A number of hormones and growth factors stimulate target cells through receptors which are coupled to second messenger pathways. The second messenger cAMP, for example, mediates a wide variety of cellular responses to hormonal signals, including changes in intermediary metabolism, cellular proliferation and cellular motility. In mammalian cells, all of these biological responses are triggered by the activation of the cAMP-dependent protein kinase A, a heterotetramer consisting of paired catalytic and regulatory subunits. Upon hormonal stimulation, cAMP binds tightly to the regulatory subunits, thereby liberating catalytic subunits and promoting the phosphorylation of cellular substrates. In the liver, cAMP functions as a starvation state signal, mediating hormonal cues from the pancreas and adrenal gland to stimulate glucose production. cAMP stimulates glucose production, in part, by regulating transcription of the gene for phosphoenolpyruvate carboxykinase (PEPCK), a rate-limiting enzyme in gluconeogenesis. Following hormonal stimulation, cAMP induces PEPCK gene expression 10-fold within 20-30 min. This induction appears to be independent of new protein synthesis.