Induction of CYP1A and glutathione S-transferase activities with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was studied in human hepatocytes in primary culture to investigate the variability of inducibility and the potency of TCDD. Determining induction of 7-ethoxyresorufin O-deethylase activity, preferentially catalyzed by CYP1A isozymes, we obtained concentration-response diagrams in TCDD-treated hepatocyte cultures from transplant donors and patients undergoing hepatic surgery. At a concentration of 10(-10) M TCDD approximately half-maximal induction of CYP1A was observed. Northern analysis of CYP1A gene expression showed a similar concentration-response relationship. In comparison with rat hepatocytes, human hepatocytes were about 10-fold less sensitive towards the CYP1A-inducing effect of TCDD. No pronounced interindividual differences in the inducing potency of TCDD (concentration which leads to half-maximal induction) were obvious in the six human individuals studied, whereas the efficacy of CYP1A induction was highly variable. In addition, inducibility of glutathione S-transferase (GST) activity also revealed a considerable degree of interindividual variation, i.e. a complete lack of induction in three out of six hepatocyte preparations and a highly variable efficacy of GST induction among responders which was not related to CYP1A inducibility.

Induction of PAI-2 by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been studied in human primary hepatocytes, hepatoma HepG2 cells and monocytic U937 cells, extending recent findings in human keratinocytes. PAI-2 represents a serpine-type protease inhibitor with wide-ranging implications in fibrinolysis, extracellular matrix proteolysis, growth factor activation and carcinogenesis. PAI-2 was induced by >10(-9) M TCDD in hepatocytes and HepG2 cells and by >10(-10) M TCDD in U937 cells. In the latter cell line, PAI-2 induction by TCDD and by 12-O-tetradecanoyl phorbol-13-acetate (TPA) has been compared. TCDD appeared to be less efficient than TPA as an inducer of PAI-2. In contrast to induction by TPA, PAI-2 induction by TCDD was found to be biphasic, with an early peak of mRNA at 1-3 h and a late peak at 12-24 h. A biphasic response was also seen at the protein level although production of PAI-2 protein lagged behind the corresponding mRNA. PAI-2 is known to contain AP-1 sites, i.e. Jun/Fos protein-binding sites, in its promotor region. Hence, PAI-2 induction by TCDD has originally been conceived to be due to an indirect response, secondary to the induction of Jun/Fos proteins. Therefore, expression of jun/fos genes and their AP-1 activity were studied at the early phase of PAI-2 induction by TCDD. TCDD did not increase mRNA of c-fos, c-jun, junB or junD (in contrast to TPA which markedly increased the expression of c-fos and junB), nor did TCDD increase AP-1 activity. In conclusion, the findings suggest that PAI-2 induction by TCDD is not restricted to human keratinocytes but includes liver cells and monocytic U937 cells. The induction mechanism is complex but the early phase does not appear to involve Jun/Fos proteins.