Down‐regulation of human CYP3A4 by the inflammatory signal interleukin 6: molecular mechanism and transcription factors involved

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Abstract
The hepatic drug‐metabolizing cytochrome P‐450 (CYP) enzymes are down‐regulated during inflammation. In vitro studies with hepatocytes have shown that the cytokines released during inflammatory responses are largely responsible for this CYP repression. However, the signaling pathways and the cytokine‐activated factors involved remain to be properly identified. Our research has focused on the negative regulation of CYP3A4 (the major drug‐metabolizing human CYP) by interleukin 6 (IL‐6) (the principal regulator of the hepatic acute‐phase response). CYP3A4 down‐regulation by IL‐6 requires activation of the glycoprotein receptor gp130; however, it does not proceed through the JAK/STAT pathway, as demonstrated by the overexpression of a dominant‐negative STAT3 factor by means of an adenoviral vector. The involvement of IL‐6‐activated kinases such as extracellular signal‐regulated kinase ERK1/2 or p38 is also unlikely, as evidenced by the use of specific chemical inhibitors. It is noteworthy that IL‐6 caused a moderated induction in the mRNA of the transcription factor C/EBPβ (CCAAT‐ enhancer binding protein β) and a marked increase in the translation of C/EBPβ‐LIP, a 20‐kDa C/EBPβ isoform lacking a transactivation domain. Adenovirus‐mediated expression of C/EBPβ‐ LIP caused a dose‐dependent repression of CYP3A4 mRNA, whereas overexpression C/EBPα and C/EBPβ‐LAP (35 kDa) caused a significant induction. Our results support the idea that IL‐6 down‐regulates CYP3A4 through translational induction of C/EBPβ‐LIP, which competes with and antagonizes constitutive C/EBP transactivators. From a clinical point of view, these findings could be relevant in the development of therapeutic cytokines with a less repressive effect on hepatic drug‐metabolizing enzymes.