Developmental expression of elements

The expression of several key enzymes and receptors of rat hepatic cholesterol metabolism was studied during development. Among major findings were: acyl coenzyme A:cholesterol acyltransferase, the cholesteryl ester hydrolases, cholesterol7a-hydroxylase and the a2-macroglobulin receptor (LRP) were very low in fetal livers, but all were induced shortly before birth, suggesting that these elements are important for extrauterine life. Although the other elements continued to increase, by day 6 of postnatal life, cholesterol-7a-hydroxylase had reached undetectable levels. It reappeared by day 12 of suckling, placing it in the group of late-appearing activities necessary for the fully mature hepatic phenotype. Changes in acyl coenzyme A:cholesterol acyltransferase activity appeared due predominantly to changes in amount of active protein. The cholesteryl ester hydrolase (CEH) activities all showed different developmental patterns, suggesting that each was a unique activity. The bile saltdependent CEH activity was much higher in the suckling period than in the adult where it was almost undetectable, suggesting that this CEH may have its major importance in the suckling period of development. Low density lipoprotein receptors exhibited a pattern very different from that of the a2-macroglobulin receptors and did not show consistent correlation with any other elements. At some developmental time points, the relationships amongst the elements differed significantly from the adult pattern. These studies provide for the first time an integrated picture of developmental expression of key elements of hepatic cholesterol metabolism and set the stage for further studies on their modes of regulation.-Smith, J. L., S. R. Lear, and S. K. Erickson. Developmental expression of elements of hepatic cholesterol metabolism in the rat.J Lipid Res. 1995. 36: 641-652. Supplementary key words acyl coenzyme A:cholesterol acyltransferase cholesteryl ester hydrolase cholesterol-7a-hydroxylase 3hydroxy-3-methylglutaryl coenzyme A reductase low density lipoprotein receptor cY2-macroglobulin receptor (LRP) The liver plays an important role in maintaining cholesterol homeostasis in the body. In the adult this is achieved by integrated regulation of a group of hepatic enzymes, receptors, and other proteins key for cholesterol, lipoprotein, and biliary metabolism. However, to date the interactions and regulation of these pathways to maintain cholesterol homeostasis during development have not been systematically studied in an integrated fashion. The developmental pattern of activity (1-5) and specific mRNA content (6) for 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, a rate-limiting enzyme for mevalonate and isopentenoid synthesis, including cholesterol, has been reported in rat liver during development. Limited data are available on the response of hepatic acyl coenzyme A:cholesterol acyltransferase (ACAT) activity (7, 8) as are limited data on expression of rat 0-very low density lipoprotein (VLDL) binding sites (recognizing both apolipoprotein (apo) E and apoB) during development (9). The mRNA level for the hepatic low density lipoprotein (LDL) receptor has been examined at some postnatal developmental points in rats (lo), but no data have been reported on the expression of LDL receptor protein during development. No data have been reported on ontogeny of the LDL receptor-related protein (LRP, a2-macroglobulin receptor), proposed to be involved in clearance of apoE-containing lipoproteins (11-13). The patterns of rat liver cholesteryl ester hydrolase (CEH) activities during development also have not been reported. The developmental expression of mRNA for a variety of apolipoproteins and 1ecithin:cholesterol acyltransferase (LCAT) has been determined in rat liver (14-19), and changes in plasma lipoprotein patterns (9, 20-23) and plasma LCAT (9) during some stages of development have been described. Data also have been published on developmental expression of elements of bile acid synthesis, secretion, and metabolism (24-31). We hypothesized that changes in the relationships or expression of these elements occur at key points in development, for example, when the animal as a whole is known to undergo dietary and hormonal changes, e.g., at Abbreviations: HMG-CoA, 3-hydroxy-3-methylglutaryl coenzyme A; CEH, cholesteryl ester hydrolase; ACAT, acyl coenzyme A:cholesterol acyltransferase; LDL, low density lipoprotein; LRP, low density lipoprotein receptor-related protein. 'Present address: Lipid Metabolism Laboratory, Department of Surgery, University of Queensland, Royal Brisbane Hospital, Herston, QLD 4029, Australia. *To whom correspondence should be addressed. Journal of Lipid Research Volume 36, 1995 641 at P E N N S T A T E U N IV E R S IT Y , on F ebuary 1, 2013 w w w .j.org D ow nladed fom birth, after first suckling, at weaning (for general review see ref. 32). Further, we hypothesized that during the suckling period, (when liver morphology and hepatospecific functions begin to mature, with minimal known changes in diet and hormonal pattern) changes in expression of these elements are the result of an intrinsic liver differentiation program. As a first step in understanding how the elements of cholesterol metabolism are regulated developmentally, we investigated the expression of activities of ACAT, cytosolic and microsomal CEH (acidic, neutral, and bile aciddependent activities), cholesterol-7a-hydroxylase (both activity and protein level), the LDL receptor, and a2macroglobulin receptor (LRP) in rat livers from 19-dayold fetuses at time intervals through to 28-day-old animals (1 week post weaning) and compared these values with those in the adult rat liver. The rat was chosen because the developing rat liver provides a good model for studying human hepatic development (33). HMG-CoA reductase activity was used as a developmental reference point in hepatic cholesterol metabolism because the most complete studies to date have been reported for this enzyme (1-6). EXPERIMENTAL PROCEDURES