FATE OF THE ADIPOCYTE

新生哺乳动物的脂肪主要是棕色脂肪

The fat in the newborn mammal is principally brown fat in order to satisfy the acute requirement for heat which occurs at birth. As the animal ages, brown fat tends to disappear to varying degrees, depending on the species, while white fat concurrently increases. This normal developmental process has been drastically altered in a transgenic mouse that overexpresses the structural gene for glycerol 3-phosphate dehydrogenase (Gdc-1). In this transgenic mouse brown fat does not involute while white fat growth is arrested; overall the total fat of the animal has been reduced. The effect of the Gdc-1 transgene is most evident in db/db mice which normally can have 25 gm of body fat, but when carrying the transgene the body fat can be virtually undetectable. The specific aims of this proposal seek to determine the mechanism by which overexpression of Gdc-1 controls the deposition of body fat. The working hypothesis is that the high levels of Gdc-1 expression have formed a futile ATPase which expends excess energy in the animal. This hypothesis will be tested by first establishing that increased thermogenesis occurs in Gdc-1 transgenic mice - by indirect calorimetry in whole animals and by measurements of respiration in cell cultures in vivo. We will then analyze intermediates and products of glycerol metabolism and carbon flux through pathways which are affected by over-expression of Gdc-1 in order to identify a mechanism for the futile ATPase. Assuming that the data is confirmatory, the second specific aim will characterize a glycerol 3- phosphate phosphatase which we hypothesize to be a component of a futile ATPase. The third specific aim will breed new congenic lines of mice carrying the Gdc-1 transgenes in combination with the obesity mutants db and ob so that the effects of the transgene on obesity can be analyzed in controlled genetic backgrounds; in addition, new transgenic lines will be created in which the Gdc-1 transgene expression is restricted to specific tissues and under hormonal control, and in which other genes of glycerol metabolism will be over-expressed. In the final specific aim we will characterize in the brown fat of transgenic mice the expression of mitochondrial mRNA and proteins which are important to thermogenesis.