The immune response to sepsis involves a series of complex, highly integrated homeostatic responses that, if prolonged and excessive, can lead to organ dysfunction and death. Nitric oxide (NO) synthesis is upregulated by sepsis in many tissues and is an essential component of the host immune response. Nitric oxide synthesis can be beneficial and improve immune and organ function, but if synthesis is excessive and prolonged, NO can promote organ injury, tissue inflammation, and death. NO is produced in hepatocytes by the inducible nitric oxide synthase (iNOS) that is stimulated by cytokines and proinflammatory stimuli. Excessive NO from iNOS produces cellular dysfunction and hepatic injury. Glucagon and cyclic adenosine monophosphate (cAMP) regulate hepatic iNOS expression in vitro and in vivo, and by doing so, decrease NO-mediated hepatic injury. Our preliminary data demonstrate that insulin also down-regulates cytokine-induced iNOS expression. Both glucagon and insulin alter specific intracellular signaling pathways in hepatocytes, but the mechanisms involved in the regulation of hepatocyte function in sepsis by glucagon and insulin, and specifically the regulation of hepatocyte iNOS expression, have not been identified. In this proposal, we will determine the mechanisms responsible for the regulation of hepatocyte iNOS expression by glucagon and insulin.
In Aim I, we will continue our work in determining the mechanism for the glucagon and cAMP-induced inhibition of hepatocyte iNOS expression. We will focus on protein kinase A (PKA)-independent pathways induced by cAMP and evaluate the role of the guanine nucleotide exchange factor Epac and the role of calcium.
In Aim II, we will determine the mechanisms responsible for the inhibition of iNOS by insulin. By defining how these hormones regulate hepatocyte iNOS expression, we will provide a framework for understanding the basic pathophysiologic cellular events in shock and sepsis that may lead to novel cellular-based therapies for critically ill patients. Project Narrative: Nitric oxide is synthesized in critically ill patients during septic shock, and when overproduced, can increase cellular dysfunction, tissue injury, and death. Glucagon and insulin primarily regulate blood glucose, which has become an important facet of the care of critically ill patients, but we have found that they also regulate hepatic nitric oxide production. We will determine the mechanisms responsible for the regulation of hepatocyte inducible nitric oxide synthase (iNOS) expression by glucagon and insulin. By defining these mechanisms, we will provide a framework for understanding the basic cellular events in shock and sepsis, which may lead to novel cellular-based therapies for critically ill patients.
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