A malignant transformation of human cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin exhibits altered expressions of growth regulatory factors.

The neoplastic transformation of human cells in culture with exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has recently been reported. In this study, expressions of growth regulatory factors were analyzed to examine their possible roles in TCDD-induced malignant transformation of human cells. Reverse transcription-polymerase chain reaction (RT-PCR) and immunoblot analysis were performed to detect altered expressions of genes associated with dioxin responses. The RT-PCR analysis showed that expressions of the growth regulatory factors, such as transforming growth factor-beta1 (TGF-beta1), plasminogen activator inhibitor-2 (PAI-2) and tumor necrosis factor-alpha (TNF-alpha), were significantly changed in the transformed cells as compared with the parental cells. Whereas parental cells showed a dose-dependent increase of PAI-2 mRNA levels following TCDD exposure, the transformed cells did not show any significant induction. In addition, constitutive levels of PAI-2 mRNA were 25 times lower in the transformed cells than in the parental cells. The mRNA stability assay suggests that downregulation of PAI-2 mRNA in the transformed cells may be associated with the posttranscriptional control. Expression of TGF-beta1 mRNA in the transformed cells was also four times lower than the parental cells. However, levels of TNF-alpha mRNA in the transformed cells were increased 3-fold. These results suggest that dysregulation of growth regulatory factors may be involved in TCDD-induced cellular transformation. Whereas plenty of studies demonstrated a number of immediate toxic effects by TCDD, this study revealed an initial evidence that altered expression of growth regulatory genes, such as PAI-2, TGF-beta1 or TNF-alpha, are some of the genetic events fixed in the genome following the successive cell divisions of TCDD-damaged cells. It is suggested that these changes may be associated with TCDD-induced malignant transformation of human cells.