The effect of mild cold exposure on UCP3 mRNA expression and UCP3 protein content in humans

OBJECTIVE: In rodents, adaptive thermogenesis in response to cold exposure and high-fat feeding is accomplished by the activation of the brown adipose tissue specific mitochondrial uncoupling protein, UCP1. The recently discovered human uncoupling protein 3 is a possible candidate for adaptive thermogenesis in humans. In the present study we examined the effect of mild cold exposure on the mRNA and protein expression of UCP3. SUBJECTS: Ten healthy male volunteers (age 24.4 +/- 1.6 y; height 1.83 +/- 0.02 m; weight 77.3 +/- 3.0 kg; percentage body fat 19 +/- 2). DESIGN: Subjects stayed twice in the respiration chamber for 60 h (20.00-8.00 h); once at 22 degrees C (72 degrees F), and once at 16 degrees C (61 degrees F). After leaving the respiration chamber, muscle biopsies were taken and RT-competitive-PCR and Western blotting was used to measure UCP3 mRNA and protein expression respectively. RESULTS: Twenty-four-hour energy expenditure was significantly increased at 16 degrees C compared to 22 degrees C (P<0.05). At 16 degrees C, UCP3T (4.6 +/- 1.0 vs 7.7 +/- 1.5 amol/microg RNA, P=0.07), UCP3L (2.0 +/- 0.5 vs 3.5 +/- 0.9 amol/microg RNA, P=0.1) and UCP3S (2.6 +/- 0.6 vs 4.2 +/- 0.7 amol/microg RNA, P=0.07) mRNA expression tended to be lower compared with at 22 degrees C, whereas UCP3 protein content was, on average, not different. However, the individual differences in UCP3 protein content (16-22 degrees C) correlated positively with the differences in 24 h energy expenditure (r=0.86, P<0.05). CONCLUSION: The present study suggests that UCP3 protein content is related to energy metabolism in humans and might help in the metabolic adaptation to cold exposure. However, the down-regulation of UCP3 mRNA with mild cold exposure suggests that prolonged cold exposure will lead to lower UCP3 protein content. What the function of such down-regulation of UCP3 could be is presently unknown.