Untersuchung zur Toxizitaet und Kanzerogenitaet von 2,3,7,8-Tetrachlordibenzo-p-dioxin Schlussbericht

SIGLEAvailable from TIB Hannover: D.Dt.F.QN1(20,40) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Forschung und Technologie (BMFT), Bonn (Germany)DEGerman

Dexamethasone-mediated potentiation of P450IA1 induction in H4IIEC3/T hepatoma cells is dependent on a time-consuming process and associated with induction of the <em>Ah</em> receptor.

The synergistic effect of dexamethasone (DEX) and polycyclic aromatic hydrocarbons on the induction of cytochrome P450IA1 (P450IA1) was examined in H4IIEC3/T Reuber hepatoma cells. P450IA1 activity was determined by the hydroxylation of benzo[a]pyrene (AHH) and deethylation of 7-ethoxy-resorufin (EROD). The amount of Ah receptors, i.e. the specific cytosolic binding protein of 3-methylcholanthrene or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in H4IIEC3/T cells was characterized and quantitated by high performance gel filtration. Benz[a]anthracene and TCDD induced AHH and EROD activities, respectively, about 20-fold within 4 h. The increase was about 100-fold when cells were pretreated with DEX. The glucocorticoid alone induced P450IA1 activities 3-4-fold. DEX elicited half maximum AHH induction at a concentration of 20 nM in the presence or absence of benz[a]anthracene. Maximal potentiation of AHH induction required treatment with DEX for at least 32 h prior to the exposure to benz[a]anthracene. Treatment of H4IIEC3/T cells with DEX for 20 h caused a 2-3-fold increase in the amount of Ah receptor. The results suggest that the synergistic effect of DEX and polycyclic aromatic hydrocarbons on P450IA1 induction involves a time-consuming process which may consist of the synthesis or modification of a factor, possibly the Ah receptor

Inhibition of growth by 2,3,7,8-tetrachlorodibenzo-p-dioxin in 5L rat hepatoma cells is associated with the presence of Ah receptor.

The role of the Ah receptor in mediating the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was investigated in 5L rat hepatoma cells containing TCDD-inducible cytochrome P450IA1 activity and in variants lacking cytochrome P450IA1 and Ah receptor. TCDD inhibited growth of the wild-type 5L cells, but not of the Ah receptor deficient variants. The two strong Ah receptor ligands 3,3&#39;,4,4&#39;-tetrachlorobiphenyl (3,3&#39;,4,4&#39;-TCB) and benz[a]anthracene (BA) exerted toxic effects in 5L cells that resembled those of TCDD. The poor Ah receptor ligand 2,2&#39;,4,4&#39;-tetrachlorobiphenyl was not toxic in 5L cells. The concentrations of TCDD, 3,3&#39;,4,4&#39;-TCB or BA required for the toxic response were similar to those that elicited P450IA1 induction. The present results suggest strongly that interaction with the Ah receptor is a necessary link in the chain of events leading to toxic effects in 5L cells upon exposure to TCDD

The level of aryl hydrocarbon (Ah) receptor and of 4S polycyclic aromatic hydrocarbon (PAH) binding protein in diploid and polyploid hepatocytes of 2-acetylaminofluorene-treated rats.

Sequential treatment of partially hepatectomized male Wistar rats with diethylnitrosamine (DEN) and 2-acetylaminofluorene (AAF) induces the emergence of diploid hepatocyte populations. These carcinogen-induced hepatocytes are thought to include the precursor cells of liver carcinomas that arise later in this treatment protocol. The growth of the diploid hepatocytes is promoted by AAF and it has been suggested that the action of the arylamine may be receptor-mediated. AAF has been shown to bind specifically to the aryl hydrocarbon (Ah) receptor and the so-called 4S polycyclic aromatic hydrocarbon (PAH) binding protein. The present study addresses the question of whether the concentrations of the two binding proteins differ in diploid and polyploid hepatocytes from DEN/AAF-treated rats. Hepatocytes from carcinogen-treated rats were isolated and diploid, and tetraploid hepatocytes separated by means of centrifugal elutriation. Whereas Ah receptor concentrations in diploid hepatocytes were insignificantly lower (21.8 +/- 5.9 versus 29.2 +/- 6.6 fmol/mg cytosolic protein; n = 4; P = 0.1), levels of the 4S PAH binding protein in diploid hepatocytes were twice as high as in tetraploid hepatocytes (252.3 +/- 93.6 versus 124.0 +/- 18.5 fmol/mg cytosolic protein; n = 4; P = 0.04). We conclude from our results that the differences in growth control in polyploid and carcinogen-induced diploid hepatocytes are not associated with changes in the levels of the Ah receptor. The role of the 4S PAH binding protein in the process of hepatocarcinogenesis remains to be established