Phospholipase Cε Activates Nuclear Factor-κB Signaling by Causing Cytoplasmic Localization of Ribosomal S6 Kinase and Facilitating Its Phosphorylation of Inhibitor κB in Colon Epithelial Cells

Phospholipase C epsilon (PLC epsilon), an effector of Ras and Rap small GTPases, plays a crucial role in inflammation by augmenting proinflammatory cytokine expression. This proinflammatory function of PLC epsilon is implicated in its facilitative role in tumor promotion and progression during skin and colorectal carcinogenesis, although their direct link remains to be established. Moreover, the molecular mechanism underlying these functions of PLC epsilon remains unknown except that PKD works downstream of PLC epsilon. Here we show by employing the colitis-induced colorectal carcinogenesis model, where Apc(Min/+) mice are administered with dextran sulfate sodium, that PLC epsilon knock-out alleviates the colitis and suppresses the following tumorigenesis concomitant with marked attenuation of proinflammatory cytokine expression. In human colon epithelial Caco2 cells, TNF-alpha induces sustained expression of proinflammatory molecules and sustained activation of nuclear factor-kappa B (NF-kappa B) and PKD, the late phases of which are suppressed by not only siRNA-mediated PLC epsilon knockdown but also treatment with a lysophosphatidic acid (LPA) receptor antagonist. Also, LPA stimulation induces these events in an early time course, suggesting that LPA mediates TNF-alpha signaling in an autocrine manner. Moreover, PLC epsilon knockdown results in inhibition of phosphorylation of I kappa B by ribosomal S6 kinase (RSK) but not by I kappa B kinases. Subcellular fractionation suggests that enhanced phosphorylation of a scaffolding protein, PEA15 (phosphoprotein enriched in astrocytes 15), downstream of the PLC epsilon-PKD axis causes sustained cytoplasmic localization of phosphorylated RSK, thereby facilitating I kappa B phosphorylation in the cytoplasm. These results suggest the crucial role of the TNF-alpha-LPA-LPA receptor-PLC epsilon-PKD-PEA15-RSK-I kappa B-NF-kappa B pathway in facilitating inflammation and inflammation-associated carcinogenesis in the colon.