ABSTRACT
The accumulation of unfolded protein in the endoplasmic reticulum (ER) attenuates protein synthesis initiation through phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2alpha) at Ser51. Subsequently, transcription of genes encoding adaptive functions including the glucose-regulated proteins is induced. We show that eIF2alpha phosphorylation is required for translation attenuation, transcriptional induction, and survival in response to ER stress. Mice with a homozygous mutation at the eIF2alpha phosphorylation site (Ser51Ala) died within 18 hr after birth due to hypoglycemia associated with defective gluconeogenesis. In addition, homozygous mutant embryos and neonates displayed a deficiency in pancreatic beta cells. The results demonstrate that regulation of translation through eIF2alpha phosphorylation is essential for the ER stress response and in vivo glucose homeostasis.
Subject(s)
DNA-Binding Proteins/genetics , Glucose/metabolism , Heat-Shock Proteins , Homeostasis/physiology , Hypoglycemia/metabolism , Protein Biosynthesis/physiology , Transcription Factors/genetics , Activating Transcription Factors , Animals , Animals, Newborn , Blood Proteins/genetics , Blood Proteins/metabolism , CCAAT-Enhancer-Binding Proteins/genetics , CCAAT-Enhancer-Binding Proteins/metabolism , Carrier Proteins/genetics , Carrier Proteins/metabolism , Cell Survival/physiology , DNA-Binding Proteins/chemistry , DNA-Binding Proteins/metabolism , Endoplasmic Reticulum/metabolism , Endoplasmic Reticulum Chaperone BiP , Gene Expression/physiology , Germ-Line Mutation , Gluconeogenesis/physiology , Homozygote , Hypoglycemia/genetics , Islets of Langerhans/cytology , Islets of Langerhans/metabolism , Mice , Mice, Mutant Strains , Molecular Chaperones/genetics , Molecular Chaperones/metabolism , Mutagenesis/physiology , Phosphorylation , Protein Folding , RNA, Messenger/analysis , Transcription Factor CHOP , Transcription Factors/chemistry , Transcription Factors/metabolism , Transcriptional Activation/physiologySubject(s)
Islets of Langerhans/physiology , Proto-Oncogene Proteins c-myc/genetics , Adaptation, Physiological , Animals , Cells, Cultured , DNA-Binding Proteins/genetics , Dose-Response Relationship, Drug , Gene Expression Regulation/drug effects , Gene Expression Regulation/physiology , Islets of Langerhans/cytology , Islets of Langerhans/metabolism , RNA, Messenger/drug effects , RNA, Messenger/genetics , RNA, Messenger/metabolism , Rats , Reverse Transcriptase Polymerase Chain Reaction , TATA-Box Binding Protein , Transcription Factors/geneticsSubject(s)
Hyperglycemia/physiopathology , Islets of Langerhans/metabolism , Acyl-CoA Oxidase , Adaptation, Physiological , Animals , Carnitine O-Palmitoyltransferase/genetics , Gene Expression Regulation/drug effects , Islets of Langerhans/physiopathology , Islets of Langerhans/surgery , Oxidoreductases/genetics , Pancreatectomy , Phlorhizin/pharmacology , RNA, Messenger/genetics , RNA, Messenger/metabolism , Rats , Receptors, Cytoplasmic and Nuclear/genetics , Transcription Factors/geneticsABSTRACT
Differentiated pancreatic beta cells are unique in their ability to secrete insulin in response to a rise in plasma glucose. We have proposed that the unique constellation of genes they express may be lost in diabetes due to the deleterious effect of chronic hyperglycemia. To test this hypothesis, Sprague-Dawley rats were submitted to a 85-95% pancreatectomy or sham pancreatectomy. One week later, the animals developed mild to severe chronic hyperglycemia that was stable for the next 3 weeks, without significant alteration of plasma nonesterified fatty acid levels. Expression of many genes important for glucose-induced insulin release decreased progressively with increasing hyperglycemia, in parallel with a reduction of several islet transcription factors involved in beta cell development and differentiation. In contrast, genes barely expressed in sham islets (lactate dehydrogenase A and hexokinase I) were markedly increased, in parallel with an increase in the transcription factor c-Myc, a potent stimulator of cell growth. These abnormalities were accompanied by beta cell hypertrophy. Changes in gene expression were fully developed 2 weeks after pancreatectomy. Correction of blood glucose by phlorizin for the next 2 weeks normalized islet gene expression and beta cell volume without affecting plasma nonesterified fatty acid levels, strongly suggesting that hyperglycemia triggers these abnormalities. In conclusion, chronic hyperglycemia leads to beta cell hypertrophy and loss of beta cell differentiation that is correlated with changes in c-Myc and other key transcription factors. A similar change in beta cell differentiation could contribute to the profound derangement of insulin secretion in human diabetes.
Subject(s)
Diabetes Mellitus, Experimental/pathology , Hyperglycemia/pathology , Islets of Langerhans/pathology , Animals , Blood Glucose/metabolism , Body Weight/drug effects , Cell Differentiation , Chronic Disease , DNA Primers , Disease Models, Animal , Fatty Acids, Nonesterified/blood , Humans , Insulin/blood , Male , Pancreatic Hormones/genetics , Pancreatic Hormones/metabolism , Phlorhizin/pharmacology , RNA, Messenger/metabolism , Rats , Rats, Sprague-DawleyABSTRACT
The insulin-responsive glucose transporter GLUT-4 is found in muscle and fat cells in the trans-Golgi reticulum (TGR) and in an intracellular tubulovesicular compartment, from where it undergoes insulin-dependent movement to the cell surface. To examine the relationship between these GLUT-4-containing compartments and the regulated secretory pathway we have localized GLUT-4 in atrial cardiomyocytes. This cell type secretes an antihypertensive hormone, referred to as the atrial natriuretic factor (ANF), in response to elevated blood pressure. We show that GLUT-4 is targeted in the atrial cell to the TGR and a tubulo-vesicular compartment, which is morphologically and functionally indistinguishable from the intracellular GLUT-4 compartment found in other types of myocytes and in fat cells, and in addition to the ANF secretory granules. Forming ANF granules are present throughout all Golgi cisternae but only become GLUT4 positive in the TGR. The inability of cyclohexamide treatment to effect the TGR localization of GLUT-4 indicates that GLUT-4 enters the ANF secretory granules at the TGR via the recycling pathway and not via the biosynthetic pathway. These data suggest that a large proportion of GLUT-4 must recycle via the TGR in insulin-sensitive cells. It will be important to determine if this is the pathway by which the insulin-regulatable tubulo-vesicular compartment is formed.
