TRANSIENT INDUCTION OF CYTOCHROME-P450-1A1 MESSENGER-RNA BY CULTURE-MEDIUM COMPONENT IN PRIMARY CULTURES OF ADULT-RAT HEPATOCYTES

Because the metabolic environment can alter gene expression in culture d cells, we examined the effects of change of medium on the levels of several cytochrome P450 mRNAs in primary cultures of rat hepatocytes m aintained on Matrigel. The amounts of P450 1A2, 2B1/2, or 3A1 MRNA wer e unaffected by changing the medium. In contrast, P450 1A1 mRNA levels were increased 1 to 2 h after media change, reached maximum levels by 6 h, and declined to baseline by 24 h. Supplementing day-old media wi th components of the medium revealed that only addition of amino acids resulted in 1A1 mRNA induction. From the results of direct additions and omissions, we showed that tryptophan, but not histidine, was large ly responsible for the 1A1 mRNA induction. Moreover, mild photoactivat ion of tryptophan resulted in a substantially increased magnitude of 1 A1 mRNA induction. The time course for IAI mRNA induction by treatment with photoactivated tryptophan was identical to that observed after m edium change. Treatment of hepatocyte cultures with beta-naphthoflavon e, which is metabolized by 1A1, also resulted in a transient 1A1 mRNA induction time-course profile over a 24-h period, whereas treatment wi th 2,3,7,8-tetrachlorodibenzo-p-dioxin, which is relatively stable to metabolic transformation, produced sustained elevations of 1A1 mRNA, s uggesting that the transient response to tryptophan also may involve m etabolism of the inducer. Our results extend previous data showing tha t oxidized products of tryptophan induce 1A1, and suggest that the tra nsient nature of the induction may be due to elimination of the activa ted tryptophan molecule.