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Protein acetylation plays a crucial role in regulating transcriptional activity. Acetylation complexes (such as CBP/p300) or deacetylation complexes (such as HDAC) can be recruited to DNA-bound transcription factors (TF) in response to signaling pathways. Histone hyperacetylation by histone acetyltransferases (HATs) is associated with transcriptional activation, presumably by remodeling nucleosomal structure into an open conformation more accessible to transcription complexes. Conversely, histone deacetylation is associated with transcriptional repression reversing the chromatin remodeling process. Several transcriptional coactivators and corepressors possess intrinsic acetylase or deacetylase enzymatic activities, respectively. Site-specific acetylation of a growing list of nonhistone proteins, including p53 and E2F, has been shown to play an important role in transcriptional regulation and cell proliferation.

2004-10-30 21:10

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The Wnt pathway (named as a hybrid of Wingless and Int ) regulates cell fate decisions during development of a vide variety of animal species. Secreted Wnt glycoproteins bind to the Frizzled receptor, a family of serpentine receptors, to activate Dishevelled, a PDZ domain protein. Dishevelled acts to inhibit a cytoplasmic complex involving GSK-3, axin and APC that acts to degrade beta- catenin. GSK-3 phosphorylates beta-catenin leading to ubiquitination and degradation by the proteosome. Activation of the Wnt pathway inhibits degradation of beta-catenin allowing its nuclear transport and gene induction via binding to TCF. During the elaboration of cell types and tissues, the Wnt pathway often interacts with the FGF and TGF-beta pathways.

2004-10-30 21:11

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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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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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