Silvia Q. Giraudo, Ph.D.
Department of Food Science and Nutrition, University of Minnesota
The hypothesis is that the a-MSH/MC4-R signal in the hypothalamic paraventricular nucleus (PVN) interacts with neuropeptide Y (NPY) in a neural network regulating feeding behavior. The studies proposed herein will directly determine the brain sites of action for a-MSH/MC4-R and explore the interaction with other brain signals participating in the food/energy regulatory circuitry. Each set of experiments will employ MC4-R agonists, a-MSH and MT II, and MC4-R antagonists, agouti-related protein (Agrp) and SHU9119.
The first objective is to define neural sites activated in response to an injection of a MC4-R agonist or antagonist to answer the following questions:
What neural sites are activated after single injection of a-MSH or MT II (MC4-R agonists) in the PVN, as determined by cFos-immunoreactivity?
What neural sites are activated after a single injection of Agrp or SHU9119 (MC4-R antagonists) in the PVN, as determined by cFos-immunoreactivity?
Once the areas of the brain that are activated/inhibited after injection of a-MSH, MT II, Agrp or SHU9119 in the PVN are determined (Part I), feeding studies with double cannulated rats will be performed to determine the neuronal melanocortin pathways and the relationship to other feeding-related regulators present in the same brain areas (Part II).
Ann Vogel Hertzel, Ph.D.
Department of Biochemistry, Molecular Biology, and Biophysics
University of Minnesota
Obesity is a disorder with multiple etiologies, creating a disruption of energy balance in which energy intake exceeds expenditure. This results in many complications including insulin resistance and type II diabetes (about 90% of type II diabetics are overweight or obese). The excess energy is stored as triglycerides in adipocytes and is mobilized as fatty acids to other cells as required. Fatty acid-binding proteins (FABP) are small cytoplasmic proteins which bind and presumably transport fatty acids within a cell thereby affecting the steady state levels of lipids. Recently, in an attempt to understand the function of FABPs in adipocytes, a targeted disruption of the murine adipocyte lipid-binding protein (ALBP/aP2) gene was created (Hotamisligil G.S., et al. Science 274:1377-9, 1996). Although the ALBP/aP2 deficient mice become obese on a high fat diet, they do not become insulin resistant like the wild type mice. Therefore, the ALBP/aP2 disrupted mice have lost the connection that leads from obesity to insulin resistance.
Due to the complete lack of ALBP/aP2, the adipocytes compensate by increasing the expression of the keratinocyte lipid-binding protein (KLBP) and its mRNA 14- and 40-fold, respectively. In order to begin to understand the physiological alterations that result in the maintenance of insulin sensitivity irrespective of obesity, it is necessary to determine whether the loss of ALBP/aP2 or the gain of KLBP is responsible for the differences in these mice. To study this, transgenic mice will be created that express KLBP under the control of the strong adipocyte specific ALBP/aP2 promoter in both wild type (wt+K) and ALBP/aP2 disrupted (A-/-+K) mice. This will allow testing whether the differences (including glucose and insulin levels in obese mice) seen in the ALBP/aP2 disrupted mice are due to a gain of function (through increased KLBP) or a loss of function (through loss of ALBP/aP2). If a loss of function is responsible for the differences, then the wt+K transgenic mice that express both FABPs will remain like wild type and show obesity induced insulin resistance.
However, if the differences are due to a gain of function, then the wt+K transgenic mice that express both ALBP and KLBP will show the differences seen in the ALBP/aP2 deficient mice, including maintenance of insulin sensitivity. To investigate whether this same phenomenon occurs in humans, the levels of ALBP/aP2 and KLBP in adipocytes of obese humans will be measured to see if there is a correlation (positive for ALBP/aP2 and/or negative for KLBP) between the levels of these FABPs and obesity-induced insulin resistance.
Intramuscular triglyceride (imTG) is an important energy source for skeletal muscle and is abnormally high in obesity, type I and type II diabetes. Increased imTG content is associated with impaired insulin-stimulated skeletal muscle glucose uptake (insulin resistance), and correlated with other insulin resistance indices such as decreased muscle glycogen synthesis and high waist to hip ratios. Thus, a greater imTG store in skeletal muscle is related to some major metabolic abnormalities. Among them, impairment of the ability for skeletal muscle to utilize glucose is especially a health concern because it contributes to development of hyperglycemia and may play roles in transition from obesity to type II diabetes.
The pathway(s) responsible for the increase in imTG store is unknown, nor has the link between increase imTG store and skeletal insulin resistance been established. A further understanding of the roles of insulin in imTG metabolism is needed before the mechanism can be identified. Synthetic and hydrolytic kinetics of imTG with or without insulin infusion in normal and obese rat models will be measured to determine whether and how insulin plays roles in these processes.
This project will test whether:
insulin stimulates imTG synthesis and inhibits imTG hydrolysis in normal and obese rats;
imTG synthesis rate is higher in obese rats than in normal rats; and
imTG hydrolysis is resistant to insulin action, and hydrolysis rate is higher in obese rats.
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