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 capable of regulating diverse cellular processes. In this study we investigated the effect of autocrine TGF-beta signaling on tumor necrosis factor (TNF) alpha-induced cell death. We abrogated the TGF-beta autocrine loop by overexpression of a truncated TGF-beta type II receptor in MCF-7 breast carcinoma cells and found that this generated resistance to TNF-alpha-induced cytotoxicity. To elucidate the molecular basis of the influence of TGF-beta on TNF-alpha-induced cytotoxicity, we evaluated the expression levels or activities of proteins involved in TNF-alpha signal transduction or the regulation of apoptosis in general in TGF-beta-responsive and TGF-beta-nonresponsive MCF-7 cells. We observed no significant difference in the expression of TNF-alpha receptors or the TNF receptor-associated death domain protein. In addition, downstream activation of nuclear factor kappaB by TNF-alpha was not altered in cells that had lost TGF-beta responsiveness. Analysis of members of the Bcl-2 family of apoptosis-regulatory proteins revealed that Bcl-X(L) and Bax expression levels were not changed by disruption of TGF-beta signaling. In contrast, the TGF-beta-nonresponsive cells expressed much 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 reduction 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 derepression of Bcl-2 expression.
Tobin
S W
SW
Department of Medicine, Dartmouth Medical School, Hanover, New Hampshire 03755, USA.
Brown
M K
MK
Douville
K
K
Payne
D C
DC
Eastman
A
A
Arrick
B A
BA
D017209:Apoptosis / Q000502:physiology*; D001943:Breast Neoplasms / Q000503:physiopathology*; D002460:Cell Line; D016207:Cytokines / Q000378:metabolism; D005260:Female; D006801:Humans; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000096:biosynthesis*; D015398:Signal Transduction / Q000502:physiology; D014162:Transfection; D016212:Transforming Growth Factor beta / Q000378:metabolism*; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism*; D015854:Up-Regulation
D016207:Cytokines; D019253:Proto-Oncogene Proteins c-bcl-2; D016212:Transforming Growth Factor beta; D014409:Tumor Necrosis Factor-alpha