Steroid hormone receptors receptor classes

In the absence of steroid hormones the receptors remain in an inactive complex, designated the apo-receptor complex (review Pratt, 1993 Bohen, 1995). In the apore-ceptor complex the receptor is boimd to proteins belonging to the chaperone class. Chaperones are proteins whose levels are increased as a result of a stress situation, such as a rise in ambient temperature. The chaperones assume a central function in the folding process of proteins in the cell. Chaperones aid proteins in avoiding incorrectly folded states. They participate in the folding of proteins during and after ribosomal protein biosynthesis, during membrane transport of proteins, as well as in the correct assembly of protein complexes. 

But how do these ovarian and testicular hormones exert their effects All cells that interact with these steroid hormones have receptors for them within the cytoplasm. The presence of oestrogen receptors was first demonstrated by Jensen and Jacobsen in 1962. The hormones pass through the cell membrane and then encounter receptors that are specific for each of the classes of hormones the oestrogens, androgens, progesterone and glucocorticoids... 

The signal is what starts everything off. Signals take a variety of forms, but for our purposes there are only two. The first type are signals that go into the cell, bind to internal receptors, and exert their effects. Steroid hormones, vitamin D, thyroid hormone, and retinoids are the only members of this class. All of the intracellular receptors ultimately activate the transcription of regulated genes. The common feature of signals that enter the cell is that they are all small lipophilic molecules that can cross the cell membrane. 

Nuclear hormone receptors, including those for the principal classes of steroids, retinoids, vitamin D, and thyroid hormones, are transcription factors that influence gene expression. 

Several biologic ligands are sufficiently lipid-soluble to cross the plasma membrane and act on intracellular receptors. One class of such ligands includes steroids (corticosteroids, mineralocorticoids, sex steroids, vitamin D), and thyroid hormone,... 

Water-insoluble hormones (steroid, retinoid, and thyroid hormones) readily pass through the plasma membrane of their target cells to reach their receptor proteins in the nucleus (Fig. 23-4). With this class of hormones, the hormone-receptor complex itself carries the message it interacts with DNA to alter the expression of specific genes, changing the enzyme complement of the cell and thereby changing cellular metabolism (see Fig. 12 10). 

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