The complex formation between pirin and IkappaBalpha supports the prolonged NFκB activation in tumor cells

Abstract

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The constant activation of NFkB makes tumor cells resistant to pro-apoptotic stimulus and hypoxia. The activation of NFkB stimulates the transcription from the IkB alpha (IkBa) gene, and this regulatory loop decreases the NFkB activity after stimulation in the normal cells. This mechanism leads to the fast down regulation of NFkB activity after stimulation by TNF and IL1, and prevents the persistent activation of NFkB, even when the NFkB-activating factors are present in the media. The additional mechanisms should be present in the cells to keep NFkB constantly activated. Many tumor cells have an increased level of NFkB activity without additional stimulation. In our previous experiments, we isolated the protein pirin as a cellular interactor with poliovirus protein 3A. Pirin was described as a protein, which can be found in the complex with transcription factor NF1 and with an ankyrin repeat-containing protein bcl3 from IkB family (Dechend R. et al. 1999). The other member of IkB family is IkBa, an ankyrin repeat-containing cytoplasmic chaperon and a major suppressor of NFkB. We found that IkBa and pirin form a complex in vivo, and are co-localized in the cells. The level of pirin is elevated in tumor cells like HeLa, 293T, and melanomas in comparison with primary fibroblasts, primary epithelial cells, and benign cells from the moles. There is a good correlation between the level of pirin expression and the level of NFkB activity in the cells. The over-expression of pirin in fibroblasts increased NFkB-specific transcription. The depletion of pirin by siRNA-specific mechanism decreases NFkB activity and activates apoptosis in HeLa cells. We hypothesize that pirin may have a function as an IkB -binding protein that supports NFkB activity in tumor cells.