Tissue-specific response of carbohydrate-responsive element binding protein (ChREBP) to mammalian hibernation in 13-lined ground squirrels

Abstract

Mammalian hibernation is characterized by a general suppression of energy expensive processes and a switch to lipid oxidation as the primary fuel source. Glucose-responsive carbohydrate responsive element binding protein (ChREBP) has yet to be studied in hibernating organisms, which prepare for the cold winter months by feeding until they exhibit an obesity-like state that is accompanied by naturally-induced and completely reversible insulin resistance. Studying ChREBP expression and activity in the hibernating 13-lined ground squirrel is important to better understand the molecular mechanisms that regulate energy metabolism under cellular stress. Immunoblotting was used to determine the relative expression level and subcellular localization of ChREBP, as well as serine phosphorylation at 95 kDa, comparing euthermic and late torpid ground squirrel liver, kidney, heart and muscle. DNA-binding ELISAs and RT-PCR were used to explore ChREBP transcriptional activity during cold stress. ChREBP activity seemed generally suppressed in liver and kidney. During torpor, ChREBP total protein levels decreased to 44% of EC in liver, phosphoserine levels increased 2.1-fold of EC in kidney, and downstream Fasn/Pkl transcript levels decreased to <60% of EC in liver. By contrast, ChREBP activity generally increased during torpor in cardiac and skeletal muscle, where ChREBP total protein levels increased over 1.5-fold and 5-fold of EC in muscle and heart, respectively; where DNA-binding increased by ∼2-fold of EC in muscle; and where Fasn transcript levels increased over 3-fold and 7-fold in both muscle and heart, respectively. In summary, ChREBP has a tissue-specific role in regulating energy metabolism during hibernation.