Question
Steroid hormones exert their action by ________: what completes this sentence in endocrinology?
Solution
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Steroid hormones exert their action by entering cells and directly interacting with intracellular receptors to modulate gene expression.
Here is a detailed, step-by-step explanation of how steroid hormones function:
1. Synthesis and Release: Steroid hormones are synthesized from cholesterol in the endocrine glands, such as the adrenal cortex (which produces cortisol, aldosterone, and androgens) and the gonads (which produce estrogen, progesterone, and testosterone).
Here is a detailed, step-by-step explanation of how steroid hormones function:
1. Synthesis and Release: Steroid hormones are synthesized from cholesterol in the endocrine glands, such as the adrenal cortex (which produces cortisol, aldosterone, and androgens) and the gonads (which produce estrogen, progesterone, and testosterone).
1. Synthesis and Release: Steroid hormones are synthesized from cholesterol in the endocrine glands, such as the adrenal cortex (which produces cortisol, aldosterone, and androgens) and the gonads (which produce estrogen, progesterone, and testosterone).
2. Transport in the Bloodstream: Once synthesized, steroid hormones are released into the bloodstream. Because they are lipophilic (fat-soluble), they travel through the bloodstream bound to specific carrier proteins, which maintain their solubility and extend their half-life.
3. Diffusion through Cell Membranes: Upon reaching their target cells, steroid hormones can easily diffuse across the lipid bilayer of cell membranes due to their lipophilic nature.
4. Binding to Intracellular Receptors: Inside the cell, steroid hormones bind to specific intracellular receptors located in the cytoplasm or nucleus. These receptors are part of the nuclear receptor family, which are ligand-activated transcription factors.
5. Formation of Hormone-Receptor Complex: The binding of the hormone to its receptor induces a conformational change in the receptor, activating it. The activated hormone-receptor complex then translocates to the cell nucleus if it is not already there.
6. DNA Binding and Gene Regulation: In the nucleus, the hormone-receptor complex binds to specific DNA sequences called hormone response elements (HREs) located in the promoter region of target genes.
7. Transcriptional Activation or Repression: The binding of the hormone-receptor complex to HREs can either increase (upregulate) or decrease (downregulate) the transcription of the target genes, depending on the response elements and co-regulatory proteins involved.
8. mRNA Translation and Protein Synthesis: The mRNA transcripts produced from the upregulated genes are exported from the nucleus to the cytoplasm, where they are translated into proteins by ribosomes.
9. Physiological Response: The newly synthesized proteins result in a specific physiological response, which can include changes in metabolism, regulation of salt and water balance, inflammatory responses, and reproductive functions, among others.
10. Termination of Hormone Action: The action of steroid hormones is terminated when they are metabolized by enzymes in the liver and other tissues into inactive metabolites, which are then excreted from the body.
This mechanism of action is characteristic of steroid hormones and distinguishes them from peptide hormones and amine hormones, which typically act through cell surface receptors and second messenger systems due to their inability to cross the cell membrane directly.
2. Transport in the Bloodstream: Once synthesized, steroid hormones are released into the bloodstream. Because they are lipophilic (fat-soluble), they travel through the bloodstream bound to specific carrier proteins, which maintain their solubility and extend their half-life.
3. Diffusion through Cell Membranes: Upon reaching their target cells, steroid hormones can easily diffuse across the lipid bilayer of cell membranes due to their lipophilic nature.
4. Binding to Intracellular Receptors: Inside the cell, steroid hormones bind to specific intracellular receptors located in the cytoplasm or nucleus. These receptors are part of the nuclear receptor family, which are ligand-activated transcription factors.
5. Formation of Hormone-Receptor Complex: The binding of the hormone to its receptor induces a conformational change in the receptor, activating it. The activated hormone-receptor complex then translocates to the cell nucleus if it is not already there.
6. DNA Binding and Gene Regulation: In the nucleus, the hormone-receptor complex binds to specific DNA sequences called hormone response elements (HREs) located in the promoter region of target genes.
7. Transcriptional Activation or Repression: The binding of the hormone-receptor complex to HREs can either increase (upregulate) or decrease (downregulate) the transcription of the target genes, depending on the response elements and co-regulatory proteins involved.
8. mRNA Translation and Protein Synthesis: The mRNA transcripts produced from the upregulated genes are exported from the nucleus to the cytoplasm, where they are translated into proteins by ribosomes.
9. Physiological Response: The newly synthesized proteins result in a specific physiological response, which can include changes in metabolism, regulation of salt and water balance, inflammatory responses, and reproductive functions, among others.
10. Termination of Hormone Action: The action of steroid hormones is terminated when they are metabolized by enzymes in the liver and other tissues into inactive metabolites, which are then excreted from the body.
This mechanism of action is characteristic of steroid hormones and distinguishes them from peptide hormones and amine hormones, which typically act through cell surface receptors and second messenger systems due to their inability to cross the cell membrane directly.
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