Intrahepatic assembly of very low density lipoproteins. Varied synthetic response of individual apolipoproteins to fasting.

Hepatocytes obtained from rats fed for 3 days chow (control) or drinking water only (fasted) were used to examine how metabolic state affects lipogenesis, apolipoprotein synthesis, and the capacity to secrete de novo synthesized triacylglycerol. The secretion of triacylglycerol (mass and 3H-labeled via 3H2O incorporation) by both groups of cells was constant for 30 h. Moreover, cells from fasted rats secreted triacylglycerol at rates which were markedly reduced (mass -84%; 3H-labeled -91%). To assess the relative capacities of the two groups of hepatocytes to augment triacylglycerol secretion in response to stimulated lipogenesis, cells were incubated with increasing concentrations of glucose. Control cells responded to glucose by increasing equally the synthesis and secretion of [3H] triacylglycerol. When cells from fasted rats were challenged with glucose, triacylglycerol secretion was not increased. Rather, it accumulated intracellularly. Double-reciprocal plot analysis of the capacity to augment triacylglycerol secretion in response to glucose showed that cells from fasted rats had a greater than 10-fold decrease in V'max. Moreover, fasting changed the synthesis and secretion of apolipoproteins selectively: secretion of low molecular weight apo-B was decreased 50%, large molecular weight apo-B was unchanged, and apo-E was increased 2-4-fold. Analysis of the lipoproteins from both groups of cells on Bio-Gel A-50m showed that the very low density lipoprotein secreted by cells from fasted rats was smaller. In addition, all of the increased de novo synthesized apo-E secreted by cells from fasted rats eluted after the triacylglycerol-rich lipoproteins. The combined data show that: 1) the synthesis of individual very low density lipoprotein apolipoproteins is independently regulated, and 2) the synthesis (availability) of apo-B determines the capacity of the hepatocyte to assemble/secrete triacylglycerol-rich very low density lipoprotein.

Dietary carbohydrate induces lipogenesis and very-low-density lipoprotein synthesis.

Hepatocytes obtained from sucrose-fed rats secreted triacylglycerol at a rate that was about twice that of cells from control rats. The increased rate of triacylglycerol secretion by cells from sucrose-fed rats was accompanied by a twofold increase in its rate of synthesis as determined by 3H2O incorporation. In addition, cells from sucrose-fed rats had a two- to threefold increase in apolipoprotein synthesis. Differences between the two groups became even more marked when cells were challenged in vitro with glucose. Double-reciprocal analysis showed that compared with controls, cells from sucrose-fed rats had a fourfold increase in the Vmax that described the glucose stimulation of [3H]triacylglycerol secretion. In contrast to in vivo carbohydrate (sucrose) induction of apolipoprotein synthesis, glucose added in vitro did not affect apolipoprotein synthesis. These data suggest that in vivo induction by dietary carbohydrate requires factors in addition to increased hexose that are not contained within the isolated hepatocyte system. The coinduction by dietary carbohydrate of both lipogenesis and apolipoprotein synthesis is likely to play a role in the increased capacity of cells from sucrose-fed rats to both assemble and secrete triacylglycerol-rich lipoproteins.