TNFR1-induced activation of the classical NF-κB pathway

TNFR1 and TNFR2 regulate the extrinsic apoptotic pathway in myeloma cells by multiple mechanisms

The huge majority of myeloma cell lines express TNFR2 while a substantial subset of them failed to show TNFR1 expression. Stimulation of TNFR1 in the TNFR1-expressing subset of MM cell lines had no or only a very mild effect on cellular viability. Surprisingly, however, TNF stimulation enhanced cell death induction by CD95L and attenuated the apoptotic effect of TRAIL. The contrasting regulation of TRAIL- and CD95L-induced cell death by TNF could be traced back to the concomitant NFκB-mediated upregulation of CD95 and the antiapoptotic FLIP protein. It appeared that CD95 induction, due to its strength, overcompensated a rather moderate upregulation of FLIP so that the net effect of TNF-induced NFκB activation in the context of CD95 signaling is pro-apoptotic. TRAIL-induced cell death, however, was antagonized in response to TNF because in this context only the induction of FLIP is relevant. Stimulation of TNFR2 in myeloma cells leads to TRAF2 depletion. In line with this, we observed cell death induction in TNFR1-TNFR2-costimulated JJN3 cells. Our studies revealed that the TNF-TNF receptor system adjusts the responsiveness of the extrinsic apoptotic pathway in myeloma cells by multiple mechanisms that generate a highly context-dependent net effect on myeloma cell survival.

Transforming Growth Factor-β1 (TGF-β1) Driven Epithelial to Mesenchymal Transition (EMT) is Accentuated by Tumour Necrosis Factor α (TNFα) via Crosstalk Between the SMAD and NF-κB Pathways

Epithelial to mesenchymal transition (EMT) is a process by which an epithelial cell alters its phenotype to that of a mesenchymal cell and plays a critical role in embryonic development, tumour invasion and metastasis and tissue fibrosis. Transforming growth factor-β1 (TGF-β1) continues to be regarded as the key growth factor involved in driving EMT however recently tumour necrosis factor α (TNFα) has been demonstrated to accentuate TGF-β1 driven EMT. In this study we investigate how various signalling pathways contribute to this accentuated effect. A549 cells were treated with TGF-β1 (10 ng/ml), TNFα (20 ng/ml) or a combination of both for 72 h and EMT assessed. The effect of selective inhibition of the SMAD, MAPK and NF-κB pathways on EMT was assessed. A549 cells treated with TGF-β1 downregulate the expression of epithelial markers, increase the expression of mesenchymal markers, secrete matrix-metalloproteinases and become invasive. Significantly, TGF-β1 driven EMT is accentuated by co-treatment with TNFα. SMAD 3 inhibition attenuated TGF-β1 driven EMT but has no effect on the accentuation effect of TNFα. However, inhibiting IKKβ blocked both TGF-β1 driven EMT and the accentuating action of TNFα. Inhibiting p38 and ERK signalling had no effect on EMT. TNFα accentuates TGF-β1 driven EMT in A549 cells via a SMAD 2/3 independent mechanism involving the NF-κB pathway independent of p38 and ERK 1/2 activation.

Tumor necrosis factor receptor cross-talk

Extensive research has been performed to unravel the mechanistic signaling pathways mediated by tumor necrosis factor receptor 1 (TNFR1), by contrast there is limited knowledge on cellular signaling upon activation of TNFR2. Recently published data have revealed that these two receptors not only function independently, but also can influence each other via cross-talk between the different signaling pathways initiated by TNFR1 and TNFR2 stimulation. Furthermore, the complexity of this cross-talk is also dependent on the different signaling kinetics between TNFR1 and TNFR2, by which a delicate balance between cell survival and apoptosis can be maintained. Some known signaling factors and the kinetics that are involved in the receptor cross-talk between TNFR1 and TNFR2 are the topic of this review.