The specific cells that respond to a given hormone have receptor sites for that hormone. This is sort of a lock and key mechanism. If the key fits the lock, then the door will open.
If a hormone fits the receptor site, then there will be an effect. All the cells that have receptor sites for a given hormone make up the target tissue for that hormone. In some cases, the target tissue is localized in a single gland or organ.
In other cases, the target tissue is diffused and scattered throughout the body so that many areas are affected. Hormones bring about their characteristic effects on target cells by modifying cellular activities.
Protein hormones react with receptors on the surface of the cell, and the sequence of events that results in hormone action is relatively rapid.
Steroid hormones typically react with receptor sites inside the cell. Because this method of action actually involves synthesis of proteins, it is relatively slow.
These hydrophilic (and mostly large) hormone molecules bind to receptors on the surface of “target” cells: that is, cells able to respond to the presence of the hormone. These receptors are transmembrane proteins. Binding of the hormone to its receptor initiates a sequence of intracellular signals that may
i. Alter the behaviour of the cell (such as by opening or closing membrane channels) or
ii. Stimulate (or repress) gene expression in the nucleus by turning on (or off) the promoters and enhancers of the genes
The hormone binds to a site on the extracellular portion of the receptor.
i. The receptors are transmembrane proteins.
ii. Many (but not all) pass through the plasma membrane 7 times, with their N-terminal exposed at the exterior of the cell and their C-terminal projecting into the cytoplasm.
Binding of the hormone to the receptor
i. Activates a G protein associated with the cytoplasmic C-terminal
ii. This initiates the production of a ”second messenger”. The most common of these are
(a) cyclic AMP, (cAMP) which is produced by adenyl cyclase from ATP and
(b) inositol 1, 4, 5-triphosphate (IP3)
iii. The second messenger, in turn, initiates a series of intracellular events such as –
(a) phosphorylation and activation of enzymes
(b) release of Ca2+ stores within the cytoplasm
iv. In the case of cAMP, these enzymatic changes activate the transcription factor CREB (cAMP response element binding protein)
v. Bound to its response element 5? TGACGTCA 3? in the promoters of genes that are able to respond to the hormone, activated CREB turns on gene transcription.
vi. The cell begins to produce the appropriate gene products in response to the hormonal signal it had received at its surface.
Steroid hormones, being hydrophobic molecules, diffuse freely into all cells. However, their “target” cells contain cytoplasmic and/or nuclear proteins that serve as receptors of the hormone.
The hormone binds to the receptor and the complex binds to hormone response elements – stretches of DNA within the promoters of genes responsive to the hormone.
The hormone/receptor complex acts as a transcription factor turning target genes “on” (or “off’). mRNA is formed and with translation, protein is synthesized. This protein formed is responsible for the biological action of that steroid.
Hormones are required in specific amount. Their hyposecretion (secretion in lesser amount than normal), as well as hypersecretion (secretion in large amount than normal) produces serious physiological disturbances in the body.