Modulation of apoptotic pathways in intestinal mucosa during hibernation

Mammalian hibernation is associated with several events that can affect programmed cell death (apoptosis) in nonhibernators, including marked changes in blood flow, extended fasting, and oxidative stress. However, the effect of hibernation on apoptosis is poorly understood. Here, we investigated apoptosis and expression of proteins involved in apoptotic pathways in intestinal mucosa of summer and hibernating ground squirrels. We used terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) to identify possible apoptotic enterocytes in small intestine of summer squirrels and hibernating squirrels throughout the winter. Nuclear TUNEL staining increased as hibernation progressed, but the staining pattern was diffuse and not accompanied by chromatin condensation or apoptotic bodies. Electrophoresis of mucosal DNA revealed no ladders typical of apoptosis. Nuclear levels of proapoptotic p53 protein were fourfold less in hibernators compared with summer squirrels. A 12-fold increase in anti-apoptotic Bcl-x(L) compared with a 2-fold increase in proapoptotic Bax suggested a balance in favor of antiapoptotic signaling in hibernators. There was no change in Bcl-2 protein expression but phospho-Bcl-2 increased in mucosa of hibernators. Hibernation had minimal effects on expression of active caspase-8 or -9, whereas caspase-3-specific activity was lower in hibernators during an interbout arousal compared with summer squirrels. Expression of the prosurvival protein Akt increased 20-fold during hibernation, but phospho-Akt was not altered. These data provide evidence for enhanced expression of antiapoptotic proteins during hibernation that may promote enterocyte survival in a pro-oxidative, proapoptotic environment.