Fractalkine (CX3CL1) stimulated by nuclear factor kappaB (NF-kappaB)-dependent inflammatory signals induces aortic smooth muscle cell proliferation through an autocrine pathway

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Abstract
Fractalkine (also known as CX3CL1), a CX3C chemokine, activates and attracts monocytes/macrophages to the site of injury/inflammation. It binds to CX3C receptor 1 (CX3CR1), a pertussis toxin-sensitive G-protein-coupled receptor. In smooth muscle cells (SMCs), fractalkine is induced by proinflammatory cytokines [tumour necrosis factor-α (TNF-α) and interferon-γ (IFN-γ)], which may mediate monocyte adhesion to SMCs. However, the mechanisms underlying its induction are unknown. In addition, it is unlear whether SMCs express CX3CR1. TNF-α activated nuclear factor κB (NF-κB) and induced fractalkine and CX3CR1 expression in a time-dependent manner in rat aortic SMCs. Transient transfections with dominant-negative (dn) inhibitory κB (IκB)-α, dnIκB-β, dnIκB kinase (IKK)-γ, kinase-dead (kd) NF-κB-inducing kinase (NIK) and kdIKK-β, or pretreatment with wortmannin, Akt inhibitor, pyrrolidinecarbodithioc acid ammonium salt (‘PDTC’) or MG-132, significantly attenuated TNF-α-induced fractalkine and CX3CR1 expression. Furthermore, expression of dn TNF-α-receptor-associated factor 2 (TRAF2), but not dnTRAF6, inhibited TNF-α signal transduction. Pretreatment with pertussis toxin or neutralizing anti-CX3CR1 antibodies attenuated TNF-α-induced fractalkine expression, indicating that fractalkine autoregulation plays a role in TNF-α-induced sustained fractalkine expression. Fractalkine induced its own expression, via pertussis toxin-sensitive G-proteins, phosphoinositide 3-kinase (PI 3-kinase), phosphoinositide-dependent kinase 1 (PDK1), Akt, NIK, IKK and NF-κB activation, and induced SMC cell–cell adhesion and cellular proliferation. Taken together, our results demonstrate that TNF-α induces the expression of fractalkine and CX3CR1 in rat aortic SMCs and that this induction is mediated by NF-κB activation. We also show that fractalkine induces its own expression, which is mediated by the PI 3-kinase/PDK1/Akt/NIK/IKK/NF-κB signalling pathway. More importantly, fractalkine increased cell–cell adhesion and aortic SMC proliferation, indicating a role in initiation and progression of atherosclerotic vascular disease.