Summary
To study the role of c-Src kinase in prooxidant-induced stimulation of TLR4, we used LPS-EK and MPLA as TLR4 specific agonists and positive controls, and SIN-1 and PPC as prooxidant sources. We used HEK-Blue mTLR4 cell line that is stably transfected with mouse TLR4 and that expresses optimized SEAP reporter under the control of a promoter inducible by NF-κB transcription factor. The level of SEAP released due to TLR4 stimulation was a measure of NF-κB activation. Treatment with either the prooxidants or LPS-EK increased SEAP release and TNF-α production in these cells. These treatments also increased intracellular ROS accumulation with an enhanced production of nitric oxide and TBARS to confirm oxidant stress in these cells. Pretreatment with c-Src kinase inhibitors, PP2 and CA-pY, which act by different mechanisms, decreased these parameters. Pretreatment with SSG, a c-Src activator, enhanced the effects promoted by LPS-EK and prooxidants, and rescued cells from PP2- and Ca-pY-induced effects. Curiously, prooxidants but not TLR4 agonist increased the ratio of TNFα to IL-10 released suggesting that prooxidants can initiate and maintain an imbalance of TNFα production over IL-10. To different degrees, both prooxidant and TLR4 agonist increased formation of c-Src complexes with TLR4 and IκB-α as coimmunoprecipitates. Both prooxidant and TLR4 agonist increased c-Src phosphorylation of Tyr-42 residue in IκB-α, but prooxidant-induced effect was more robust and much longer lasting. Taken together, these studies provide a mechanism whereby c-Src assumes a central role in prooxidant-induced NF-κB activation in TLR4 signaling. Prooxidant-induced activation of TLR4 through c-Src/NFκB/IκB-α coupling provides a basis for a molecular dissection of the initiation and maintenance of sterile inflammation that may serve as a “pathophysiologic primer” for many diseases.