New Animal Models for Study of Metabolism

This minireview series will present several specific examples of the surprising new information that has been generated by the application of molecular genetics to the study of metabolism. The ability to manipulate the genome of mice has provided a major tool for the study of metabolism and its regulation in normal, physiological states and in disease. It is now possible, for example, to determine the metabolic function of a specific gene product in animals by ablating the expression of the gene in a tissue of interest and determining the resulting phenotype. As a result of this technology, the metabolic functions of many proteins that were once thought to be well characterized are being re-examined in these animal models. In addition, it is possible to alter the tissuespecific expression of a gene of metabolic interest during development to allow the determination of the impact of that change on the appearance of metabolic function. The recent literature on the biological function of the transcription factor C/EBP provides an excellent example of the potential of genetic manipulation to provide new insights into energy metabolism. It is known from a number of studies on the role of C/EBP that two members of this family of transcription factors, C/EBP and C/EBP , are required for the expression of a number of genes involved in lipid and carbohydrate metabolism in the liver and adipose tissue of mammals. Deleting the gene for C/EBP results in mice that die in the immediate perinatal period with profound hypoglycemia and a failure of the urea cycle to develop normally (1). The livers of these mice have no glycogen at birth and the genes for the enzymes that are involved in hepatic glucose synthesis, the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) (PEPCK-C) and glucose 6-phosphatase, are not induced in the liver in the normal fashion (1). A deletion in the gene for C/EBP , on the other hand, produces litters of mice with two phenotypes; one dies within an hour after birth whereas the other lives until adulthood with major metabolic anomalies (2, 3). The genes for both C/EBP and C/EBP are first expressed in the liver during the last trimester of pregnancy, and their appearance is linked to the development of critical metabolic pathways (4). This type of information could only be generated using animal models where the development of specific processes can be determined within the physiological context of normal metabolic development. The work of Chen et al. (5) has greatly extended this story. They have generated a C/EBP null mouse in which the structural gene for C/EBP was replaced by the structural gene for C/EBP . These mice are healthy and fertile and demonstrate none of the abnormalities noted with the C/EBP -deficient mouse. However, these animals fail to develop white adipose tissue in the normal manner, indicating that C/EBP is required for adipose tissue development but not for the other metabolic processes in the liver and other tissues. Thus, C/EBP can replace C/EBP in controlling hepatic gene transcription. This fact was not evident from a number of previous studies that stressed the importance of C/EBP in the response of the gene for PEPCK-C to hormones such as glucagon (acting via cAMP) (6, 7). This finding underlines the importance of the timing and context of the expression of members of the C/EBP gene family, rather than a specific individual role for either isoform (except for the development of adipose tissue). What is clearly important is the control of the developmental and tissue-specific expression of the gene (either C/EBP or C/EBP ) that is regulated by the gene promoter. This important insight regarding the mechanisms by which transcription factors regulate the development of metabolic processes would not have been possible without the ability to substitute one gene for another and to preserve the genomic context in which the gene of interest is expressed. This minireview series will present articles from a number of laboratories that have had important roles in generating animal models to study metabolism. These reviews range from studies of the function of glucose transport and the role of insulin receptor in this process to insights that these animal models provide for understanding the development of genes for critical enzymes in metabolism. The minireview by Haruka Okamoto and Domenico Accili entitled “In Vivo Mutagenesis of the Insulin Receptor” provides a new look at the role that the insulin receptor plays in tissues that are not normally considered acutely insulin sensitive (non-canonical insulin-responsive tissues) such as the liver, brain, and pancreas. The review also assesses the relative roles of the insulin receptor and the insulin-like growth factor receptor in controlling growth and metabolic processes in mice. As discussed above, the availability of powerful techniques for manipulating the genome permits the tissue-specific ablation of the insulin receptor gene in mice; from this has come a detailed physiological analysis of the response of the animals. This genetic mapping of the insulin receptor function in mice has a number of metabolic surprises and makes for fascinating reading. A major area of metabolic research in obesity involves the role of thermogenesis in controlling the rate of energy expenditure in all mammals. Studies of human obesity have demonstrated that individuals reach a set point for body fat and that food intake and energy expenditure are balanced by a complex set of hormonal and neural signals that control this processes. A critical factor in the regulation of energy expenditure involves the control of thermogenesis. It was known for many years that brown adipose tissue contains an uncoupling protein (UCP1) that is activated by norepinephrine that is induced in mammals by exposure to the cold. This in turn causes an elevation of cAMP in the tissue and the subsequent induction of lipolysis. The fatty acids generated by this process activate UCP1, resulting in the insertion of this protein into the inner mitochondria membrane causing a partial collapse of the proton gradient and an uncoupling of respiration from phosphorylation. The end result of this metabolic alteration is a rise in body temperature; this process is often referred to as “non-shivering thermogenesis.” More recently, isoforms of UCP1, termed UCP2 and UCP3, have been discovered in a number of tissues including muscle and the -cells of the pancreas. Interestingly, transcription for the gene for UCP2 in muscle can be induced by fatty acids (8, 9) suggesting that this protein could be responsible for an increased rate of fatty acid oxidation in muscle of individuals that express the gene at significant levels and could be a factor in weight regulation. The research of Bradford B. Lowell and his colleagues (10, 11) has provided a number of interesting clues regarding the importance of the regulation of thermogenesis in the control of energy expenditure. The minireview by Lowell and Eric S. Bachman entitled “ -Adrenergic Receptors, Diet-induced Thermogenesis, and Obesity” reviews the literature regarding the metabolic consequence of ablating expression of the genes for members of the UCP family on energy metabolism in mice. The critical factor, often ignored in our considerations of metabolic control, is the rate of futile cycling of metabolic fuels that takes place in all organisms (12). An example of futile cycling that * These minireviews will be reprinted in the 2003 Minireview Compendium, which will be available in January, 2004. ‡ To whom correspondence should be addressed. E-mail: rwh@po. cwru.edu. Minireview Prologue THE JOURNAL OF BIOLOGICAL CHEMISTRY Vol. 278, No. 31, Issue of August 1, pp. 28357–28358, 2003 © 2003 by The American Society for Biochemistry and Molecular Biology, Inc. Printed in U.S.A.