Biochemistry - The Chemical Reactions of Living Cells, Volume 3 - D. Metzler 1980

Cell Growth, Differentiation, and Chemical Communication
Hormones

The regulation of vital activity in a complex multicellular Organism depends to a vast extent on chemical signals transmitted from Cell to Cell. One of the primary modes of communication is the secretion of Hormones into the bloodstream. Considerably less studied is The process of chemical information exchange via intercellular contacts (Ch. 1, Sec. E, 3, c). This process has been best investigated in Nerve Cells, and Neurochemistry has now become one of the main branches of biochemistry. Cell-to-cell communication plays a major role in embryonic development and tissue differentiation. However, cell GROWTH AND DEVELOPMENT are regulated not only by external factors but also by internal ones; the latter are determined by developmental programs encoded in DNA. In this chapter, we will briefly review both of these topics as well as communication between organisms, i.e., the biochemistry of ecological interactions.

Most hormones exert their effects via one of two mechanisms. In the first case, the hormone binds to a receptor on The Cell membrane. For example, Glucagon, epinephrine, and ACTH bind to the cell surface and stimulate the synthesis of cAMP (Ch. 5, Sec. B, 5), which in turn triggers the Chemical modification of Proteins. It is quite likely that The stimulation of prostaglandin synthesis (Ch. 12, Sec. E, 3) is mediated in this exact manner. The second Mechanism of hormone Action involves their binding to cytoplasmic receptors, ultimately influencing the process of RNA METABOLISM/31.html">Transcription. Steroid Hormones, thyroxine, and Growth Hormone (somatotropin) belong to the group of compounds that appear to act in this fashion. Steroid Hormone Receptors, localized in the Cytoplasm, tightly bind the Steroids entering the cell [2]. Following an "activation" step, the hormone-receptor complex enters The Nucleus, where it binds to specific regions of Chromatin (binding sites), a process that apparently involves certain non-histone proteins [3]. The chemical basis of these interactions is not yet fully elucidated. It can only be said that ultimately this leads to the initiation of transcription of individual genes in hormone-sensitive cells [3a].

In general, hormone action is governed by feedback principles. For instance, Insulin is known to stimulate glucose uptake by Tissues. However, a decrease in Blood glucose concentration causes The rate of insulin secretion by the Pancreas to drop via a feedback mechanism. A similar regulatory interdependence can be observed for the majority of hormones. Sometimes this feedback operates in several stages and involves sensitive structures of the Central Nervous system. In such cases, nerve impulses stimulate the Hypothalamus (Sec. B, 2), leading to the release of neurohormones that travel to the anterior pituitary. The pituitary, in turn, produces hormones such as corticotropin (adrenocorticotropic hormone, ACTH), which stimulates the synthesis of steroid hormones in the adrenal cortex. In addition to other effects, corticosteroids exert an inhibitory feedback influence on the hypothalamus, thereby lowering ACTH secretion by the pituitary. Using autoradiography with ³H-labeled steroid hormones, researchers have successfully identified the localization of specific Brain cells sensitive to the hormone under study [3b].

A characteristic feature of hormone action is the uniqueness of its effect. Moreover, the Introduction/43.html">Action of Certain hormones is generally counterbalanced by the opposing action of others. For example, both glucagon and epinephrine induce The breakdown of Liver Glycogen and the release of glucose into the bloodstream. Glucocorticoids increase the rate of glucose formation from other sources (Ch. 11, Sec. E, 7). Growth hormone promotes an increase in blood glucose levels by suppressing glucose utilization in tissues. On the other hand, insulin increases tissue glucose uptake and enhances the efficiency of its utilization. Thyroid hormone, which raises the overall rate of cellular metabolism, also contributes to a decrease in blood glucose concentration.

Tomkins [4] proposed a generalized model of hormone-mediated Intercellular Communication (interaction). According to this model, a small number of intracellular "symbols," i.e., compounds such as cAMP or ppGpp in Prokaryotic Cells, participate in regulatory mechanisms governing specific metabolic domains within cells. The Role of Cyclic NUCLEOTIDES as symbols has been firmly established. Furthermore, in Eukaryotic cells, Ca²⁺, Na⁺, and K⁺ ions are also likely candidates for this role. In Bacteria, ppGpp serves as a symbol indicating nitrogen or amino acid deficiency. In A wide variety of cells—ranging from bacterial to animal cells—an elevated level of cAMP acts as a symbol of carbon source deficiency. Thus, in E. coli experiencing a shortage of carbon sources in the medium, the cAMP concentration rises, which stimulates the transcription of numerous bacterial operons (Ch. 15, Sec. B.1, b). In Dictyostelium (Ch. 1, Sec. D, 1), cells secrete cAMP upon starvation. In the latter case, the cyclic nucleotide Functions as a hormone, mediating signal transmission from cell to cell.

However, while cAMP is utilized as a hormone in lower organisms, such usage becomes impossible in higher animals due to the high metabolic lability of this compound. Consequently, in our bodies, hormones such as glucagon and epinephrine Relay signals to the cell surface, where they bind to receptors and stimulate cAMP production. This, in turn, leads to the mobilization of the cell's metabolic resources, notably glycogen and triglycerides, which precisely mirrors the cellular response to starvation. According to Tomkins' scheme, hormones are produced by "sensory" cells in direct response to environmental signals; then, transported via the blood to more distant "responder" cells, they activate them. The picture can be made even more generalized by considering that Neurotransmitters are largely Amino Acid Derivatives. Tomkins suggests that these Amino acids initially served as intracellular symbols reflecting changes in amino acid concentrations in the environment, but subsequently came to be used for short-range signaling between cells of The Nervous System.

Supplement 16-A

Radioimmunoassay Method

One of the methodological breakthroughs that ensured rapid progress in studying The Mechanism of hormone action was the application of specific Antibodies raised against hormones or hormone-Structure/178.html">Protein Complexes. Utilizing radioisotope-labeled hormones, radioimmunoassay makes it possible to detect the substance under study at extraordinarily low concentrations—on the order of femtomoles (i.e., The amount of substance contained in 1 mL of a solution with a total concentration of 10^-12 M)^a. Currently, Methods for the radioimmunoassay of virtually any purified hormone have been developed^b.

The assay is usually performed as follows. Various volumes of the sample in which the hormone concentration (e.g., insulin) is to be determined are added to a series of test tubes. Simultaneously, control samples containing known amounts of the hormone are prepared. A standard amount of labeled hormone (usually hormones labeled with gamma-ray-emitting ¹²⁵I) and a hormone-specific antibody are then added to each tube. The solution is incubated for a certain period (from several minutes to hours) to allow equilibrium between the hormone (antigen) and the antibody-hormone complex to be reached. Next, the hormone-antibody complex is separated, for example, by Gel filtration or ammonium sulfate precipitation, and the radioactivity of the resulting complex is measured. If the hormone is present in high concentration in the test sample, the dilution of the labeled hormone will be higher, and the radioactivity of the hormone-antibody complex correspondingly lower compared to a sample where the given hormone is present in a lower concentration. Using known hormone concentrations, a standard curve is constructed, which is then used to directly determine the hormone concentration in the test sample.

^a Brooker B., Terasaki W. L., Price M. G., Science, 194, 270–276 (1976).

^b Jaffe B. M., Behrmann H. R., eds., Methods of Hormone Radioimmunoassay, Acad. Press, New York, 1974.