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Pennington Biomedical Research Center, Louisiana State University, Baton Rouge, Louisiana 70808
Address all correspondence and requests for reprints to: Leslie P. Kozak, Pennington Biomedical Research Center, 6400 Perkins Road, Baton Rouge, Louisiana 70808. E-mail: kozaklp{at}pbrc.edu.
We investigated the role of leptin in regulating energy metabolism through induction of uncoupling protein (UCP)-1-based brown fat thermogenesis by comparing phenotypes of energy balance in ob/ob and double-mutant ob/ob.Ucp1–/– mice. Measurements of adiposity and lean body mass (nuclear magnetic resonance), energy expenditure (indirect calorimetry), body weight, food intake, and core body temperature were determined in the two mutant stocks of 3-month-old mice maintained at an initial ambient temperature of 28 C for 21 d and then at 21 C for 16 d, and finally with leptin administration for 8 d at 21 C. No phenotypic differences between ob/ob and ob/ob.Ucp1–/– mice were detected, suggesting that UCP1-based thermogenesis is not essential for the regulation of adiposity in ob/ob mice at temperatures between 21 and 28 C. Although both Ucp1–/– and ob/ob mice can survive in extreme cold at 4 C, provided they are adapted to the cold by gradually lowering ambient temperature, ob/ob.Ucp1–/– mice could not adapt and survive at temperatures lower than 12 C unless they were administered leptin. As the ambient temperature was reduced from 20 to 16 C, ob/ob.Ucp1–/– mice treated with leptin have elevated levels of circulating T3 that correlate with elevated sarcoendoplasmic reticulum Ca2+ ATPase 2a mRNA levels in gastrocnemius muscle. Furthermore, ob/ob.Ucp1–/– mice, treated with T3, were able to maintain body temperature and stimulate sarcoendoplasmic reticulum Ca2+ ATPase 2a expression when the ambient temperature was gradually reduced to 4 C. Thus, in the absence of UCP1, leptin-induced thermogenesis protects body temperature in part through its action on the thyroid hormone axis.
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