Obesity is a major health concern in the United States and abroad. Recent estimates within the US classify more than 30% of the adult population as obese (BMI >30). Since obesity increases the health risks for other associated metabolic and non-metabolic disorders, the increasing obesity epidemic is a major concern for public health. The brain functions as an organized and integrative center in the control of body weight, regulating both food intake and energy expenditure. Leptin is a hormone secreted from the peripheral adipose tissue that acts primarily in the brain to suppress food intake and increase energy expenditure. Mice with a mutation in the gene encoding leptin (ob/ob) are obese and diabetic. Protein tyrosine phosphatase 1B (PTP1B) has been shown to negatively regulate leptin signaling, and whole body and brain-specific PTP1B -/- mice are resistant to diet-induced obesity and are leptin hyper-sensitive. Interestingly, leptin and PTP1B double knockouts (PTP1B-/- ob/ob) mice are leaner than ob/ob single knockouts, suggesting that PTP1B may regulate additional non-leptin signaling pathways which regulate body weight and metabolism. Thus, whether PTP1B requires intact leptin signaling for its metabolic effects remains unclear. Other non-leptin circulating factors have been implicated in the control of energy homeostasis. Interleukin-6 (IL-6) is a circulating cytokine that, like leptin, is found at levels which correlate with adipose mass. Central injection of low dose IL-6 was shown to increase energy expenditure in rats, and chronic daily injections of IL-6 suppressed body weight. IL- 6 has been shown to signal through an intracellular pathway similar to that of leptin. This pathway involves downstream activity by Janus kinase 2 (JAK2), a known substrate of PTP1B. The goal of this proposal is to train the fellowship applicant in a variety of molecular biology techniques, mouse genetics, and in vivo physiology in order to test the hypothesis that PTP1B's metabolic effects are mediated by both leptin- dependent and leptin-independent signaling. Additionally, this proposal aims to test the hypothesis that PTP1B regulates IL-6 signaling.
The first aim i s to determine whether PTP1B regulates energy balance through a leptin receptor-dependent or -independent manner in the CNS. This will be done with mouse genetics by generating CNS-specific leptin receptor and PTP1B double mutants and comparing their metabolic phenotypes with those of single mutants and wild type controls.
The second aim i s to determine whether PTP1B regulates IL-6 signaling involved in the central control of metabolism. PTP1B expression will be knocked down in cultured cells via viral-mediated shRNA, and activation of downstream effectors of IL-6 (pJAK, pSTAT) will be measured. Additionally, IL-6 sensitivity in vivo will be assessed based upon the in vitro results.
The proposed work is basic research designed to elucidate the role of protein tyrosine phosphatase 1B (PTP1B) in the central control of energy homeostasis. Understanding the function and pathways of PTP1B in the brain may shed light on new targets and novel pathways for treating metabolic disorders such as obesity and type 2 diabetes.