DESCRIPTION Energy homeostasis is maintained through complex feedback loops integrated via circulating humoral factors such as insulin, glucagon, glucocorticoids and leptin. The breakdown of this system results in a plethora of health risks causally related to obesity including diabetes, cardiovascular disease and certain types of cancer. Leptin expression is dynamically regulated at the level of transcription in response to overall changes in adipose tissue levels and feeding state. The primary objective of this proposal is to identify and characterize novel humoral factors that regulate expression of the mouse obese (ob) gene, encoding for the hormone leptin. A highly sensitive and biologically sound cell- based assay will e developed for the detection of ob expression utilizing luciferase as a reporter gene. This assay will be used to screen extracts of calf and fetal bovine serum for small organic compounds and novel polypeptide hormones that induce or repress expression of the ob gene. Humoral factors will be purified and characterized by standard methods. Once identified, the levels of these factors in human, mouse, and rat serum will be measured as a function of general nutrition and feeding state. Finally, an effort will be made to determine if the effects of these factors are mediated via transcription factors known to regulate leptin expression.