Melanoma Differentiation-Associated Gene-7/IL-24 Gene Enhances NF-κB Activation and Suppresses Apoptosis Induced by TNF

Melanoma differentiation-associated gene-7 (mda-7), also referred to as IL-24, is a novel growth regulatory cytokine that has been shown to regulate the immune system by inducing the expression of inflammatory cytokines, such as TNF, IL-1, and IL-6. Whether the induction of these cytokines by MDA-7 is mediated through activation of NF-kappaB or whether it regulates cytokine signaling is not known. In the present report we investigated the effect of MDA-7 on NF-kappaB activation and on TNF-induced NF-kappaB activation and apoptosis in human embryonic kidney 293 cells. Stable or transient transfection with mda-7 into 293 cells failed to activate NF-kappaB. However, TNF-induced NF-kappaB activation was significantly enhanced in mda-7-transfected cells, as indicated by DNA binding, p65 translocation, and NF-kappaB-dependent reporter gene expression. Mda-7 transfection also potentiated NF-kappaB reporter activation induced by TNF receptor-associated death domain and TNF receptor-associated factor-2. Cytoplasmic MDA-7 with deleted signal sequence was as effective as full-length MDA-7 in potentiating TNF-induced NF-kappaB reporter activity. Secretion of MDA-7 was not required for the potentiation of TNF-induced NF-kappaB activation. TNF-induced expression of the NF-kappaB-regulated gene products cyclin D1 and cyclooxygenase-2, were significantly up-regulated by stable expression of MDA-7. Furthermore, MDA-7 expression abolished TNF-induced apoptosis, and suppression of NF-kappaB by IkappaBalpha kinase inhibitors enhanced apoptosis. Overall, our results indicate that stable or transient MDA-7 expression alone does not substantially activate NF-kappaB, but potentiates TNF-induced NF-kappaB activation and NF-kappaB-regulated gene expression. Potentiation of NF-kappaB survival signaling by MDA-7 inhibits TNF-mediated apoptosis.