The role of UCP3 in brown adipose tissue mitochondrial bioenergetics is complementary to that of UCP1

Mitochondrial uncoupling protein 3 (UCP3) plays a complementary role in uncoupling protein 1 (UCP1)-dependent uncoupling in mammalian thermogenesis. Using mouse strains that lack UCP1 or UCP3, plus a UCP1/UCP3 double knockout, we have previously demonstrated that UCP3 is necessary for sustained heat production. However, how and where UCP3 is essential for heat production remains unknown. Here we use brown adipocyte mitochondria from mice lacking UCP1, UCP3, or UCP1 plus UCP3 (double knockout) we found that in the absence of UCP1, UCP3 does not act as a free fatty acid-inducible uncoupling protein; however, UCP3 is necessary for maximal GDP-sensitive respiration. We additionally confirm that the loss of UCP1 is a dominant regulator of mitochondrial respiratory chain activity, assembly, and composition in brown adipose tissue. Our data suggest without UCP1, brown adipose tissue is not a significant source of heat production, and that UCP3-dependent heat generation serves a regulatory function outside of adipose tissues. Together these findings suggest that in brown adipose tissue, UCP3s thermogenic function is independent of its role as a bona fide uncoupling protein.