Abstract
Introduction
A severe burn elicits a systemic hypermetabolic response that substantially alters the function of multiple organs and contributes to increased morbidity and mortality. A consequence of hypermetabolism is the activation of UCP1-mediated browning of white adipose tissue (WAT), which may further facilitate the hypermetabolic response. In this study, we aimed to provide comprehensive characterization of the acute and long term pathophysiological responses to burns to determine the persistence of adipose tissue browning and its potential contribution to the hypermetabolic response.
Methods
Mice were subjected to either a 30% total body surface area (TBSA) scald burn or were denoted sham. Body weight and food intake were monitored throughout the duration of the study. Cohorts were sacrificed at 6hrs, 1, 3, 5, 7, 14, 30 and 60d post-burn and adipose tissue depots were harvested. Mitochondrial respiration, protein expression, and morphology in adipose tissues were assessed.
Results
Despite consuming considerably more food, the burn group lost significantly more weight throughout the duration of the study. We also detected increases in free fatty acids and interleukin-6, markers of whole-body lipolysis and inflammation, respectively. At the tissue level, eWAT mass significantly decreased over time, suggesting that this depot provides substrate to fuel the hypermetabolic response. This was further supported by a decrease in adipocyte area and an increase in lipolytic markers which remains significant up until 60d post-burn relative to sham. There were no significant difference in iWAT mass, however we detected significant increases in the protein content of UCP1, the master regulator of adipose tissue browning, as early as day 3 which persisted until day 60. This was corroborated by the presence of UCP1+ adipocytes.
Conclusions
Consistent with previous human studies, a burn injury elicits a dynamic response that cannot be simply characterized by a single timepoint. The alterations that occur in adipose tissue are depot-specific, time-dependent, and this notion likely extends to other metabolic tissues. Further, we demonstrate that in our 30% TBSA burn murine model, the effects of the hypermetabolic response persist for up to 60 days following initial injury.
Applicability of Research to Practice
Our data indicate the hypermetabolic response persists for up to 60 days, the equivalent of approximately 7 years in humans. This underscores the severity of adipose tissue browning and potentially provides an explanation as to how the hypermetabolic response persists even after the wound has healed. Moreover, providing a comprehensive map of the time-dependent changes in a murine model gives clinicians a better indication of the metabolic effects in a burn patient and will contribute to the development of effective, targeted treatments.
Influences of childhood exercise on adipose tissue browning in OLETF rats