An interaction between free fatty acids and UCP1 (uncoupling protein-1) leading to de-energization of mitochondria was assumed to be a key event for triggering heat production in brown fat. Recently, Matthias et al., finding indistinguishable de-energization of isolated brown fat mitochondria by fatty acids in UCP1-deficient mice and control mice, challenged this assumption (Matthias, A., Jacobsson, A., Cannon, B., and Nedergaard, J. (1999) J. Biol. Chem. 274, 28150 -28160). Since their results were obtained using UCP1-deficient and control mice on an undefined genetic background, we wanted to determine unambiguously the phenotype of UCP1 deficiency with the targeted Ucp1 allele on congenic C57BL/6J and 129/SvImJ backgrounds. UCP1-deficient congenic mice have a very pronounced cold-sensitive phenotype; however, deficient mice on the F1 hybrid background were resistant to cold. We propose that heterosis provides a mechanism to compensate for UCP1 deficiency. Contrary to the results of Matthias et al., we found a significant loss of fatty acid-induced de-energization, as reflected by membrane potential and oxygen consumption, in brown fat mitochondria from UCP1-deficient mice. Unlike cold sensitivity, fatty acid-induced uncoupling of mitochondria was independent of the genetic background of UCP1-deficient mice. We propose that intracellular free fatty acids directly regulate uncoupling activity of UCP1 in a manner consistent with models described in the literature. Brown adipose tissue (BAT)1 plays an important role in heat production and is considered to contribute to energy balance (5, 6). Stimulation by the sympathetic nervous system causes an up-regulation in the metabolic rate of BAT that is reflected in an increase of heat production (7,8). The ability to generate heat is attributed to the high number of mitochondria containing UCP1 (uncoupling protein-1) (9). This transmembrane protein is thought to be an ion carrier that uncouples respiration from ATP synthesis, allowing mitochondria to produce heat. The importance of UCP1 for thermogenesis was proven by the observation that UCP1-deficient mice are cold-sensitive (10). However, the fact that some UCP1-deficient mice are resistant to cold and that adiposity is not increased led us to postulate that additional thermogenic mechanisms can compensate for UCP1 deficiency. In this respect, UCP1-deficient mice can help not only to understand the mechanisms that control BAT thermogenesis, but can also be used to identify alternative pathways for heat production.Although it is known that the sympathetic nervous system controls heat production of brown adipocytes (11, 12), the intracellular signaling pathway remains unclear. It has been proposed that free fatty acids (FFAs), released by the action of hormone-sensitive lipase, serve both as an energy substrate and as an activator of the proton carrier function of UCP1, thereby triggering heat production. It has also been shown that FFAs can increase respiration of isolated brown adipocytes (13) and that isolated BAT m...

To identify novel regulatory factors controlling induction of the brown adipocyte-specific mitochondrial uncoupling protein (Ucp1) mRNA in the retroperitoneal white fat depot, we previously mapped quantitative trait loci (QTLs) that control this trait to chromosomes 2, 3, 8, and 19. Since the peroxisome proliferator activator receptor-gamma coactivator-1alpha (PGC-1alpha) regulates Ucp1 and other genes of energy metabolism, we have evaluated whether the QTLs controlling Ucp1 mRNA levels also modulate Pgc-1alpha mRNA levels by analysis of backcross progeny from the A/J and C57BL/6J strains of mice. The results indicate that a locus on chromosome 3 orchestrates expression of Pgc-1alpha and Ucp1 in retroperitoneal fat of mice fed a low-fat diet; however, the effect of this locus on Pgc-1alpha is lost, and a significant correlation between Ucp1 and Pgc-1alpha is severely reduced in mice fed a high-fat diet. An additional QTL located on chromosome 5 has also been identified for the selective regulation of Ucp1 mRNA levels. Similar to the effects of a high-fat diet on the chromosome 3 QTL, linkage of the chromosome 5 QTL is also lost in mice on a high-fat diet. Thus dietary fat has a profound influence on PGC-1alpha-regulated pathways controlling energy metabolism in white fat. The allelic variation observed in the regulation of Ucp1 and Pgc-1alpha expression in brown adipocytes of white fat but not interscapular brown fat suggests that fundamentally different regulatory mechanisms exist to control the thermogenic capacities of these tissues.