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 bloodstream.
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.