Structure of the hamster low density lipoprotein receptor gene

The metabolism of low density lipoprotein (LDL) in the hamster is substantially similar to that of the human. To extend the usefulness of the hamster as an experimental model, the hamster LDL receptor gene was isolated and characterized. The gene is composed of 18 exons and 17 introns which span 26 kilobases. The introns occur at precisely the same positions as those previously determined for the human LDL receptor gene. The 18 exons of the hamster gene predict an LDL receptor protein of 854 amino acids that is similar in organization and sequence to those predicted from the cDNAs of rat, rabbit, cow, and human. Within the 5'-flanking region of the hamster LDL receptor gene are three highly conserved imperfect direct repeat sequences of 16 nucleotides each that in the human gene have been demonstrated to regulate transcription. In addition, a similar arrangement of direct repeat sequences was also isolated from the 5'-flanking region of the rat LDL receptor gene using the polymerase chain reaction. These results indicate a strong sequence and structural conservation of the LDL receptor among several species and further support the hamster as an experimental model for the study of human LDkholesterol metabolism.-Bishop, R. W. Structure of the hamster low density lipoprotein receptor gene. J. Lipid Res. 1992.33: 549-557. Supplementary key words LDL receptor promoter transcriptional regulation rat cholesterol homeostasis sterol regulatory element * nucleotide sequence * cloning * RNA The cellular uptake of low density lipoprotein (LDL) through receptor-mediated endocytosis has been characterized by thorough genetic and biochemical analysis of the LDL receptor (1). Numerous mutants have been described that affect the synthesis, processing, ligand binding, and recycling of the LDL receptor protein (2). These mutations occur naturally and are found in humans, rabbits, and monkeys with the genetic disorder familial hypercholesterolen$a (24). The physiological consequence of such defects is to impair the efficient removal of LDL particles from the blood which in turn can lead to increased serum LDL concentrations and atherosclerosis (5, 6). While the characterization of these mutants has been productive, the inherent physiological limitations illustrate the need for an animal model that a p proximates human cholesterol metabolism. The various species currently in use include the monkey, pigeon, pig, guinea pig, rat, rabbit, and hamster (4, 79). Among these, the metabolism of cholesterol in the hamster most closely resembles that in humans (9-13). Support for the hamster model comes from kinetic analysis of LDL production and degradation (9-13). These studies demonstrate similarities between the male hamster and humans with regard to rates of hepatic cholesterol synthesis (9, lo), the relative significance of receptordependent and receptor-independent LDL transport ( l l ) , and the response of plasma LDL cholesterol to dietary additions (12) or the administration of cholesterol-lowering agents ( 10, 13). The usefulness of the hamster model is further enhanced by the availability of established cell lines such as the Chinese Hamster Ovary (CHO) line which contain defects in the pathways of cholesterol synthesis (14), regulation (15), esterification (16), and uptake by the LDL receptor protein (17). The isolation of cDNA clones for the bovine, human, rabbit, and rat LDL receptors has been reported (3, 18-20). In addition, the gene for the human LDL receptor has been isolated and characterized (21). To complement these studies and to extend the genetic analysis of the LDL receptor in the hamster, the cloning and structural analysis of the hamster LDL receptor gene is presented, including a highly conserved transcriptional the immediate 5'-flanking DNA. regulatory region in Abbreviations: LDL, low density lipoprotein; CHO, Chinese hamster ovary; EGF, epidermal growth factor; SRE, sterol regulatory element; PCR, polymerase chain reaction; SDS, sodium dodecyl sulfate; cDNA, complementary DNA. 'Present address: Department of Metabolic Diseases, Bristol-Myers Squibb Pharmaceutical Research Institute, P.O. Box 4000, Princeton, NJ 08543-4000. Journal of Lipid Research Volume 33, 1992 549 by gest, on N ovem er 6, 2017 w w w .j.org D ow nladed fom