3.5
Internal Receptors
Many cellular signals are hydrophilic and therefore cannot pass through the plasma membrane. However, small or hydrophobic signaling molecules can cro…
Internal receptors are soluble proteins found in either the cytoplasm or the nucleus. They bind signaling molecules called ligands inside the cell and trigger cellular responses, such as changes in gene expression.
Typically, ligands that bind to internal receptors are hydrophobic, nonpolar molecules that can diffuse through the plasma membrane without needing a membrane receptor. Some signaling molecules can also be synthesized inside the target cell.
Once a ligand binds, the receptor changes shape and interacts with other regulatory elements in the cell.
For example, the hormone testosterone, or its converted form dihydrotestosterone, also called DHT, binds to androgen receptors located in the cytoplasm.
After binding, the receptor-ligand complex changes structure and exposes its DNA-binding region. This change also allows the receptors to form dimers and move into the nucleus.
Inside the nucleus, the activated receptor binds to specific DNA sequences called hormone-response elements. In androgen signaling, these sequences are called androgen-response elements.
By binding to these DNA sequences, the receptor recruits other cellular machinery that helps regulate gene activity by increasing or decreasing the expression of specific target genes.
These changes in gene expression lead to androgen-related effects in target tissues, such as muscles, the prostate, and hair follicles.
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Q1: What are internal receptors and where are they located in cells?
Internal receptors are signaling proteins located inside cells, typically in the cytoplasm or nucleus, that bind to small hydrophobic ligands like steroid hormones and thyroid hormones. Unlike cell surface receptors, internal receptors allow ligands to cross the cell membrane and directly interact with receptor proteins within the cell, initiating intracellular signaling cascades that regulate gene expression and cellular responses.
Q2: How do internal receptors differ from cell surface receptors?
Internal receptors bind ligands inside the cell and typically function as transcription factors that directly regulate gene expression, while cell surface receptors bind extracellular ligands and activate signaling pathways through intermediate proteins. Internal receptors are suited for small, lipid-soluble molecules that can cross membranes, whereas surface receptors detect large, hydrophilic signaling molecules that cannot penetrate the cell membrane.
Q3: What types of ligands activate internal receptors?
Internal receptors are activated by small, hydrophobic ligands including steroid hormones like estrogen and testosterone, thyroid hormones, and vitamin A derivatives. These ligands can cross the lipid bilayer of the cell membrane due to their lipophilic nature, allowing them to reach and bind internal receptor proteins in the cytoplasm or nucleus.
Q4: What happens after an internal receptor binds its ligand?
Once an internal receptor binds its ligand, the receptor-ligand complex typically translocates to the nucleus and functions as a transcription factor. The complex binds to specific DNA sequences called response elements, recruiting coactivators or corepressors to regulate the transcription of target genes, ultimately producing physiological responses through altered gene expression.
Q5: How do internal receptors contribute to cellular signaling pathways?
Internal receptors initiate signaling by directly modulating gene transcription rather than activating intermediate signaling cascades. This mechanism allows cells to respond to hormonal signals by altering the expression of specific genes, which can influence metabolism, development, and homeostasis. Internal receptor signaling represents a direct pathway from ligand binding to changes in cellular function through altered gene expression.
Q6: What is the role of heat shock proteins in internal receptor function?
Heat shock proteins, such as HSP90, bind to unliganded internal receptors in the cytoplasm, maintaining them in an inactive conformation and preventing premature nuclear translocation. When a ligand binds, heat shock proteins dissociate, allowing the receptor-ligand complex to enter the nucleus and function as a transcription factor, enabling proper regulation of internal receptor signaling.
Q7: How do internal receptors regulate physiological responses compared to other signaling mechanisms?
Internal receptors provide direct transcriptional control of gene expression, enabling sustained physiological responses to hormonal signals. Unlike rapid cell surface receptor signaling that produces immediate effects through protein phosphorylation, internal receptor signaling produces longer-lasting changes by altering the expression of genes involved in metabolism, growth, and differentiation, making them essential for maintaining homeostasis and coordinating complex physiological processes.