| Abstract: | Previously, we reported that several of the toxic effects of 2,3,7,8-tetrachlorodibenzo-p -dioxin (TCDD) were not fully expressed in c-src −/− and −/+ mice (1). In the current study, we studied the basic molecular mechanism of their differential responses. First, we could show that chemical inhibition of c-src kinase could produce practically the same phenomenon of reduced toxicity of TCDD in wild-type mice cotreated with geldanamycin, a specific chemical inhibitor known to suppress c-src kinase. Second, we established that the level of the Ah receptor associated with c-src kinase (2) is indeed low in c-src deficient mice as well as geldanamycin-treated mice. We could show, at the same time, that the effect of c-src deficiency on the toxicity of TCDD is very selective; that is, despite the reduction of many of its toxic signs, the enlargement of liver, induction of cytochrome P450, and other drug-metabolizing enzymes took place normally in those c-src–deficient mice. Apparently, induction of these detoxification enzymes are independent of c-src–mediated pathway. Based on the known signaling pathways of c-src, we tested c-fos–deficient mice and found that some of the c-src–dependant toxic signs of TCDD such as thymic atrophy and decrease in adipose tissue weight were also reduced in c-fos–deficient mice, indicating that these two toxic effects are likely to be mediated through both c-src and c-fos. However, other TCDD responses noticeable in c-fos–deficient mice; downregulation of receptors for epidermal growth factor (EGF), tumor necrosis factor (TNFα), and retinoic acid; and up-regulation of the T3 receptor. These findings clearly show that c-fos mediates only a part of c-src signaling pathway in transducing these specific toxic actions of TCDD as mediated by c-fos. © 1998 John Wiley & Sons, Inc. J Biochem Toxicol 12: 263–274, 1998 |
