A cell-based assay for IκBα stabilization using a two-color dual luciferase-based sensor

A cell-sensor assay for stabilization of I kappa B alpha was developed in the activated B cell-like diffuse large B-cell lymphoma cell line OCI-Ly3. This cell line expresses known nuclear factor kappa B (NF kappa B) target genes due to high constitutive activity of I kappa B kinase (IKK), which phosphorylates the protein I kappa B alpha leading to proteasomal degradation of I kappa B alpha and activation of NF kappa B. The cell-sensor assay uses green and red light-emitting beetle luciferases, with the green luciferase fused to I kappa B alpha (I kappa B alpha-CBG68) and the red luciferase (CBR) present in its native state. The I kappa B alpha-CBG68 reporter functions as a sensor of IKK and proteasome activity, while CBR serves to normalize for cell number and nonspecific effects. Both reporter constructs were stably integrated and placed under the control of an inducible promoter system, which increased fold responsiveness to inhibitors when assay incubations were performed simultaneous to reporter induction by doxycycline. The assay was miniaturized to a 1,536-well plate format and showed a Z' of 0.6; it was then used to panel 2,677 bioactive compounds by a concentration-response-based screening strategy. The concentration-effect curves for the I kappa B alpha-CBG68 and CBR signals were then used to identify specific stabilizers of I kappa B alpha, such as IKK inhibitors or proteasome inhibitors, which increased the doxycycline-induced rise in I kappa B alpha-CBG68 without affecting the rise in CBR. Known and unexpected inhibitors of NF kappa B signaling were identified from the bioactive collection. We describe here the development and performance of this assay, and discuss the merits of its specific features.