Reply to The Letter to The Editor: Adipocyte Browning in Response to Trauma: Some Important Methodological Considerations

To the Editor:

Thank you Dr Porter for taking the time to thoughtfully read through our study and publication. While we agree that state 3 respiration is not a measure of UCP1-dependent thermogenesis, we use succinate as a driver of mitochondrial respiration as it initiates a robust response via Seahorse analysis, and it supports our measurements of mitochondrial biogenesis and UCP-1 protein. Indeed, UCP-1 function would be better measured directly, however, it is not justified to say succinate cannot drive thermogenesis in BAT as there is evidence to the contrary (1).

As is widely known, BAT lacks ATP synthase, as evidenced by the poor response to oligomycin in our studies. However, oxygen consumption still increases when given succinate as a fuel source, further supporting the notion that succinate can drive thermogenesis. Moreover, metabolic caging and UCP-1 KO mice were not used here, so we cannot conclude that UCP-1-mediated thermogenesis itself is a contributor to hypermetabolism. However, our group has recently demonstrated that UCP-1 KO mice are protected from systemic complications such as burn-induced browning and hepatic steatosis (2). Rather, the purpose of this article was to highlight that alterations in adipose physiology post-burn are chronic and associated with the systemic dysfunction seen in burn patients. For example, we detected long-term increases in circulating IL-6 and liver mass, and significant decreases in body weight despite an increase in food consumption. We believe these data are valuable, highlighting the role of adipose tissue in chronic systemic complications following trauma, regardless of the functionality of UCP-1. Indeed, this is why we complimented our UCP-1 expression with measures of lipolysis, histology and systemic circulating factors.

We agree our studies are inconsistent with those of Dr Fischman’s group, and we addressed this discrepancy in our discussion. Indeed, consistency in murine burn studies appears to be a widespread problem. For instance, one group reported that a severe burn induces a decrease in hepatic mitochondrial respiratory capacity at 24 h after the injury (3), which was subsequently followed by a report that burn injury increases hepatic mitochondrial respiratory capacity (4). Furthermore, while cold stress provokes a substantial increase in BAT thermogenesis in murine models, we find it controversial to assume that a severe burn injury promotes an identical response with regards to WAT and BAT physiology due to differences in skeletal muscle responses (i.e., Colds stress leads to increase shivering while burn injury leads to muscle atrophy), systemic inflammation, etc. For example, Qing et al. demonstrated that endotoxemia-induced increases in β₃-adrenergic receptor activity leads to BAT IL-6 production in the absence of any increases in the thermogenic program (5). This suggests that BAT may have multiple roles beyond heat generation depending on the external stimuli that may not necessarily involve UCP-1 induction. Moreover, a recent report by Dr Porter’s group demonstrated that UCP-1 function in BAT following a severe burn is only elevated at 24 h post-burn (6). While we do not doubt the validity of these findings despite our contradictory results, given that hypermetabolism is a delayed yet long-term response of severe burns, it calls into question the physiological significance of this acute increase in activity. However, one could argue that the true contribution of burn-induced increases in UCP-1 in either WAT or BAT has still not yet been elucidated as all previous studies have been conducted at sub-thermoneutral temperatures that poorly mimic burn patient conditions, which we have highlighted as a caveat in our paper. Overall, many questions remain to be explored with regards to WAT and BAT physiology following a sever burn.

We also find it interesting that Dr Porter believes WAT UCP-1 plays a minimal role, as it contradicts the stance from a previous manuscript by the same group (7) where they reported:

“Mitochondrial respiration uncoupled from ATP production in UCP1 positive mitochondria increases metabolic rate by converting energy stored in glucose and fatty acids to heat. Our data suggest a 2- to 3-fold increase in sWAT thermogenic capacity following burn trauma. Since sWAT is thought to account for ~4.5% of REE in healthy individuals (8), we speculate that sWAT may account for up to 13.5% of REE in burn survivors. In the current patient cohort, total REE increased from ~1,400 to ~1,950 kcal/day, which represents a 550 kcal/day (40%) increase. In the early bun group, 4.5% of 1,400 kcal is ~60 kcal; in the late burn 13.5% of 1,950 kcal equals 263 kcal.”

To that end, we stand by the conclusions of our manuscript, which is that

1. burn induces a heterogeneous impact on adipose tissue depots and

2. that the role played by adipose tissue in systemic dysfunction post-burn is crucial and often overlooked.

Further studies focusing on the activation of BAT thermogenesis mechanistically will be key to delineate whether this phenomenon is a contributor burn-induced sequelae.