ABSTRACT
Sphingolipids represent a diverse group of bioactive lipid species that are generated intracellularly in response to tumor necrosis factor-alpha (TNF-alpha) and are implicated as potential mediators of acute lung injury. The purpose of these studies was to determine whether there was an extracellular, TNF-alpha-regulated pool of sphingolipids in the alveolus that modulates the surface tension lowering capacity of pulmonary surfactant. Intratracheal instillation of TNF-alpha in adult rats led to a twofold increase in the amount of surfactant-associated ceramide and tended to decrease levels of sphingomyelin without significantly altering sphingosine or sphinganine content. TNF-alpha induction of alveolar ceramide was associated with nearly an 80% increase in acid sphingomyelinase activity recovered in cell-free alveolar lavage. Ceramide administered in a dose-dependent manner potently antagonized the surface tension lowering effects of natural surfactant in vitro. Intratracheal TNF-alpha and ceramide treatment of rats significantly increased lung permeability, as was evidenced by extravasation of Evans blue dye into alveolar lavage and lung tissue. Thus these studies are the first to demonstrate the existence of a cytokine-regulated alveolar pool of sphingomyelin hydrolysis products that impairs the biophysical properties of the alveolar surfactant film. The results also suggest the presence of a secretory alveolar sphingomylinase that is TNF-alpha responsive and mediates effects of the cytokine on alveolar sphingolipid metabolism.
Subject(s)
Pulmonary Alveoli/metabolism , Pulmonary Surfactants/metabolism , Sphingolipids/physiology , Tumor Necrosis Factor-alpha/pharmacology , Animals , Biophysical Phenomena , Biophysics , Capillary Permeability/drug effects , Ceramides/pharmacology , Coloring Agents/pharmacokinetics , Evans Blue/pharmacokinetics , Male , Pulmonary Circulation/drug effects , Rats , Rats, Sprague-Dawley , Sphingomyelin Phosphodiesterase/metabolism , Surface TensionABSTRACT
We examined whether administration of very low-density lipoproteins (VLDL) to pregnant rats increases surfactant phosphatidylcholine (PtdCho) content in fetal pre-type II alveolar epithelial cells. VLDL-triglycerides are hydrolyzed to fatty acids by lipoprotein lipase (LPL), an enzyme activated by heparin. Fatty acids released by LPL can incorporate into the PtdCho molecule or activate the key biosynthetic enzyme cytidylyltransferase (CCT). Dams were given BSA, heparin, VLDL, or VLDL with heparin intravenously. Radiolabeled VLDL given to the pregnant rat crossed the placenta and was distributed systemically in the fetus and incorporated into disaturated PtdCho (DSPtdCho) in pre-type II cells. Maternal administration of VLDL with heparin increased DSPtdCho content in cells by 45% compared with control (P < 0.05). VLDL produced a dose-dependent, saturable, and selective increase in CCT activity. VLDL did not significantly alter immunoreactive CCT content but increased palmitic, stearic, and oleic acids in pre-type II cells. Furthermore, hypertriglyceridemic apolipoprotein E knockout mice contained significantly greater levels of DSPtdCho content in alveolar lavage and CCT activity compared with either LDL receptor knockout mice or wild-type controls that have normal serum triglycerides. Thus the nutritional or genetic modulation of serum VLDL-triglycerides provides specific fatty acids that stimulate PtdCho synthesis and CCT activity thereby increasing surfactant content.
Subject(s)
Lipoproteins, VLDL/pharmacology , Lung/embryology , Pregnancy, Animal/drug effects , Pulmonary Surfactants/biosynthesis , Animals , Apolipoproteins E/deficiency , Apolipoproteins E/genetics , Choline-Phosphate Cytidylyltransferase/metabolism , Fatty Acids/metabolism , Female , Fetus/metabolism , Immunoblotting , Injections, Intravenous , Lipoproteins, VLDL/administration & dosage , Lipoproteins, VLDL/pharmacokinetics , Mice , Mice, Knockout/genetics , Mice, Knockout/metabolism , Phosphatidylcholines/metabolism , Pregnancy , Rats , Rats, Sprague-Dawley , Receptors, LDL/geneticsABSTRACT
Tumor necrosis factor (TNF)-alpha is a major cytokine implicated in inducing acute and chronic lung injury, conditions associated with surfactant phosphatidylcholine (PtdCho) deficiency. Acutely, TNF-alpha decreases PtdCho synthesis but stimulates surfactant secretion. To investigate chronic effects of TNF-alpha, we investigated PtdCho metabolism in a murine transgenic model exhibiting lung-specific TNF-alpha overexpression. Compared with controls, TNF-alpha transgenic mice exhibited a discordant pattern of PtdCho metabolism, with a decrease in PtdCho and disaturated PtdCho (DSPtdCho) content in the lung, but increased levels in alveolar lavage. Transgenics had lower activities and increased immunoreactive levels of cytidylyltransferase (CCT), a key PtdCho biosynthetic enzyme. Ceramide, a CCT inhibitor, was elevated, and linoleic acid, a CCT activator, was decreased in transgenics. Radiolabeling studies revealed that alveolar reuptake of DSPtdCho was significantly decreased in transgenic mice. These observations suggest that chronic expression of TNF-alpha results in a complex pattern of PtdCho metabolism where elevated lavage PtdCho may originate from alveolar inflammatory cells, decreased surfactant reuptake, or altered surfactant secretion. Reduced parenchymal PtdCho synthesis appears to be attributed to CCT enzyme that is physiologically inactivated by ceramide or by diminished availability of activating lipids.
