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丁香园版主

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Insulin is the major hormone controlling critical energy functions such as glucose and lipid metabolism. Insulin activates the insulin receptor tyrosine kinase, which phosphorylates and recruits different substrate adaptors such as the IRS family of proteins. Tyrosine-phosphorylated IRS then displays binding sites for numerous signaling partners. PI3K has a major role in insulin functions, mainly via the activation of the Akt/PKB and the PKCzeta cascades. Activated Akt induces glycogen synthesis, through inhibition of GSK-3; protein synthesis via mTOR and downstream elements; cell survival, through inhibition of several proapoptotic agents (Bad, Forkhead family transcription factors, GSK-3); and inhibits lipolysis via activation of PDE3. A major consequence of insulin stimulation is the stimulation of glucose uptake in muscle and adipocytes, which is mediated by translocation of GLUT4 vesicles to the plasma membrane. While the PI3K/Akt cascade participates in this process, another major pathway leading to GLUT4 translocation involves the insulin receptor–mediated phosphorylation of CAP and formation of the CAP:Cbl:CrkII complex. This complex, through its interaction with flotillin, localizes to lipid rafts facilitating GLUT4 translocation, using in the final step a Synip-containing specialized SNARE complex. Insulin signaling also has growth and mitogenic effects, which are mostly mediated by the Akt cascade, as well as by activation of the Ras/MAPK pathway. A negative feedback signal emanating from Akt/PKB, PKzeta, p70 S6K and perhaps the MAPK cascades results in serine phosphorylation and inactivation of IRS signaling.

2004-10-30 21:13

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• 教育部：今年国家定向培养免费医学生，你会报考吗？——回帖

xiaoxuanzi

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Jaks and Stats are necessary components of cytokine receptor signaling: regulating growth, survival, differentiation and pathogen resistance. An example of these pathways is shown for the IL-6 (or gp130) family of receptors. Cytokine binding induces receptor dimerization, activating the associated Jaks, which phosphorylate the receptor itself. The phosphorylated receptor then serves as a docking site for the SH2-containing Stats. Receptor-bound Stats are phosphorylated by Jaks, dissociate from the receptor, dimerize and translocate into the nucleus. Once in the nucleus, Stat dimers bind specific enhancers, regulating the transcription of target genes. The suppressor of cytokine signaling (SOCS) family of proteins dampen receptor signaling via homologous or heterologous feedback regulation. In addition to activating Stats, Jak kinases phosphorylate other signaling/adaptor proteins, linking Jak signaling to other pathways such as the MAP kinases. Jaks or Stats can also participate in signaling through other receptor classes, as outlined in the ?Jak/Stat Utilization Table?.

2004-10-30 21:15

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• 综合医院全科门诊中乏力患者特征及就诊原因分析

xiaoxuanzi

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Transforming growth factor-beta (TGF-beta) superfamily signaling plays a critical role in the regulation cell growth, differentiation, and development in a wide range of biological systems. Signaling is initiated with ligand-induced oligomerization of serine/threonine receptor kinases and phosphorylation of the cytoplasmic signaling molecules Smad2 and Smad3 for the TGF-beta/activin pathway, or Smad1/5/8 for the bone morphogenetic pathway, or BMP. C-terminal phosphorylation of Smads by activated receptors results in their partnering with the common signaling transducer Smad4 and translocation to the nucleus. Activated Smads regulate diverse biological effects by partnering with transcription factors resulting in cell-state specific modulation of transcription. The activin and BMP pathways are themselves attenuated by MAPK signaling at a number of levels, while the expression of inhibitory, or I-Smads 6 and 7 is induced by both activin/TGF-beta and BMP signaling as part of a negative feed-back route.

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