Costford SR, Chaudhry SN, Crawford SA, Salkhordeh M, Harper ME. Long-term high-fat feeding induces greater fat storage in mice lacking UCP3. Am J Physiol Endocrinol Metab 295: E1018-E1024, 2008. First published August 19, 2008 doi:10.1152/ajpendo.00779.2007.-Uncoupling protein-3 (UCP3) is a mitochondrial inner-membrane protein highly expressed in skeletal muscle. While UCP3's function is still unknown, it has been hypothesized to act as a fatty acid (FA) anion exporter, protecting mitochondria against lipid peroxidation and/or facilitating FA oxidation. The aim of this study was to determine the effects of long-term feeding of a 45% fat diet on whole body indicators of muscle metabolism in congenic C57BL/6 mice that were either lacking UCP3 (Ucp3 Ϫ/Ϫ ) or had a transgenically induced approximately twofold increase in UCP3 levels (UCP3tg). Mice were fed the high-fat (HF) diet for a period of either 4 or 8 mo immediately following weaning. After long-term HF feeding, UCP3tg mice weighed an average of 15% less than wild-type mice (P Ͻ 0.05) and were 20% less metabolically efficient than both wild-type and Ucp3 Ϫ/Ϫ mice (P Ͻ 0.01). Additionally, wild-type mice had 21% lower, whereas UCP3tg mice had 36% lower, levels of adiposity compared with Ucp3 Ϫ/Ϫ mice (P Ͻ 0.05 and P Ͻ 0.001, respectively), indicating a protective effect of UCP3 against fat gain. No differences in whole body oxygen consumption were detected following long-term HF feeding. Glucose and insulin tolerance tests revealed that both the UCP3tg and Ucp3 Ϫ/Ϫ mice were more glucose tolerant and insulin sensitive compared with wild-type mice after short-term HF feeding, but this protection was not maintained in the long term. Findings indicate that UCP3 is involved in protection from fat gain induced by long-term HF feeding, but not in protection from insulin resistance. uncoupling protein-3; obesity; insulin resistance; type 2 diabetes mellitus; skeletal muscle BECAUSE OF ITS HIGH SEQUENCE HOMOLOGY to the original uncoupling protein [uncoupling protein-1 (UCP1)] expressed in brown adipose tissue (BAT), UCP3 was first hypothesized to dissipate the proton gradient across the mitochondrial inner membrane in skeletal muscle, thereby uncoupling oxidative phosphorylation and releasing energy as heat. Since its discovery in 1997, several studies have shown that UCP3 is likely not responsible for basal proton leak in muscle (1, 6). In fact, the adenine nucleotide translocator is probably responsible for the majority of proton leak in this tissue (5). The novel uncoupling proteins (UCPs 2-5) have been studied over the last decade; however, their physiological functions are still unclear. UCP3 is expressed highly selectively in skeletal muscle, BAT, and at low levels in the heart (3, 35). Early studies of UCP3 overexpression in experimental systems ranging from yeast to mice were confounded by supraphysiological levels of expression (9,14,16,31,39). These high levels of protein expressed on the mitochondrial inner membrane induced artifactual proton leak (14, 31)...