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mRNA by retinoic acid in hepatic
macrophages: implications for alcoholic liver disease
Departments of 1 Medicine and 2 Pathology, University of Southern California School of Medicine, Los Angeles 90033; 3 Department of Nutrition, University of California, Davis 95616; and 4 Veterans Affairs Greater Los Angeles Healthcare System, Sepulveda, California 91343
Retinoic
acid (RA) inhibits hepatic macrophage (HM) cytokine expression, and
retinoids are depleted in alcoholic liver disease (ALD). However,
neither the causal link between the two nor the mechanism underlying
RA-mediated HM inhibition is known. The aim of the present study was to
determine the mechanism of RA-induced inhibition of HM tumor necrosis
factor (TNF)-
expression and the relevance of this regulation to
ALD. Treatment with all-trans RA (500 nM) caused a 50%
inhibition in lipopolysaccharide (LPS)-stimulated TNF-
expression by
cultured normal rat HM. The mRNA levels for inducible nitric oxide
synthase, interleukin (IL)-6, IL-1
, and IL-1
were also reduced,
whereas those for transforming growth factor-
1, MMP-9, and membrane
cofactor protein-1 were unaffected. The inhibitory effect on TNF-
expression was reproduced by LG268, a retinoid X receptor
(RXR)-specific ligand, but not by TTNPB, an RA receptor (RAR)-specific
ligand. RA did not alter LPS-stimulated NF-kB and activation protein-1
binding but significantly decreased TNF-
mRNA stability in HM. HM
isolated from the ALD model showed significant decreases in
all-trans RA (
48%) and 9-cis RA (
61%) contents, RA response element (RARE) binding, and mRNA levels for
RAR
, RXR
, and cytosolic retinol binding protein-1, whereas TNF-
mRNA expression was induced. TNF-
mRNA stability was
increased in these cells, and an ex vivo treatment with
all-trans RA normalized both RAR
and TNF-
mRNA levels.
These results demonstrate the RA-induced destabilization of TNF-
mRNA by cultured HM and the association of RA depletion with increased
TNF-
mRNA stability in HM from experimental ALD. These findings
suggest that RA depletion primes HM for proinflammatory cytokine
expression in ALD, at least in part, via posttranscriptional regulation.
Kupffer cells; tumor necrosis factor-
mRNA stability; retinoic
acid receptors; retinoid X receptors; retinoic acid response element
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