Inhibition of transforming growth factor beta signaling in MCF-7 cells results in resistance to tumor necrosis factor alpha: A role for Bcl-2

Transforming growth factor beta (TGF-beta) is a multifunctional cytokine ca pable of regulating diverse cellular processes. In this study we investigat ed the effect of autocrine TGF-beta signaling an tumor necrosis factor (TNF ) alpha -induced cell death. We abrogated the TGF-beta autocrine loop by ov erexpression of a truncated TGF-beta type II receptor in MCF-7 breast carci noma cells and found that this generated resistance to TNF-alpha -induced c ytotoxicity. To elucidate the molecular basis of the influence of TGF-beta on TNF-alpha -induced cytotoxicity, we evaluated the expression levels or a ctivities of proteins involved in TNF-alpha signal transduction or the regu lation of apoptosis in general in TGF-beta -responsive and TGF-beta -nonres ponsive MCF-7 cells. We observed no significant difference in the expressio n of TNF-alpha receptors or the TNF receptor-associated death domain protei n. In addition, downstream activation of nuclear factor kappaB by TNF-alpha was not altered in cells that had lost TGF-beta responsiveness. Analysis o f members of the Bcl-2 family of apoptosis-regulatory proteins revealed tha t Bcl-X-L and Bax expression levels were not changed by disruption of TGF-b eta signaling. In contrast, the TGF-beta -nonresponsive cells expressed muc h higher levels of Bcl-2 protein and mRNA than did cells with an intact TGF -beta autocrine loop. Furthermore, restoration of a TGF-beta signal to MCF- 7 cells that had spontaneously acquired resistance to TGF-beta caused a red uction in Bcl-2 protein expression. Taken together, our data indicate that loss of autocrine TGF-beta signaling results in enhanced resistance to TNF- alpha -mediated cell death and that this is likely to be mediated by derepr ession of Bcl-2 expression.