Cytosolic steroids

Steroid hormones act in a different manner from most hormones we have considered. In many cases, they do not bind to plasma membrane receptors, but rather pass easily across the plasma membrane. Steroids may bind directly to receptors in the nucleus or may bind to cytosolic steroid hormone receptors, which then enter the nucleus. In the nucleus, the hormone-receptor complex binds directly to specific nucleotide sequences in DNA, increasing transcription of DNA to RNA (Chapters 31 and 34). 

Corticosteroids a chronic painless myopathy associated with the long-term use of corticosteroids is a particularly common example of drug-induced muscle disorder. It is almost certain that mild cases are overlooked because steroids are so frequently used to treat inflammatory myopathies such as polymyositis. Fluorinated steroids are particularly frequently implicated, and the incidence of drug-induced muscle disease is dose and time-related. The presence of muscle weakness can even complicate topical steroid therapy. Corticosteroid-induced myopathy is mediated via intramuscular cytosolic steroid receptors. The steroid-receptor complexes inhibit protein synthesis and interfere with oxidative phosphorylation. The myopathy is associated with vacuolar changes in muscle, and the accumulation of cytoplasmic glycogen and mitochondrial aggregations. 

Table 11.1 Agonists and antagonists of cytosolic steroid hormone receptors...

Anderson KM, Pheian J, Marogil M, Hendrickson C and Economou S (1980) Sodium molybdate increases the amount of progesterone and estrogen receptor detected in certain human breast cancer cytosols. Steroids 35 273-280. 

Stimulation of C.P450 synthesis occurs at the transcriptional level, and different cytosolic receptors have been identified for benzpyrene, dioxin, isosafrole and 3-methylcholanthrene. The analogy with the mcxle of action of steroid hormones is striking. It must be remembered, however, that the existence of cytosolic steroid receptors in vivo is doubted [J.Gors-ki etal. Molec. Cell Endocrinol 36 (1984) 11-15]. Cytosolic receptors are probably artifacts of cell disruption, and steroid receptors are thought to reside only in the nucleus This may also apply to cytosolic receptors of C.P450 substrates [S.K.Yang etal. Science 196 (1977) 1199-1201, D.PfeU J.Friedrich Pharmazie 46 (1985) 217-221, J.Friedrich D.Pfeil... 

Carilla E, Briley M, Fauran F, et al. Binding of Permixon, a new treatment for prostatic benign hyperplasia, to the cytosolic androgen receptor in the rat prostate. J Steroid Biochem 20 521-523, 1984. 

Understanding of the intracellular localization of steroid receptors has gone through a number of phases, beginning with the view that receptors translocated from cytoplasm to nucleus in the presence of hormone. Indeed, with the exception of thyroid hormone receptors, which are exclusively nuclear in location, cell fractionation studies have revealed that in the absence of hormone, steroid receptors are extracted in the soluble or cytosolic fraction. However, when steroid is present in the cell, many occupied receptors are retained by purified cell nuclei. Histological procedures, such as immunocytochemistry, have confirmed the largely nuclear localization of occupied receptors, but... 

Estradiol. The first neuroactive steroid receptor type to be recognized was that for estradiol [3]. In vivo uptake of [3H] estradiol, and binding to cell nuclei isolated from hypothalamus, pituitary and other brain regions, revealed steroid specificity closely resembling that of the uterus, where steroid receptors were first discovered [3]. Cytosolic estrogen receptors isolated from pituitary and brain tissue closely resemble those found in uterus and mammary tissue. A hallmark of the estrogen receptor is its existence... 

The intracellular distribution of steroid hormone receptors has long been the object of controversy. The first theoretical formulation on the intracellular location of the ERs was elaborated by Jensen in 1968 and is known as the two-step theory. Its execution was based entirely on biochemical observations obtained by means of tritium-marked estradiol. The ERs, in cells not exposed to hormones, are found abundantly in the soluble cell fraction, or cytosol (Fig. 1.1). Treatment with hormones confines the receptors to the particulated or nuclear fraction and causes their disappearance from the cytosol. The two-step theory established that the receptor is found in the cytoplasm naturally and upon the arrival of a hormone it is transformed into a complex hormone-receptor (first step) capable of translocating itself to the nucleus and of modifying gene expression (second step). 

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...

Navarro D, Ledn L, Chirino R, Fernandez L, Pestano J, Dlaz-Chico BN (1998) The two native estrogen receptor forms of 8S and 4S present in cytosol from human uterine tissues display opposite reactivity with tamoxifen aziridine and the estrogen response element. J Steroid Biochem Mol Biol 64 49... 

Meyers CY, Kolb VM, Gass GH, et al. 1988. Doisynolic-type acids - uterotropically potent estrogens which compete poorly with estradiol for cytosolic estradiol receptors. J Steroid Biochem 31 (4A) 393-404. 

Like all steroids, aldosterone enters the target cell and combines with cytosolic mineralocorticoid receptor. Such receptors are not entirely specific for aldosterone and will also bind cortisol, the principal glucocorticoid hormone. The receptors are protected from cortisol activation by 11 3 hydroxysteroid dehydrogenase which... 

Receptor-effector mechanisms include (1) enzymes with catalytic activities, (2) ion channels that gate the transmembrane flux of ions (ionotropic receptors), (3) G protein-coupled receptors that activate intracellular messengers (metabotropic receptors), and (4) cytosolic receptors that regulate gene transcription. Cytosolic receptors are a specific mechanism of many steroid and thyroid hormones. The ionotropic and metabotropic receptors are discussed in relevance to specific neurotransmitters in chapter 2. 

Here are some insights into how l,25(OH)2D works. Like steroid hormones and retinoic acid, l,25(OH)2D binds to and activates a cytosolic receptor present in most cells of the human body. The activated receptor migrates to the cell nucleus, binds to a specific nucleotide sequence in the nuclear DNA, and acts as a transcription factor. Directly or indirectly, the expression of some 200 genes is affected as a result. 

Figure 12.8 Effector mechanism activation of a specific gene by hormone-receptor complex binding to DNA. A steroid is used to illustrate the mechanism. The hormone enters the cell and binds to its receptor (R) in the cytosol, the hormone-receptor complex enters the nucleus and binds to a specific sequence in the DNA that stimulates transcription of a gene or genes the resultant increase in mRNA increases the synthesis of specific proteins. The binding site on the DNA is specific and is usually termed a response element. Thyroxine (i.e. triiodothyronine) also uses this effector mechanism. Activation of genes, RNA processing to produce mRNA and translation are described in Chapter 20 (see Figures 20.20, 20.21 and 20.22).

An account of the principles which help to understand how hormones achieve their roles in the body is given in Chapter 12. The understanding is based on separation of the effects of hormones into three components the action, the effects (biochemical and physiological) and the function. A steroid hormone binds to a cytosolic intracellular receptor, which then moves into the nucleus where it binds to DNA at a specific site (the steroid response element) and activates genes which result in the formation of proteins that elicit biochemical and physiological effects. This is discussed for cortisol in Chapter 12 and aldosterone in Chapter 22. Much of the interest in the reproductive steroid hormones is in the physiological effects and how these account for their functions. 

Protein synthesis-regulating receptors (D) for steroids, thyroid hormone, and retinoic acid are found in the cytosol and in the cell nucleus, respectively. 

S.M. Abel, D.J. Back, J.L. Maggs, B.K. Park, Cortisol metabolism by human liver in vitro. IV. Metabolism of 9ot-fluorocortisol by human liver microsomes and cytosol, J. Steroid Biochem. Mol. Biol. 46 (1993) 833-839. 

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