Two key temporally distinguishable molecular and cellular components of white adipose tissue browning during cold acclimation

Conversion of white into brown adipose tissue may have important implications in obesity resistance and treatment. Several browning agents or conditions ignite thermogenesis in white adipose tissue (WAT). To reveal the capacity of WAT to function in a brownish/burning mode over the long term, we investigated the progression of the rat retroperitoneal WAT (rpWAT) browning during 45days of cold acclimation. During the early stages of cold acclimation, the majority of rpWAT adipocytes underwent multilocularization and thermogenic-profile induction, as demonstrated by the presence of a multitude of uncoupling protein 1 (UCP1)-immunopositive paucilocular adipocytes containing peroxisome proliferator-activated receptor (PPAR) coactivator-1 (PGC-1) and PR domain-containing 16 (PRDM16) in their nuclei. After 45days, all adipocytes remained PRDM16 immunopositive, but only a few multilocular adipocytes rich in mitochondria remained UCP1/PGC-1 immunopositive. Molecular evidence showed that thermogenic recruitment of rpWAT occurred following cold exposure, but returned to starting levels after cold acclimation. Compared with controls (22 +/- 1 degrees C), levels of UCP1 mRNA increased in parallel with PPAR (PPAR from days 1 to 7 and PGC-1 on day 1). Transcriptional recruitment of rpWAT was followed by an increase in UCP1 protein content (from days 1 to 21). Results clearly showed that most of the adipocytes within rpWAT underwent transient brown-fat-like thermogenic recruitment upon stimulation, but only a minority of cells retained a brown adipose tissue-like phenotype after the attainment of cold acclimation. Therefore, browning of WAT is dependent on both maintaining the thermogenic response and retaining enough brown-like thermogenically competent adipocytes in the long-term. Both aspects of browning could be important for long-term energy homeostasis and body-weight regulation.