Cytidine monophosphate-dependent synthesis of phosphatidylglycerol in permeabilized type II pneumonocytes

Abstract

Results of previous investigations support the proposition that, in type II pneumonocytes, CMP is involved in integration of the synthesis of phosphatidylcholine and phosphatidylglycerol for lung surfactant. In the present investigation, the amount of CMP in rat type II pneumonocytes was altered directly and resultant changes in the synthesis of phosphatidylglycerol were examined. Type II pneumonocytes were made permeable to CMP by treatment with Ca2+-free medium, and phosphatidylglycerol synthesis was then assessed by measurement of the incorporation of a radiolabelled precursor, [14C]glycerol 3-phosphate, that was not effectively utilized by cells that resisted permeabilization. Incorporation of [14C]glycerol 3-phosphate into phosphatidylglycerol (but not into other lipids) was stimulated greatly by CMP (half-maximal stimulation at approx. 0.1 mM). CMP stimulated the incorporation of [14C]glycerol 3-phosphate into both the phosphatidyl moiety and the head group of phosphatidylglycerol. Incorporation of [14C]palmitate into phosphatidylglycerol was also stimulated by CMP. myo-Inositol, at concentrations found in foetal-rat serum (0.2-2.0 mM), inhibited CMP-dependent incorporation of [14C]glycerol 3-phosphate into phosphatidylglycerol and promoted, instead, CMP-dependent incorporation into phosphatidylinositol. These data, when extrapolated to foetal type II pneumonocytes, are consistent with the view that the developmental increase in the synthesis of phosphatidylglycerol for surfactant by foetal lungs is promoted by the increase in intracellular CMP and the declining availability of myo-inositol that were found previously to be associated with this period of development.

Altered lipid synthesis in type II pneumonocytes exposed to lung surfactant

“…The apparently selective inhibition of dihydroxyacetonephosphate acyltransferase activity in type II cells exposed to surfactant is consistent with the observation that the incorporation into lipids of [2-3H]glycerol relative to [U-14C]glycerol is increased in type II cells exposed to surfactant. The observation that total incorporation ofradiolabelled glycerol into phosphatidylglycerol is increased in the presence of surfactant, but the ratio of 14C to 3H is unchanged, is consistent with the finding that, after short incubation periods, most of the glycerol is incorporated into the head group of this lipid [24] and so does not involve dihydroxyacetone-phosphate acyltransferase. The finding that inhibition of dihydroxyacetone-phosphate acyltransferase activity in type II cells exposed to surfactant is small may be indicative that only one of the subcellular forms of this enzyme [30] is involved in surfactant lipid synthesis and is inhibited when type II cells are exposed to surfactant.…”

“…Glycerophospholipids in lipid extracts were separated by use of two-dimensional t.l.c. [25] and radioactivity associated with individual lipids was measured as described previously [23,24]. When disaturated phosphatidylcholine was analysed, total lipid extracts were treated with OS04 to oxidize unsaturated lipids [31], and disaturated phosphatidylcholine was then separated by use of t.l.c.…”

“…Washed cells were detached from culture dishes by scraping three times into ice-cold ethanol (1 x 1 ml, 2 x 0.5 ml). Suspensions of broken cells in ethanol were evaporated to dryness under N2 and lipids were extracted from the residues as described previously [23,24]. In some experiments, after the cells were exposed to radiolabelled surfactant and then washed, further attempts were made to remove radioactive surfactant that remained associated with the cells.…”

Simultaneous measurement of phosphatidylglycerol and disaturated‐phosphatidylcholine palmitate kinetics from alveolar surfactant. Study in infants with stable isotope tracer, coupled with isotope ratio mass spectrometry

“…This conclusion is in agreement with the results of animal studies [35,36] and with the fact that DSPC, PG and phosphatidyl inositol (PI) originate from interconnected synthetic pathways. [31] Indeed, in type II pneumocytes isolated from rabbits at late gestation, [37][38][39] an increase in DSPC synthesis is accompanied by an increase in PG synthesis and a decrease in PI synthesis. [39] The time to maximum incorporation of tracer was significantly shorter for PG with respect to DSPC ( Table 2).…”