Date of Award
obesity, adipocytes, KLF4, leptin, fat tissue, regulation of body weight
The increased white adipose tissue mass associated with obesity is the result of both hyperplasia and hypertrophy of adipocytes. While adipocyte development and transcriptional processes are well studied in vitro, regulation of in vivo genes (such as leptin), the identity of the adipocyte progenitor cells and the development of the adipose organ have not been defined invivo. In this thesis, firstly KLF4 was discovered to be an essential early regulator of adipogenesis. KLF4 together with Krox20 cooperatively transactivates C/EBPÎ², suggesting that KLF4 and Krox20 are part of an immediate early transcriptional network. This network is upregulated in a lipodystrophic animal model, encoding a dominant negative transgene against C/EBP factors, suggesting that these animals carry hyper-adipogenic residual fat pads, which provide a niche for transplantation experiments for identifying possible adipocyte progenitors. When injected into the residual fat pads of lipodystrophic mouse, indeed, a cell population, sorted from stromavascular fraction reconstitutes a functional white adipose tissue. Next, through a leptin-luciferase animal model, where luciferase is expressed only in adipocytes (see below), the location and timing of embryonic adipose development were determined. Identification of the location and timing led the way to study gene regulation and morphology of the developing adipose tissue in embryos. Leptin is an in vivo regulated adipocyte hormone, which is the afferent signal in a negative feedback loop controlling body weight and energy expenditure. Leptin is secreted in proportion to adipose tissue mass. This suggests the possibility that cellular lipid content is sensed and that a fuller understanding of the mechanisms of leptin production could lead to the delineation of a lipid sensing mechanism in fat cells and possibly other cell types. To discover this mechanism, we searched for the region/s in the leptin gene promoter that control the transcription of the leptin gene using a deletion series of BAC transgenic mouse lines that express luciferase under the control of overlapping leptin regulatory sequences. Cis elements that confer qualitative and quantitative control of the leptin gene are located between â€“ 762B and +8kb relative to the transcription start site. Since luciferase is driven by leptin regulatory sequences in leptin-luciferase animals and leptin levels are highly correlated with the amount of body fat, luciferase expression can be used as a surrogate for studying changes in the amount of adipose tissue. To study the responses of adipocytes to changes in weight, leptin-luciferase animals were used to show that weight loss induced by fasting or leptin treatment results in the retention of lipid-depleted adipocytes in adipose depots. This work led to the identification of a cellular program that controls the recovery of adipose fat mass after weight loss.
Birsoy, Kivanc, "Transcriptional Regulation Of Adipocyte Function" (2009). Student Theses and Dissertations. 106.