Synthesis and secretion of B-100 and A-I apolipoproteins in response to the changes of intracellular cholesteryl ester content in chick liver.

We investigated in the chick whether the diet-induced changes of the hepatic content of cholesteryl esters (CE) influence the synthesis and the secretion of apoBand apoA-Icontaining lipoproteins. Control chicks received a low cholesterol diet for 2 (SD-I), 4 (SD-2), or 7 (SD-3) weeks; the chicks in the experimental groups received a cholesterol-rich diet for 2 weeks and were killed at the end of the cholesterol feeding (CH-F), and after 2 (CH-D) or 5 (CH-DD) weeks of a low cholesterol diet. Hepatic CE content in CH-F chicks was 30-fold that observed in controls, but returned to the control level after 5 weeks of cholesterol depletion (CH-DD). The incorporation of "S-labeled amino acids into cell and medium apoB and apoA-I was measured in liver slices. Intracellular 35S-labeled apoB was similar in all groups whereas medium 35S-labeled apoB was 2-fold higher in CH-F than in controls (SDI). Pulse-chase experiments showed that radioactive apoB secreted by CH-F chicks at 120 min of chase was 2 times that of SD-1 chicks. This increased secretion of apoB was not found in CH-D chicks. In CH-F chicks, the intracellular and medium 35S-labeled apoA-1 were 2-fold the values found in controls (SD-1); apoA-I production returned to the control level only after 5 weeks of cholesterol depletion (CHDD). The increased secretion of apoB and apoA-I in CH-F chicks was associated with an increased secretion of very low, intcrmediate, and low density lipoproteins containing newly synthesized apoB and apoA-I and of high density lipoproteins containing predominantly apoA-I. I Thus, in response to hepatic CE accumulation induced by cholesterol feeding, a larger proportion of newly synthesized apoB is driven to the secretory pathway and more apoA-I is synthesized. This promotes an increased secretion of plasma lipoproteins that contribute to the removal of CE from the liver.-Tarugi, P., S. Nicolini, G. Ballarini, L. Marchi, C. Duvigneau, P. Tartoni, and s. Calandra. Synthesis and secretion of B-100 and A-I apolipoproteins in response to the changes of intracellular cholesteryl ester content in chick liver.]. Lipid Res. 1996. 37: 493-507. Supplementary key words esters lipoprotein secretion cholesterol feeding hepatic cholesteryl In recent years there has been renewed interest in the changes of hepatic cholesterol content that occur during late embryonic and early post-natal life of the chick (1-4). A large amount of cholesteryl esters (CE) progressively accumulates in the liver during the embryonic life and the first 2 days after hatching, reaching a level that is 30-fold that found in the adult animal (1-4). It has been suggested that CE accumulation derives from the uptake of remnants of cholesterol-rich lipoproteins secreted by the yolk sac membrane as well as the inefficiency of the embryonic liver in disposing of this excess material (1, 2). This enormous hepatic CE store decreases within a few days (days 2-7) of post-natal life (3, 4). This rapid depletion is thought to be the result of the interaction of several factors, such as the local utilization of cholesterol for growth, the conversion of cholesterol into bile acids, or its direct excretion into the bile, and the secretion of CE-rich lipoproteins into the plasma (1, In a recent study (4) we demonstrated that the depletion of hepatic CE that occurs from day 2 to day 7 of post-natal life was associated with an increased production of apoB-100 and apoA-I by the liver and an increased secretion of CE-rich lipoproteins containing these apolipoproteins. These results led us to propose that CE accumulation in the liver of the newborn chick might be the trigger for the increased production of apoA-I and apoB (4). Within this context it might be expected that other in vivo conditions known to cause a substantial accumulation of CE in the liver are associ2,4). Abbreviations: apoB, apolipoprotein B100; apoA-I, apolipoprotein A-I; VLDL, very low density lipoproteins; IDL, intermediate density lipoproteins; LDL, low density lipoproteins; HDL, high density lipoproteins; SDS, sodium dodecyl sulfate; PAGE, polyacrylamide gel electrophoresis; CE, cholesteryl ester; CH, cholesterol; HSS, hi h speed supernatant. 'To whom correspondence should be addressed. 'This work is part of the PhD thesis of Dr. S. Nicolini. Journal of Lipid Research Volume 37, 1996 493 by gest, on July 4, 2011 w w w .j.org D ow nladed fom ated with an overproduction of apoB and apoA-I, leading to an enhanced secretion of plasma lipoproteins that, in turn, contributes to the removal of CE from the liver. To test this hypothesis we have chosen the model of cholesterol feeding, a dietary manipulation known to cause an accumulation of cholesteryl esters in the liver (5, 6) whose magnitude might be comparable to that found in the liver of the newborn chick. In order to mimic the situation found in the newborn chick (accumulation and depletion of hepatic CE), our study was performed in chicks that, after being fed a cholesterolrich diet, (hepatic cholesterol accumulation), received a low cholesterol diet for variable periods of time (hepatic cholesterol depletion). MATERIALS AND METHODS