Regulation of hepatic MAPK phosphatase by nuclear Ca2+

Yale University, New Haven CT

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Abstract

The broad goal of this research is to define the signal transduction pathways that regulate MKP-1 and to assess its role in liver function. The mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) serves to inactivated by dephosphorylation the MAPKs in the nucleus. MKP-1 was identified as an immediate early gene that becomes induced in response to oxidative stress and liver regeneration. How dephosphorylation of the MAPKs by MKP-1 in the nucleus is regulated and whether MKP1- plays a role in liver function is unknown. Cellular calcium (Ca2+) also plays an important signaling role and serves to control gene expression, proliferation, secretion and apoptosis in the liver. Evidence now suggests that Ca2+ in the nucleus is regulated independently of Ca2+ in the cytosol. How Ca2+, the MAPKs and MKP- 1 integrate in the nucleus to regulate hepatic physiological function needs to be addressed in order to appreciate the diversity and spatial complexity of these signaling pathways. Disruption of these pathways often leads to a loss of normal liver homeostasis, resulting in aberrant liver regeneration and repair, metabolic dysfunction and an increased propensity to develop hepatocellular carcinomas. We demonstrate using a novel approach to disrupt selectively, either nuclear or cytosolic Ca2+ signals, that nuclear but not cytosolic Ca2+ regulates MKP-1 gene expression. We also shown that nuclear, but not cytosolic Ca2+ affects transactivation of Elk-1, an Ets family transcription factor and MAPK substrate. These data suggest a role for a novel signaling mechanism in which MKP-1, and the MAPKs, are regulated by nuclear Ca2+. We will test the hypothesis that nuclear Ca2+ is a critical component for signaling events mediated by MKP-1 and the MAPKs in liver nuclei. Specific Aim 1, will define how nuclear Ca2+ regulates MKP-1 gene expression, by identifying the Ca2+- responsive regulatory elements within the human MKP-1 promoter. Liver cell lines genetically engineered so that either nuclei or to cytosolic Ca2+ signals can be disrupted will be generated to evaluate the role of nuclear vs. cytosolic Ca2+ signals can be disrupted will be generated to evaluate the role of nuclear vs. cytosolic Ca2+ on MKP-1/MAPK activity. Specific Aim 2 will determine how nuclear Ca2+ regulates MAPK- mediated localizes to the nucleus to regulate this pool of MAPKs is unknown. We provide data suggesting that the cdc25 homology domain of MKP-1 is required for its localization to the nucleus. Specific Aim 3, will establish the basis for how MKP-1 localizes to the nucleus. Finally, in Specific Aim 4, mice that contain a targeted disruption of the MKP-1 gene will be used to investigate the functional role of MKP-1 during liver regeneration. The sensitivity of the liver from MKP-1(-/-) mice to stress- induced apoptosis will also be assessed. Together, these studies will provide new information regarding the role of a novel nuclear Ca2+ signaling mechanism for the regulation of MKP-1 and the MAPKs in hepatocyte function. Elucidating the role of MKP-1 in liver pathophysiology may provide insight into new animal models for the study of liver diseases.