A Study On The Roles Of The Ras Activation Pathway During Interferonγ Mediated Functional Responses And Acetaminophen-induced Liver Injury In Mice

Abstract

Interferons (IFNs) perform a wide range of biological activities: anti-microbial, anti-proliferative, immunomodulatory etc. The IFN family includes three main classes: Type I, Type II and the recently identified Type III. The two main members of Type I class are IFNα and IFNβ, which are well known for their anti-viral roles. IFNλ, a member of the Type III class of IFNs, also exhibits antiviral activity. IFNγ, also known as immune IFN, is a Type II IFN which is secreted, primarily, by activated T cells, NK cells and macrophages. IFNγ is a potent immunomodulator which plays important roles in host defense. The diverse functions of this cytokine are demonstrated in Ifnγ-/- mice which display increased sensitivity to several pathogens, high incidences of tumors, reduced inflammatory response etc. IFNγ binds to its cognate receptors, which consist of two subunits, IFNγ receptor (IFNGR) 1 and IFNGR2. IFNγ mediates its multifarious biological actions by activating the Janus activated kinase (Jak)-Signal transducer and activator of transcription (Stat) 1 signaling pathway. Jaks belong to a family of non-receptor protein tyrosine kinases and phosphorylate the IFNγ receptor and the transcriptional co-activator, Stat. IFNGR1, the larger subunit, is required for ligand binding and its carboxyl terminus is involved in binding to Jak1, which in turn phosphorylates Stat1. The smaller subunit, IFNGR2, is required for signaling and contains the Jak2 binding site. After binding of IFNγ to its receptor, a series of phosphorylation events occur, resulting in Stat1 phosphorylation and homodimerization of Stat1 to form the gamma activating factor (GAF). These activated molecules translocate to the nucleus and bind to gamma activating sequence (GAS) present in the promoters of several IFNγ-modulated genes. Thus, the cellular responses mediated by IFNγ are, primarily, due to modulation of gene expression. Therefore, the identification and study of IFNγ stimulated genes, signaling mediators and their cross talk with other cellular pathways is an active area of research. The system of our study was a hepatoma cell line, H6, which is derived from a spontaneous tumor from B10.A mice and selected for in vitro cell culture. It is an IFNγ inducible system and has been used to study IFNγ-induced gene expression and functional responses. Treatment of H6 cells with IFNγ greatly enhanced MHC class I levels but also reduced cell growth. High amounts of reactive oxygen species (ROS) and reactive nitrogen intermediates (RNI) play crucial roles in the growth suppressive effect of IFNγ. To better understand the signaling pathways involved in the generation of ROS and RNI, the involvement of Ras was investigated. Ras-GTP levels were determined by pull down assays using GST-Raf1-Ras binding domain fusion protein bound to glutathione agarose. Ras activation (conversion of Ras-GDP to Ras-GTP) was observed in H6 cells upon IFNγ treatment by ~12 hr. To assess the functional role of Ras activation, studies with Manumycin A, a farnesyl transferase inhibitor (FTI), were performed. The formation of functional Ras requires farnesylation, a post-translational modification, which is inhibited by FTIs. Treatment with Manumycin A blocked Ras activation but did not significantly modulate the IFNγ-induced MHC class I. However, the inhibitor reduced ROS amounts leading to increased cell growth in the presence of IFNγ. Together, these results delineated the role of Ras and ROS in modulating some functions of IFNγ. To further understand the mechanisms by which Ras mediates its functions during IFNγ mediated growth suppression, the activation and function of Ras effectors was evaluated. In particular, the role of Ras-like (Ral) guanyl nucleotide-binding proteins, RalA and RalB, was investigated. IFNγ induced transcripts of RalA but not RalB. Also, the induction of RalA and IFNγ induced growth suppression were Stat1-dependent. Studies involving chemical inhibitors and genetic studies revealed that Ras played a role in the induction of RalA during IFNγ treatment. The role of c-Jun N-terminal kinase (JNK), a stress induced kinase, was also elucidated in this system. Together, IFNγ induced activation of Ras and its effectors RalA and JNK, leading to high amounts of ROS that suppressed cell growth. To evaluate the physiological significance of Ras activation during inflammatory responses, the mouse model of acetaminophen (APAP) induced liver injury was established. Hepatotoxicity due to overdose of the analgesic and antipyretic, APAP, is a major cause of liver failure in adults. APAP is metabolized into a reactive metabolite which binds to glutathione. Consequently, the depletion of intracellular glutathione stores leads to oxidative stress and liver injury. Notably, Ifnγ-/- mice are resistant to APAP-induced liver damage demonstrating a crucial role for this cytokine. The role of Ras activation was evaluated after oral dosing of BALB/c mice with APAP. Ras-GTP was induced early and decreased amounts were observed upon treatment with L-methionine, which replenished glutathione amounts. Injection with L-methionine or Manumycin A rescued liver injury as assessed by lowered serum alanine aminotransferase amounts and histological analysis. Kinetic studies were also performed, under different treatment conditions, to estimate different biochemical parameters: glutathione amounts, JNK activation, protein carbonylation, ROS amounts, serum amounts of cytokines, TNFα and IFNγ etc. This study reveals a role of Ras activation in stimulating proinflammatory responses and demonstrates the therapeutic efficacy of FTIs during APAP-induced liver injury. In addition the role of RalA during APAP-induced liver injury was also studied. In summary, this study, involving in vitro cell culture and in vivo liver injury model systems, sheds light on the significant contributions of Ras and its effector, RalA, during IFNγ mediated growth suppression and APAP-induced liver injury.