2,3,7,8-Tetrachlorodibenzo-p-dioxin suppresses apoptosis and leads to hyperphosphorylation of p53 in rat hepatocytes

Inhibition of apoptosis of preneoplastic cells is thought to represent a ma jor mechanism of action of tumor promoters. 2,3,7,8-tetrachlorodibenzo-p-di oxin (TCDD), the most potent promoter of liver carcinogenesis in rodents, s uppressed apoptosis in rat hepatocytes pretreated in vitro with an apoptoge nic dose of UV light. This effect, which was also observable in DNA fragmen tation analysis, coincided with a pronounced inhibition of the p53 increase usually seen after UV irradiation of rat hepatocytes. Interestingly, TCDD also led to a very minor but consistent enhancement of DNA fragmentation an d to a slight increase in p53. Furthermore, TCDD resulted in a dose-depende nt increase in p53 phosphorylation in intact cells. The concentration-respo nse curves of the effects of TCDD on p53 phosphorylation and aromatic hydro carbon receptor (AhR)-dependent induction of cytochrome P450 1A1 activity w ere almost superimposable, suggesting that TCDD induces p53 phosphorylation an AhR-linked kinase activity. In an extract prepared from rat liver homog enate, 1 nM TCDD also stimulated p53 phosphorylation. Since the tyrosine ki nase c-src was previously shown by others to be activated upon binding of T CDD to the AhR, extracts were pretreated with anti-sir-antibodies. This tre atment almost completely abrogated the effect of TCDD on p53 phosphorylatio n suggesting a key role for AhR-associated c-src. This mode of action may r esult in the observed suppression of the p53 response to apoptogenic UV irr adiation, and may contribute to the inhibition of apoptosis. (C) 1998 Elsev ier Science B.V. All rights reserved.