METABOLIC ARREST DURING HIBERNATION! CARDIAC REGULATION OF PYRUVATE DEHYDROGENASE (PDH) IN A HIBERNATING GROUND SQUIRREL (ICTIDOMYS TRIDECEMLINEATUS)

Abstract

Many mammals use hibernation to survive freezing temperatures and lack of food during winter months by cycling through periods of torpor-arousal. Hibernating 13-line ground squirrels (Ictidomys tridecemlineatus) native to the North American prairies utilize variety of physiological adaptations to survive the harsh winter conditions. Pyruvate dehydrogenase (PDH) is a vital regulatory enzyme that links glycolysis and tricarboxylic acid cycle. Inhibition of PDH activity via three serine phosphorylation sites (p-S293, p-S300, and p-S232) regulates the metabolic flux through the TCA cycle, decrease glucose utilization, facilitate lipid metabolism, and overall provide metabolic adaptation to nutrient availability. The purpose of this study was to explore the post-translational regulation of pyruvate dehydrogenase mediated by mitochondrial serine/threonine kinases, PDHKs (1-4) during topor-arousal cycle in cardiac tissues of 13-line ground squirrels. Liver and muscle tissues were also included in the study for multi-tissue comparison purpose. A combination of Luminex Multiplex Assay and western immunoblotting were used to measure the protein expression levels of total PDH, three phospho-PDH modifications, and four PDH kinases in euthermic control, entrance, late torpor, and interbout arousal cycles (n=4, p<0.05). Heart showed significant 2-3 fold upregulation in PDH-pS293 and PDH-pS300 levels during torpor compared to the euthermic-control, indicating a possible shut down of PDH activity during hibernation. Subsequently, cardiac PDHK4 (with high affinity for pS293 and pS300) showed a 2-fold increase during torpor. Liver showed 30-fold increase in PDH-pS300 during torpor with PDHK1 showing 2-fold increase during torpor and arousal. As expected skeletal muscle showed limited PDH regulation during torpor-arousal cycle. Overall, heart and liver showed strong inhibitory PDH regulation, indicating a possible decrease in glucose utilization through glycolysis and an increase in β-oxidation of lipids during times of extended starvation in 13-line ground squirrels. This study provides us with greater insight into the biochemical mechanisms controlling the teeter-totter between glucose oxidation and β-oxidation of fatty acids, a central theme behind recent cardiac hypertension and type-II diabetes studies.