Biosynthesis of dipalmitoyl- sn-glycero-3-phosphocholine by adenoma alveolar type II cells

Abstract

Adenomas induced in lungs of BALB/c mice by urethane treatment consist almost exclusively of pulmonary type II cells and, like normal type II cells, produce dipalmitoylphosphatidylcholine, the major lipid component of lung surfactant [Snyder, C., Malone, B., Nettesheim, P., and Snyder, F. (1973) Cancer Res. 33, 2437–2443]. In the present investigation, these adenomas were used for studying the biosynthesis of dipalmitoylphosphatidylcholine in the intact cells. Whole adenomas were incubated in Krebs-Ringer phosphate buffer or Medium 199 with [1- 14C]palmitate, [1- 14C]acetate, [U- 14Clglycerol, or [1,3- 14C,2- 3H]glycerol and the synthesis of [ 14C]dipalmitoylphosphatidylcholine was subsequently determined after its isolation by cryochromatography. The [1- 14C]acetate was converted to [ 14C]palmitate, which was incorporated into disaturated phosphatidylcholine in the same manner as exogenous [1- 14C]palmitate. After a 1-h incubation of either [1- 14C]palmitate or [1- 14C]acetate, approximately 60% of the [ 14C]phosphatidylcholine was disaturated while 75–80% of the label in the disaturated species was in the acyl group at the 2 position. The same results were obtained when minced lung was used instead of adenomas, except the incorporation rate was higher in the adenomas. It was concluded from these studies that palmitate, whether synthesized de novo or provided exogenously, enters a common pool before being used by the type II cells for the synthesis of dipalmitoylphosphatidylcholine. When [1,3- 14C,2- 3H]glycerol was used to label phosphatidylcholine in the system, all molecular species, based on the degree of unsaturation, retained the same 3 H 14 C ratio. This indicated that the dihydroxyacetone phosphate pathway is of no more significance for the synthesis of disaturated phosphatidylcholine than for the other molecular species. The study demonstrates the usefulness of the adenoma system as a model for studying surfactant biosynthesis in pulmonary type II cells and indicates that the major pathway by which palmitate enters dipalmitoylphosphatidylcholine is by the deacylation and reacylation of phosphatidylcholine.