Targeting the NF-kappa B 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 Gö6976, 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-kappaB, 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-kappaB inhibition in MDA-MB-231 breast cancer cells using small inhibitory RNAs (siRNA). siRNAs designed against NF-kappaB protein p65 (RelA) and IKKalpha, IKKbeta, and IKKgamma, strongly decreased the target proteins. But, unlike Gö6976, they did not decrease basal NF-kappaB or cause apoptosis. In particular, the decrease in p65 protein had no effects on apoptosis or cell proliferation, thus questioning the importance of NF-kappaB alone in the maintenance of these cells. Furthermore, the proteasome inhibitor MG-132 caused loss of IkappaBalpha, and an increase of it is phosphorylated form, but basal NF-kappaB was unchanged, whilst activation of NF-kappaB by TNFalpha was completely inhibited, suggesting that MG-132 activity is independent of constitutive NF-kappaB 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-kappaB and activating other basal pathways that are blocked by Gö6976 but not by IKK and p65 siRNAs.