Differences in regulation of gene expression between Cyp1a-1 and Cyp1a-2 in adult mouse hepatocytes in primary culture.

The regulation of expression of Cyp1a-1 and Cyp1a-2 genes was investigated in adult C57BL/6NCrj mouse hepatocytes for up to 5 days after transferring to either monolayer or spheroid (multi-cellular aggregate) primary culture. The expression of 3-methylcholanthrene (MCA)-induced CYP1A1 mRNA remained high during the observation period under both monolayer and spheroid culture conditions. In contrast, while levels of CYP1A2 mRNA in spheroid culture were also appreciable throughout, they rapidly decreased in monolayer culture to become negligible. An increase in intracellular cyclic nucleotide content induced CYP1A1 mRNA in the later culture period in either spheroid or monolayer cultures. A significant elevation of both basal and MCA-induced 7-ethoxycoumarin-O-deethylase, and MCA-induced aryl hydrocarbon hydroxylase and 7-methoxyresorufin-O-demethylase activities was observed in the presence of intracellular cyclic nucleotide content-increasing agents, although the extent of enhancement far exceeded that expected from the scarce changes in MCA-induced CYP1A1 mRNA levels. Basal and MCA-induced CYP1A2 mRNA expression were not changed by altering intracellular cyclic nucleotide content. The level of CYP1A1 mRNA after MCA treatment was elevated in the presence of cycloheximide. Furthermore, with increasing culture time, addition of this agent caused expression of Cyp1a-1 gene in MCA-untreated cells. In contrast, the presence of cycloheximide did not increase constitutive or MCA-induced CYP1A2 mRNA. These observations indicate that expression of Cyp1a-1 and Cyp1a-2 genes may be regulated by different mechanisms.