Regulation of Liver by Nuclear Ca2+ Signaling

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The liver manages a wide range of metabolic functions, which are controlled by interrelated signaling pathways. One such pathway involves cytosolic Ca2+ signaling in hepatocytes, which regulates activities such as bile secretion, glucose metabolism, and cytoskeletal organization. The goal of this Program Project is to examine the mechanisms and effects of a complementary Ca2+ signaling system, within the nucleus of hepatocytes. During the current award it was found that free Ca2+ in the nucleus and cytosol can be controlled separately, and that gene transcription, hepatocyte growth, and development of steatosis are regulated by nuclear Ca2+ through downstream activation of the mitogen-activated protein kinase (MAPK) and MAPK phosphatase-1 (MKP-1) pathways. During the next award period we will test the hypothesis that growth factors regulate liver growth by release of nuclear Ca2+ from inositol 1,4,5-trisphosphate receptors (InsPSRs) that reside on the nucleoplasmic reticulum, and that activation of nuclear Ca2+ signals controls gene expression involved in hepatic growth and regeneration by dynamically acting on both positive (MAPK) and negative (MKP-1) regulatory signaling pathways. This will be tested through the following projects: Project by Nathanson will determine whether and how receptor tyrosine kinases regulate growth of hepatocytes by inducing lnsP3-mediated Ca2+ signals within the nucleus. Project by Ehrlich will investigate whether and how MAPK and the MAPK phosphatase, MKP-1, differentially regulate the function and distribution of nuclear and cytosolic InsPSRs. Project by Bennett will investigate how local Ca2+ signals within the cytosol or nucleus control MKP-1 expression in order to modulate the magnitude and kinetics of gene activity required for liver growth and metabolism. To help carry out these projects, core facilities will be established for cell and molecular biology, cell imaging, and administration. These projects will collectively provide a comprehensive investigation of how nuclear Ca2+ provides a local conduit in the nucleus for the control of MAPK-mediated gene expression in the liver. The results of these studies will have broad clinical implications for the treatment of liver diseases in which regulation of hepatic growth is impaired, including cirrhosis and hepatocellular carcinoma, as well as associated metabolic syndromes such as non-alcoholic fatty liver disease (NAFLD).