REGULATION OF LIVER FUNCTION BY MAPK/MKP-1

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The broad goal of this project is to understand the molecular mechanisms that control liver function in health and disease. Specifically, the aim of this project is to understand how local nuclear and cytosolic calcium signals integrate with the mitogen-activated protein kinase (MARK) signal transduction pathway in the control of liver growth, regeneration, and metabolism. The MAPKs are negatively regulated by a family of enzymes known as the MARK phosphatases (MKPs). Recent work from this laboratory has shown that the nuclear localized MKP family member, MKP-1, is essential for the negative regulation of the MAPKs. These observations are supported in vivo where we have discovered that mice lacking expression of MKP-1 exhibit enhanced MAPK activation in the liver which correlates with enhanced hepatic lipid metabolism and resistance to the acquisition of a fatty liver. We present preliminary data to support a new signaling paradigm in which cytosolic and nuclear calcium signals differentially regulate the transcription of MKP-1 in a positive and negative manner, respectively. This discrete regulation of MKP-1 in the nucleus by calcium controls MAPK-mediated gene expression. We hypothesize that local calcium signals regulates MKP-1 expression which serves to limit the magnitude and spatio-temporal kinetics of MAPK-mediated gene activity required for liver growth, regeneration, and metabolic homeostasis. We will test this hypothesis in three specific aims. Aim 1 will define the molecular basis for the differential effects of nuclear and cytosolic calcium on the regulation of MKP-1 gene transcription by disrupting calcium signaling in these sub-cellular compartments using previously developed targeted calcium-binding proteins. Aim 2 will define the importance of MKP-1 nuclear localization for its physiological function in the liver. To test this, a novel genetic mutation in MKP-1 that targets it to the cytosol will be "knocked-in" to mice using an inducible CreLoxP approach. Aim 3 will determine the role of MKP-1 on liver growth, regeneration, and stress management using MKP 1 "knock-out" mice. Together with projects by Nathanson and Ehrlich, the proposed molecular, biochemical and genetic approaches in this project will establish a new signaling paradigm between local calcium signals, MKP-1 and MAPK-mediated gene expression in the control of liver function.