FLCs (κ2 and λ3) induce Src kinase-dependent serine and tyrosine phosphorylation of IκBα in renal epithelial cells. Renal epithelial cells were incubated with 2 different FLCs (κ2 and λ3) and vehicle. At different time points (0, 2, 4, and 24 hours), the experiments were stopped and cell lysates were produced to immunoprecipitate IκBα. Incubation of cells with the FLCs κ2 and λ3, but not the vehicle, promoted a > 2-fold increase in serine phosphorylation of IκBα (top panels), indicating activation of the canonical NF-κB pathway. This effect, which was observed as early as after the first 2 hours of incubation, was inhibited by the Src kinase inhibitor PP2. Both FLCs, but not the vehicle, generated a 4-fold increase in tyrosine phosphorylation of IκBα (middle panels); this was also inhibited by PP2. The bottom panels demonstrate the presence of IκBα in all of the samples studied. The other major band in the gels represents the heavy-chain component of the antibody used to precipitate the protein complexes.