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Department of Physiology, University of Texas Health Science Center at San Antonio, San Antonio, Texas 78229-3900
Tissue injury in
inflammation involves the release of several cytokines that activate
sphingomyelinases and generate ceramide. In the lung, the impaired
metabolism of surfactant phosphatidylcholine (PC) accompanies this
acute and chronic injury. These effects are long-lived and extend
beyond the time frame over which tumor necrosis factor (TNF)-
and
interleukin-1
are elevated. In this paper, we demonstrate that in
H441 lung cells these two processes, cytokine-induced metabolism of
sphingomyelin and the inhibition of PC metabolism, are directly
interrelated. First, metabolites of sphingomyelin hydrolysis themselves
inhibit key enzymes necessary for restoring homeostasis between
sphingomyelin and its metabolites. Ceramide stimulates
sphingomyelinases as effectively as TNF-
, thereby amplifying the
sphingomyelinase activation, and TNF-
, ceramide, and sphingosine all
inhibit PC:ceramide phosphocholine transferase (sphingomyelin
synthase), the enzyme that restores homeostasis between sphingomyelin
and ceramide pools. Second, ceramide inhibits PC synthesis, probably
because of its effects on CTP:phosphocholine cytidylyltransferase, the
rate-limiting enzymatic step in de novo PC synthesis. The data
presented here suggest that TNF-
may be an inhibitor of phospholipid
metabolism in inflammatory tissue injury. These actions may be
amplified because of the ability of metabolites of sphingomyelin to
inhibit the pathways that should restore the normal
ceramide-sphingomyelin homeostasis.
sphingosine; tumor necrosis factor-
; pulmonary surfactant; cytidine 5'-triphosphate
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