EGF activates an inducible survival response via the RAS-> Erk-1/2 pathway to counteract interferon-α-mediated apoptosis in epidermoid cancer cells

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Abstract
The mechanisms of tumor cell resistance to interferon-α (IFNα) are at present mostly unsolved. We have previously demonstrated that IFNα induces apoptosis on epidermoid cancer cells and EGF antagonizes this effect. We have also found that IFNα-induced apoptosis depends upon activation of the NH2-terminal Jun kinase-1 (Jnk-1) and p38 mitogen-activated protein kinase, and that these effects are also antagonized by EGF. At the same time, IFNα increases the expression and function of the epidermal growth factor receptor (EGF-R). Here we report that the apoptosis induced by IFNα occurs together with activation of caspases 3, 6 and 8 and that EGF also antagonizes this effect. On the basis of these results, we have hypothesized that the increased EGF-R expression and function could represent an inducible survival response that might protect tumor cells from apoptosis caused by IFNα via extracellular signal regulated kinase 1 and 2 (Erk-1/2) cascades. We have found an increased activity of Ras and Raf-1 in IFNα-treated cells. Moreover, IFNα induces a 50% increase of the phosphorylated isoforms and enzymatic activity of Erk-1/2. We have also demonstrated that the inhibition of Ras activity induced by the transfection of the dominant negative Ras plasmid RASN17 and the inhibition of Mek-1 with PD098059 strongly potentiates the apoptosis induced by IFNα. Moreover, the selective inhibition of this pathway abrogates the counteracting effect of EGF on the IFNα-induced apoptosis. All these findings suggest that epidermoid tumor cells counteract the IFNα-induced apoptosis through a survival pathway that involves the hyperactivation of the EGF-dependent Ras->Erk signalling. The selective targeting of this pathway appears to be a promising approach in order to enhance the antitumor activity of IFNα.