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 major mechanism of action of tumor promoters. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most potent promoter of liver carcinogenesis in rodents, suppressed apoptosis in rat hepatocytes pretreated in vitro with an apoptogenic dose of UV light. This effect, which was also observable in DNA fragmentation 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 and to a slight increase in p53. Furthermore, TCDD resulted in a dose-dependent increase in p53 phosphorylation in intact cells. The concentration-response curves of the effects of TCDD on p53 phosphorylation and aromatic hydrocarbon receptor (AhR)-dependent induction of cytochrome P450 1A1 activity were almost superimposable, suggesting that TCDD induces p53 phosphorylation via an AhR-linked kinase activity. In an extract prepared from rat liver homogenate, 1 nM TCDD also stimulated p53 phosphorylation. Since the tyrosine kinase c-src was previously shown by others to be activated upon binding of TCDD to the AhR, extracts were pretreated with anti-src-antibodies. This treatment almost completely abrogated the effect of TCDD on p53 phosphorylation suggesting a key role for AhR-associated c-src. This mode of action may result in the observed suppression of the p53 response to apoptogenic UV irradiation, and may contribute to the inhibition of apoptosis.