Role of Toll‐like receptor signaling in hepatocytes in regulating the gene expression of drug metabolizing enzymes

Abstract

In inflammation, drug metabolism and clearance is altered due to suppression of drug metabolizing enzymes (DMEs) and their regulatory nuclear receptors (NRs: PXR, CAR and RXRα). Inflammatory responses are mediated by cytokines produced by activation of Toll‐like receptors (TLRs) on liver macrophages or Kupffer cells (KCs). Cytokines act on hepatocytes to suppress gene expression. However, hepatocytes also express TLRs, and we hypothesize that TLRs on hepatocytes are directly involved in suppression of hepatic DMEs. Primary mouse hepatocytes from TLR4+/+ (C3HeB/FeJ) and TLR4‐mutant (C3H/HeJ) mice were treated with LPS (1 μg/ml) or saline. RNA and protein levels were analyzed by real‐time PCR and immunoblotting, respectively. The purity of the hepatocytes was assessed by CD68 staining for KC contamination. RNA levels of the major DMEs, Cyp3a11 and Ugt1a1 were reduced ~60% by LPS in TLR4+/+, but not TLR4‐mutant hepatocytes. The cell‐signaling components, JNK and NF‐κB, known regulators of NRs and DMEs were activated in TLR4+/+ and not TLR4‐mutant cells. Nuclear protein levels of RXRα were rapidly reduced after LPS treatment of TLR4+/+, but not TLR4‐mutant hepatocytes. Treatment of hepatocytes with the KC inhibitor, Gadolinium chloride did not attenuate the effect of LPS. Thus, both cytokine‐dependent and independent pathways are involved in the regulation of hepatic gene expression in response to LPS signals.