Specific pools of phospholipids are used for lipoprotein secretion by cultured rat hepatocytes.

J E Vance,D E Vance

doi:10.1016/s0021-9258(17)38526-5

Abstract

Since phospholipids are major components of all serum lipoproteins, the role of phospholipid biosynthesis in lipoprotein secretion from cultured rat hepatocytes has been investigated. In liver, phosphatidylcholine is made both by the CDP-choline pathway and by the methylation of phosphatidylethanolamine, which in turn is derived from both serine (via phosphatidylserine) and ethanolamine (via CDP-ethanolamine). Monolayer cultures of rat hepatocytes were incubated in the presence of [methyl-3H]choline, [1-3H] ethanolamine, or [3-3H]serine. The specific radioactivity of the phospholipids derived from each of these precursors was measured in the cells and in the secreted lipoproteins of the cultured medium. The specific radioactivities of phosphatidylcholine and phosphatidylethanolamine derived from [1-3H]ethanolamine were markedly lower (approximately one-half and less than one-tenth, respectively) in the secreted phospholipids than in the cellular phospholipids. Thus, ethanolamine was not an effective precursor of the phospholipids in lipoproteins. On the contrary, the specific radioactivity of phosphatidylcholine made from [methyl-3H]choline was approximately equal in cells and lipoproteins. In addition, over the first 4 h of incubation with [3-3H]serine, the specific radioactivities of phosphatidylcholine and phosphatidylethanolamine were significantly higher in the lipoproteins than in the cells. These data indicate that there is not a random and homogeneous labeling of the phospholipid pools from the radioactive precursors. Instead, specific pools of phospholipids are selected, on the basis of their routes of biosynthesis, for secretion into lipoproteins.

Highlights

Since phospholipids are major components of all been investigated in several studies [3,4,5]
PE itself is derived from either ethanolamine [10] or by the decarboxylation of phosphatidylserine, a reaction which occurs in the mitochondrion [11].In an earlier study: we investigated whether or not the methylation pathway for PC synthesis from ethanolamine was required for lipoprotein secretion
Phospholipids, phosphatidylcholine (PC1),are Using the adenosine analogue 3-deazaadenosine,which inhibprominent components of all plasma lipoproteins, yet little is itsPE methylation by greater than 90% in cultured rat known of the function and sequence of assembly of phospho- hepatocytes [12],we demonstrated that thesecretion of apolipids into nascentlipoprotein particles [1].In human plasma, proteins, PC and P E into lipoprotein fractions VLDL, LDL, 20% of the lipid components of VLDL is phospholipid, of HDL, and a fraction of density greater than 1.18 g/ml, was which 60% is PC

Summary

Source of phospholipid

SpecificPhospholipid Pools and LipoSpreoctreeitnion lipoprotein fractions of the culture medium. One might have anticipated that any difference in specific radioactivity between the cells and medium, which was the result of the lag time takenfor the lipoprotein particle to be assembled and secreted, would have been abolished by 24 h. These experiments (Table 11, Fig. lA)show that PC made from ethanolamine was secreted into the medium, this secreted PC was not so highly labeled from ethanolamine asthe total cellular pool of PC. Specific radioactivities of thePCand PE were determined from radioactivity incorporated and chemical amounts of the phospholipids, as described under "Materials and Methods." Each value of specific radioactivity is the mean f S.D. of at least three analyses

Specific radioactivity of phospholipid

Specific radioactivityof phospholipid

Findings

VLDL LDL