Role of Transforming Growth Factor β in the Proliferative Effect of 2,3,7,8-Tetrachlorodibenzo-p-dioxin on Human Squamous Carcinoma Cells

Abstract

The highly toxic environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has recently been shown to stimulate proliferation of two human squamous carcinoma cell lines, SCC-15G and SCC-25, by decreasing the sensitivity of the cells to high density growth arrest. TCDD is known to alter the activity of several endogenous growth-regulatory compounds, and this study was undertaken to investigate the possibility that modulation of transforming growth factor β (TGF-β) activity might be involved in the mechanism of growth stimulation by TCDD. TGF-β inhibited monolayer growth and DNA synthesis of SCC-15G and SCC-25 cells equally well in the presence and the absence of 10 nm TCDD. TCDD alone stimulated proliferation and inhibited differentiation in both cell lines but had no effect on binding of ¹²⁵I-TGF-β to or secretion of TGF-β by SCC-15G cells. Inhibition of growth of SCC-15G cultures by TGF-β was incomplete, in that cell number continued to increase even in the presence of 100 pm TGF-β, although at a greatly reduced rate compared to non-TGF-β-treated controls. These cells, grown in 100 pm TGF-β alone, reached growth arrest at the same density as non-TGF-β-treated cultures but failed to reach density-dependent growth arrest when treated with TCDD. Cells treated for 12 h with TCDD exhibited maximal induction of ethoxyresorufin-O-deethylase activity. TGF-β inhibited this induction in a dose-dependent manner even when added after a 12-h TCDD pretreatment. The inhibition was rapidly reversible after removal of TGF-β, indicating that it occurred via a mechanism which did not involve inhibition of very early steps in the TCDD response pathway. These results demonstrate that the stimulatory effect of TCDD on keratinocyte proliferation is not mediated through alterations in TGF-β activity and that TCDD and TGF-β appear to exert their opposite effects on cellular proliferation by independent mechanisms.