IKK-ε directly phosphorylated FOXO3, while IKK-ε-KA had no effect (Fig. 2D). IKK-ε frequently induces multiple phosphorylations, such as at the C-terminus of IRF-3 protein []. IKK-ε IWR-1 in vitro phosphorylates serine and threonine residues of FOXO3 as indicated by immunostaining with pan-phospho-serine or pan-phospho-threonine antibodies that correspond to the top band of the HA-stained panel as indicated by the asterisk (Fig. 2E). Surprisingly, we failed to detect IKK-β-induced
FOXO3 phosphorylation using the same phospho-serine antibodies (Fig. 2E), suggesting that FOXO3 is phosphorylated more efficiently by IKK-ε, possibly at multiple serine/threonine residues, and independently of the described AKT and IKK-β phosphorylation sites (Supporting Information Fig. 2C). Further analysis is needed to formally identify residues targeted by IKK-ε. Finally, as the data indicates that IKK-ε induces lower levels of FOXO3 in ABT 263 both the nuclear and
cytoplasmic fraction, unlike IKK-β (Fig. 1B), consistent with the lower level observed in co-expression experiments (Fig. 2A, 2E, Supporting Information Fig. 2A.), we then tested if IKK-ε induces FOXO3a degradation. HA-FOXO3 was expressed in the 293-TLR4 cells together with FLAG-IKK-ε or FLAG-IKK-ε-KA in presence of cycloheximide (CHX), a protein synthesis inhibitor, and the protein stability was monitored by WB. We observed that in the IKK-ε expressing cells FOXO3, and especially its highly phosphorylated forms, decreased more quickly than in IKK-ε-KA expressing cells, suggesting that IKK-ε triggers FOXO3 degradation (Supporting Information Fig. 3A). In addition, this mechanism seems to be proteasome dependent as the treatment with the proteasome inhibitor MG-132 increased protein stability (Supporting Information Fig. 3B). Together, our data point towards
IKK-ε as a regulator of FOXO3 activity, nuclear localization, and stability. To understand the functional consequences of FOXO3 inactivation by IKK-ε, we assessed the role of FOXO3 in regulation Sirolimus mw of IKK-ε-dependent genes, such as type I IFNs, during immune response to microbial stimuli. We examined the effect of FOXO3 expression on the transcriptional activity of IFN genes in response to TLR4 stimulation. IFN-β is the only type I IFN expressed in human MDDCs stimulated with LPS []. Co-expression of FOXO3 together with the luciferase-reporter construct driven by the IFN-β promoter in 293-TLR4 cells blocked its LPS-induced transcriptional activity (Fig. 3A). Similar results were obtained for the luciferase-reporter construct driven by the promoter of IFN-λ1, type III IFN which is co-ordinately expressed with IFN-β in MDDCs in response to TLR4 stimulation [] (Supporting Information Fig. 4).