21.2
Intracellular Hormone Receptors
Lipid-soluble hormones diffuse across the plasma and nuclear membrane of target cells to bind to their specific intracellular receptors. These recepto…
Hormones can also bind to intracellular receptors, protein receptors found inside the target cell, to alter cellular functions.
First, lipid-soluble hormones like the steroid testosterone easily diffuse across the cell membrane of the endocrine cell of the testes. Once outside in the extracellular fluid, these hormones attach to transport proteins to remain soluble, which is particularly important in the aqueous blood stream.
At the target cell, the lipid-soluble hormone detaches from the transport protein and then diffuses through the cell membrane. Now inside the cell, it binds to its intracellular hormone receptors, in this case, androgen receptors. This complex can enter the nucleus and then bind to a specific DNA sequence called a hormone response element, which triggers gene transcription and translation, thus regulating the production of mRNA and gene expression.
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Q1: How do lipid-soluble hormones enter target cells?
Lipid-soluble hormones like testosterone diffuse directly across the cell membrane because of their hydrophobic nature. In the bloodstream, these hormones attach to transport proteins to remain soluble in the aqueous environment. At the target cell, the hormone detaches from the transport protein and diffuses through the plasma membrane to reach intracellular receptors.
Q2: What is the role of hormone response elements in gene regulation?
Hormone response elements are specific DNA sequences that bind hormone-receptor complexes in the nucleus. Once the hormone-receptor complex attaches to these regulatory sequences, it triggers gene transcription and translation. This process regulates mRNA production and controls gene expression in the target cell.
Q3: How do Type I and Type II intracellular receptors differ?
Type I receptors, such as androgen receptors, reside in the cytoplasm and move to the nucleus only after hormone binding. Type II receptors, like thyroid hormone receptors, are already bound to DNA response elements in the nucleus. Without hormone, Type II receptors actively repress transcription; hormone binding activates transcription of target genes.
Q4: Why do lipid-soluble hormones require transport proteins in the bloodstream?
Lipid-soluble hormones are hydrophobic and cannot dissolve in the aqueous blood environment. Transport proteins keep these hormones soluble and prevent them from aggregating or being lost. This ensures hormones remain stable and available to reach their target cells throughout the body.
Q5: What happens when a hormone-receptor complex enters the nucleus?
Once inside the nucleus, the hormone-receptor complex binds to specific DNA sequences called hormone response elements. This binding activates gene transcription, leading to mRNA production and protein synthesis. The receptor essentially acts as a transcription factor that regulates which genes are expressed in response to the hormone signal.
Q6: How do intracellular receptors function as transcription factors?
Intracellular receptors regulate gene expression by binding to hormone response elements and controlling transcription. Type I receptors activate transcription after hormone binding, while Type II receptors switch from repressing to activating transcription upon hormone binding. This mechanism allows hormones to directly influence which proteins a cell produces.
Q7: How does the intracellular hormone signaling pathway differ from cell surface signaling?
Intracellular receptors bind lipid-soluble hormones inside the target cell and directly regulate gene transcription in the nucleus. In contrast, cell surface signaling and GPCRs involve water-soluble hormones binding to external receptors that trigger intracellular signaling cascades. Intracellular receptors provide direct nuclear access, while cell surface signaling relies on secondary messenger systems.