Destabilization of TNF-alpha mRNA by retinoic acid in hepatic macrophages: implications for alcoholic liver disease

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)-alpha 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-alpha expression by cultured normal rat HM. The mRNA levels for inducible nitric oxide synthase, interleukin (IL)-6, IL-1alpha, and IL-1beta were also reduced, whereas those for transforming growth factor-beta1, MMP-9, and membrane cofactor protein-1 were unaffected. The inhibitory effect on TNF-alpha 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-alpha 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 RARbeta, RXRalpha, and cytosolic retinol binding protein-1, whereas TNF-alpha mRNA expression was induced. TNF-alpha mRNA stability was increased in these cells, and an ex vivo treatment with all-trans RA normalized both RARbeta and TNF-alpha mRNA levels. These results demonstrate the RA-induced destabilization of TNF-alpha mRNA by cultured HM and the association of RA depletion with increased TNF-alpha 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.