Hormonal mechanisms of gene regulation

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Adiponectin is a hormone secreted by fat tissue that helps keep the liver, skeletal muscle, pancreas and blood vessels healthy. When a person becomes obese, their fat cells no longer make as much adiponectin. Because of this, it has been suggested that the negative effects of obesity are caused by a lack of adiponectin. One of the key functions of adiponectin is to help control blood sugar (glucose) levels. This means that adiponectin works with insulin in controlling glucose production by the liver. Less adiponectin in obesity may be the reason insulin levels have to increase to keep glucose levels in check. We have been studying how insulin works to control glucose for over 25 years, but no one has looked at how adiponectin helps. We want to look at this question because some of our work showed adiponectin activates a key protein called FoxO6, which is also activated by insulin. Interestingly, both together inactivate FoxO6. We wish to determine how adiponectin and insulin work to coordinate glucose production by the liver via FoxO6. Based on our results, we propose the following hypothesis: the reduction in hepatic FoxO6 phosphorylation that occurs as a result of the combined actions of insulin and adiponectin is due to a decrease in the binding of the 14-3-3 scaffold protein, which blocks the action of FoxO6 on specific genes. What is being suggested in the hypothesis is that adiponectin and insulin together interfere with the activation of FoxO6 by changing the ability of another protein called 14-3-3 to bind to FoxO6. The resulting inactivation of FoxO6 leads to a decrease in the expression of certain genes involved in the production of glucose by the liver. To address the hypothesis, we propose completing three separate objectives, each providing us with information regarding how insulin and adiponectin affect FoxO6. Objective 1 will identify the factors present in the cell that send signals from the insulin and adiponectin receptors to FoxO6. Objective 2 will examine the phosphorylation state of FoxO6 (which relates to its activation) using specific antibodies to this modified form of FoxO6 in response to insulin and adiponectin. We will include in this analysis the different forms of adiponectin that are known to be present in the blood. These are the full-length, which represents the form of adiponectin that is secreted by fat cells, and the globular form, a shorter version of adiponectin that is produced in the blood by an enzyme capable of removing a small piece of the protein. Objective 3 will investigate if 14-3-3 binding to FoxO6 is affected by insulin and adiponectin. These experiments will provide the first mechanistic evidence of a direct interaction between insulin and adiponectin in the regulation of glucose production by the liver. Overall, the project being proposed will delineate the key points of cross-talk and add to our understanding of the cellular processes that are critical for efficient liver function.