Leptin signaling in the hypothalamus: emphasis on energy homeostasis and leptin resistance

Leptin, the long-sought satiety factor of adipocytes origin, has emerged as one of the major signals that relay the status of fat stores to the hypothalamus and plays a significant role in energy homeostasis. Understanding the mechanisms of leptin signaling in the hypothalamus during normal and pathological conditions, such as obesity, has been the subject of intensive research during the last decade. It is now established that leptin action in the hypothalamus in regulation of food intake and body weight is mediated by a neural circuitry comprising of orexigenic and anorectic signals, including NPY, MCH, galanin, orexin, GALP, alpha-MSH, NT, and CRH. In addition to the conventional JAK2-STAT3 pathway, it has become evident that PI3K-PDE3B-cAMP pathway plays a critical role in leptin signaling in the hypothalamus. It is now established that central leptin resistance contributes to the development of diet-induced obesity and ageing associated obesity. Central leptin resistance also occurs due to hyperleptinimia produced by exogenous leptin infusion. A defective nutritional regulation of leptin receptor gene expression and reduced STAT3 signaling may be involved in the development of leptin resistance in DIO. However, leptin resistance in the hypothalamic neurons may occur despite an intact JAK2-STAT3 pathway of leptin signaling. Thus, in addition to defective JAK2-STAT3 pathway, defects in other leptin signaling pathways may be involved in leptin resistance. We hypothesize that defective regulation of PI3K-PDE3B-cAMP pathway may be one of the mechanisms behind the development of central leptin resistance seen in obesity.     

Leptin signalling and leptin-mediated activation of human platelets: importance of JAK2 and the phospholipases Cgamma2 and A2

Leptin enhances agonist-induced platelet aggregation, and human platelets have been reported to express the leptin receptor. However, the pathways and mediators lying downstream of leptin binding to platelets remain, with few exceptions, unknown. In the present study, we sought to gain further insight into the possible role of leptin as a platelet agonist. Stimulation of platelets with leptin promoted thromboxane generation and activation of alpha(IIb)beta(3), as demonstrated by PAC-1 binding. Furthermore, it increased the adhesion to immobilised fibrinogen (p<0.001) and induced cytoskeletal rearrangement of both platelets and Meg01 cells. Leptin time- and dose-dependently phosphorylated the intracellular signalling molecules JAK2 and STAT3, although the importance of STAT3 for leptin-induced platelet activation remains to be determined. Important intracellular mediators and pathways activated by leptin downstream of JAK2 were found to include phosphatidylinositol-3 kinase, phospholipase Cgamma2 and protein kinase C, as well as the p38 MAP kinase-phospholipase A(2) axis. Accordingly, incubation with the specific inhibitors AG490, Ly294002, U73122, and SB203580 prevented leptin-mediated platelet activation. These results help delineate biologically relevant leptin signalling pathways in platelets and may improve our understanding of the mechanisms linking hyperleptinaemia to the increased thrombosis risk in human obesity.     

AG490阻断STAT3信号通路对人胶质瘤细胞增殖和周期的抑制作用

目的 探讨AG490阻断STAT3信号通路对人胶质瘤细胞增殖和细胞周期的影响.方法 用不同浓度的AG490作用于体外培养的人胶质瘤细胞株U87、U251;用免疫荧光细胞化学染色观察肿瘤细胞STAT3和激活态p-STAT3的表达;Western blot验证AG490对STAT3信号通路的阻断情况;Sulforhodamine B染色观察肿瘤细胞增殖的改变;流式细胞技术分析细胞周期的变化. 结果 STAT3蛋白在胶质瘤细胞胞浆中表达,而P-STAT3则在细胞核中表达.AG490作用胶质瘤细胞后可使p-STAT3表达下降,而STAT3表达不受影响.AG490阻断STAT3信号通路后,胶质瘤细胞的增殖受到显著抑制,该抑制作用与剂量及时间存在一定关系.AG490作用后胶质瘤的细胞周期出现阻滞.结论 应用AG490阻断STAT3通路能够导致胶质瘤细胞增殖的抑制和细胞周期的阻滞.针对STAT3信号通路的研究可能为胶质瘤的治疗提供更加有效的方法.     

STIP1对胶质瘤U251细胞增殖、侵袭和凋亡的影响

目的 探讨下调磷酸化应激诱导蛋白1（STIP1）表达对胶质瘤U251细胞增殖、侵袭和凋亡的影响,及其对JAK2/STAT3信号通路的调控作用。方法 免疫印迹法检测体外培养的正常胶质细胞（SVG）和人胶质瘤细胞（U251、U87和U37）STIP1蛋白表达水平。NC-siRNA或STIP1-siRNA质粒转染U251细胞,CCK-8法检测U251细胞增殖;Transwell实验检测U251细胞侵袭能力;流式细胞术检测U251细胞凋亡率;免疫印迹法检测JAK2/STAT3信号通路蛋白表达水平。结果 与正常胶质细胞SVG比较,胶质瘤细胞U251、U87和U373的STIP1蛋白表达水平均明显增高（P<0.05）。与NC-siRNA组比较,STIP1-siRNA组STIP1蛋白表达水平、细胞增殖活力和细胞侵袭力明显明显降低（P<0.05）,细胞凋亡率明显增高（P<0.05）,而且,p-JAK2和p-STAT3蛋白表达水平明显降低（P<0.05）。结论 STIP1在胶质瘤细胞中呈高表达,抑制STIP1表达可以抑制胶质瘤细胞的增殖和侵袭、促进凋亡,机制可能与抑制JAK2/STAT3信号通路有关。     

In vitro neuroprotective effects of ciliary neurotrophic factor on dorsal root ganglion neurons with glutamate-induced neurotoxicity

Ciliary neurotrophic factor has neuroprotective effects mediated through signal transducer and Janus kinase (JAK) 2/activator of transcrip-tion 3 (STAT3) and phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathways. Whether ciliary neurotrophic factor is neuroprotective for glutamate-induced excitotoxicity of dorsal root ganglion neurons is poorly understood. In the present study, the in vitro neuroprotective effects of ciliary neurotrophic factor against glutamate-induced excitotoxicity were determined in a primary culture of dorsal root ganglion neurons from Wistar rat embryos at embryonic day 15. Whether the JAK2/STAT3 and PI3K/Akt signaling pathways were related to the protective effects of ciliary neurotrophic factor was also determined. Glutamate exposure inhibited neurite outgrowth, cell viability, and growth-associated protein 43 expression and promoted apoptotic neuronal cell death, all of which were reversed by the administration of exogenous ciliary neurotrophic factor. Additionally, preincubation with either JAK2 inhibitor AG490 or PI3K inhibitor LY294002 blocked the neuroprotective effect of ciliary neurotrophic factor. These data indicate that the two pathways JAK2/STAT3 and PI3K/Akt play major roles in mediating the in vitro neuroprotective effects of ciliary neurotrophic factor on dorsal root ganglion neurons with glutamate-in-duced neurotoxicity.     

PI3K/akt, JAK/STAT and MEK/ERK pathway inhibition protects retinal ganglion cells via different mechanisms after optic nerve injury

Recently we unexpectedly found that PI3K/akt, JAK/STAT and MEK/ERK pathway inhibitors enhanced retinal ganglion cell (RGC) survival after optic nerve (ON) axotomy in adult rat, a phenomenon contradictory to conventional belief that these pathways are pro-survival. In this study we showed that: (i) the RGC protection was pathway inhibition-dependent; (ii) inhibition of PI3K/akt and JAK/STAT, but not MEK/ERK, activated macrophages in the eye, (iii) macrophage removal from the eye using clodronate liposomes significantly impeded PI3K/akt and JAK/STAT inhibition-induced RGC survival and axon regeneration whereas it only slightly affected MEK/ERK inhibition-dependent protection; (iv) in the absence of recruited macrophages in the eye, inhibition of PI3K/akt or JAK/STAT did not influence RGC survival; and (v) strong PI3K/akt, JAK/STAT and MEK/ERK pathway activities were located in RGCs but not macrophages after ON injury. In retinal explants, in which supply of blood-derived macrophages is absent, MEK/ERK inhibition promoted RGC survival whereas PI3K/akt or JAK/STAT inhibition had no effect on RGC viability. However, MEK/ERK inhibition exerted opposite effects on the viability of purified adult RGCs at different concentrations in vitro, suggesting that this pathway may be bifunctional depending on the level of pathway activity. Our data thus demonstrate that inhibition of the PI3K/akt or JAK/STAT pathway activated macrophages to facilitate RGC protection after ON injury whereas the two pathways per se did not modulate RGC viability under the injury conditions (in the absence of the pathway activators). In contrast, the MEK/ERK pathway inhibition protected RGCs via macrophage-independent mechanism(s).     

STAT3信号通路影响人胶质瘤细胞株存活和凋亡的研究

目的 了解STAT3在星形细胞瘤中的表达情况并研究STAT3通路在胶质瘤细胞株生长、凋亡过程中的作用和相关机制。方法 通过免疫组化的方法显示星形细胞瘤手术标本中STAT3的表达情况。用AG490作用于体外培养的人胶质瘤细胞株U251、U87，通过Western blot了解STAT3蛋白在肿瘤细胞株中的表达和AG490对STAT3通路的阻断情况。观察AG490作用后胶质瘤细胞存活率的变化，并用流式细胞分析技术检测STAT3阻断后肿瘤细胞的凋亡情况。通过RT-PCR了解STAT3通路阻断后部分相关基因mRNA表达的改变。结果 STAT3在星形细胞瘤中有广泛表达，瘤内表达明显高于瘤周组织（P〈0．01）。STAT3在不同病理分级、性别、年龄组之间，差异无统计学意义（P〉0．05）。AG490作用胶质瘤细胞株后可使细胞内p-STAT3表达下降，同时肿瘤细胞存活率明显下降，肿瘤细胞凋亡比例明显升高。AG490对U251细胞存活率和凋亡的影响与AG490的浓度和作用时间有关。在AG490阻断STAT3信号通路的过程中，Mcl-1、Bcl-XL和Survivin的mRNA表达有所下降。结论 STAT3在星形细胞瘤中有广泛表达并与肿瘤细胞生长凋亡的生物学特性有着内在联系，STAT3通路有可能成为胶质瘤治疗的有用靶点。     

Activation of the JAK/STAT pathway following transient focal cerebral ischemia: signaling through Jak1 and Stat3 in astrocytes

JAK/STAT is one of the pathways bearing signals from the cell membrane to the nucleus in response to extracellular growth factors and cytokines. In the present study, we examined the cellular distribution of Jak1 and Stat3, and activation of the JAK/STAT pathway following transient focal cerebral ischemia in the rat. Jak1 was mainly seen in white matter astrocytes and in certain neurons. Notably, large pyramidal neurons of cortical layer V showed the highest neuronal Jak1 expression within cerebral cortex and, in addition, expressed Stat3 indicating that the JAK/STAT pathway is involved in signaling in the corticofugal projection system. Shortly following ischemia, Jak1 immunoreactive astrocytes located in the ipsilateral neighbouring white matter and ischemic cortex and striatum showed nuclear translocation of Stat3. These features were maintained in large reactive astrocytes that surrounded the infarct from 3 to 7 days. At these later times, the abundant reactive microglia/macrophages were strongly immunoreactive to Stat3 and, to a lesser extent, Jak1. Two main protein complexes showing DNA binding activity at the sis-inducible element site were found under basal conditions, followed by changes in this pattern following ischemia concomitant with neuronal cell loss and activation of glia. This study showed basal cerebral activity of JAK/STAT signaling pathway, involving Jak1 and Stat3 proteins, and selective activation following ischemia. It is suggested that the kinase activity of Jak1 mediates nuclear translocation of Stat3 in astrocytes, and that this signaling pathway is involved in the astroglial response to focal cerebral ischemia.     

Knockdown of eukaryotic translation initiation factors 3B (EIF3B) inhibits proliferation and promotes apoptosis in glioblastoma cells

Eukaryotic initiation factors 3 (EIF3) complex is essential for initiation of protein synthesis for both cells and virus. It consists of 13 subunits (EIF3A to M), among which EIF3B serves as a major scaffolding subunit. However, its functions in human glioblastoma have not been explored yet. Here, we showed that EIF3B was expressed in human glioblastoma (Grade I-IV) and human glioblastoma cell lines (U251, U87, A172 and U373). Loss of function analysis was performed on U87 cells using lentivirus-mediated siRNA against EIF3B. EIF3B-shRNA expressing lentivirus could effectively infect U87 glioma cells and downregulate EIF3B expression. Knockdown of EIF3B expression significantly inhibited proliferation of U87 cells. Further study showed that the proliferation inhibitory effect was associated with accumulation in G0/G1-phase cell number and an increased rate of apoptosis. In conclusion, EIF3B promotes the proliferation of U87 cells and may play an important role in human glioblastoma development.     

Amyloid-β Precursor Protein Induces Glial Differentiation of Neural Progenitor Cells by Activation of the IL-6/gp130 Signaling Pathway

Although amyloid precursor protein (APP) due to the cytotoxicity of Aβ peptides, has been intensively studied, the physiological role of APP still remains wrapped up in veil. In this article, we propose that α-cleaved ectodomain of APP (sAPPα) stimulates the IL-6/gp130 signaling pathway for induction of gliogenesis within neural progenitor cells (NPCs). In our previous study, a high dose of APP differentiated NPCs into glial fibrillary acidic protein (GFAP) positive cells. In order to elucidate the mechanism of APP-induced glial differentiation, we examined the effects of sAPPα on the IL-6/gp130 signaling pathway. Application of sAPPα promoted mRNA expression of gp130, ciliary neurotrophic factor (CNTF), and Janus kinase 1 (JAK1). sAPPα stimulated the glial differentiation by upregulating the expression and phosphorylation of gp130. While mRNA expression of STAT3 was unchanged, phosphorylation of STAT3-Tyr705 gradually increased. Application of small interference RNA (siRNA) for STAT3 suppressed GFAP expression even in the presence of APP. Treatment with siRNA or inhibitor, AG490, of JAK1 efficiently suppressed STAT3 phosphorylation and GFAP expression. Upregulation of CNTF was observed in either short- or long-term treatment with sAPPα. RNA’s interference of CNTF dose-dependently inhibited GFAP expression upregulated by treatment with sAPPα. This study suggests that the IL-6/gp130 signaling pathway is involved in sAPPα-induced glial differentiation of NPCs. Although further investigation is needed, this study may provide insight into the mechanism of glial differentiation of NPCs under pathological conditions in Alzheimer’s disease or Down syndrome.     

Oligodendroglial survival factors,PDGF-AA and CNTF,activate similar JAK/STAT signaling pathways

Platelet-derived growth factor (PDGF) and ciliary neurotrophic factor (CNTF) have pleiotropic actions on many cell types. In the presence of these factors, oligodendroglia respond by enhanced survival when deprived of trophic factors or in the presence of the cytotoxic cytokine, tumor necrosis factor-α (TNF-α). To determine whether these two oligodendroglial survival factors converge in their signaling cascades, we examined their JAK/STAT pathways in enriched oligodendrocyte (OL) progenitors and in the progenitor OL cell line, central glia-4 (CG-4). Cytokine pathways such as JAK/STAT have been characterized extensively in hematopoietic cells; however, it is increasingly evident that the same cytokines that play a role in hematopoiesis also play a role during development and injury of the central nervous system. This is the first study that clearly defines the presence and activation of JAK/STAT proteins in OL progenitors and compares the signal transduction pathway of two well-known oligodendroglial survival factors. In this study, we report that PDGF- and CNTF-induced OL progenitors responded with a rapid tyrosine phosphorylation of JAK1, JAK2, STAT1α/β, and STAT3. We feel that these identified JAK/STAT signaling molecules play a large role in the cellular response to these factors. Because both PDGF and CNTF enhance OL progenitor survival, these JAK/STATs may play a role in regulating this important cellular process. J. Neurosci. Res. 54:191–205, 1998. © 1998 Wiley-Liss, Inc.     

Lentivirus介导RNAi抑制人胶质瘤干细胞STAT3基因生物学效应观察

目的了解慢病毒载体对胶质瘤干细胞的亲嗜性及对靶基因表达的干扰效率，初步探索干扰STAT3基因对胶质瘤干细胞增殖的影响。方法构建STAT3基因shRNA的慢病毒表达载体，经293T细胞包装后，获得可表达STAT3基因shRNA的慢病毒颗粒；活性载体病毒感染人原代胶质瘤干细胞后流式细胞分析细胞感染效率；实时定量多聚酶链反应（PCR）和Westernblot检测细胞STAT3基因mRNA和蛋白表达及活化水平；增殖分析试剂盒测定细胞生长曲线，流式细胞分析细胞周期分布。结果在病毒感染比率值20：1时慢病毒载体对人原代胶质瘤干细胞的感染效率为98．6％；细胞感染可表达STAT3基因shRNA的载体慢病毒后，STAT3基因mRNA显著下降，抑制率为84．3％，STAT3蛋白表达下降81．5％，活化的pSTAT3下降97．9％；胶质瘤干细胞STAT3表达、活化受抑后细胞生长显著变慢，G1期细胞比例显著增高。结论慢病毒载体对人原代胶质瘤干细胞有着很高的感染效率，介导的RNAi可显著抑制靶基因的表达与活化，是对人胶质瘤干细胞基因功能研究的理想工具。人胶质瘤干细胞STAT3基因受抑后细胞生长显著变慢，出现G1期阻滞。     

Crkl Efficiently Mediates Cell Proliferation, Migration, and Invasion Induced by TGF-β Pathway in Glioblastoma

Crk-like (CrkL) is an adapter protein that has crucial roles in cell proliferation, adhesion, and migration. However, the expression pattern and potential mechanism of CrkL protein in glioblastoma multiforme (GBM) have not been fully elucidated. To determine roles of CrkL in cell signaling, proliferation, and migration, small interfering RNAs and plasmids transfection were used to suppress or overexpress CrkL in U87 and U251; soft-agar assay and wound-healing assay were used to observe cell invasiveness, migration, and proliferation. Erk1/2, Smad2, and matrix metalloproteinase 9 (MMP9) were also analyzed by western blot. CrkL was expressed in U87 and U251 cell lines and can be activated by transforming growth factor-beta 1 (TGF-β1) in vitro; CrkL knockdown significantly suppressed the expression of phosph-ERK1/2 and MMP9 but enhanced phosph-Smad2 expression compared with control (p?<?0.001). Overexpression of CrkL against control upregulated phosph-ERK1/2 and MMP9 and, at the same time, downregulated phosph-Smad2 (p?<?0.01). On the other hand, CrkL knockdown could significantly affect U87 and U251 invasiveness (p?<?0.01) and wound closure (p?<?0.01) using soft-agar assay and wound-healing assay. These studies suggest that CrkL efficiently mediates cell proliferation, migration, and invasion induced by TGF-β pathway in glioblastoma. Furthermore, CrkL can be used as a potential and efficient therapeutic target of GBM and may also mediate other signaling pathway.