Secretion of cytokines and growth factors as a general cause of constitutive NFκB activation in cancer

Abstract

The constitutive activation of nuclear factor B (NFB) helps a variety of tumors to resist apoptosis and desensitizes them to chemotherapy, but the causes are still largely unknown. We have analysed this phenomenon in eight mutant cell lines derived from human 293 cells, selected for NFB-dependent expression of a marker gene, and also in seven tumor-derived cell lines. Conditioned media from all of these cells stimulated the activation of NFB (up to 30-fold) in indicator cells carrying an NFB-responsive reporter. Therefore, secretion of extracellular factors as the cause of constitutive activation seems to be general. The mRNAs encoding several different cytokines and growth factors were greatly overexpressed in the tumor and mutant cells. The pattern of overexpression was distinct in each cell line, indicating that the phenomenon is complex. Two secreted factors whose roles in the constitutive activation of NFB are not well defined were investigated further as pure proteins: transforming growth factor 2 (TGF2) and fibroblast growth factor 5 (FGF5) were both highly expressed in some mutant clones and tumor cell lines, each activated NFB alone, and the combination was synergistic. Our data indicate that a group of different factors, expressed at abnormally high levels, can contribute singly and synergistically to the constitutive activation of NFB in all of the mutant and tumor cell lines we studied. Since several NFB target genes encode secreted proteins that induce NFB, autocrine loops are likely to be ubiquitously important in the constitutive activation of NFB in cancer. We provide the first evidence of the general, complex, and synergistic activation of NFB in tumor and mutant cell lines through the action of secreted factors and suggest that the same explanation is likely for the constitutive activation of NFB in cancers.