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CD14-dependent internalization and metabolism of extracellular phosphatidylinositol by monocytes
1Department of Internal Medicine, The University of Texas Southwestern Medical Center, Dallas, Texas 75235-9113, USA.
Abstract:
We report that membrane CD14 (mCD14), a cell surface receptor found principally on leukocytes, can mediate the uptake and metabolism of extracellular phosphatidylinositol (PtdIns). mCD14 facilitates PtdIns internalization, targeting it to intracellular sites where, following stimulation with a calcium ionophore, it can be acted upon by cytosolic phospholipase A(2). The [(14)C]arachidonate released from mCD14-acquired [(14)C]arachidonyl-PtdIns is either esterified to triacylglycerol and retained in the cell or secreted as free arachidonate or leukotrienes. Although less than 10% of the arachidonate-derived lipids secreted from endogenous cellular stores are 5-lipoxygenase metabolites, over one-half of the secreted (14)C-lipids derived from mCD14-acquired PtdIns are hydroxyeicosatetraenoic acids or leukotriene B(4). mCD14 may allow these highly active blood cells to acquire and use extracellular PtdIns as a source of arachidonate for leukotriene synthesis.
Insights
Membrane CD14 (mCD14) on leukocytes facilitates uptake of extracellular phosphatidylinositol (PtdIns). This process provides arachidonate for leukotriene synthesis, impacting inflammatory responses.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Leukocytes utilize cell surface receptors for nutrient uptake.
- Phosphatidylinositol (PtdIns) is a key lipid involved in cellular signaling and metabolism.
- Arachidonate is a precursor for important inflammatory mediators like leukotrienes.
Purpose of the Study:
- To investigate the role of membrane CD14 (mCD14) in the uptake and metabolism of extracellular phosphatidylinositol (PtdIns).
- To determine the fate of arachidonate derived from mCD14-mediated PtdIns acquisition.
- To assess the contribution of mCD14-acquired PtdIns to leukotriene synthesis.
Main Methods:
- Utilized radiolabeled [(14)C]arachidonyl-PtdIns to trace uptake and metabolism.
- Stimulated cells with a calcium ionophore to activate phospholipase A(2).
- Analyzed secreted lipids, including free arachidonate, hydroxyeicosatetraenoic acids, and leukotrienes.
Main Results:
- mCD14 mediates the internalization of extracellular PtdIns into leukocytes.
- Intracellular PtdIns is acted upon by cytosolic phospholipase A(2) upon stimulation.
- A significant portion of arachidonate from mCD14-acquired PtdIns is converted to hydroxyeicosatetraenoic acids and leukotriene B(4).
Conclusions:
- mCD14 serves as a pathway for leukocytes to acquire extracellular PtdIns.
- This acquisition provides a source of arachidonate for the synthesis of potent inflammatory mediators like leukotrienes.
- mCD14 plays a crucial role in lipid metabolism and inflammatory signaling in leukocytes.