Steroid hormones target tissues

In humans, most steroids function as hormones, chemical messengers that are secreted by endocrine glands and carried through the bloodstream to target tissues. There are two main classes of steroid hormones the sex hormones, which control maturation, tissue growth, and reproduction, and the adrenocortical hormones, which regulate a variety of metabolic processes. 

Fig. 1.1. General mechanism of action of steroid hormones. Steroid hormones cross through the plasmatic membrane without apparent difficulty favored by gradient. Some, which can be considered prohormones, are metabolized and transformed into more active products. This is the case with testosterone, which becomes dihydrotestosterone (DHT) in the target tissues of androgens, through the 5-alfa-reductase enzyme. The hormone binds to the receptor, a soluble protein of the cellular cytosol that, in the absence of hormone, is found associated with other proteins (hsp90 and others) that maintain the receptor in an inactive state. The hormone-receptor bond causes the other proteins to separate and a homodimer to be formed. The homodimer is the activated form of the receptor since it is capable of recognizing the genes that depend on that steroid hormone as well as of activating its expression, which leads to the synthesis of specific proteins...

Enzymes that are involved in steroid hormone biosynthesis or in steroid metabolism are also targets of anti-hormonal therapy. Recently, it was discovered that certain co-factors modulate the signalling of steroid hormone receptors in a tissue-selective fashion. By binding the receptor ligand complex, these co-activators and co-repressors are capable of either activating or repressing transcription, respectively [5j. 

Pharmacological knowledge, gained with the discovery of steroid hormones, their nuclear receptors and their function in normal and malignant tissues, has been successfully translated into the first targeted drugs in oncology. 

H)2D3 exerts its influence within target tissues through high-affinity sterol-specific intracellular receptor proteins. The D3 receptor, similar to steroid receptor systems, translocates the hormone from the cell cytoplasm to the nucleus, where biological response is initiated via transcription and translation (Fig. 66.3). 

Steroid receptors are highly specific macromolecules found in central regulatory organs (e.g., pituitary, hypothalamus), in various end-point target tissues (e.g., uterus, vagina, prostate), and in lower concentrations in the brain, liver, kidney, ovary, and many other organs. Steroid hormones exhibit remarkable tissue selectivity when binding to these... 

While the macromolecules and target tissues involved show extreme specificity for the appropriate steroid hormones and their congeners, the general scheme of the steroid-receptor mechanism is remarkably uniform. We can therefore deal with this receptor model in a general way, mentioning specific details as appropriate in the subsections of this chapter. 

Steroids are chemical messengers, also known as hormones. They are synthesized in glands and dehvered by the bloodstream to target tissues to stimulate or inhibit some process. Steroids are nonpolar and therefore lipids. Their nonpolar character allows them to cross cell membranes, so they can leave the cells in which they are synthesized and enter their target cells. 

Reproductive development and function depend on endocrine communication throughout the mammalian life cycle. Protein and steroid hormones, growth factors and other signalling molecules affect gene expression and protein synthesis in target cells of different tissues. In particular, fetal development, development of the reproductive tract,... 

Most steroid hormones exist in part as sulfate esters and may also become esterified with fatty acids.256 The fatty acid esters may have relatively long lives within tissues.256 A special sex hormone-binding globulin transports sex hormones in the blood and regulates their access to target cells.256a b... 

Vitamin D. The term vitamin D refers to a group of seco-steroids that possess a common conjugated triene system of double bonds. Vitamin I), (10a) and vitamin D, (10b) are the best-known examples (Fig. 2). Vitamin D (10a) is found primarily in vertebrates, whereas vitamin 11 (10b) is found primarily in plants. The term vitamin is a misnomer. Vitamin I) is a prohormonc that is converted into physiologically active form, primarily 1.25-dihydroxy vitamin D3 (11), by successive hydroxylalions in the liver and kidney. This active form is part of a hormonal system that regulates calcium and phosphate metabolism in the target tissues. 

All classes of steroid hormones bind to specific cytoplasmic receptors in their respective target tissues, and are then translocated to the nucleus. For example, testosterone, a lipid-soluble substance, enters the cell and is enzymatically reduced to dihydrotestosterone by 5-a reductase. Dihydrotestosterone then becomes bound to a specific androgen receptor site located in the cytoplasm. This complex becomes activated and is then translocated to the nucleus, where it binds to the chromatin acceptor site consisting of DNA and nonhistone chromosomal proteins. This interaction results in the transcription of a specific messenger RNA that is then relocated to the cytoplasm and translated on the cytoplasmic ribosomes, resulting in the synthesis of a new protein that sponsors the androgenic functions (Figure 61.6). 

Steroid hormones achieve their effects on target tissues through intracellular receptor proteins. According to recent views, oestrogen and progestin receptors are localized in the nuclear compartment of the cells, whereas glucocorticoid receptors may reside in both the cytoplasm and the nucleus. Determination of the intracellular localization of androgen receptors awaits the development of (monoclonal) antibodies which will enable immunohistochemical studies. The molecular aspects of the mechanism of action of steroid hormones will be covered in other chapters [1-3] in this volume. The present chapter deals with the characterization, assay and purification of steroid receptors. 

Untransformed 8-10S receptors are stabilized in vitro by sodium molybdate. Several laboratories have purified native PR in both transformed and untransformed states and have examined their protein composition. Molybdate-stabilized PR contain, in addition to A- or B-proteins, a non-steroid binding of 90 kDa, which is a heat-shock protein (hsp). This has been observed for PR of different target tissues and species [55-58] and also for molybdate stabilized glucocorticoid receptors [59,60]. Since the 90 kDa hsp associates only with molybdate-stabilized receptors it has been suggested that transformation and receptor conversion from 8-10S to 4S is due to 90 kDa dissociation and unmasking of receptor DNA binding sites. If so, the 90 kDa component of 8S receptors would function to maintain receptors in an inactive state in the absence of hormone. As with other hsps, 90 kDa is a ubiquitous and abundant protein and only a small fraction is found associated with 8-10S receptors. Because of this there is concern that 90 kDa-receptor associations may be in vitro artifacts it is not known whether these associations occur in vivo and whether these interactions are of physiological relevance to receptor function. 

---