A systems-level approach to understanding transcriptional regulation by p53 during mammalian hibernation

Abstract

Presumably to conserve energy, many mammals enter into hibernation during the winter. Homeostatic processes such as transcription and translation are virtually arrested. To further elucidate transcriptional regulation during hibernation, we studied the transcription factor p53. Here, we demonstrate that changes in liver mRNA and protein concentrations of known regulators of p53 are consistent with activation. p53 mRNA and protein concentrations are unrelated. Importantly, p53 protein concentration is increased ~2-fold during the interbout arousal that punctuates bouts of torpor. As a result, both the interbout arousal and the torpid state are characterized by high levels of nuclear-localized p53. Chromatin immunoprecipitation assays indicate that p53 binds DNA during the winter. Furthermore, p53 recruits RNA polymerase II, as indicated by nuclear run-on data. However, and consistent with previous data indicating an arrest of transcriptional elongation during torpor, p53 'activity' does not result in expected changes in target gene transcripts. These data demonstrate the importance of using a systems level-approach in understanding a complex phenotype such as mammalian hibernation. Relying on interpretations of data that are based on steady-state regulation in other systems may be misleading in the context of non-steady-state conditions such as torpor. KEY WORDS: Torpor, Nuclear run-on, Metabolic depression INTRODUCTIONDuring the winter, hibernating mammals such as ground squirrels oscillate between bouts of metabolic depression (torpor), wherein core body temperature may be below −2°C, and euthermic periods (interbout arousals), wherein body temperature approaches euthermic values (~36°C) (van Breukelen and Martin, 2002; Carey et al., 2003). Oxygen consumption during torpor may be as low as 1/100th of the active rate (Carey et al., 2003). Hibernation represents a unique metabolic constraint. Hibernators must balance a need to conserve energy with maintenance of cellular structure. Many energetically expensive physiological processes normally vital to homeostasis are dramatically reduced during torpor. Processes such as transcription and translation are not sustainable given the severe metabolic depression. Earlier, we demonstrated marked depressions of both hepatic transcription and translation (van Breukelen and Martin, 2001;van Breukelen and Martin, 2002). Nuclear run-on data from hibernating golden-mantled ground squirrels demonstrate a moderate inhibition of transcriptional initiation (~50%), but indicate RESEARCH ARTICLESchool of Life Sciences, University of Nevada, Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154, USA. that elongation of transcripts is essentially arrested during torpor (van Breukelen and Martin, 2002). Translational regulation during hibernation is more elaborate. Polysome analyses reveal that the bulk of initiation is depressed as animals enter into torpor (van Breukelen and Martin, 2001). 4E-binding protein 1, a potent regulator of the cap binding protein, eIF4E, is absent...

Hibernation impacts lysine methylation dynamics in the 13‐lined ground squirrel, Ictidomys tridecemlineatus

“…Increased methylation of p53K372me interferes with SMYD2‐mediated methylation of K370 leading to reduced levels of p53K370me2 (Huang et al, , ). Increased regulation of p53 activity during IA in 13LGS liver has been reported previously (Hefler, Wu, & Storey, ; Pan, Treat, & van Breukelen, ), and differential methylation patterns of p53, as seen here during torpor and arousal, would seem to agree with these studies in that stabilized binding of p53 to chromosomes during IA would be promoted and poise the system for enhanced transcriptional activity while squirrels are euthermic during interbout arousals.…”

Effects of hibernation on bone marrow transcriptome in thirteen-lined ground squirrels

“…Thus it is plausible that decreased p53 ubiquitination leads to increased p53-regulated transcription of p21 and arrest of the cell cycle in bone marrow during hibernation. A similar process has been reported in golden-mantled ground squirrel livers during torpor and IBA (68).…”

Waking the sleeping dragon: gene expression profiling reveals adaptive strategies of the hibernating reptile Pogona vitticeps

“…Recently, the use of steady state abundances of mRNA and proteins during hibernation has been scrutinized given that hibernation is a non-steady state condition. Some protiens do not function during hibernation as they do in steady state conditions, notably regulation of transcription by p53 [32]. However, for this study, we assume functional equivalence of biological processes during hibernation and after arousal.…”