Abstract

Hepatic cytochrome P450 (P450) enzymes metabolize exogenous and endogenous compounds, and many are inducible by xenobiotics. Their synthesis is tightly regulated, particularly through nuclear receptors. Expression of murine CYP2B genes is strongly activated by treatment with phenobarbital or phenobarbital-like inducers, and a detectable response requires the presence of the constitutive androstane receptor (CAR). However, other compounds can also induce murine CYP2B proteins. For example, dexamethasone is known to induce rat CYP2B1 and CYP2B2 and mouse CYP2B10. Using human HepG2 and rat H4IIEC3 hepatoma cell lines, we found that dexamethasone induction of CYP2B2 and Cyp2b10 luciferase reporters required the glucocorticoid receptor. Given the well known observation that CYP2B genes are not phenobarbital-responsive in cultured cell lines, the dexamethasone responsiveness of CYP2B reporter constructs in cell lines demonstrates in itself that the mechanism of dexamethasone induction is distinct from that of phenobarbital. We also analyzed the relative importance of the phenobarbital response unit (PBRU) and of a known glucocorticoid response element in this response. Both sites contributed to the response, but other sites were required for maximal induction. CAR was also found to act as an accessory factor to stimulate the response to dexamethasone by the glucocorticoid receptor. Furthermore, in H4IIEC3 cells, CAR activated the PBRU in the natural sequence context of the CYP2B2 and Cyp2b10 5' flanks. In summary, there are at least two independent mechanisms of CYP2B induction: one involving phenobarbital and phenobarbital-like inducers and another involving glucocorticoids that induce via the glucocorticoid receptor with CAR acting as an accessory factor.