticulum and that translocation occurs in this organelle, but retrograde transport of endocytosed material beyond the trans-Golgi network has never been demonstrated. Here we show that in butyric acid-treated A431 cells endocytosed Shiga toxin is not only transported to the trans-Golgi network, but also to all Golgi stacks, to the endoplasmic reticulum and to the nuclear envelope. Furthermore, butyric acid sensitizes the cells to Shiga toxin, which is consistent with the possibility that retrograde transport is required for translocation of the toxin to the cytosol.     

Restoration of antigen presentation to the mutant cell line RMA-S by an MHC-linked transporter.

In mammalian cells, short peptides derived from intracellular proteins are displayed on the cell membrane associated with class I molecules of the major histocompatibility complex (MHC). The surface presentation of class I-peptide complexes presumably alerts the immune system to intracellular viral protein synthesis. Peptides derived from the cytosol must reach the cisternae of the endoplasmic reticulum where they are required for the assembly of stable class I molecules, and it has been proposed that the products of the two MHC-encoded ATP-binding cassette (ABC) transporter genes function to deliver the peptides across the membrane of the endoplasmic reticulum. This idea is supported by experiments in which transfection of a human cell line defective in class I expression with a complementary DNA of one of these genes restored cell surface expression levels. Here we show that the complete phenotype of the mouse mutant cell line RMA-S, in which lack of surface expression of stable class I molecules correlates with an inability to present viral peptides originating in the cytosol, is repaired by the cDNA of the other transporter gene. These results are consistent with the possibility that the two transporter polypeptides form a heterodimer.     

RNA interference screen for human genes associated with West Nile virus infection

West Nile virus (WNV), and related flaviviruses such as tick-borne encephalitis, Japanese encephalitis, yellow fever and dengue viruses, constitute a significant global human health problem. However, our understanding of the molecular interaction of such flaviviruses with mammalian host cells is limited. WNV encodes only 10 proteins, implying that it may use many cellular proteins for infection. WNV enters the cytoplasm through pH-dependent endocytosis, undergoes cycles of translation and replication, assembles progeny virions in association with endoplasmic reticulum, and exits along the secretory pathway. RNA interference (RNAi) presents a powerful forward genetics approach to dissect virus-host cell interactions. Here we report the identification of 305 host proteins that affect WNV infection, using a human-genome-wide RNAi screen. Functional clustering of the genes revealed a complex dependence of this virus on host cell physiology, requiring a wide variety of molecules and cellular pathways for successful infection. We further demonstrate a requirement for the ubiquitin ligase CBLL1 in WNV internalization, a post-entry role for the endoplasmic-reticulum-associated degradation pathway in viral infection, and the monocarboxylic acid transporter MCT4 as a viral replication resistance factor. By extending this study to dengue virus, we show that flaviviruses have both overlapping and unique interaction strategies with host cells. This study provides a comprehensive molecular portrait of WNV-human cell interactions that forms a model for understanding single plus-stranded RNA virus infection, and reveals potential antiviral targets.     

The presence of malfolded proteins in the endoplasmic reticulum signals the induction of glucose-regulated proteins

Two glucose-regulated proteins, GRP78 and GRP94, are major constituents of the endoplasmic reticulum (ER) of mammalian cells. These proteins are synthesized constitutively in detectable amounts under normal growth conditions; they can also be induced under a variety of conditions of stress including glucose starvation and treatment with drugs that inhibit cellular glycosylation, with calcium ionophores or with amino-acid analogues. Unlike the closely-related heat shock protein (HSP) family, the GRPs are not induced significantly by high temperature. Recently, GRP78 has been identified as the immunoglobulin heavy chain binding protein (BiP) (ref. 5 and Y.K. et al., in preparation) which binds transiently to a variety of nascent, wild-type secretory and transmembrane proteins and permanently to malfolded proteins that accumulate within the ER. We have tested the hypothesis that the presence of malfolded proteins may be the primary signal for induction of GRPs by expressing wild-type and mutant forms of influenza virus haemagglutinin (HA) in simian cells. Only malfolded HAs, whose transport from the ER is blocked, induced the synthesis of GRPs 78 and 94. Additional evidence is presented that malfolding per se, rather than abnormal glycosylation, is the proximal inducer of this family of stress proteins.     

Quality control in the endoplasmic reticulum protein factory

The endoplasmic reticulum (ER) is a factory where secretory proteins are manufactured, and where stringent quality-control systems ensure that only correctly folded proteins are sent to their final destinations. The changing needs of the ER factory are monitored by integrated signalling pathways that constantly adjust the levels of folding assistants. ER chaperones and signalling molecules are emerging as drug targets in amyloidoses and other protein-conformational diseases.