Hormones activate a cellular response in the target cell by binding to a specific receptor in the target cell.
Distinguish between the location and function of hydrophilic and lipophilic hormone receptors
- For water-soluble proteins, the receptor will be at the plasma membrane of the cell.
- The ligand-bound receptor will trigger a cascade of secondary messengers inside the cell.
- For lipid-soluble hormones, the receptor is typically located within the cytoplasm or nucleus of the cell.
- The binding of the hormone allows the receptor to influence transcription in the nucleus, either alone or in association with other transcription factors.
- The number of hormone molecules is usually the key factor for determining hormone action and it is determined by the concentration of circulating hormones, which in turn is influenced by the rate and level of secretion.
- Another limiting factor for hormone action is the effective concentration of hormone-bound receptor complexes that are formed within the cell. This is determined by the number of hormone/receptor molecules available for complex formation and the binding affinity between the hormone and receptor.
- secondary messenger: These are molecules that relay signals from receptors on the cell surface to target molecules inside the cell, in the cytoplasm, or the nucleus.
A hormone receptor is a molecule that binds to a specific hormone. Receptors for peptide hormones tend to be found on the plasma membrane of cells, whereas receptors for lipid-soluble hormones are usually found within the cytoplasm.
Upon hormone binding, the receptor can initiate multiple signaling pathways that ultimately lead to changes in the behavior of the target cells.
The hormone activity within a target cell is dependent on the effective concentration of hormone-receptor complexes that are formed. The number of these complexes is in turn regulated by the number of hormone or receptor molecules available, and the binding affinity between hormone and receptor.
Many hormones are composed of polypeptides—such as thyroid -stimulating hormones, follicle-stimulating hormones, luteinizing hormones, and insulin. These molecules are not lipid-soluble and therefore cannot diffuse through cell membranes.
The receptors for these hormones need to be localized to the cells’ plasma membranes. Following an interaction with the hormones, a cascade of secondary effects within the cytoplasm of the cell is triggered, often involving the addition or removal of phosphate groups to cytoplasmic proteins, changes in ion channel permeability, or an increase in the concentrations of intracellular molecules that may act as secondary messengers, such as cyclic AMP.
Lipophilic hormones—such as steroid or thyroid hormones—are able to pass through the cell and nuclear membrane; therefore receptors for these hormones do not need to be, although they sometimes are, located in the cell membrane.
The majority of lipophilic hormone receptors are transcription factors that are either located in the cytosol and move to the cell nucleus upon activation, or remain in the nucleus waiting for the steroid hormone to enter and activate them.
Upon binding by the hormone the receptor undergoes a conformational change, and the receptor together with the bound hormone influence transcription, either alone or in association with other transcription factors.
Example hormone receptor: The thyroid hormone receptor (TR) heterodimerized to the RXR. In the absence of a ligand, the TR is bound to a corepressor protein. Ligand binding to the TR causes a dissociation of co-repressor and recruitment of co-activator proteins, which in turn recruit additional proteins (such as RNA polymerase) that are responsible for the transcription of downstream DNA into RNA, and eventually into protein that results in a change in cell function.