• Sara Huerta-Yepez, Mario Vega, Hermes Garban, and Benjamin Bonavida.
• Department of Microbiology, Immunology, and Molecular Genetics, Jonsson Comprehensive Cancer Center, University of California-Los Angeles, 10833 Le Conte Ave., Los Angeles, CA 90095-1747, USA.
• Clin. Immunol. 2006 Sep 1; 120 (3): 297-309.

AbstractMany tumors are resistant to Fas ligand (FasL)-induced apoptosis. This study examined the role of tumor-derived TNF-alpha, via an autocrine/paracrine loop, in the regulation of tumor-cell resistance to FasL-induced apoptosis. We have reported that Fas expression and sensitivity to FasL is negatively regulated by the transcription repressor factor Yin Yang 1 (YY1). Thus, we hypothesized that tumor-derived TNF-alpha induces the activation of NF-kappaB and the transcription repressor YY1, both of which negatively regulate Fas expression and sensitivity to FasL-induced apoptosis. This hypothesis was tested in PC-3 prostate cancer cells which synthesize and secrete TNF-alpha and express constitutively active NF-kappaB and YY1. Treatment of PC-3 cells with TNF-alpha (10 units) resulted in increased NF-kappaB and YY1 DNA-binding activity, upregulation of YY1 expression, downregulation of surface and total Fas expression and enhanced resistance of PC-3 to apoptosis induced by the FasL agonist antibody CH-11. In contrast, blocking the binding of secreted TNF-alpha on PC-3 cells with soluble recombinant sTNF-RI resulted in significant inhibition of constitutive NF-kappaB and YY1 DNA-binding activity, downregulation of YY1 expression, upregulation of Fas expression and sensitization of tumor cells to CH-11-induced apoptosis. The regulation of YY1 expression and activity by NF-kappaB was demonstrated by the use of the NF-kappaB inhibitor Bay 11-7085 and by the use of a GFP reporter system whereby deletion of the YY1-tandem binding site in the promoter significantly enhanced GFP expression. The direct role of YY1 expression in the regulation of PC-3 resistance to CH-11-induced apoptosis was shown in cells transfected with siRNA YY1 whereby such cells exhibited upregulation of Fas expression and were sensitized to CH-11-induced apoptosis. These findings demonstrate that the TNF-alpha autocrine-paracrine loop is involved in the constitutive activation of the transcription factors NF-kappaB and YY1 in the tumor cells and this loop leads to inhibition of Fas expression and resistance to FasL-induced apoptosis. Further, these findings identify new targets such as TNF-alpha, NF-kappaB and YY1, whose inhibition can reverse tumor cell resistance to FasL-mediated apoptosis.

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