Potent anorexic and lipopenic effects of central leptin gene therapy are blocked by diet-induced obesity: Evidence for impaired leptin receptor expression/signal transduction in obesity and reversal by caloric restriction

Obesity is estimated to result in 300,000 deaths each year in the United States alone. The purpose of this dissertation was to study the molecular basis of obesity. Specifically, we studied how the brain's response to the hormone “leptin” is impaired in obese animals.; First, we demonstrated that young adult F344XBN rats are highly responsive to a single intracerebroventricular injection of adeno-associated virus encoding leptin (rAAV-leptin). Within 1 week of leptin gene delivery, animals exhibited a significant anorectic response without the concomitant decrease in resting energy expenditure that is normally recorded during anorexia. This anorectic response persisted for more than two months. As a result of this negative shift in energy balance, rAAV-leptin treated animals lost significant weight with respect to controls. More impressively, there was a near complete disappearance of visceral white fat in these animals.; We then incorporated regulation into rAAV-leptin by placing leptin under the control of the tetracycline transactivator (rtTA) and operon (tetR). Using this system, we showed that all of the physiological and biochemical responses to leptin gene therapy are completely reversible. We also demonstrated that central leptin may negatively regulate the expression of leptin receptor in the hypothalamus, and that this is reversed upon transgene inactivation. This was the first ever experiment to use an externally regulated transgene to control energy balance.; Next, we showed that diet-induced obese (DIO) animals are non-responsive to the anorectic, lipopenic, and biochemical effects of rAAV-leptin whereas high-fat fed animals that did not become obese (diet resistant) retained leptin responsiveness. It was found that the leptin resistance in the DIO animals was associated with reduced hypothalamic leptin receptor expression along with an absence of signal transduction response to rAAV-leptin. DIO animals were then tested with an acute supramaximal bolus of i.c.v. leptin, and were found to have reduced leptin receptor signal transduction capacity compared to non-obese controls. Finally, the deficits in leptin receptor expression and signaling capacity in DIO were completely reversed by 30 days of caloric restriction. Thus, short-term calorie restriction may be a viable strategy to restore leptin sensitivity in previously leptin-resistant models of acquired obesity.