HIV infection is associated with abnormal lipid metabolism, body fat redistribution, and altered energy expenditure. The pathogenesis of these complex abnormalities is unclear. Vpr, an HIV-1 accessory protein, can regulate gene transcription mediated by the glucocorticoid receptor and peroxisome proliferator activator receptor and affect mitochondrial function, in vitro. To test the hypothesis that expression of Vpr in liver and adipocytes can alter lipid metabolism in vivo, mice were engineered to express Vpr under control of the phosphoenolpyruvate carboxykinase promoter in a tissue-specific and inducible manner. These animals were used to investigate the effects of dietary fat, indinavir, and dexamethasone on energy metabolism and body composition. The transgenic mice expressed Vpr mRNA in white and brown adipose tissues and liver. An ultramicro immunoaffinity capillary electrophoresis immunoassay was developed to study the concentrations of Vpr in these animals and revealed that they had free Vpr protein in the plasma. Compared to wild-type (WT) animals, Vpr mice had lower plasma triglyceride levels after 6 weeks (P<0.05), but not after 10 weeks of high fat diet, and lower plasma cholesterol levels after 10 weeks of high fat diet (P<0.05). Treatment with dexamethasone obviated group differences, while indinavir had no significant independent effect on lipids. In the fasted state, Vpr mice had a higher respiratory quotient than WT mice (P<0.05). These data provide the first in vivo evidence that HIV-1 Vpr expressed at low levels in adipose tissues and liver can: a) circulate in the blood, b) regulate lipid and fatty acid metabolism, and c) alter fuel selection for oxidation in the fasted state. These studies are now being extended to human subjects with HIV infections and clinical changes in their fat metabolism.