Effect of transforming growth factor-beta1 on cytochrome P450 expression: inhibition of CYP1 mRNA and protein expression in primary rat hepatocytes.

Primary hepatocytes are a widely used cell model to analyse the expression and regulation of hepatic cytochrome P450 (CYP) isoenzymes. Transforming growth factor-beta1 (TGF-beta1) was previously shown to inhibit constitutive and induced CYP1 expression in human cell lines and primary hepatocytes but not in rat cells. In the present study we examined the effect of TGF-beta1 on constitutive and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced expression of CYP1 isoenzymes in primary rat hepatocytes in order to address the species-specificity of CYP1 down-regulation by TGF-beta1. The results show an inhibition of TCDD-induced CYP1-related 7-ethoxyresorufin-O-deethylase (EROD), 7-methoxyresorufin-O-demethylase (MROD) activities and mRNA expression (determined by reverse transcriptase polymerase chain reaction, RT-PCR) by 100 pM TGF-beta1 in cells co-treated for 24 h with 1 nM TCDD. However, while TGF-beta1 also down-regulated constitutive EROD and MROD activities as well as CYP1A2 protein expression, it did not change the constitutive mRNA expression of CYP1 isoenzymes. The down-regulation seemed to be specific for CYP1 isoenzymes since constitutive expression of other CYP isoenzymes was unaffected concerning protein levels, as determined by Western blot for CYP2B1/2 and CYP3A1, as well as mRNA levels, as determined by RT-PCR for CYP2B1/2, CYP2E1 and CYP3A1. Thus, TGF-beta1 not only inhibits CYP1 expression in humans but also in rats, indicating that regulation of CYP1 expression in these two species is similar.