Evidence for a second pathway in the action mechanism of 2,3,7,8-Tetrachlorodibenzo- p-dioxin (TCDD) : Significance of Ah-receptor mediated activation of protein kinase under cell-free conditions

2,3,7,8-Tetrachlorodibenzo- p-dioxin (TCDD) when administered directly to a nuclear-free subcellular homogenate of guinea pig adipose tissue, caused a significant rise in protein kinase activities within 1–10 min. Such a rapid response was not expected, based on the classic transcriptional mechanism of action for TCDD, i.e. TCDD first binds with its cytosolic Ah-receptor, translocates into the nucleus, dimerizes with “arnt” (a nuclear transcription factor), and activates genes containing “xenobiotic-responsive element” (XRE). The above actions of TCDD on protein kinases were clearly blocked by two specific Ah-receptor blockers, even under cell- and nucleus-free conditions. TCDD-induced increases in protein phosphorylation occurred mainly in cytosolic preparations (i.e. 100,000 g supernatant) devoid of nucleus, microsomes and plasma membranes and were still observed in the presence of inhibitors of protein phosphatases. Furthermore, TCDD caused a rise in protein tyrosine kinase activity in a purified Ah-receptor preparation, as well as in an isolated heat shock protein 90 complex preparation containing the Ah-receptor. This activation took place in the presence of actinomycin D and cycloheximide, indicating a portion of TCDD's action that is unrelated to de novo protein synthesis in this process. We have also obtained evidence indicating that this action of TCDD triggers the protein kinase mediated growth factor signal transduction pathway, such as stimulation of mitogen activated protein kinase 2 and tyrosine kinase activity. These results clearly support the view that the basic action pathway for such a TCDD-induced activation of protein kinases is distinctly different from its conventional action pathway involving changes in gene transcription in the nucleus.