Received for publication August 8, 2007.
Revised October 8, 2007.
Accepted for publication October 9, 2007.

Regulation of hepatic drug metabolizing enzyme genes by Toll-like receptor 4 signaling is independent of TIR-domain containing adaptor protein (TIRAP)

Abstract

During inflammation, drug metabolism and clearance is altered due to suppression of hepatic drug metabolizing enzyme genes (DMEs), and their regulatory nuclear receptors (NRs: PXR, CAR and RXR). The bacterial endotoxin, lipopolysaccharide (LPS) induces expression of pro-inflammatory cytokines in the liver, which contributes towards altered DME expression. LPS binds to the cell-surface receptor, TLR4, which initiates a signal transduction cascade, including recruitment of the toll-interleukin 1 receptor domain containing adaptor protein (TIRAP). However, the role of TLR4 and TIRAP in LPS-mediated regulation of hepatic DME gene expression is not known. Wild-type (C3HeB/FeJ), TLR4-mutant (C3H/HeJ), TIRAP^+/+ and TIRAP^-/- mice were injected intraperitoneally with LPS. RNA levels of the major hepatic DMEs, Cyp3a11 and Ugt1a1 and the NRs were suppressed ~60-70% by LPS in wild-type, but not the TLR4-mutant mice. The nuclear protein levels of RXR were reduced by LPS in wild-type, but not the TLR4-mutant mice. Induction of hepatic cytokines (IL-1, TNF and IL-6), c-jun N-terminal kinase (JNK) and NF- was blocked in TLR4-mutant mice. Surprisingly, LPS had the same effect on cytokines, kinases, NRs and DME genes in livers of both TIRAP^+/+ and TIRAP^-/- mice, indicating that TIRAP is not essential for TLR4-mediated suppression of NRs and DMEs in liver. However, TIRAP^-/- mice have reduced serum cytokine expression compared to TIRAP^+/+ mice in response to LPS. This shows that although TIRAP mediates inflammatory responses induced by LPS, it is not essential in regulating LPS-mediated alterations of gene expression in liver.

Key words: CAR, CYP gene regulation, CYP3A, hepatocytes, Kupffer cells, nuclear receptors, PXR, RXR, UDP glucuronyltransferases