Abnormal liver phosphatidylcholine synthesis revealed in patients with acute respiratory distress syndrome

Abstract

Acute respiratory distress syndrome (ARDS) is associated with a severe pro-inflammatory response; although decreased plasma cholesterol concentration has been linked to systemic inflammation, any association of phospholipid metabolic pathways with ARDS has not been characterized. Plasma phosphatidylcholine (PC), the major phospholipid of circulating lipoproteins, is synthesized in human liver by two biologically diverse pathways: the cytidine diphosphocholine (CDP):choline and phosphatidylethanolamine N-methyltransferase (PEMT) pathways. Here, we used ESI-MS/MS both to characterize plasma PC compositions and to quantify metabolic fluxes of both pathways using stable isotopes in patients with severe ARDS and in healthy controls. Direct incorporation of methyl-D9-choline estimated CDP:choline pathway flux, while PEMT flux was determined from incorporations of one and two methyl-D3 groups derived from methyl-D9-choline. The results of MS/MS analysis showed significant alterations in plasma PC composition in patients with ARDS versus healthy controls. In particular, the increased overall methyl-D9-PC enrichment and, most importantly, the much lower methyl-D3-PC and methyl-D6-PC enrichments suggest increased flux through the CDP:choline pathway and reduced flux through the PEMT pathway in ARDS. To our knowledge, this study is the first to demonstrate significant plasma PC molecular compositional changes combined with associated alterations in the dynamics of PC synthetic pathways in patients with ARDS.

Insight into erythrocyte phospholipid molecular flux in healthy humans and in patients with acute respiratory distress syndrome

“…Comparison between plasma and erythrocyte PC, LPC and SM compositions in ARDS patients indicates that previously reported alterations of plasma PC between ARDS patients and healthy volunteers are generally reflected in erythrocyte lipid compositions in the patient group (Table 2) [23]. ARDS patients exhibited significantly increased fractional compositions of monounsaturated (PC16:0_18:1) and disaturated (PC16:0/16:0) PC species in erythrocytes compared with healthy controls, with a similarly increased content of LPC18:1.…”

“…The incorporation of methyl -D 9 choline into the labeled plasma fraction and later into the erythrocyte PC provides a novel mechanism to monitor PC exchange between these two highly phospholipid enriched compartments. We have previously demonstrated the specificity and kinetics of methyl -D 9 choline into plasma PC in healthy controls and patients with ARDS [23]. The combination of these two parameters provides the mechanistic approach to quantify exchange of PC, LPC and SM between plasma and erythrocytes (Fig 3).…”

Postnatal adaptations of phosphatidylcholine metabolism in extremely preterm infants: implications for choline and PUFA metabolism

“…The alternative PEMT pathway, which synthesizes PUFA-enriched PC by 3 sequential methylations of phosphatidylethanolamine (PE) ( Figure 1 ) ( 15 ), is the sole means of endogenous choline production and accounts for ∼30% of total liver PC synthesis ( 16 ) in adults. Stable isotope labeling with methyl -D 9 -choline chloride, combined with electrospray ionization tandem MS (ESI-MS/MS), has been used to quantify the synthesis and turnover of individual molecular species of PC in lung surfactant in human volunteers ( 17 ), of hepatic PC in vivo ( 16 ), and in adult patients ventilated for acute respiratory distress syndrome ( 18 ). Here, we have applied this technology to characterize PC metabolism and turnover in preterm infants and have demonstrated the molecular specificity of PC synthesis by the immature preterm liver in vivo, monitored by the appearance of label in plasma PC.…”

Analytical Considerations of Stable Isotope Labelling in Lipidomics

“…Reversed phase (RP) chromatography is often used for analysis of stable isotope-labelled lipidomes [27,28,57,58,59], and hydrophilic interaction liquid chromatography (HILIC) has also been employed [60]. RP separates lipids according to their overall hydrophobicity (governed largely by hydrocarbon chain length), whereas the separation in HILIC is driven by the lipid head groups, with only minimal separation within individual classes [46].…”

“…For isotope labelling studies, triple quadrupole instruments or high resolution instruments such as Orbitraps and QTOFs (quadrupole-time-of-flight) are most widely used. Triple quadrupole instruments are best suited for determining the incorporation of labelled building blocks (e.g., of head groups such as labelled choline [49,60] or ethanolamine [62], or of fatty acids [19,57,63]), as these can be readily detected using precursor or neutral loss scans. In contrast, high resolution mass spectrometers rely on the acquisition of full scan spectra with high resolution and high mass accuracy to identify compounds [29,48].…”