Targeting the NF-κB pathway in estrogen receptor negative MDA-MB-231 breast cancer cells using small inhibitory RNAs

Cancer cells in order to survive are often mutated to block apoptosis. One chemotherapeutic option is the re-establishment of apoptosis. An example of such a therapy is the PKC inhibitor Go6976, which activates apoptosis and shrinks in vivo tumors in estrogen receptor-negative breast cancers. We proposed as a mechanism blockage of activation of the transcription factor NF-kappa B, which is anti-apoptotic and often elevated in cancers. Over recent years, questions have arisen regarding the specificity of these "small-molecule inhibitors." We have therefore explored the role of NF-kappa B inhibition in MDA-MB-231 breast cancer cells using small inhibitory RNAs (siRNA). siRNAs designed against NF-kappa B protein p65 (RelA) and IKK alpha, IKK beta, and IKK gamma, strongly decreased the target proteins. But, unlike Go6976, they did not decrease basal NF-kappa B or cause apoptosis. In particular, the decrease in p65 protein had no effects on apoptosis or cell proliferation, thus questioning the importance of NF-kappa B alone in the maintenance of these cells. Furthermore, the proteasome inhibitor MG-132 caused loss of I kappa B alpha, and an increase of it is phosphorylated form, but basal NF-kappa B was unchanged, whilst activation of NF-kappa B by TNF alpha was completely inhibited, suggesting that MG-132 activity is independent of constitutive NF-kappa B activation. We ascribe these differences to the specificity of inhibition by siRNAs as compared to the well-known non-specificity of small-molecule inhibitors. We conclude that the mutations in these cancer cells made them resistant to apoptosis, by elevating their NF-kappa B and activating other basal pathways that are blocked by Go6976 but not by IKK and p65 siRNAs.