The role of membrane-bound LBP, endotoxin aggregates, and the MaxiK channel in LPS-induced cell activation.
We have previously shown in patch-clamp experiments on excised outside-out cytoplasmic membrane patches from human macrophages that the activation of a high-conductance Ca(2+)- and voltage-dependent potassium channel, the MaxiK channel, is an early step in LPS-induced transmembrane signal transduction in macrophages. MaxiK can be activated by agonistically active LPS, and activation can be completely inhibited by LPS antagonists (e.g. synthetic compound 406) and by anti-CD14 antibodies. Furthermore, by inhibiting MaxiK with the specific MaxiK blocker paxilline, we could show that activation of MaxiK is essential for LPS-induced cytokine production. As shown by RT-PCR, blockade of MaxiK by paxilline also inhibits induction of the mRNA of TNF-alpha and IL-6. This observation together with the fact that all patch-clamp experiments were done on excised outside-out patches reveal that MaxiK activation is an early step in cell activation by endotoxins. Thus, since cells lacking TLR4 on their surface can also not be activated to produce cytokines, these data allow the conclusion that TLR4 and MaxiK are both essential for activation by LPS and may form a co-operative signaling complex. We have also shown that LBP not only exists as a soluble acute-phase serum protein, but is also incorporated as a transmembrane protein (mLBP) in the cytoplasmic membrane of MNC; in this configuration, it is obviously involved in the binding of endotoxin and its transfer to the transmembrane signaling proteins finally triggering cell activation. Complexation of soluble LBP and LPS in the serum prior to binding of LPS to mLBP, in contrast, leads to neutralization of LPS. Here, we provide evidence from fluorescence resonance energy transfer spectroscopy that endotoxin aggregates are intercalated into reconstituted membranes by mLBP. In addition, cell culture assays and patch-clamp experiments demonstrate that endotoxin activates macrophages and the MaxiK channel in the aggregated, but not in the monomeric, state at similar concentrations.
Müller
M
M
Division of Biophysics, Research Center Borstel, Center for Medicine and Biosciences, Parkallee 10, D-23845 Borstel, Germany.
Scheel
O
O
Lindner
B
B
Gutsmann
T
T
Seydel
U
U
D000209:Acute-Phase Proteins; D019138:Antibodies, Blocking / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects* / Q000378:metabolism; D002478:Cells, Cultured; D006017:Glycolipids / Q000494:pharmacology; D006801:Humans; D007211:Indoles / Q000494:pharmacology; D015850:Interleukin-6 / Q000235:genetics / Q000378:metabolism; D051037:Large-Conductance Calcium-Activated Potassium Channel alpha Subunits; D051036:Large-Conductance Calcium-Activated Potassium Channels; D008050:Lipid A / Q000031:analogs & derivatives* / Q000494:pharmacology; D018950:Lipopolysaccharide Receptors / Q000276:immunology; D008070:Lipopolysaccharides / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D046911:Macromolecular Substances; D008262:Macrophage Activation / Q000187:drug effects* / Q000276:immunology; D008264:Macrophages / Q000187:drug effects / Q000276:immunology / Q000378:metabolism; D008562:Membrane Glycoproteins; D018408:Patch-Clamp Techniques; D024681:Potassium Channels, Calcium-Activated / Q000037:antagonists & inhibitors / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D019779:Salmonella enterica / Q000737:chemistry; D015398:Signal Transduction; D014409:Tumor Necrosis Factor-alpha / Q000235:genetics / Q000378:metabolism; D000099231:Lipopolysaccharide-Binding Protein
D000209:Acute-Phase Proteins; D019138:Antibodies, Blocking; D002352:Carrier Proteins; D006017:Glycolipids; D007211:Indoles; D015850:Interleukin-6; D051037:Large-Conductance Calcium-Activated Potassium Channel alpha Subunits; D051036:Large-Conductance Calcium-Activated Potassium Channels; D008050:Lipid A; D018950:Lipopolysaccharide Receptors; D008070:Lipopolysaccharides; D046911:Macromolecular Substances; D008562:Membrane Glycoproteins; D024681:Potassium Channels, Calcium-Activated; D012333:RNA, Messenger; D014409:Tumor Necrosis Factor-alpha; D000099231:Lipopolysaccharide-Binding Protein; C494841:KCNMA1 protein, human; C040142:lipid A precursors, bacterial; C048220:paxilline