Influence of Extracellular Matrix Overlay on Phenobarbital-Mediated Induction of CYP2B1, 2B2, and 3A1 Genes in Primary Adult Rat Hepatocyte Culture

To obtain efficient induction by phenobarbital (PB) of cytochrome P450 (CYP) genes in primary hepatocyte culture, previous studies have demonstrated the importance of culturing hepatocytes on a substratum derived from extracellular matrix (ECM or Matrigel), or in highly enriched medium formulations such as Chee′s. In the present study, we have reexamined a variety of hepatocyte culture conditions and their relative abilities in preserving the PB-induction response within the CYP2B and 3A gene subfamilies. A modified culture system was developed that combines a highly effective medium formulation in conjunction with dilute concentrations of a Matrigel overlay. Specifically, hepatocytes were attached to a Collagen I substratum and overlaid with either daily (50 μg/ml medium) or single (233 μg/ml) additions of Matrigel. The PB-induction response obtained in vitro closely resembled that occurring in vivo. Induction in culture by 1 mM PB of CYP2B1, 2B2, and 3A1 mRNA levels was highly dependent on a variety of factors, including medium formulation and 0.1 μM dexamethasone addition. The response to dexamethasone addition on this gene battery varied in a medium-specific manner. Cells maintained a higher level of PB-induction response when maintained in Williams′ E medium than with Chee′s, Waymouth′s, or Ultraculture media. The kinetics of PB induction also were more rapid in cells cultured in Williams′ E medium. PB exposures in Chee′s medium resulted in elevation of two CYP2B-immunoreactive proteins as detected by Western blot analysis, together with increased rates of O-dealkylation of benzyloxy- and pentoxyresorufin. However, Chee′s formulation produced an abnormal PB-induced expression of CYP1A1, as determined by mRNA analysis, high rates of O-dealkylation of 7-ethoxyresorufin, and inhibition of enzymatic activity by 1 μM α-naphthoflavone. This paradoxical expression of CYP1A1 was not observed in PB-treated cultures grown in Williams′ E medium. Thus, these studies demonstrated that the use of a physiologically balanced medium, i.e., Williams′ E formulation, together with an overlay of ECM, preserves PB-induction responsiveness closely resembling that observed in vivo, and should better facilitate mechanistic investigations into the molecular nature of PB induction.