Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on protein kinase C and inositol phosphate metabolism in primary cultures of rat hepatocytes.

Adult rat hepatocytes, after maintenance for 24 h in serum-free culture, were treated with the tumor promoters, 12-O-tetradecanoylphorbol-13-acetate (TPA) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Short-term treatment (15 min) with TPA, 1 microM, increased protein kinase C (PKC) activity in the particulate fraction of hepatocytes and, concomitantly, decreased the vasopressin (100 nM)-stimulated synthesis of inositol phosphates. The latter effect of TPA could be prevented by prior addition of the PKC inhibitor, H7 (100 microM). After short-term treatment (15 min) with TCDD, 1 pM, no effects on PKC or inositol phosphate metabolism were observed. However, after prolonged exposure to TCDD (3-48 h), the particulate PKC was significantly activated (1.5-fold). In contrast to the effect of TPA (24 h), no down-regulation was found. Moreover, long-term treatment with TCDD significantly enhanced vasopressin-stimulated inositol 1,3,4,5-tetrakisphosphate synthesis, while TPA treatment (24 h) stimulated the synthesis of inositol trisphosphates and inositol 1,3,4,5-tetrakisphosphate. The results suggest that the tumor promoters, TPA and TCDD, act differently on the signal transduction pathways in hepatocytes. Thus, the effects of TCDD on PKC and inositol phosphate metabolism might be mediated by a yet unknown mechanism rather than by direct activation of PKC as seen with TPA.