Glucose homeostasis is regulated by hormones and cellular energy status. Elevations of blood glucose during feeding stimulate insulin release from pancreatic b-cells through a glucose sensing pathway. Feeding also stimulates release of gut hormones such as glucagon-like peptide-1 (GLP-1), which further induces insulin release, inhibits glucagon release and promotes b-cell viability. CREB-dependent transcription likely plays a role in both glucose sensing and GLP-1 signaling. The protein Torc2 (transducer of regulated CREB activity 2) functions as a CREB co-activator and is implicated in mediating the effects of these two pathways. In quiescent cells, Torc2 is phosphorylated at Ser171 and becomes sequestered in the cytoplasm via an interaction with 14-3-3 proteins. Glucose and gut hormones lead to the dephosphorylation of Torc2 and its dissociation from 14-3-3 proteins. Dephosphorylated Torc2 enters the nucleus to promote CREB-dependent transcription. Torc2 plays a key role in the regulation of hepatic gluconeogenic gene transcription in response to hormonal and energy signals during fasting. Torc1 and Torc3, a pair of Torc2-related proteins, also act as CREB coactivators. Torc1, Torc2 and Torc3 associate with the HTLV Tax protein to promote Tax-dependent transcription of HTLV-1 long terminal repeats. CRTC1/TORC1 is highly phosphorylated at Ser151 in mouse hypothalamic cells under basal conditions. When these cells are exposed to cAMP or a calcium activa- tor, CRTC1/TORC1 is dephosphorylated and translocates into the nucleus. CRTC1/TORC1 is essential for energy balance and fertility.
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