ABSTRACT
Alzheimer's disease (AD) and type 2 diabetes appear to share similar pathogenic mechanisms. dsRNA-dependent protein kinase (PKR) underlies peripheral insulin resistance in metabolic disorders. PKR phosphorylates eukaryotic translation initiation factor 2α (eIF2α-P), and AD brains exhibit elevated phospho-PKR and eIF2α-P levels. Whether and how PKR and eIF2α-P participate in defective brain insulin signaling and cognitive impairment in AD are unknown. We report that ß-amyloid oligomers, AD-associated toxins, activate PKR in a tumor necrosis factor α (TNF-α)-dependent manner, resulting in eIF2α-P, neuronal insulin receptor substrate (IRS-1) inhibition, synapse loss, and memory impairment. Brain phospho-PKR and eIF2α-P were elevated in AD animal models, including monkeys given intracerebroventricular oligomer infusions. Oligomers failed to trigger eIF2α-P and cognitive impairment in PKR(-/-) and TNFR1(-/-) mice. Bolstering insulin signaling rescued phospho-PKR and eIF2α-P. Results reveal pathogenic mechanisms shared by AD and diabetes and establish that proinflammatory signaling mediates oligomer-induced IRS-1 inhibition and PKR-dependent synapse and memory loss.
Subject(s)
Amyloid beta-Peptides/toxicity , Brain/drug effects , Insulin Receptor Substrate Proteins/metabolism , Polymers/toxicity , Tumor Necrosis Factor-alpha/metabolism , eIF-2 Kinase/metabolism , Alzheimer Disease/metabolism , Alzheimer Disease/pathology , Amyloid beta-Peptides/chemistry , Animals , Brain/metabolism , Disease Models, Animal , Haplorhini/metabolism , Hypoglycemic Agents/pharmacology , Insulin Receptor Substrate Proteins/antagonists & inhibitors , Memory Disorders/metabolism , Memory Disorders/pathology , Mice , Mice, Knockout , Neurons/drug effects , Neurons/metabolism , Phosphorylation/drug effects , Polymers/chemistry , Receptors, Tumor Necrosis Factor, Type I/deficiency , Receptors, Tumor Necrosis Factor, Type I/genetics , Receptors, Tumor Necrosis Factor, Type I/metabolism , Signal Transduction/drug effects , Synapses/drug effects , Synapses/metabolism , Tumor Necrosis Factor-alpha/antagonists & inhibitors , eIF-2 Kinase/deficiency , eIF-2 Kinase/geneticsABSTRACT
Inflammation plays an important pathogenic role in a number of metabolic diseases such as obesity, type 2 diabetes, and atherosclerosis. The activation of inflammation in these diseases depends at least in part on the combined actions of TLR4 signaling and endoplasmic reticulum stress, which by acting in concert can boost the inflammatory response. Defining the mechanisms involved in this phenomenon may unveil potential targets for the treatment of metabolic/inflammatory diseases. Here we used LPS to induce endoplasmic reticulum stress in the human monocyte cell-line, THP-1. The unfolded protein response, produced after LPS, was dependent on CD14 activity but not on RNA-dependent protein kinase and could be inhibited by an exogenous chemical chaperone. The induction of the endoplasmic reticulum resident chaperones, GRP94 and GRP78, by LPS was of a much lower magnitude than the effect of LPS on TLR4 and MD-2 expression. In face of this apparent insufficiency of chaperone expression, we induced the expression of GRP94 and GRP78 by glucose deprivation. This approach completely reverted endoplasmic reticulum stress. The inhibition of either GRP94 or GRP78 with siRNA was sufficient to rescue the protective effect of glucose deprivation on LPS-induced endoplasmic reticulum stress. Thus, insufficient LPS-induced chaperone expression links TLR4 signaling to endoplasmic reticulum stress.
Subject(s)
Endoplasmic Reticulum Stress/physiology , Molecular Chaperones/metabolism , Signal Transduction/physiology , Toll-Like Receptor 4/metabolism , Animals , Cell Line , DNA-Binding Proteins/metabolism , Endoplasmic Reticulum Chaperone BiP , Endoplasmic Reticulum Stress/drug effects , Glucose/pharmacology , HSP70 Heat-Shock Proteins/genetics , HSP70 Heat-Shock Proteins/metabolism , Heat-Shock Proteins/genetics , Heat-Shock Proteins/metabolism , Humans , Immunoblotting , Lipopolysaccharide Receptors/metabolism , Lipopolysaccharides/pharmacology , Lymphocyte Antigen 96/metabolism , Male , Membrane Proteins/genetics , Membrane Proteins/metabolism , Mice , Mice, Inbred C57BL , Mice, Knockout , Molecular Chaperones/genetics , Monocytes/cytology , Monocytes/drug effects , Monocytes/metabolism , Phosphorylation/drug effects , RNA Interference , Signal Transduction/drug effects , Transcription Factors/metabolism , Unfolded Protein Response/drug effects , Unfolded Protein Response/physiology , eIF-2 Kinase/deficiency , eIF-2 Kinase/genetics , eIF-2 Kinase/metabolismABSTRACT
Interferons (IFNs) are a family of cytokines that have many biological functions in the cell, including regulation of cellular growth, differentiation, immunomodulation, and viral replication by inducing a set of interferon stimulated genes (ISGs). Based on their structure and biological activities IFNs are subdivided into two groups: type I IFNs, which includes IFN-alpha and IFN-beta and type II IFNs, represented by IFN-gamma. The aim of this work was to investigate whether integrin alpha 11 (ITGA-11), a novel collagen-binding integrin, is responsive to type I IFN treatment. Our findings indicated that type I IFNs were able to induce the ITGA-11 mRNA levels in T98G cells. Increased levels of ITGA-11 protein were also observed in IFN-treated cells. The in vivo induction of ITGA-11 was detected in spleen and lungs of IFN-treated BALB/c mice. T98G cells infected with Murine encephalomyocarditis virus showed increased levels of ITGA-11 mRNA and protein. We observed that the ITGA-11 promoter has binding sites for transcriptional factors regulated by IFNs and the double-stranded RNA dependent protein kinase (PKR). Therefore we investigated the role of PKR in the induction of ITGA-11 by using a PKR deficient mouse embryo fibroblast cell line (MEFs). PKR(-/-) MEFs treated with IFN did not show increased levels of ITGA-11 protein nor mRNA although that could be promptly detected in wild type MEFs. Taken together our data suggest that ITGA-11 is a new interferon stimulated gene.