21.2
Intracellular Hormone Receptors
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.
Lipid-soluble hormones diffuse across the plasma and nuclear membrane of target cells to bind to their specific intracellular receptors. These receptors act as transcription factors that regulate gene expression and protein synthesis in the target cell
Based on their mode of action, intracellular hormone receptors are classified as Type I or Type II receptors. Type I receptors, including steroid hormone receptors such as the androgen receptor, are present in the cytoplasm. Hormone binding transports the hormone-receptor complex to the nucleus, where it binds to regulatory DNA sequences called hormone response elements and activates gene transcription.
Type II receptors, such as the thyroid hormone receptor, are bound to their DNA response elements within the nucleus even in the absence of hormone. In this state, the receptor acts as an active repressor of transcription. However, upon hormone binding, the receptor-hormone complex activates transcription of thyroid hormone-inducible genes.
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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