The choline-depleted type II pneumocyte. A model for investigating the synthesis of surfactant lipids

When type II pneumocytes from adult rats were maintained in a medium that lacked choline, the incorporation of [¹⁴C]glycerol into phosphatidylcholine was not greatly diminished during the period that the cells displayed characteristics of type II pneumocytes. Cells that were maintained in choline-free medium that contained choline oxidase and catalase, however, became depleted of choline and subsequent synthesis of phosphatidylcholine by these cells was responsive to choline in the extracellular medium. Incorporation of [¹⁴C]glycerol into phosphatidylcholine by choline-depleted cells was stimulated maximally (approx. 6-fold) by extracellular choline at a concentration (0.05 mM) that also supported the greatest incorporation into phosphatidylglycerol. The incorporation of [¹⁴C]glycerol into other glycerophospholipids by choline-depleted cells was not increased by extracellular choline. When cells were incubated in the presence of [³H]cytidine, the choline-dependent stimulation of the synthesis of phosphatidylcholine and phosphatidylglycerol was accompanied by an increased recovery of [³H]CMP. This increased recovery of [³H]CMP reflected an increase in the intracellular amount of CMP from 48±9 to 76±16 pmol/10⁶ cells. Choline-depleted cells that were exposed to [³H]choline contained [³H]CDP-choline as the principal water-soluble choline derivative. As the extracellular concentration of choline was increased, however, the amount of ³H in phosphocholine greatly exceeded that in all other water-soluble derivatives. Choline-depletion of cells resulted in an increase in the specific activity of CTP:phosphocholine cytidylyltransferase in cell homogenates (from 0.40±0.15 to 1.31±0.20 nmol.min^-1.mg of protein^-1). These data are indicative that the biosynthesis of phosphatidylcholine is integrated with that of phosphatidylglycerol and are consistent with the proposed involvement of CMP in this integration. The choline-depleted type II pneumonocyte provides a new model for investigating the regulation of CTP:phosphocholine cytidylyltransferase activity.