Enzyme-linked and Intracellular Receptor Signalling Quiz

See Intracellular Receptors. Option A is correct because a steroid receptor works by changing gene expression. Steroid, thyroid, vitamin D and retinoid hormones are lipid-soluble, so they cross the plasma membrane and bind receptors inside the cell. The ligand-bound receptor acts as a ligand-regulated transcription factor (a protein that switches genes on or off) and binds specific DNA sequences called hormone response elements (HREs). This changes which mRNAs, and therefore which proteins, the cell makes, and the physiological effect appears over hours because new RNA and protein have to be made. If transcription is pharmacologically blocked, the steroid can still enter the cell, bind the receptor and activate it. The step that produces the effect, however, is blocked, so the delayed, gene-dependent responses are greatly reduced. The word most matters here because some rapid, non-genomic actions of steroids at the membrane have been described. For the exam, the canonical output of an intracellular receptor is altered gene expression, and a block on that step leaves binding intact but the response largely missing.

• Location: Lipid-soluble hormones such as steroids cross the membrane and bind receptors inside the cell.
• Action: The ligand-bound receptor is a transcription factor that binds hormone response elements and changes which mRNAs and proteins are made.
• Time course: The effect appears over hours because new RNA and protein have to be made.
• Transcription block: Blocking transcription leaves binding intact but the gene-dependent response largely missing.