PLCε cooperates with the NF-κB pathway to augment TNFα-stimulated CCL2/MCP1 expression in human keratinocyte

Phospholipase Cε (PLCε) is a unique class of PLC regulated by both Ras family small GTPases and heterotrimeric G proteins. We previously showed by using mice bearing its null or transgenic allele that PLCε plays a crucial role in various forms of skin inflammation through upregulation of proinflammatory cytokine production from keratinocytes. However, molecular mechanisms how PLCε augments cytokine production were largely unknown. We show here using cultured human keratinocyte PHK16-0b cells that induction of the expression of chemokine (C-C motif) ligand 2 (CCL2) following stimulation with tumor necrosis factor (TNF)α, which primarily depends on the activation of the NF-κB pathway, is abrogated by small interfering RNA-mediated knockdown of PLCε. Enforced expression of PLCε causes substantial CCL2 expression and cooperates with low level TNFα stimulation to induce marked overexpression of CCL2, both of which are only partially blocked by pharmacological inhibition of the NF-κB signaling. However, PLCε knockdown exhibits no effect on both the NF-κB-cis-element-mediated transcription per se and the post-translational modifications of NF-κB implicated in transcriptional regulation, suggesting that PLCε constitutes a yet unknown signaling pathway distinct from the NF-κB pathway. This pathway can cooperate with the NF-κB pathway to achieve a synergistic TNFα-stimulated CCL2 induction in keratinocytes.