Proline-dependent expression of aryl hydrocarbon hydroxylase in C57BL/6 mouse hepatocytes in primary culture

Induction of aryl hydrocarbon hydroxylase (AHH) was studied in mouse hepatocytes in primary culture and compared with that in rat hepatocytes. Enzyme activity in hepatocytes from C57BL/6 mice was found to increase dose dependently after treatment with benz(a)anthracene. However, the induction was strictly dependent on culture medium. Although appreciable levels of AHH activity were inducible in Sprague-Dawley rat hepatocytes cultivated in either Dulbecco's minimal essential medium (DMEM) or Waymouth's medium, C57BL/6 mouse cells cultivated in DMEM responded to the inducer only very slightly, whereas those in Waymouth's or Ham's F-12 medium demonstrated a marked increase. Proline, but not glutamic acid or cysteine, all of which were lacking in DMEM but present in Waymouth's and Ham's F-12 medium, restored the potential for response to the cells in DMEM. While increased amounts of P450 mRNA in C57BL/6 cells cultivated in DMEM were transient and decreased after a peak observed at 24 h, levels of mRNA in Waymouth continued to demonstrate an increase at 48 h. Addition of proline to mouse hepatocytes in DMEM increased the generation of transcripts without, however, influencing the decrease observed from 24 h to 48 h. Timing of treatment with benz(a)anthracene and incubation in Waymouth greatly influenced the eventual AHH activity. Thus, while enzyme activities measured at 48 h were in the same range after treatment with benz(a)anthracene for either the whole period or only for the initial 24 h, and prominent induction was observed with cells in Waymouth for 24 to 48 h regardless of whether they were at first cultivated in DMEM or Waymouth, levels remained low if the cells were incubated in DMEM during the 24- to 48-h period. These observations suggest that induction of AHH in mouse hepatocytes is regulated by both transcriptional and posttranscriptional events and that proline-dependent events are required for expression of the enzyme activity.