Retinoic acid attenuates lipopolysaccharide-induced inflammatory responses by suppressing TLR4/NF-κB expression in rat mammary tissue

The retinoids, a group of natural or synthetic derivatives of vitamin A, exert various anti-neoplastic and immunomodulatory actions. Recent studies have demonstrated that retinoic acid protects rats against lipopolysaccharide (LPS)-induced mastitis, but the mechanism of action is unclear. In the present study, an LPS-induced rat mastitis model and primary cultures of rat mammary epithelial cells were used to investigate the effect of retinoic acid on the TLR4/NF-κB signaling pathway. The data indicated that toll-like receptor 4 (TLR4) gene expression reached its peak value earlier in retinoic acid-treated rats than in the control group, and that retinoic acid significantly decreased NF-κB DNA binding activity and the level of IL-1β in the mammary gland. The animal study result was confirmed by an in vitro cell culture system trial. TLR4 protein expression and NF-κB DNA binding activity were significantly decreased in primary rat mammary epithelial cells pretreated with 1 μmol/l retinoic acid at 1 h post-LPS stimulation. IL-1β gene expression was also significantly decreased at 2, 4 and 8 h post-LPS stimulation. These findings demonstrate that direct action by retinoic acid leads to attenuation of the LPS-induced inflammatory response by suppression of the TLR4/NF-κB signalling system, thereby providing a novel explanation for the underlying effect proposed for retinoic acid in the protection of mammary tissue during LPS-induced acute mastitis.