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High-Yield Terms

  • Signal transduction: refers to the movement of signals from outside the cell to inside, resulting in a change in the “state” of the cell

  • Growth factor: any of a family of proteins that bind to receptors with the primary result of activating cellular proliferation and/or differentiation

  • Cytokine: any of unique family of growth factor proteins primarily secreted from leukocytes

  • Chemokine: a subfamily of cytokines (chemotactic cytokines) that is capable of inducing chemotaxis

  • Interleukin: any of a family of multifunctional cytokines that are produced by a variety of lymphoid and nonlymphoid cells

  • Serpentine receptors: any of a family of G-protein–coupled receptors (GPCRs) so called because they span the plasma membrane 7 times

  • G-protein: any of a large family of proteins that bind and hydrolyze GTP in the act of transmitting signals, includes the heterotrimeric and the monomeric G-protein families

  • Nuclear receptors: intracellular receptors that bind lipophilic ligands and then bind to specific DNA sequences in target genes regulating their expression

Mechanisms of Signal Transduction

Signal transduction at the cellular level refers to the movement of signals from outside the cell to inside. The movement of signals can be simple, like that associated with receptor molecules of the acetylcholine class: receptors that constitute channels which, upon ligand interaction, allow signals to be passed in the form of small ion movement, either into or out of the cell. These ion movements result in changes in the electrical potential of the cells that, in turn, propagates the signal along the cell. More complex signal transduction involves the coupling of ligand-receptor interactions to many intracellular events. These events include phosphorylations by tyrosine kinases and/or serine/threonine kinases. Protein phosphorylations change enzyme activities and protein conformations. The eventual outcome is an alteration in cellular activity and changes in the program of genes expressed within the responding cells.

Growth Factors

Growth factors are classified as substances that bind to receptors with the primary result of activating cellular proliferation and/or differentiation. The vast majority of growth factors are proteins that bind to extracellular ligand-binding domains of transmembrane receptors (Table 40-1). However, some steroid hormones have growth factor activity. Steroid hormones bind intracellular receptors (see later). Many growth factors are quite versatile, stimulating cellular division in numerous different cell types; while others are specific to a particular cell type. Individual growth factor proteins tend to occur as members of larger families of structurally and evolutionarily related proteins. For example, there are at least 18 members of the fibroblast growth factor (FGF) family of proteins.

TABLE 40-1:Representative List of Well-Characterized Growth Factors

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