The discrepancy between morphological and functional activation of brown adipose tissue in the absence of functional Nrf2

Abstract

The precise regulatory mechanisms of interscapular brown adipose tissue (IBAT) thermogenesis are not fully elucidated, especially those in relation to Nuclear factor erythroid 2–related factor 2 (Nrf2). We have recently shown that mice lacking functional Nrf2 (Nrf2KO) at room temperature (RT) depict thermogenic IBAT activity at the structural level. However, gene and protein expression of the main IBAT thermogenic marker uncoupling protein 1 (UCP1), and other redox-metabolic functional parameters, did not align with this apparent thermogenic activation. Here, we investigated the functional activation of IBAT in wild-type (WT) and Nrf2KO mice maintained at RT (24±1°C) or after cold acclimation (45 days, 4±1°C). We performed immunogold labelling of Nrf2 and its closely related transcription factors Nrf1, Nrf3, and nuclear respiratory factor 1 (NRF1) to determine their spatial expression patterns. Next, we measured succinate-based mitochondrial respiration as an indicator of IBAT mitochondrial oxidative function and uncoupling capacity. Electron microscopy confirmed that Nrf2KO mice at RT had the same ultrastructural characteristics as cold-exposed mice, indicating IBAT thermogenic activity. This was associated with increased mitochondrial immunogold reaction of the above redox-sensitive transcription factors indicating their activation in Nrf2KO mice at RT. In contrast to such activated phenotype, respirometry results showed that Nrf2KO mice at RT had no uncoupling activity compared to cold-exposed WT mice. Furthermore, similar respiratory pattern was observed in Nrf2KO mice acclimated to cold, regardless of increased UCP1 expression. These results demonstrate that despite the (ultra)structural recruitment of IBAT in Nrf2KO mice at RT and after cold acclimation, Nrf2 is essential for the full functional thermogenic activation of IBAT and the lack of functional Nrf2 could not be compensated with Nrf1, Nrf3, and NRF1.