| Intracellular receptors: steroid hormone receptors
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| Steroid hormones traverse cell membranes
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| Because of the cholesterol-based nature of their structure, steroid hormones, such as cortisol (made in the cortex of the adrenal gland), sex hormones, and vitamin D, can directly traverse the plasma membrane of cells to initiate their responses via cytoplasmically located receptors called steroid hormone receptors (see Fig. 38.1). These receptors belong to a superfamily of cytoplasmic receptors called the intracellular receptor superfamily, which also transduce signals from other small hydrophobic signaling molecules such as the tyrosine-derived thyroid hormones (e.g. thyroxine) and the vitamin A-derived retinoids (e.g. retinoic acid).
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| Intracellular receptors are transcription factors
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| Figure 38.1 Mechanisms of cell signaling. NO, nitric oxide; TF, transcription factor. |
| The intracellular receptors for these steroid and thyroid hormones and retinoids are transcription factors; they bind to regulatory regions of the DNA of genes that are responsive to the particular steroid/thyroid hormone. Such 'ligand binding' (ligation) induces a conformational change in the
transcription factor that allows it to activate, or repress, gene induction. Although all the target cells have specific receptors for the individual hormones, they express distinct combinations of cell-type-specific regulatory proteins that cooperate with the intracellular hormone receptor to dictate the precise repertoire of genes that are induced. Hence the hormones induce distinct sets of responses in different target cells.
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