Another possibility is that Yersinia interacts with lipid rafts containing c-KIT in the plasma membranes of host cells during the infection process [46, 47]. Activation of receptor tyrosine kinases by bacterial LPS has been reported previously. For example, EGFR transactivation by LPS was induced by p38 and matrix metalloproteases upon TLR4-LPS interaction

and was essential for COX-2 gene expression [48]. Increased phosphorylation of EGFR was observed 5–60 min of treatment with purified LPS. In the search for host factors whose functions are required by pathogenic Yersinia to suppress the host innate immune response, we identified additional genes that belong to common GDC 0449 functional networks. For example, the SGK and WNK families directly regulate each other to control osmotic stress and cellular ion balance. During Yersinia infection, the

needle-like T3SS injects effector proteins into the host, increasing membrane permeability and introducing osmotic stress to the host [49]. Osmotic stress caused by ion IWP-2 in vitro imbalance can activate SGK1/WNK1 function and modulate downstream MAPK-ERK signaling pathways [50, 51], thus potentially providing Yersinia with another signaling pathway to manipulate gene expression. WNK1 is a substrate of SGK1 during insulin activation of PI3K [52] and can activate SGK1 during ENaC regulation [53]. WNK1 also participates in an epidermal growth factor receptor (EGFR)-ERK pathway that includes two signaling molecules, MAP3K3 and MEK1/2, which were also identified as hits from our RNAi screen (Figure 8). A direct protein-protein interaction between Phospholipase D1 WNK1 and MAP3K3 has been previously demonstrated [54]. MAP3K3 regulates ERK signaling through MEK1/2 and is required for NF-κB activation [55–57]. The Yersinia effector YopJ has been reported to catalyze the acetylation of target kinases to inhibit MEK and NF-κB signaling [9, 10]. Similar to c-KIT inactivation, downregulation of WNK1 and MAP3K3 may shunt the activation of transcription

factors that regulate inflammatory cytokine release to an alternative signaling pathway. Several of the RNAi screen hits that impact signal transduction can be directly linked to regulation of NF-κB signaling. For example, the catalytic α subunit of CKII was found to phosphorylate IKKα with high specificity and to stabilize targeting of IκB for proteosomal degradation in response to such cell stressors as UV radiation and TNF-α [58–60]. NIK/MAP3K14 regulates the alternative NF-κB signaling pathway [61]. PIK3R2, a regulatory subunit of PI3K, functions in AKT activation, which leads to phosphorylation of p50 or activation of IKKα through multiple signaling pathways [61]. Conclusions Collectively, our studies have identified multiple host kinases, that when downregulated, mitigated Yersinia-mediated suppression of the host primary immune response.