Cell Signaling Systems

What are the primary components of cell signaling pathways?

● Receptor protein activation by ligand binding results in one or more of three biochemical responses: (1) covalent modification of target proteins, (2) protein conformational changes, and (3) altered rates of protein expression.

● A signaling pathway is a linked set of biochemical reactions that consists of upstream events occurring at or near the plasma membrane and downstream events that alter the activity or level of target proteins. Hormones are biologically active compounds that bind to receptor proteins and function as first messengers through endocrine, paracrine, or autocrine mechanisms.

● The functional role of second messengers is to amplify receptor-generated signals that are initiated by a single receptor binding event; many types of second messengers are generated by catalytic enzymes with high turnover rates such as adenylate cyclase, guanylate cyclase, and nitric oxide synthase.

● Higher eukaryotes contain five abundant classes of receptor proteins: (1) G protein–coupled receptors (GPCRs), (2) receptor tyrosine kinases (RTKs), (3) tumor necrosis factor (TNF) family receptors, (4) nuclear receptors, and (5) ligand-gated ion channels.

What is the mechanism of G protein-coupled receptor (GPCR) signaling?

● GPCRs transmit extracellular signals to the cytoplasm through direct interaction with a membrane-bound protein complex called a heterotrimeric G protein, which consists of one each of Gα, Gβ, and Gγ subunits (Gαβγ).

● Gα is a member of the G protein family of signaling proteins, which contain an intrinsic GTP hydrolyzing activity (GTPase) that converts the active GTP-bound protein into the inactive GDP-bound protein.

● GPCR-mediated activation of the associated heterotrimeric complex occurs when the GDP bound to the Gα subunit is replaced by GTP, resulting in dissociation of the Gα subunit from the heterotrimeric complex.

● GPCR activation by ligand binding results in a conformational change in the receptor cytoplasmic domain that facilitates binding of the GDP-bound Gαβγ complex and subsequent stimulation of GDP–GTP exchange in the Gα subunit and dissociation of the Gαβγ complex into Gα– GTP and Gβγ signaling complexes.

● The dissociated membrane-associated Gα–GTP subunit activates several target proteins depending on the specific Gα subtype; the membrane-bound Gβγ complexes also activate target proteins.

What is the mechanism of receptor tyrosine kinase signaling?

● Receptor tyrosine kinases (RTKs) function as homodimers that bind extracellular ligands. Ligand binding activates a cytoplasmic tyrosine kinase function. RTK-mediated autophosphorylation of tyrosine residues in the cytoplasmic tail generates phosphotyrosine binding sites for adaptor proteins, which link downstream signaling pathways to activated receptors.

● Ras is a G protein that is similar to the Gα subunit of heterotrimeric G proteins and is characterized by three features: (1) it is attached to the cytoplasmic face of the plasma membrane by a lipid anchor; (2) it is activated by GEFs such as SOS; and (3) it is deactivated by stimulation of its intrinsic GTPase activity by GAPs.

● Ras activation initiates a downstream signaling pathway, which consists of a phosphorylation cascade mediated by kinases in the mitogen-activated protein (MAP) family of signaling proteins. This results in gene regulation and increased rates of cell division.

● Insulin binds to the insulin receptor, a disulfide-linked RTK. The insulin receptor consists of a transmembrane α₂β₂ complex that is activated by a single insulin molecule binding to one of the α subunits; this mode of ligand binding is an example of negative cooperativity.

● Insulin receptor signaling initiates two downstream signaling pathways: one signals through Ras to activate the MAP kinase phosphorylation cascade, and the other activates the PI3K pathway, leading to glucose uptake.

What is the mechanism of tumor necrosis factor receptor signaling?

● Binding of tumor necrosis factor-α (TNF-α) to the trimeric TNF receptor initiates two opposing pathways: one that leads to programmed cell death (apoptosis) and the other that promotes cell survival. The net cellular response to TNF-α signaling is determined by the relative abundance of downstream signaling proteins.

● TNF receptor signaling is mediated by the assembly of an adaptor protein complex, which consists of signaling proteins that share protein binding modules called death domains (DDs) and death effector domains (DEDs).

● TNF receptor–induced cell death involves recruitment of the adaptor proteins TRADD and FADD, leading to the activation of caspase 8 and initiation of a downstream proteolytic cascade that activates caspase 3; the executioner protease.

● The TNF receptor–mediated cell survival pathway involves recruitment of the adaptor kinases TRAF2–NIK and RIP to the receptor complex, which stimulates a phosphorylation cascade leading to increased expression of anti-apoptotic genes.

What is the mechanism of nuclear receptor signaling?

● Nuclear receptors are ligand-activated transcription factors that control a wide range of physiologic responses, as governed by (1) ligand bioavailability, (2) cell-specific expression of nuclear receptors and coregulatory proteins, and (3) accessibility of target gene DNA sequences in chromatin to nuclear receptor binding.

● There are two major types of nuclear receptor proteins, which are categorized on the basis of their mode of DNA binding: (1) steroid receptors bind as head-to-head homodimers to inverted repeat DNA sequences, and (2) metabolite receptors bind as head-to-tail heterodimers to direct repeat DNA sequences.

● Glucocorticoids are steroid hormones that are synthesized in the adrenal glands. They bind to the glucocorticoid receptor and regulate a variety of cellular responses, including inflammation, lung cell development, and carbohydrate metabolism.

● The pharmaceutical drugs prednisone, triamcinolone, and dexamethasone are glucocorticoid agonists that function as anti-inflammatory agents by activating glucocorticoid receptor signaling in target cells, leading to increased expression of the annexin I gene and decreased expression of the cyclooxygenase-2 gene.