Effects of Genetic Background on Thermoregulation and Fatty Acid-induced Uncoupling of Mitochondria in UCP1-deficient Mice

Abstract

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...

UCP1 deficiency causes brown fat respiratory chain depletion and sensitizes mitochondria to calcium overload-induced dysfunction

“…Considering our findings, these discrepancies may be predictive of differential mitochondrial adaptation in different UCP1-KO mice colonies to mitigate ROS sensitivity owing to a genetic absence of UCP1. Such differences might be expected to arise on congenic (i.e., C57BL/6J and 129/SvImJ) backgrounds, which are particularly sensitive to the ablation of UCP1 (48) and thus may be prone to selection against enhanced ROS production, depending on breeding strategy. More generally, the functional effects that arise from distinct ROS sites in BAT on thermogenesis is an interesting avenue for future research.…”

Native UCP1 Displays Simple Competitive Kinetics between the Regulators Purine Nucleotides and Fatty Acids

“…Fatty acids can induce uncoupling in all mitochondria (14) and not just UCP1-containing brown-fat mitochondria, but the presence of UCP1 should nonetheless be expected to induce dramatic effects in the fatty acid sensitivity of brown-fat mitochondria. However, the reported effects of UCP1 on the fatty acid sensitivity of brown-fat mitochondria have been minor (12,15,16), making it difficult to reconcile the qualitative and dramatic effects of fatty acids on brown-fat cells and the suggested mediatory role of fatty acids in norepinephrine stimulation of the cells with the apparent relative insensitivity of isolated brown-fat mitochondria to fatty acids. For example, when fatty acid-induced de-energization was investigated, the reported difference between brown-fat mitochondria with and without UCP1 was only ϳ1.5-fold (but with a 50-fold difference between brown-fat mitochondria without UCP1 and liver mitochondria) (12,15,16).…”

“…However, the reported effects of UCP1 on the fatty acid sensitivity of brown-fat mitochondria have been minor (12,15,16), making it difficult to reconcile the qualitative and dramatic effects of fatty acids on brown-fat cells and the suggested mediatory role of fatty acids in norepinephrine stimulation of the cells with the apparent relative insensitivity of isolated brown-fat mitochondria to fatty acids. For example, when fatty acid-induced de-energization was investigated, the reported difference between brown-fat mitochondria with and without UCP1 was only ϳ1.5-fold (but with a 50-fold difference between brown-fat mitochondria without UCP1 and liver mitochondria) (12,15,16). Also, when thermogenesis was assessed as the fatty acid-induced increase in the rate of oxygen consumption, the total increase caused by fatty acid corresponded only to an increase of 25-75% compared with the 5-fold increase seen in brown-fat cells, and the difference in sensitivity to added fatty acids was also small (approximately a factor 2) (12,15,16).…”

“…For example, when fatty acid-induced de-energization was investigated, the reported difference between brown-fat mitochondria with and without UCP1 was only ϳ1.5-fold (but with a 50-fold difference between brown-fat mitochondria without UCP1 and liver mitochondria) (12,15,16). Also, when thermogenesis was assessed as the fatty acid-induced increase in the rate of oxygen consumption, the total increase caused by fatty acid corresponded only to an increase of 25-75% compared with the 5-fold increase seen in brown-fat cells, and the difference in sensitivity to added fatty acids was also small (approximately a factor 2) (12,15,16). Thus, the data so far published comparing brown-fat mitochondria with and without UCP1 are not adequate to explain the role of fatty acids as a regulator of UCP1.…”

UCP1 deficiency increases susceptibility to diet‐induced obesity with age

“…Ucp tm1 knockout mice on a congenic C57BL /6 J background were established by backcrossing and typing with microsatellite markers as described (Hofmann et al, 2001). The mice used in this study were N12-N15 generations.…”