Sustained NF-kappa B Activation Produces a Short-Term Cell Proliferation Block in Conjunction With Repressing Effectors of Cell Cycle Progression Controlled by E2F or FoxM1

NF-kappa B transcription factors induce a host of genes involved in pro-inflammatory/stress-like responses; but the collateral effects and consequences of sustained NF-kappa B activation on other cellular gene expression programming remain less well understood. Here enforced expression of a constitutively active IKK beta T-loop mutant (IKK beta ca) drove murine fibroblasts into transient growth arrest that Subsided within 2-3 weeks of continuous culture. Proliferation arrest was associated with a GI/S phase block in immortalized and primary early passage MEFs. Molecular analysis in immortalized MEFs revealed that inhibition of cell proliferation in the initial 1-2 weeks after their IKK beta ca retroviral infection was linked to the transient, concerted repression of essential cell cycle effectors that are known targets of either E2F or FoxMI. Co-expression of a phosphorylation resistant I kappa B alpha super repressor and IKK beta ca abrogated growth arrest and cell cycle effector repression, thereby linking IKK beta ca's effects to canonical NF-kappa B activation. Transient growth arrest of IKK beta ca cells was associated with enhanced p21 (cyclin-dependent kinase inhibitor IA) protein expression, due in part to transcriptional activation by NF-kappa B and also likely due to strong repression of Skp2 and Csk1, both of which are FoxMI direct targets mediating proteasomal dependent p21 turnover. Ablation of p21 in immortalized MEFs reduced their IKK beta ca mediated growth suppression. Moreover, trichostatin A inhibition of HDACs alleviated the repression of E2F and FoxMI targets induced by IKK beta ca, Suggesting chromatin mediated gene silencing in IKK beta ca's short term repressive effects on E2F and FoxMI car-get gene expression.