Cyclic AMP—A Second Messenger Molecule

Cyclic AMP (cAMP), full name adenosine 3',5'-monophosphate, is a second messenger molecule widely found in living organisms. It is produced by cells in response to hormones and nutrients. Research indicates that cAMP is present in most animal cells, typically at very low concentrations. In the human body, cAMP is synthesized from ATP—a reaction catalyzed by adenylate cyclase—and is subsequently degraded into 5'-AMP by phosphodiesterases. Its fundamental functions include mediating the intracellular effects of various hormones, regulating hormone release, activating PKA (thereby modulating gene transcription), and participating in the regulation of diverse metabolic pathways [1-2].

Functions of Cyclic AMP as a Second Messenger

Acting as a "second messenger," cyclic AMP converts extracellular signals from hormones (the first messengers) into intracellular signals to elicit biological effects. This mediation is cell-specific; different cells respond to different hormones based on the specific receptors they express. Upon hormonal stimulation, cAMP functions through a classic signaling cascade: the binding of a ligand (such as a hormone, neurotransmitter, or growth factor) to a specific receptor on the plasma membrane activates a receptor-coupled GTP-binding protein (G protein). The Gs protein promotes the activation of adenylate cyclase, which converts ATP into cAMP. The resulting cAMP activates protein kinase A (PKA), which subsequently phosphorylates various proteins. Beyond mediating hormonal actions, cAMP extensively regulates hormone secretion and mediates the metabolic regulatory effects of various hormones. For instance, glucagon stimulates the transcription of genes associated with hepatic gluconeogenesis by elevating cAMP levels. Its classic role is also evident in the regulation of glycogenolysis.

cAMP-mediated signaling is typically transient, subject to precise regulation by mechanisms such as homologous and heterologous desensitization, degradation by phosphodiesterases, and the action of PKA inhibitors and phosphatases, all of which ensure the timely termination of the signal.

References

[1] Daniel, P. B., Walker, W. H., Habener, J. F. (1998). Cyclic AMP signaling and gene regulation. Annual Review of Nutrition, 18, 353–383. https://doi.org/10.1146/annurev.nutr.18.1.353

[2] Sutherland, E. (1970). On the biological role of cyclic AMP. JAMA Neurology, 14 1, 1281–1288. https://doi.org/10.1001/JAMA.1970.03180070047009