Several kinds of receptors have no intrinsic catalytic activity (e.g., immune receptors, some cytokine recep- tors, and integrins). For these, a separate intracellular protein—known as a nonreceptor tyrosine kinase— interacts with receptors after ligand binding and phos- phorylates specific motifs on the receptor or other proteins. The cellular homolog of the transforming protein of the Rous sarcoma virus, called SRC, is the prototype for an important family of such nonreceptor tyrosine kinases (Src-family kinases). SRC contains unique functional regions called Src-homology (SH) domains; SH2 domains typically bind to receptors phosphorylated by another kinase, allowing the aggre- gation of multiple enzymes, whereas SH3 domains mediate protein–protein interactions, often involving proline-rich domains. #229 в контекст | | |
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G-protein coupled receptors are polypeptides that charac- teristically traverse the plasma membrane seven times (hence their designation as seven-transmembrane or serpentine receptors); more than 1500 such receptors have been identified. After ligand binding, the receptor associates with an intracellular guanosine triphosphate (GTP)-binding protein (G protein). At baseline, these G proteins contain guanosine diphosphate (GDP); interac- tion with a receptor-ligand complex results in G protein activation through the exchange of GDP for GTP. Down- stream signaling typically involves the generation of cAMP, and inositol-1,4,5,-triphosphate (IP3), the latter releasing calcium from the ER. #231 в контекст | | |
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Nuclear receptors. Lipid-soluble ligands can diffuse into cells where they interact with intracellular proteins to form a receptor-ligand complex that directly binds to nuclear DNA; the results can be either activation or repression of gene transcription. #233 в контекст | | |
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Other classes of receptors. Other receptors—originally rec- ognized as important for embryonic development and cell fate determination—have since been shown to par- ticipate in the functions of mature cells, particularly within the immune system. These pathways rely on protein:protein interactions, rather than enzymatic activities, to transduce signals, which may serve to allow for very precise control. #235 в контекст | | |
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Receptor proteins of the Notch family: ligand binding to Notch receptors leads to proteolytic cleavage of the receptor and subsequent nuclear translocation of the cytoplasmic domain (intracellular Notch) to form a transcription complex. #237 в контекст | | |
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Wnt protein ligands act through a pathway involving transmembrane Frizzled family receptors, which reg- ulate the intracellular levels of β-catenin. In the absence of Wnt, β-catenin is targeted for ubiquitin- directed proteasome degradation. Wnt binding to Frizzled (and other coreceptors) recruits other pro- teins that disrupt the degradation-targeting complex. This stabilizes β-catenin, allowing it to translocate to the nucleus and form a transcription complex. #239 в контекст | | |
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The traditional linear view of signaling—that receptor activation triggers an orderly sequence of biochemical inter- mediates that ultimately leads to changes in gene expression and the desired biological response—is oversimplified. Instead, it is increasingly clear that any initial signal results in multiple primary and secondary effects, each of which contributes in varying degrees to the final outcome. This is particularly true of signaling pathways that rely on enzymatic activities, which typically modulate a web of polypeptides with complex interactions. For example, phosphorylation of any given protein can allow it to associ- ate with a host of other molecules, resulting in multiple effects such as: #241 в контекст | | |
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Nuclear (or cytoplasmic) localization of transcription factors (see later) #245 в контекст | | |
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Activation of feedback inhibitory (or stimulatory) loops Adaptor proteins play a key role in organizing intracel- #253 в контекст | | |
lular signaling pathways. These proteins function as molecular connectors that physically link different enzymes and promote the assembly of complexes; adaptors can be integral membrane proteins or cytosolic proteins. A typical adaptor may contain a few specific domains (e.g., SH2 or SH3) that mediate protein–protein interactions. By influ- encing the proteins that are recruited to signaling com- plexes, adaptors can determine downstream signaling events. #254 в контекст | | |
By analogy with computer networks, the protein– protein complexes can be considered nodes and the bio- chemical events feeding into or emanating from these nodes can be thought of as hubs. Signal transduction can therefore be visualized as a kind of networking phenom- enon; understanding this higher-order complexity is the province of systems biology, involving a “marriage” of biology and computation. #255 в контекст | | |
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Most signal transduction pathways ultimately influence cellular function by modulating gene transcription through the activation and nuclear localization of tran- scription factors. Conformational changes of transcription factors (e.g., following phosphorylation) can allow their translocation into the nucleus or can expose specific DNA or protein-binding motifs. Transcription factors may drive the expression of a relatively limited set of genes or may have much more widespread effects on gene expression. Among the transcription factors that regulate the expres- sion of genes that are needed for growth are MYC and JUN, whereas a transcription factor that triggers the expression of genes that lead to growth arrest is p53. Transcription factors have a modular design, often containing domains that bind DNA and others that interact with other proteins, such as components of the RNA polymerase complex required for transcription. #257 в контекст | | |
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DNA-binding domains permit specific binding to short DNA sequences. Whereas some transcription factor binding sites are found in promoters, close to the site where transcription starts, it is now appreciated that most transcription factors bind widely throughout genomes, including to long-range regulatory elements such as enhancers. Enhancers function by looping back to gene promoters, and therefore are spatially located close to the genes that they regulate, even though it terms of genomic sequence they may appear to be far away. These insights highlight the importance of chro- #259 в контекст | | |
matin organization in regulating gene expression, both normal and pathologic. #260 в контекст | | |
For a transcription factor to induce transcription, it must also possess protein:protein interaction domains that directly or indirectly recruit histone-modifying enzymes, chromatin-remodeling complexes, and (most importantly) RNA polymerase—the large multipro- tein enzymatic complex that is responsible for RNA synthesis. #261 в контекст | | |
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A major role of growth factors is to stimulate the activity of proteins that are required for cell survival, growth and division. Growth factor activity is mediated through binding to specific receptors, ultimately influencing the expression of genes that can: #265 в контекст | | |
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Relieve blocks on cell-cycle progression (thus promot- ing replication) #269 в контекст | | |
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Enhance biosynthesis of cellular components (nucleic acids, proteins, lipids, carbohydrates) required for a mother cell to give rise to two daughter cells #273 в контекст | | |
Although some growth factors are proteins that “just” stimulate cell proliferation and/or survival, it is important to remember that they also can drive a host of other activi- ties, including migration, differentiation, and synthetic capacity. Some of the important growth factors relevant to tissue regeneration and repair are listed in Table 1.1 and described further in Chapter 3. #274 в контекст | | |
Growth factors can be involved in the proliferation of cells at steady state as well as after injury, when irrevers- ibly damaged cells must be replaced. Uncontrolled proliferation can result when the growth factor activity is #275 в контекст | | |
dysregulated, or when growth factor signaling pathways are altered to become constitutively active. Thus, many growth factor pathway genes are proto-oncogenes, and gain- of-function mutations in these genes can convert them into oncogenes capable of driving unfettered cell proliferation and tumor formation. The following discussion summa- rizes selected growth factors that are involved in the important proliferative processes of tissue repair and regeneration; by virtue of their proliferative effects they can also drive tumorigenesis. Although the growth factors described here all involve receptors with intrinsic kinase activity, other growth factors may signal through each of the various pathways shown in Fig. 1.12. #276 в контекст | | |
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Epidermal growth factor and transforming growth factor-α. Both of these factors belong to the EGF family and bind to the same receptors, explaining their shared biologic activities. EGF and TGF-α are produced by macrophages and a variety of epithelial cells, and are mitogenic for hepatocytes, fibroblasts, and a host of epithelial cells. The “EGF receptor family” includes four membrane receptors with intrinsic tyrosine kinase activity; the best- characterized is EGFR1, also known as ERB-B1, or simply EGFR. EGFR1 mutations and/or amplification fre- quently occur in a number of cancers including those of the lung, head and neck, breast, and brain. The ERBB2 receptor (also known as HER2) is overexpressed in a subset of breast cancers. To treat malignancies, many of these receptors have been successfully targeted by anti- bodies and small molecule antagonists. #278 в контекст | | |
