MAPK信号通路与脂肪细胞分化

促分裂原活化的蛋白激酶(MAPK)通路是真核细胞重要的信号转导通路,主要有ERK、p38和JNK三条途径,参与调控多种细胞应答和生理病理过程。该文重点讨论了MAPK对脂肪细胞分化的调控。其中ERK对脂肪细胞分化的调节具有多样性,随分化进程不同表现为不同的调控功能,p38和JNK也通过不同的机制对脂肪细胞分化发挥相异的调节作用。MAPK信号转导与脂肪分化的紧密联系,使其可能成为调控与脂分化密切相关的代谢疾病如肥胖、糖尿病等的一条关键通路。     

脂肪细胞因子对脂肪细胞增殖分化的反馈调节作用

肥胖已被证实是胰岛素抵抗、2 型糖尿病、高血压、高脂血症及冠状动脉粥样硬化性心脏病等代谢性疾病发生的重要诱因.肥胖的发生主要是由于体内脂肪细胞的异常分化和增殖,最终导致脂肪细胞异常增多及细胞内脂质过度沉积产生的.脂肪细胞的增殖分化受到多种因素的调控,其中脂肪细胞因子作为脂肪组织分泌的肽类激素,也在脂肪细胞的发育分化过程中起重要的反馈调节作用.大多数肥胖患者体内存在脂肪细胞因子分泌异常及其相应的功能紊乱.本文将对几种主要的脂肪细胞因子在脂肪细胞发育分化中的作用及最新研究进展进行简要综述及讨论.     

Tmem64 Modulates Calcium Signaling during RANKL-Mediated Osteoclast Differentiation

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Highlights? Tmem64-deficient mice show increased bone volume ? Tmem64 deficiency reduces [Ca²⁺]_i oscillation in response to RANKL stimulation ? Tmem64 interacts with SERCA2 ? Tmem64 positively regulates osteoclast formation via SERCA2/Ca²⁺ signaling     

Insulin Regulates Adipocyte Lipolysis via an Akt-Independent Signaling Pathway

After a meal, insulin suppresses lipolysis through the activation of its downstream kinase, Akt, resulting in the inhibition of protein kinase A (PKA), the main positive effector of lipolysis. During insulin resistance, this process is ineffective, leading to a characteristic dyslipidemia and the worsening of impaired insulin action and obesity. Here, we describe a noncanonical Akt-independent, phosphoinositide-3 kinase (PI3K)-dependent pathway that regulates adipocyte lipolysis using restricted subcellular signaling. This pathway selectively alters the PKA phosphorylation of its major lipid droplet-associated substrate, perilipin. In contrast, the phosphorylation of another PKA substrate, hormone-sensitive lipase (HSL), remains Akt dependent. Furthermore, insulin regulates total PKA activity in an Akt-dependent manner. These findings indicate that localized changes in insulin action are responsible for the differential phosphorylation of PKA substrates. Thus, we identify a pathway by which insulin regulates lipolysis through the spatially compartmentalized modulation of PKA.The storage and mobilization of nutrients from specialized tissues requires the spatial organization of both signaling functions and energy stores. Nowhere is this more evident than in mammalian adipose tissue, which maintains the most efficient repository for readily available energy. Here, fuel is segregated into lipid droplets, once thought to be inert storehouses but now recognized as complex structures that represent a regulatable adaptation of a ubiquitous organelle (5, 40). The synthesis and maintenance of functional lipid droplets requires numerous proteins, not only fatty acid binding proteins and enzymes of lipid synthesis but also molecules critical to constitutive and specialized membrane protein trafficking (23).During times of nutritional need, triglycerides within the adipocyte lipid droplet are hydrolyzed into their components, fatty acids, acyl-glycerides, and, ultimately, glycerol. This process, termed lipolysis, is controlled dynamically by multiple hormonal signals that respond to the nutrient status of the organism. During fasting, catecholamines such as norepinephrine stimulate lipolysis via beta-adrenergic receptor activation, promoting adenylyl cyclase activity and the production of cyclic AMP (cAMP) (17). cAMP binds to the regulatory subunits of its major effector, protein kinase A (PKA), triggering the dissociation of these subunits and the subsequent activation of the catalytic subunits (62, 63). PKA is frequently sequestered into multiple parallel, intracellular signaling complexes, though such structures have not been studied in hormone-responsive adipocytes (68). Two targets of activated PKA important for lipolysis are hormone-sensitive lipase (HSL) and perilipin, the major lipid droplet coat protein (17). The phosphorylation of HSL on Ser 559/660 is crucial for its activation and translocation to the lipid droplet, where HSL catalyzes the hydrolysis of diglycerides to monoglycerides (26, 55). Another lipase, adipose triglyceride lipase (ATGL), carries out the initial cleavage of triglycerides to diglycerides and most likely is rate limiting for lipolysis, but it does not appear to be regulated directly via PKA phosphorylation (24, 73). Perilipin under basal conditions acts as a protective barrier against lipase activity; upon stimulation, the phosphorylation of least six PKA consensus sites triggers a conformational change in perilipin, permitting access to the lipid substrates in the droplet, the recruitment of HSL, and possibly the activation of ATGL (7, 8, 21, 41, 46, 58, 60, 61). Perilipin, therefore, possesses dual functions, both blocking lipolysis in the basal state as well as promoting lipolysis upon its phosphorylation (5, 58, 60).Following the ingestion of a meal, insulin stimulates the uptake of nutrients such as glucose into specialized tissues and also potently inhibits lipolysis in adipocytes (17). Insulin signaling in the adipocyte involves the activation of the insulin receptor tyrosine kinase, the phosphorylation of insulin receptor substrates, the activation of PI3K, and the subsequent production of specific phosphoinositides at the plasma membrane (59). These phosphoinositides then recruit Akt, via its pleckstrin homology domain, to the plasma membrane, where Akt becomes phosphorylated and activated by two upstream kinases. Akt stimulates the translocation of the glucose transporter GLUT4 to the plasma membrane, thereby promoting the uptake of glucose into the cell (2). The mechanism by which insulin inhibits lipolysis has been proposed to involve the reduction of cAMP levels and thus PKA activity. In this model, insulin signaling activates phosphodiesterase 3b (PDE3b) via the Akt-mediated phosphorylation of Ser273 (14, 32). Upon activation by Akt, PDE3b catalyzes the hydrolysis of cAMP to 5′AMP, thereby attenuating PKA activity and lipolysis. Recent studies of PDE3b knockout mice have highlighted the importance of PDE3b activity in the regulation of lipolysis but were uninformative regarding the mechanism of insulin action (12). Adipocytes isolated from these mice exhibit reduced responses to insulin with respect to lipolysis, but it is not clear whether this is due to the loss of the critical target enzyme or a normal mechanism being overwhelmed by supraphysiological concentrations of cAMP (12). Biochemical studies using dominant-inhibitory Akt have demonstrated that Akt can regulate PDE3b activity, and other studies also have suggested that Akt interacts directly with PDE3b, implying a direct connection to lipolysis regulation (1, 32). Nevertheless, the actual requirement for Akt in insulin action with regard to the lipolysis itself has not been demonstrated directly in, for example, genetic loss-of-function experiments.There now is substantial evidence implicating elevated free fatty acid levels as a consequence of inappropriate lipolysis as a major etiological factor for insulin resistance and type 2 diabetes mellitus (T2DM) (51). Conditions such as obesity and diabetes are characterized by a pathophysiological state in which these tissues become unresponsive to insulin, which contribute to the adverse long-term sequelae of diseases such as T2DM and the metabolic syndrome (4, 44). Thus, understanding in detail the mechanism by which insulin suppresses fat cell lipolysis is critical to identifying the underlying defect in resistant adipose tissue and ultimately developing effective therapeutics. In the present study, we investigated both Akt-dependent and -independent modes of insulin action toward lipolysis. We found the latter to predominate at low, physiological levels of adrenergic stimulation, acting via a pathway dependent on the preferential phosphorylation of downstream PKA substrates.     

Hippo信号对卵巢生殖干细胞增殖及卵巢功能的影响

已有研究表明,Hippo信号通路对干细胞的自我更新和分化至关重要,且Hippo信号通路在调控卵泡生长中起重要作用,然而,目前关于Hippo通路对卵巢生殖干细胞的增殖和分化以及卵巢功能重塑的影响相关的研究较少。为了明确Hippo信号通路效应因子YAP1与卵巢生殖干细胞体外增殖分化的关系,以及Hippo信号通路对卵巢癌的主要功能。我们采用两步法酶促分离和磁性分离技术分别鉴定卵巢生殖干细胞,通过测定MVH和OCT4标记物的表达,然后选择YAP1作为Hippo信号通路的主要效应分子,作为研究的靶基因。将含有过表达的YAP1或YAP1靶向的shRNA的慢病毒转导入卵巢生殖干细胞中。通过将过表达YAP1或YAP1 shRNA的慢病毒载体微量注射到不育小鼠模型中,观察调节Hippo信号通路对卵巢的增殖、分化和内分泌功能的影响。研究结果表明,在分离的卵巢生殖干细胞中观察到YAP1和MVH的共表达。与对照组相比,过表达YAP1的卵巢生殖干细胞中MVH和OCT4表达水平显著增加。而YAP1敲低后,MVH和OCT4水平显著降低;不育小鼠模型中YAP1过表达15 d后,E2和FSH含量显著升高,而YAP1 shRNA表达后,小鼠血清E2和FSH含量显著降低。YAP1可用于调控卵巢生殖干细胞的增殖和分化以及小鼠的卵巢功能。本研究表明,Hippo信号通路可能是调控卵巢功能重建的一个新的分子靶点。     

Tissue-Dependent Consequences of Apc Inactivation on Proliferation and Differentiation of Ciliated Cell Progenitors via Wnt and Notch Signaling

The molecular signals that control decisions regarding progenitor/stem cell proliferation versus differentiation are not fully understood. Differentiation of motile cilia from progenitor/stem cells may offer a simple tractable model to investigate this process. Wnt and Notch represent two key signaling pathways in progenitor/stem cell behavior in a number of tissues. Adenomatous Polyposis Coli, Apc is a negative regulator of the Wnt pathway and a well known multifunctional protein. Using the cre-LoxP system we inactivated the Apc locus via Foxj1-cre, which is expressed in cells committed to ciliated cell lineage. We then characterized the consequent phenotype in two select tissues that bear motile cilia, the lung and the testis. In the lung, Apc deletion induced β-catenin accumulation and Jag1 expression in ciliated cells and by lateral induction, triggered Notch signaling in adjacent Clara cells. In the bronchiolar epithelium, absence of Apc blocked the differentiation of a subpopulation of cells committed to the ciliogenesis program. In the human pulmonary adenocarcinoma cells, Apc over-expression inhibited Jag1 expression and promoted motile ciliogenic gene expression program including Foxj1, revealing the potential mechanism. In the testis, Apc inactivation induced β-catenin accumulation in the spermatogonia, but silenced Notch signaling and depleted spermatogonial stem cells, associated with reduced proliferation, resulting in male infertility. In sum, the present comparative analysis reveals the tissue-dependent consequences of Apc inactivation on proliferation and differentiation of ciliated cell progenitors by coordinating Wnt and Notch signaling.     

Hedgehog信号通路与脂肪细胞发育育

TGFβ、Wnt、FGF和Hedgehog（Hh)等信号通路是参与胚胎发育的关键信号通路.从果蝇到人类,Hh信号通路广泛存在并高度保守,在多种器官的发育过程中发挥重要作用. 脂肪细胞发育的过程包括多潜能干细胞向前脂肪细胞定向和脂肪细胞终末分化两个阶段.近年来,Hh信号通路在脂肪细胞发育过程中的作用逐渐成为研究热点.越来越多的研究表明,Hh信号通路抑制脂肪细胞发育.本文将对Hh信号通路抑制脂肪细胞发育的作用以及其发挥作用的阶段进行综述,并分析将该信号通路作为靶点治疗肥胖症及相关疾病的可行性.     

Hippo Signaling Mediates Proliferation,Invasiveness, and Metastatic Potential of Clear Cell Renal Cell Carcinoma

Recent work has identified dysfunctional Hippo signaling to be involved in maintenance and progression of various human cancers, although data on clear cell renal cell carcinoma (ccRCC) have been limited. Here, we provide evidence implicating aberrant Hippo signaling in ccRCC proliferation, invasiveness, and metastatic potential. Nuclear overexpression of the Hippo target Yes-associated protein (YAP) was found in a subset of patients with ccRCC. Immunostaining was particularly prominent at the tumor margins and highlighted neoplastic cells invading the tumor-adjacent stroma. Short hairpin RNA-mediated knockdown of YAP significantly inhibited proliferation, migration, and anchorage-independent growth of ccRCC cells in soft agar and led to significantly reduced murine xenograft growth. Microarray analysis of YAP knockdown versus mock-transduced ccRCC cells revealed down-regulation of endothelin 1, endothelin 2, cysteine-rich, angiogenic inducer, 61 (CYR61), and c-Myc in ccRCC cells as well as up-regulation of the cell adhesion molecule cadherin 6. Signaling pathway impact analysis revealed activation of the p53 signaling and cell cycle pathways as well as inhibition of mitogen-activated protein kinase signaling on YAP down-regulation. Our data suggest CYR61 and c-Myc as well as signaling through the endothelin axis as bona fide downstream effectors of YAP and establish aberrant Hippo signaling as a potential therapeutic target in ccRCC.     

Insulin-FOXO3 Signaling Modulates Circadian Rhythms via Regulation of Clock Transcription

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TLR4 Activation Promotes Bone Marrow MSC Proliferation and Osteogenic Differentiation via Wnt3a and Wnt5a Signaling

Mesenchymal stem cells (MSCs) from adult bone marrow maintain their self-renewal ability and the ability to differentiate into osteoblast. Thus, adult bone marrow MSCs play a key role in the regeneration of bone tissue. Previous studies indicated that TLR4 is expressed in MSCs and is critical in regulating the fate decision of MSCs. However, the exact functional role and underlying mechanisms of how TLR4 regulate bone marrow MSC proliferation and differentiation are unclear. Here, we found that activated TLR4 by its ligand LPS promoted the proliferation and osteogenic differentiation of MSCs in vitro. TLR4 activation by LPS also increased cytokine IL-6 and IL-1β production in MSCs. In addition, LPS treatment has no effect on inducing cell death of MSCs. Deletion of TLR4 expression in MSCs completely eliminated the effects of LPS on MSC proliferation, osteogenic differentiation and cytokine production. We also found that the mRNA and protein expression of Wnt3a and Wnt5a, two important factors in regulating MSC fate decision, was upregulated in a TLR4-dependent manner. Silencing Wnt3a with specific siRNA remarkably inhibited TLR4-induced MSC proliferation, while Wnt5a specific siRNA treatment significantly antagonized TLR4-induced MSC osteogenic differentiation. These results together suggested that TLR4 regulates bone marrow MSC proliferation and osteogenic differentiation through Wnt3a and Wnt5a signaling. These finding provide new data to understand the role and the molecular mechanisms of TLR4 in regulating bone marrow MSC functions. These data also provide new insight in developing new therapy in bone regeneration using MSCs by modulating TLR4 and Wnt signaling activity.