Editors' ChoiceMetabolism

Surviving Lean Times

Science Signaling  18 Nov 2008:
Vol. 1, Issue 46, pp. ec395
DOI: 10.1126/scisignal.146ec395

The body's response to fasting is complex, with mechanisms for maintaining energy balance changing between early and late periods of fasting. Glucagon released from the pancreas at early phases of fasting triggers a strong hepatic gluconeogenesis program through the activation of CRTC2 [cAMP-response element binding protein (CREB) regulated transcription coactivator 2, also known as TORC2], whereas later stages of fasting are associated with hepatic ketone body production, which spares protein loss. Liu et al. report a biochemical mechanism for changes in hepatic gluconeogenesis that occur during fasting. Two transcriptional regulatory mechanisms contribute to gluconeogenesis: the transcription factor CREB through the interaction with CRTC2, and the transcription factor forkhead box O1 (FOXO1), the activity of which is increased when insulin signaling decreases (as occurs during fasting). Using mice engineered with luciferase reporter genes—Ad-CRE-luc, responsive to CRTC2, or Ad-G6Pase-luc, responsive to both CRTC2 and FOXO1—the authors performed in vivo imaging of the activity of these two transcriptional regulators in response to fasting for 6 hours or 18 to 24 hours. The abundance of CRTC2 was increased after 6 hours of fasting and reduced to baseline (fed) by 24 hours, and the activity of the Ad-CRE-luc reporter mimicked this pattern. In contrast, FOXO1 abundance was unchanged at 6 hours or 24 hours after fasting and Ad-G6Pase-luc activity was only reduced to 50% of the 6-hour fasting amount after 24 hours of fasting. Knockdown experiments revealed that CRTC2 played a key role in promoting gluconeogenesis at 6 hours, but not 18 hours, of fasting, whereas knockdown of FOXO1 was important for maintaining blood glucose concentrations, G6Pase mRNA abundance, or Ad-G6Pase-luc reporter gene activity 18 hours, but not 6 hours, after fasting. CRTC2 was acetylated at Lys628 after 8 hours of fasting (or exposure of primary hepatocytes to glucagon), whereas after 24 hours of fasting, ubiquitinated CRTC2 was detectable. Eight hours of fasting increased the interaction of histone acetyltransferases (HATs) of the CREB binding protein family, CBP and p300, with CRTC2, and CRTC2 was a substrate for p300 in vitro. Knockdown of p300 or inhibition of HAT activity in vivo reduced induction of Ad-CRE-luc reporter activity in response to 6 hours of fasting, decreased blood glucose concentration, and decreased G6Pase mRNA relative to control fasted mice, consistent with a role in acetylation of CRTC2 in its activation. If acetylation activates CRTC2, but prolonged fasting decreased CRTC2 activity, then the authors reasoned that a deacetylase may play a role in this switch. The deacetylase SIRT1 has been implicated in glucose homeostasis through activation of FOXO1 and was found to accumulate in the liver after 18 hours of fasting. SIRT1 and CRTC2 coimmunoprecipitated from 18-hour fasted mice but not from 6-hour fasted mice. Pharmacological activation of SIRT1 decreased acetylated CRTC2 and Ad-CRE-luc activity in primary hepatocytes exposed to glucagons, whereas SIRT1 antagonists had the opposite effect. Genetic knockout of SIRT1 in hepatocytes resulted in a lack of degradation of CRTC2 in response to prolonged exposure of primary hepatocytes to glucagons. Thus, an acetylation switch controls the activity of CRTC2 in regulating the hepatic gluconeogenic program in response to fasting.

Y. Liu, R. Dentin, D. Chen, S. Hedrick, K. Ravnskjaer, S. Schenk, J. Milne, D. J. Meyers, P. Cole, J. Yates III, J. Olefsky, L. Guarente, M. Montminy, A fasting inducible switch modulates gluconeogenesis via activator/coactivator exchange. Nature 456, 269–273 (2008). [PubMed]