Growth factor-mediated mechanisms of nicotine-dependent carcinogenesis

The direct effect of nicotine on the expression of receptors for the tumor necrosis factor alpha (TNF alpha) and transforming growth factor beta (TGF beta) and the internalization, intracellular distribution and stability of these growth factors in cervical cancer cell line SiHa was studied. Nicotine at concentrations in the range 0.05-0.25% inhibited cell growth and it exerted strong apoptotic and cytolytic effects at concentrations above 0.5%. Nicotine at 0.1% stabilized and protected from degradation [125I]TNF alpha and [125I]TGF beta internalized by cervical cancer SiHa cell line. In the absence of nicotine, [125I]TNF alpha and [125I]TGF beta were internalized during the first hour of incubation and localized mainly in the cytoplasm and in smaller amounts in the nucleus. After 1 day of cell exposure to growth factors, only traces of radioactivity were detected inside the cells, which indicated that both growth factors were rapidly degraded. In the presence of nicotine, both [125I]TNF alpha and [125I]TGF beta were detected in high quantities and in a non-degraded form in the cytoplasm and chromatin during 5 days of incubation. In addition to the lack of growth factor degradation, the presence of nicotine induced a nuclear accumulation of growth factors, with up to 37% of the internalized [125I]TGF beta being in the chromatin. An increased intracellular accumulation of [125I]TNF alpha and [125I]TGF beta in cells exposed to nicotine occurred without changes in expression of the cell surface receptors. Nuclear accumulation of TGF beta was followed by increased RNA synthesis and a switch from the growth-promoting action of TGF beta to the strong growth inhibitory effect. Inhibition of the lysosomal degradation of growth factors by nicotine is discussed as a potential mechanism of tobacco-induced carcinogenesis.