E2 enhances CYP2B6 expression. (A) Primary human hepatocytes from 2 donors (HH3 and HH4) were treated with increasing concentrations of E2 for 72 hr. mRNA expression level of CYP2B6 was determined by qRT-PCR. (B) Primary human hepatocytes from 3 donors (HH3, HH4, and HH5) were treated with ethanol, E2 (1 μM), or CITCO (100 nM) for 72 hr and mRNA expression level of CYP2B6 was determined. (C and D) Primary human hepatocytes were treated with ethanol, E2 (1 μM), or CITCO (100 nM) for 72 hr. CYP activity was then determined by adding a CYP2B6-specific probe drug [100 μM S-mephenytoin (C) or 10 μM bupropion (D)] to the media and measuring concentrations of S-nirvanol (C) or hydroxybupropion (D) in the media. Data shown are relative S-nirvanol formation rate in drug-treated hepatocytes as compared to vehicle-treated cells (C) and cumulative hydroxybupropion production in HH8 (D). CYP2B6 activities were not determined in CITCO-treated hepatocytes #1, 2, and 6.