Rucete ✏ Lehninger Principles of Biochemistry In a Nutshell
12.7 Regulation of Transcription by Nuclear Hormone Receptors
This chapter explains how steroid hormones, thyroid hormones, retinoids, and vitamin D regulate cell behavior by directly altering gene expression through intracellular nuclear receptors. Unlike GPCRs or membrane receptor kinases, these signals act mainly in the nucleus and produce slower but long-lasting effects through changes in transcription and protein synthesis.
Hormones That Use Nuclear Receptors
• A major group of signaling molecules acts through nuclear hormone receptors.
• These include:
• Steroid hormones
• Retinoic acid (retinoids)
• Thyroid hormones
• Vitamin D
• These ligands exert at least part of their biological effects by directly regulating gene expression.
How This Pathway Differs from Other Hormones
• Most hormones discussed earlier act through plasma membrane receptors.
• They trigger second messengers, kinase cascades, or ion channel changes.
• Nuclear hormone receptor ligands instead act mainly inside the cell nucleus.
• Their mechanism is fundamentally different because they regulate transcription directly.
Examples of Steroid Hormones
• Estrogen
• Progesterone
• Cortisol
• Vitamin D–related steroid signaling molecules
Hydrophobic Nature of Steroid Hormones
• Steroid hormones are highly hydrophobic.
• Therefore they do not dissolve readily in blood plasma.
• They are transported through the bloodstream bound to specific carrier proteins.
• Carrier proteins deliver them from the site of secretion to target tissues.
Entry into Target Cells
• Because these hormones are lipid-soluble, they can cross membranes by simple diffusion.
• They pass through the plasma membrane.
• They can also pass through the nuclear membrane.
• No membrane transporter is required in the classic pathway.
Location of Receptors
• Specific receptor proteins are located in the nucleus.
• Some related receptors may also exist in cytoplasm and later move to the nucleus.
• In this chapter’s model, the receptor is shown in the nucleus.
Hormone Binding Activates the Receptor
• The hormone binds its specific receptor protein.
• Binding changes receptor conformation.
• The receptor may become capable of forming dimers or other active complexes.
• The activated receptor gains the ability to interact with DNA regulatory sequences.
Hormone Response Elements (HREs)
• Activated receptor-hormone complexes bind specific DNA sequences called hormone response elements (HREs).
• HREs are regulatory regions located near hormone-responsive genes.
• Binding to HREs influences transcription of adjacent genes.
Role of Other Proteins
• The receptor-hormone complex does not act alone.
• It works together with additional proteins required for transcription.
• These include coactivators, corepressors, chromatin regulators, and RNA polymerase-associated factors.
• Together they increase or decrease gene transcription.
Effect on Gene Expression
• Binding of the hormone-receptor complex can enhance expression of specific genes.
• In some contexts it can also suppress transcription of target genes.
• The resulting mRNA changes alter protein synthesis.
• New protein levels then modify cell metabolism or function.
Why the Response Is Slow
• Nuclear receptor signaling usually requires hours or days for full effect.
• Time is needed for:
• Changes in transcription
• mRNA production
• Translation into protein
• Accumulation or loss of functional proteins
• Observable metabolic or physiological change
Long-Lasting Nature of the Response
• Because the pathway changes gene expression, effects are often sustained.
• This differs from rapid second-messenger systems that may act within seconds or minutes.
General Mechanism Summary
• Hormone travels in blood bound to carrier protein.
• Hormone dissociates and enters target cell.
• Hormone diffuses into nucleus and binds receptor.
• Receptor changes conformation and binds HREs in DNA.
• Transcription of nearby genes is regulated.
• New proteins are synthesized.
• Cell function changes in response to the hormone.
Specificity of Hormone-Receptor Binding
• Nuclear receptor signaling is highly specific.
• Each receptor recognizes particular hormone structures.
• This specificity can be exploited pharmacologically.
Tamoxifen and Breast Cancer
• Tamoxifen is used to treat certain breast cancers.
• Some breast cancer cells require estrogen signaling for continued division.
• Tamoxifen is an estrogen antagonist.
• It competes with estrogen for binding to the estrogen receptor.
• However, the tamoxifen-receptor complex has little or no stimulatory effect on gene expression.
• As a result, estrogen-dependent tumor growth slows or stops.
Clinical Use of Tamoxifen
• Tamoxifen may be administered after surgery.
• It may also be used during chemotherapy.
• Its goal is to suppress growth of remaining hormone-dependent cancer cells.
Mifepristone (RU486)
• Mifepristone is another steroid analog.
• It binds the progesterone receptor.
• It blocks progesterone-dependent hormone actions.
• Progesterone signaling is important for implantation of a fertilized ovum in the uterus.
• Therefore mifepristone can function as a contraceptive.
Pharmacologic Importance of Nuclear Receptors
• Because these receptors bind small hydrophobic molecules, they are major drug targets.
• Synthetic agonists or antagonists can selectively alter transcriptional programs.
• This principle is widely used in endocrinology, oncology, and reproductive medicine.
Comparison with Other Signaling Systems
• GPCR pathways usually act rapidly through cAMP, Ca²⁺, or ion channels.
• Receptor tyrosine kinases activate phosphorylation cascades.
• Nuclear receptors mainly regulate transcription directly.
• Their responses are slower but often broader and longer lasting.
Biological Importance
• Nuclear receptor signaling regulates development, metabolism, reproduction, stress responses, calcium balance, and tissue differentiation.
• Improper signaling can contribute to cancer, endocrine disease, infertility, and metabolic disorders.
In a Nutshell
Steroid hormones, thyroid hormones, retinoids, and vitamin D can diffuse into target cells and bind intracellular receptors that act directly on DNA. The hormone-receptor complex binds hormone response elements (HREs) and works with transcriptional proteins to increase or decrease expression of nearby genes. Because this pathway changes RNA and protein synthesis, responses are slower than membrane signaling pathways but often long-lasting and medically important.