Adrenocorticotropic hormone part of GPCR ligand binding
Function
ACTH stimulates secretion of glucocorticoid steroid hormones from adrenal cortex cells, especially in the zona fasciculata of the adrenal glands. ACTH acts by binding to cell surface ACTH receptors, which are located primarily on adrenocortical cells of the adrenal cortex. The ACTH receptor is a seven-membrane-spanning G protein-coupled receptor. Upon ligand binding, the receptor undergoes conformation changes that stimulate the enzyme adenylyl cyclase, which leads to an increase in intracellular cAMP and subsequent activation of protein kinase A.
ACTH influences steroid hormone secretion by both rapid short-term mechanisms that take place within minutes and slower long-term actions. The rapid actions of ACTH include stimulation of cholesterol delivery to the mitochondria where the P450scc enzyme is located. P450scc catalyzes the first step of steroidogenesis that is cleavage of the side-chain of cholesterol. 
ACTH also stimulates lipoprotein uptake into cortical cells. This increases the bioavailability of cholesterol in the cells of the adrenal cortex.
The long term actions of ACTH include stimulation of the transcription of the genes coding for steroidogenic enzymes, especially P450scc, steroid 11β-hydroxylase, and their associated electron transfer proteins. This effect is observed over several hours.In addition to steroidogenic enzymes, ACTH also enhances transcription of mitochondrial genes that encode for subunits of mitochondrial oxidative phosphorylation systems. These actions are probably necessary to supply the enhanced energy needs of adrenocortical cells stimulated by ACTH.
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