Which Of The Following Best Describes Hormonal Signaling In Animals?

Many different kinds of molecules transmit information between the cells of multicellular organisms. Although all these molecules act as ligands that bind to receptors expressed past their target cells, at that place is considerable variation in the structure and function of the different types of molecules that serve as bespeak transmitters. Structurally, the signaling molecules used past plants and animals range in complexity from simple gases to proteins. Some of these molecules carry signals over long distances, whereas others human action locally to convey information between neighboring cells. In addition, signaling molecules differ in their manner of activity on their target cells. Some signaling molecules are able to cross the plasma membrane and bind to intracellular receptors in the cytoplasm or nucleus, whereas about bind to receptors expressed on the target cell surface. The sections that follow discuss the major types of signaling molecules and the receptors with which they interact. Subsequent discussion in this chapter focuses on the mechanisms by which cell surface receptors then role to regulate cell behavior.

Modes of Prison cell-Jail cell Signaling

Cell signaling can outcome either from the straight interaction of a cell with its neighbour or from the activeness of secreted signaling molecules (Figure 13.1). Signaling by direct cell-cell (or cell-matrix) interactions plays a critical office in regulating the behavior of cells in fauna tissues. For example, the integrins and cadherins (which were discussed in the previous affiliate) function non just every bit jail cell adhesion molecules simply likewise as signaling molecules that regulate prison cell proliferation and survival in response to cell-jail cell and cell-matrix contacts. In add-on, cells express a diversity of cell surface receptors that interact with signaling molecules on the surface of neighboring cells. Signaling via such direct cell-cell interactions plays a disquisitional function in regulating the many interactions between dissimilar types of cells that take place during embryonic development, equally well as in the maintenance of adult tissues.

Figure 13.1

Modes of jail cell-cell signaling. Cell signaling can accept identify either through directly jail cell-cell contacts or through the activity of secreted signaling molecules. (A) In endocrine signaling, hormones are carried through the circulatory system to act on distant (more...)

The multiple varieties of signaling past secreted molecules are frequently divided into three full general categories based on the altitude over which signals are transmitted. In endocrine signaling, the signaling molecules (hormones) are secreted past specialized endocrine cells and carried through the circulation to act on target cells at distant body sites. A archetype example is provided by the steroid hormone estrogen, which is produced by the ovary and stimulates evolution and maintenance of the female reproductive organization and secondary sex characteristics. In animals, more than 50 different hormones are produced by endocrine glands, including the pituitary, thyroid, parathyroid, pancreas, adrenal glands, and gonads.

In dissimilarity to hormones, some signaling molecules deed locally to touch on the behavior of nearby cells. In paracrine signaling, a molecule released by i cell acts on neighboring target cells. An case is provided by the action of neurotransmitters in carrying signals betwixt nerve cells at a synapse. Finally, some cells respond to signaling molecules that they themselves produce. 1 important example of such autocrine signaling is the response of cells of the vertebrate immune system to foreign antigens. Certain types of T lymphocytes respond to antigenic stimulation by synthesizing a growth factor that drives their own proliferation, thereby increasing the number of responsive T lymphocytes and amplifying the immune response. It is too noteworthy that aberrant autocrine signaling frequently contributes to the uncontrolled growth of cancer cells (see Chapter fifteen). In this situation, a cancer cell produces a growth factor to which it also responds, thereby continuously driving its own unregulated proliferation.

Steroid Hormones and the Steroid Receptor Superfamily

As already noted, all signaling molecules act by binding to receptors expressed by their target cells. In many cases, these receptors are expressed on the target cell surface, merely some receptors are intracellular proteins located in the cytosol or the nucleus. These intracellular receptors reply to small hydrophobic signaling molecules that are able to diffuse across the plasma membrane. The steroid hormones are the classic examples of this grouping of signaling molecules, which likewise includes thyroid hormone, vitamin D₃, and retinoic acid (Figure 13.2).

Figure 13.2

Structure of steroid hormones, thyroid hormone, vitamin D ₃, and retinoic acid. The steroids include the sexual activity hormones (testosterone, estrogen, and progesterone), glucocorticoids, and mineralocorticoids.

The steroid hormones (including testosterone, estrogen, progesterone, the corticosteroids, and ecdysone) are all synthesized from cholesterol. Testosterone, estrogen, and progesterone are the sex steroids, which are produced past the gonads. The corticosteroids are produced by the adrenal gland. They include the glucocorticoids, which act on a variety of cells to stimulate product of glucose, and the mineralocorticoids, which deed on the kidney to regulate salt and water balance. Ecdysone is an insect hormone that plays a key role in evolution by triggering the metamorphosis of larvae to adults.

Although thyroid hormone, vitamin D_three, and retinoic acid are both structurally and functionally distinct from the steroids, they share a mutual machinery of activeness in their target cells. Thyroid hormone is synthesized from tyrosine in the thyroid gland; it plays important roles in development and regulation of metabolism. Vitamin D _three regulates Ca²⁺ metabolism and bone growth. Retinoic acrid and related compounds (retinoids) synthesized from vitamin A play of import roles in vertebrate development.

Considering of their hydrophobic character, the steroid hormones, thyroid hormone, vitamin D₃, and retinoic acid are able to enter cells by diffusing across the plasma membrane (Figure 13.3). One time inside the cell, they bind to intracellular receptors that are expressed past the hormonally responsive target cells. These receptors, which are members of a family of proteins known every bit the steroid receptor superfamily, are transcription factors that contain related domains for ligand binding, DNA binding, and transcriptional activation. Ligand bounden regulates their role as activators or repressors of their target genes, and then the steroid hormones and related molecules direct regulate gene expression.

Effigy 13.3

Action of steroid hormones. The steroid hormones lengthened across the plasma membrane and bind to nuclear receptors, which direct stimulate transcription of their target genes. The steroid hormone receptors demark DNA as dimers.

Ligand binding has distinct effects on different receptors. Some members of the steroid receptor superfamily, such as the estrogen and glucocorticoid receptors, are unable to bind to Dna in the absence of hormone. The binding of hormone induces a conformational change in the receptor, allowing it to bind to regulatory Deoxyribonucleic acid sequences and activate transcription of target genes. In other cases, the receptor binds Dna in either the presence or absence of hormone, but hormone binding alters the activity of the receptor equally a transcriptional regulatory molecule. For example, thyroid hormone receptor acts as a repressor in the absence of hormone, merely hormone binding converts information technology to an activator that stimulates transcription of thyroid hormone-inducible genes (Figure 13.4).

Figure 13.4

Cistron regulation by the thyroid hormone receptor. Thyroid hormone receptor binds DNA in either the presence or absence of hormone. Withal, hormone binding changes the function of the receptor from a repressor to an activator of target gene transcription. (more...)

Nitric Oxide and Carbon Monoxide

The simple gas nitric oxide (NO) is a major paracrine signaling molecule in the nervous, immune, and circulatory systems. Like the steroid hormones, NO is able to diffuse directly across the plasma membrane of its target cells. The molecular footing of NO activity, however, is distinct from that of steroid action; rather than bounden to a receptor that regulates transcription, NO alters the activeness of intracellular target enzymes.

Nitric oxide is synthesized from the amino acid arginine by the enzyme nitric oxide synthase (Effigy 13.5). In one case synthesized, NO diffuses out of the jail cell and tin can act locally to bear upon nearby cells. Its activeness is restricted to such local furnishings because NO is extremely unstable, with a half-life of simply a few seconds. One well-characterized case of NO action is signaling the dilation of claret vessels. The first step in this process is the release of neurotransmitters, such every bit acetylcholine, from the terminus of nerve cells in the blood vessel wall. These neurotransmitters act on endothelial cells to stimulate NO synthesis. NO then diffuses to neighboring smooth muscle cells where information technology reacts with atomic number 26 bound to the agile site of the enzyme guanylyl cyclase. This increases enzymatic activity, resulting in synthesis of the second messenger cyclic GMP (discussed later in this chapter), which induces muscle cell relaxation and blood vessel dilation. For example, NO is responsible for signaling the dilation of claret vessels that leads to penile erection. It is besides interesting to note that the medical use of nitroglycerin in treatment of heart disease is based on its conversion to NO, which dilates coronary claret vessels and increases blood flow to the heart.

Figure 13.5

Synthesis of nitric oxide. The enzyme nitric oxide synthase (NOS) catalyzes the formation of nitric oxide from arginine.

Some other simple gas, carbon monoxide (CO), likewise functions every bit a signaling molecule in the nervous system. CO is closely related to NO and appears to act similarly as a neurotransmitter and mediator of blood vessel dilation. The synthesis of CO in brain cells, like that of NO, is stimulated by neurotransmitters. In addition, CO can stimulate guanylate cyclase, which may also represent the major physiological target of CO signaling.

Neurotransmitters

The neurotransmitters carry signals between neurons or from neurons to other types of target cells (such equally musculus cells). They are a diverse group of small-scale hydrophilic molecules including acetylcholine, dopamine, epinephrine (adrenaline), serotonin, histamine, glutamate, glycine, and γ-aminobutyric acid (GABA) (Effigy 13.6). The release of neurotransmitters is signaled past the arrival of an activity potential at the terminus of a neuron (encounter Figure 12.22). The neurotransmitters then diffuse beyond the synaptic cleft and bind to receptors on the target cell surface. Note that some neurotransmitters can likewise human activity every bit hormones. For example, epinephrine functions both as a neurotransmitter and every bit a hormone produced by the adrenal gland to betoken glycogen breakdown in musculus cells.

Figure 13.6

Structure of representative neurotransmitters. The neurotransmitters are hydrophilic molecules that bind to cell surface receptors.

Because the neurotransmitters are hydrophilic molecules, they are unable to cross the plasma membrane of their target cells. Therefore, in contrast to steroid hormones and NO or CO, the neurotransmitters act by binding to prison cell surface receptors. Many neurotransmitter receptors are ligand-gated ion channels, such as the acetylcholine receptor discussed in the preceding chapter (see Effigy 12.23). Neurotransmitter binding to these receptors induces a conformational change that opens ion channels, directly resulting in changes in ion flux in the target jail cell. Other neurotransmitter receptors are coupled to Thousand proteins—a major group of signaling molecules (discussed later on in this chapter) that link jail cell surface receptors to a variety of intracellular responses. In the case of neurotransmitter receptors, the associated Yard proteins oftentimes human action to indirectly regulate ion channel activity.

Peptide Hormones and Growth Factors

The widest diverseness of signaling molecules in animals are peptides, ranging in size from merely a few to more than a hundred amino acids. This group of signaling molecules includes peptide hormones, neuropeptides, and a diverse array of polypeptide growth factors (Table xiii.one). Well-known examples of peptide hormones include insulin, glucagon, and the hormones produced past the pituitary gland (growth hormone, follicle-stimulating hormone, prolactin, and others).

Table 13.1

Representative Peptide Hormones, Neuropeptides, and Growth Factors.

Neuropeptides are secreted past some neurons instead of the small-molecule neurotransmitters discussed in the previous department. Some of these peptides, such as the enkephalins and endorphins, function not only as neurotransmitters at synapses but also as neurohormones that human activity on distant cells. The enkephalins and endorphins take been widely studied because of their activity equally natural analgesics that decrease hurting responses in the fundamental nervous arrangement. Discovered during studies of drug addiction, they are naturally occurring compounds that bind to the aforementioned receptors on the surface of brain cells every bit morphine does.

The polypeptide growth factors include a wide variety of signaling molecules that command brute jail cell growth and differentiation. The first of these factors (nerve growth factor, or NGF) was discovered by Rita Levi-Montalcini in the 1950s. NGF is a member of a family of polypeptides (chosen neurotrophins) that regulate the development and survival of neurons. During the course of experiments on NGF, Stanley Cohen serendipitously discovered an unrelated factor (chosen epidermal growth gene, or EGF) that stimulates prison cell proliferation. EGF, a 53-amino-acid polypeptide (Figure 13.7), has served every bit the prototype of a large array of growth factors that play critical roles in controlling animal cell proliferation, both during embryonic evolution and in adult organisms.

Figure thirteen.seven

Structure of epidermal growth cistron (EGF). EGF is a single polypeptide chain of 53 amino acids. Disulfide bonds between cysteine residues are indicated. (After One thousand. Carpenter and S. Cohen, 1979. Ann. Rev. Biochem. 48: 193.)

A adept example of growth factor action is provided by the activity of platelet-derived growth factor (PDGF) in wound healing. PDGF is stored in claret platelets and released during claret clotting at the site of a wound. It then stimulates the proliferation of fibroblasts in the vicinity of the clot, thereby contributing to regrowth of the damaged tissue. Members of another large group of polypeptide growth factors (called cytokines) regulate the evolution and differentiation of blood cells and control the activities of lymphocytes during the allowed response. Other polypeptide growth factors (membrane-anchored growth factors) remain associated with the plasma membrane rather than beingness secreted into extracellular fluids, therefore functioning specifically as signaling molecules during direct cell-cell interactions.

Peptide hormones, neuropeptides, and growth factors are unable to cantankerous the plasma membrane of their target cells, then they act past bounden to jail cell surface receptors, equally discussed afterwards in this affiliate. As might exist expected from the disquisitional roles of polypeptide growth factors in decision-making cell proliferation, abnormalities in growth factor signaling are the basis for a variety of diseases, including many kinds of cancer. For example, abnormal expression of a shut relative of the EGF receptor is an important factor in the evolution of many human chest and ovarian cancers.

Eicosanoids

Several types of lipids serve as signaling molecules that, in dissimilarity to the steroid hormones, act by binding to cell surface receptors. The almost of import of these molecules are members of a class of lipids called the eicosanoids, which includes prostaglandins, prostacyclin, thromboxanes, and leukotrienes (Effigy thirteen.8). The eicosanoids are rapidly cleaved downwardly and therefore act locally in autocrine or paracrine signaling pathways. They stimulate a variety of responses in their target cells, including claret platelet aggregation, inflammation, and smooth-muscle wrinkle.

Effigy 13.eight

Synthesis and structure of eicosanoids. The eicosanoids include the prostaglandins, prostacyclin, thromboxanes, and leukotrienes. They are synthesized from arachidonic acid, which is formed by the hydrolysis of phospholipids catalyzed past phospholipase (more...)

All eicosanoids are synthesized from arachidonic acrid, which is formed from phospholipids. The first step in the pathway leading to synthesis of either prostaglandins or thromboxanes is the conversion of arachidonic acid to prostaglandin H_two. Interestingly, the enzyme that catalyzes this reaction (cyclooxygenase) is the target of aspirin and other nonsteroidal anti-inflammatory drugs. Past inhibiting synthesis of the prostaglandins, aspirin reduces inflammation and pain. Past inhibiting synthesis of thromboxane, aspirin besides reduces platelet aggregation and claret clotting. Because of this activeness, small daily doses of aspirin are often prescribed for prevention of strokes. In addition, aspirin and nonsteroidal anti-inflammatory drugs have been found to reduce the frequency of colon cancer in both animal models and humans, plain by inhibiting the synthesis of prostaglandins that act to stimulate cell proliferation and promote cancer development.

Plant Hormones

Establish growth and development are regulated by a grouping of small molecules called plant hormones. The levels of these molecules within the plant are typically modified by environmental factors, such as low-cal or infection, and then they coordinate the responses of tissues in dissimilar parts of the plant to environmental signals.

The plant hormones are generally divided into 5 major classes: auxins, gibberellins, cytokinins, abscisic acid, and ethylene (Figure xiii.9), although several additional establish hormones have recently been discovered. The kickoff plant hormone to be identified was auxin, with the early on experiments leading to its discovery having been performed past Charles Darwin in the 1880s. One of the effects of auxins is to induce found cell elongation past weakening the jail cell wall (see Figure 12.49). In addition, auxins regulate many other aspects of plant development, including cell segmentation and differentiation. The other constitute hormones likewise have multiple effects in their target tissues, including stem elongation (gibberellins), fruit ripening (ethylene), jail cell sectionalisation (cytokinins), and the onset of dormancy (abscisic acrid).

Our understanding of the molecular mechanisms of establish hormone action is less advanced than comparable studies of brute cells, and the receptors for plant hormones are just beginning to be identified and characterized. One area of noteworthy progress has been in agreement the machinery by which found cells respond to ethylene. Using the small weed Arabidopsis as a model, several of the genes required for ethylene responsiveness accept been identified. These include genes encoding the ethylene receptor, which is similar to a family of receptors commonly found in bacteria and yeast. Additional genes that have been identified in the ethylene signaling pathway include a protein related to the Raf protein kinase, which plays a cardinal function in animate being cell signaling pathways (discussed later in this chapter), and transcription factors, which regulate the expression of ethylene-responsive genes.

Source: https://www.ncbi.nlm.nih.gov/books/NBK9924/

Posted by: craneacursent.blogspot.com